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Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers

An in-depth comparison of the properties of wool and synthetic fabric for base layers yielded some surprising results.

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by | 2006-07-25 00:00:00-06

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 1
Alan backpacking in a clown shirt on a sweltering day in the Appalachians. Backpacking Light manufactured three hybrid testing tops that were half wool and half synthetic. Hybrid shirts were sent to staffers Alan Dixon, Will Rietveld, and Don Wilson. Each of us used a wool-synthetic top for several months of intensive field testing over the winter and spring. Early on in the testing process, Will dubbed these “clown shirts.”


Fifty years ago nearly every outdoor kit included a wool shirt. Ten years ago, wool had been banished in favor of lighter and more comfortable synthetic base layers. But wool is back. With finer yarns, wool base layers are now marketed as comfortable next to skin. At the same time advances have been made in synthetic fabrics that further improve performance and address some of their shortfalls. This healthy competition has saturated the industry with new technologies and performance claims regarding wool and synthetic base layers.

In this article we will take an independent look at the properties of these fabrics. We ran a series of field and laboratory tests to evaluate field comfort, drying times, fabric weights and moisture wicking performance. In the process, we revealed some unexpected performance properties and learned a few things about how these fabrics compare in a diverse range of field conditions.

The Purpose of a Base Layer

Base layers have two fundamental objectives - temperature control and moisture control. The two are closely interrelated, but for purposes of this article we will equate temperature control with insulation. All base layers perform this task reasonably well, and relatively little separates the wool and synthetic fabrics.

By its very definition a base layer is not designed to be your primary protection against wind, rain or intense cold. When conditions go bad, you should pull your outer layers out of your pack. But under many conditions it may be that your base layer is your only top garment for the vast majority of your hiking day - providing sun protection, some warmth, moisture management, and a bit of modesty. This is both a weight saving (potentially fewer garments) and a time saving (less clothing adjustments/changes). Since you could potentially wear this garment 24 /7 its comfort is a significant contributor to your overall comfort and well being on a trip.

Moisture control is where a base layer can really make a difference in maintaining comfort in the field. A base layer should help keep you warm when it is cool, and cool when it is warm. As your next to skin layer, it is primarily responsible for keeping built up moisture away from your skin. Base layers use a variety of techniques and fabric properties to help you stay dry and comfortable. They will wick water away from your skin. They will disperse moisture across a wider area to facilitate drying. They will store water and keep you cool with evaporative cooling. When soaked, a base layer should dry quickly.

The series of tests we describe here were designed to help us compare the relative performance of a sample of commonly used wool and synthetic base layers. But before we jump into the test results, let’s review the structure and properties of wool and synthetic fabrics.

Wool Base Layer Fabrics

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 2
Photo 2. Wool and synthetic fibers: standard wool (top), merino wool (center) and synthetic polyester fiber (bottom). Photo courtesy of Ibex.

Wool base layers have made a significant splash over the past few years primarily as a result of the improved comfort level of wool as a next to skin fabric. New softer wool fabrics have been developed and a growing number of manufacturers are now producing wool base layers. The critical change has been the use of Merino wool in modern outerwear. Merino wool has a softer feel than traditional wool, with fiber diameters of less than 20 microns (see Photo 2). Traditional wools are typically at least 32 microns, and frequently up to 80 or more microns. This explains why old wool fabrics felt like Brillo Pads against your skin. The thinner fibers of Merino wool are more flexible, hence softer and more comfortable against your skin. A few manufacturers are now producing merino wool fabric with fibers less than 18 microns in diameter, and efforts continue to produce affordable base layers with ever more comfortable fabrics.

To understand the properties of Merino wool fabrics you must first understand the structure and properties of wool fibers. Unlike synthetics, which have a uniform composition inside and out (i.e. it’s a hunk of plastic that may be shaped), individual wool fibers have an exterior scaly sheath, enclosing bundles of interior fibers (see photo 3). The surface of each fiber is hydrophobic (repels moisture), helping to repel water off the surface of the fabric.

Because of the complex internal structure of wool fibers, and the fact that the interior portion of the fiber is hydrophilic (attracts moisture), water can be absorbed INTO the fibers of a wool fabric, whereas in synthetic fabrics water is only held in the spaces between individual fibers since the fibers themselves do not absorb water. Merino wool fibers can absorb approximately 30 percent of their weight in water. Synthetic fibers themselves will not absorb any water. This means that wool fabrics will hold more moisture than synthetics and will take longer to dry. But in a wool fabric the water is stored away from the surface, and consequently a wool garment may not feel so clammy next to your skin when wet.

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 3
Photo 3. The internal structure of a single wool fiber is a complex matrix consisting of numerous bundles of sub-fibers. Water can be absorbed into this internal matrix and then released again. Image courtesy of Ibex.

This ability to store moisture within the structure of the fabric gives wool some of its most useful properties. As your body warms up, the moisture stored within the fabric will begin to evaporate, cooling the air between your skin and the fabric. This same cooling effect will also hold for synthetic fabrics, but the additional moisture held within the wool fibers will prolong the cooling effect. So long as the wool is not completely saturated, its surface will stay dryer than a slightly wet synthetic layer, and it will not feel cold and clammy against your skin.

An interesting part of the wool story is that as wool absorbs water, the internal structure of the wool changes, and the fabric actually releases heat in a process known as the “heat of sorption”. Although the total energy involved in this process is small in comparison to other processes such as evaporation, this heat of sorption may help to raise your skin temperature and make you feel warmer. Depending on the conditions this may or may not be beneficial. In cold weather where you may have built up a little sweat from skiing or snowshoeing, this reaction could help keep your skin warmer. Note: There is still debate about the significance of this phenomenon. The discernible effect, if any, is mostly the comfort of the fabric against the skin. The actual heat generated is too small to have a significant warming effect for your body.

Of course, wool clothing is also well known for resisting the build-up of body odor that is prevalent in many synthetic base layers. Wool is naturally resistive to the bacterial growth that can make some synthetic clothes bring tears to your eyes after a few days or weeks in the backcountry. Synthetics claim to be closing the gap with new treatments to reduce stench. While synthetics have improved their resistance to odor build-up, the consensus among the Backpacking Light staff is that wool is still ahead by a significant margin. A wool trail shirt may be your only civilized option for sitting in a restaurant after the trip, assuming you don’t have a chance for a shower and change of clothes before you dig into that hot meal.

For many people another positive for wool is that it’s a natural, renewable resource. Merino sheep are sheared once per year, and wool fibers will naturally break down over time. Wool easily absorbs natural dyes and can be processed with limited use of harmful or toxic chemicals. Some manufacturers are now processing wool with minimal chemical processes.

Synthetic Base Layer Fabrics

Many of the original synthetic base layers were polypropylene, but polypropylene has a very low melting point and requires a bit of care while washing. The melting point of polyester (260 to 270 degrees C) is much higher than polypropylene (165 degrees C) and therefore requires much less care laundering, especially in a hot drier. Note: wool too has improved. The newest merino wools require much less care than their predecessors and can be put in the drier without shrinking. Polypro also had a reputation for body odor build up. Modern polypro has improved upon both care and odor problems but its reputation persists. For example, many polypro laundering instructions still call for a cold wash and drip dry. Many synthetic base layers, including most of the high-end ones, use polyester fabrics for their ease of care and improved odor control. Other problems with polypropylene include poor abrasion resistance and poor "memory” (shape retention). The advantage to polypro is that it is a lighter fiber than polyester so you could potentially make a 40% lighter garment. (Note: The density of polypropylene (0.91g/cc) is much lower than that of polyester (1.38 g/cc). Polypropylene floats in water while polyester sinks.) Another advantage of polypro is that it absorbs almost no moisture. Finally polypro is typically a less expensive fabric. Thus polypro is still used as a base layer fabric, but primarily in lower cost garments.

Woven - as opposed to a knit fabric as is typically used in base layers - nylon fabric is often used in warmer conditions for sun blocking, but few base layer (i.e. wicking) garments are made of nylon. Since most high-end base layers use polyester we will focus on polyester base layers in this article.

The key advantages of synthetics over wool are that they are lighter, dry faster and are considerably less expensive. As we will show in the tests below, the lightest synthetic base layers are still significantly lighter than the finest wool. Because the individual fibers do not absorb water, synthetics will remain much lighter than wool when both are saturated. And the synthetics will dry more quickly too. Last but not least, synthetics are much less expensive - typically 30 to 50 percent less than wool products with similar function.

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 4
Photo 4. Two examples of polyester synthetic fibers with added surface complexity. Textured surfaces improve moisture movement and drying times; and consequently reduce odor buildup. GoLite’s C-Thru fabric (left) and MontBell’s Wickron fabric (right). Images courtesy of GoLite and MontBell.

Synthetic fibers have made numerous advances over the years, many of them mimicking the best properties of wool, but still maintaining the benefits of synthetics. Paramount among these are advances that help synthetics to move moisture more quickly across the surface of individual fibers, helping to disperse moisture, improve drying time, and keep moisture away from your skin. This is typically done by adding texture to the surface of the synthetic fibers, or changing the cross-sectional shape of the fibers, or applying a chemical finish. This texture or finish increases surface complexity, making it more hydrophilic. The objective of these shape modifications is to speed movement of water along individual fibers, away from your skin and dispersing the moisture closer to the outer surface where it will evaporate more quickly. Examples of these fibers are Patagonia’s Capilene (treated, roughened surface), Marmot Qwick (hydrophilic chemical treatment) and GoLite C-Thru (modified fiber shape).

Many synthetic garment manufacturers have taken steps to reduce the build up of body odor in their clothing. This odor problem is especially prevalent among synthetic base layers which are next to your skin and kept warm and moist as you wear them - perfect conditions to grow a festering pool of stinky bacteria. An ultralight backpacker might wear the same base layer for a week, significantly increasing bacterial breeding opportunities and stench. Wool is inherently anti-bacterial. Its protein based structure (keratin) provides natural protection against odor producing bacteria.

Synthetic garment manufacturers use numerous techniques to reduce bacteria buildup in synthetic base layers. Most of these involve applying anti-bacterial coatings to some or all of the fibers. Many treatments use silver ions which are applied or woven into the fabric. One approach is to impregnate silver ions directly into the yarn. Another approach embeds the silver ions on the surface of the fabric, making more effective use of silver’s anti-microbial properties. An example of a silver embedded fabric is VisaEndurance, by Milliken. VisaEndurance is used by many manufacturers, including Mountain Hardwear in their Extend Featherweight base layers.

Comparative Field Testing of Wool and Synthetic Base Layers

One way to compare the performance of wool and synthetic base layers is to create hybrid garments. Backpacking Light spliced and sewed three hybrid tops that were half wool and half synthetic. We chose Smartwool Microweight for the wool side and Patagonia Lightweight Capilene for the synthetic side. The insulating properties and field use of these fabrics are comparable, which made them a good pair for comparison of wool versus synthetic field performance.

Each tester used a hybrid wool-synthetic top for several months of intensive field testing over the winter and spring. Our goal was to get the top out into a variety of conditions, with a focus on conditions that challenged the performance of the fabrics. That meant using the tops in vigorous aerobic activity and a variety of temperature and weather conditions. We tested the tops in temperatures ranging from well below freezing in the Colorado mountains to 90 degrees in the Sonoran desert spring and 95 degree late spring days in the Northeast. Weather conditions ranged from wet/cold winter days in Colorado, humid spring conditions in the eastern US, and dry spring heat in the Sonoran desert. We all received occasional dubious looks while wearing these shirts, or were teased mercilessly by friends and family. Early on in the testing process, Will dubbed these “clown shirts” - and henceforth we referred to them by that moniker. They were definitely a conversation starter when we met people in the backcountry.

Field Test Conditions

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 5
Don on a winter training run, Saguaro National Park, Arizona.

Don made a habit of wearing his top on all of his trail runs and conditioning hikes during winter and spring. That usually meant a vigorous climb or run of 6 to 12 miles with elevation gains of 2000 to 4000 feet. Most of the time Don overheated while climbing, and the top was important to keeping him dry and cool. A few times conditions were cold enough to keep him cool or cold, but that was the exception in his testing. Don is a longtime user of synthetic tops, only recently spending more time in wool base layers.

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 6
Will in his clown shirt on a conditioning hike in southern Colorado.

Over a four-month period Will put his pride aside in the name of science, and wore the infamous shirt out in public (gasp!) on a total of 31 day trips while snowshoeing, cross-country skiing, and day hiking. Will wore it either as a base layer or an outer layer, depending on the temperatures, which ranged from 30 to 65 °F. On several occasions he pushed hard to build up a good sweat on his favorite conditioning hike - a 1.5 mile 1500 foot 45 minute charge up a local mountain without stopping. (Will says, “Don’t snicker, that’s good for me!”)

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 7
Alan on the spectacular Angel’s Landing trail, Zion National Park, Utah.

Alan tested his shirt in the cool spring conditions of Washington, DC, with temperatures in the 30’s to 50’s °F, and also had it out in the western US for a late spring trip to southern Utah. Finally, Alan tested his top on a muggy 95 °F backpacking trip in the Appalachians. Note that Alan’s top is a short sleeve model, while Will and Don used a complementary pair of long sleeved tops. All three tops were made from the same fabric combination.

Cool Weather Performance

All three of us tested the tops in cool conditions, from below freezing up to about 50 degrees. Alan had intense bicycle and trail running workouts in 30 to 50 degree temperatures with relative humidity around 80 percent. Under these conditions, Alan noted that the Capilene side spread moisture out over a larger area and dried more quickly than the wool side. This held true for all of our testing, in all conditions. But despite absorbing more water than the Capilene side, Alan found that the wool side was warmer and more comfortable on the skin in cool temperatures. The wool side felt especially warm when cooling off after an intense workout while the Capilene side felt decidedly chilly. But the Capilene dried much faster. An hour after one of Alan’s workouts, the Capilene side was dry while the wool side was still damp. It took the wool side approximately another hour to dry.

Don’s cool weather testing was done under drier conditions. On long uphill trail runs in cool weather, the Capilene side stayed drier, but when wet, Don also found the wool side more comfortable. The Capilene side allowed more wind to pass through to the skin, helping Don stay cool even when the climbs got serious. In cool, comfortable, windy conditions, Don found the performance difference negligible, as both sides were dry and comfortable. The more humid conditions of Alan’s testing may help to highlight the differences between the fabrics.

Will’s experience in cool conditions was interesting. His comment on the field performance difference between the fabrics was, “nada!” In the cool conditions of Colorado, he found no significant difference between the two fabrics, despite putting them through a long series of field tests. In Will’s words, “When worn as an outer layer, I built up a good sweat climbing hard, stood in the wind at the top, got chilled on both sides, put a shell on over it, and dried out quickly without further chilling. When worn as a base layer, both fabrics effectively dispersed moisture and dried out fairly quickly. I tried hard to distinguish a difference, but I can’t honestly say I could detect a difference.”

Synthetic (Capilene) Dries Faster

All three testers found that the Capilene fabric was faster to dry. Don soaked his top in a creek and wore it for a downhill hike on a 58 degree morning. Almost immediately the wool side felt warmer. After 5 minutes of hiking, the Capilene side began to be noticeably drier than the wool side. The Capilene side was dry after 20 minutes of hiking, while the wool side reached a similar level of dryness at 35 minutes. Will ran a similar test and found the drying time of the Microweight wool side about 50 percent longer than Capilene [more on Will’s tests below.]. Alan’s field experience for drying times was similar (except he exercised for a long time at an intense level and used his own sweat to soak the shirt.).

Warm Weather Performance

Don wore his top on numerous warm weather trail runs and conditioning hikes. Under all of these conditions, Don detected little difference in the performance of the two fabrics. He was surprised at the comfort level on the wool side in these conditions. Both fabrics were reasonably comfortable for Don, even when exercising in hot weather. (Note: Don is a desert dweller and others may find these fabrics uncomfortably warm under hot weather conditions.)

Alan tested his top on a 30 mile backpacking trip in the Appalachians with muggy weather, 95 degree days and many, many thousands of feet of climbing. While Alan preferred the synthetic side for comfort, it was not by a large margin. The wool side’s lack of stench in the hot weather was a real plus.

Both Don and Alan noted that the synthetic side was a bit cooler due to its lighter and more air permeable fabric. This was especially true under breezy conditions, where the synthetic side was noticeably more ventilated and cool. While Don found both fabrics reasonably comfortable in hot weather stress tests, he would not opt to use a base layer under these conditions. His preference for warm weather hiking is a loose fitting, well ventilated top such as the RailRiders Adventure Top (a woven nylon fabric with mesh side inserts). Don also notes that wool has made good strides in comfort against the skin, but still is “definitely not cotton.” Alan had no issues with wool’s next to skin comfort, even in extremely hot weather.

There is consensus that untreated synthetic garments build up a significant stink, but what about wool? Our testing provided a good opportunity to test both fabrics under identical conditions. Don did not wash his clown shirt all winter and spring. After every sweaty workout he threw it in the drawer with his other base layers. Four months later, Don can conclusively state that the Capilene side does indeed stink. The wool side is nearly stink free with some of the minimal odor possibly from loitering near the Capilene for so long. Alan also did not wash his garment for at least two months with similar results. The wool side was nearly stench free while the Capiline side was not fit to be worn withing 50 feet of civilized company (the neighbor’s Rottweiler thought it was great!). So while synthetics may have improved, wool is still superior to untreated synthetic in fighting off odor buildup. [Will was more hygienic and washed his clown shirt.]

Laboratory Tests of Select Wool and Synthetic Fabrics

To get a more controlled look at the drying rate and fabric weights of wool and synthetic fabrics we ran a soak and dry test on five leading fabrics. These tests were a simple look at drying rates under low humidity conditions and are most useful as a comparative indicator. Field drying times will vary widely based in humidity, temperature, wind, aerobic activity, and other clothing worn. Two synthetic fabrics were tested, GoLite DriMove and Patagonia Lightweight Capilene. The three wool fabrics were Ibex 17.5 micron, Icebreaker Superfine, and Smartwool Microweight.

Table 1. Fabric Specifications
Fabric Fabric type Swatch Weight (g) Fabric Weight g/m2
Patagonia Lightweight Capilene 100% polyester 12 133
GoLite DriMove 100% polyester 10 111
Ibex 17.5 micron Merino wool 14 156
Icebreaker Superfine Merino wool 18 200
Smartwool Microweight Merino wool 14 156
Note: Fabric samples were 30 cm by 30 cm. Due to relatively small swatch sizes and rounding error, weight per meter squared has approximately 10 percent (plus or minus) error.

The test procedure was a simple soak of the fabric swatches in room temperature tap water for 20 minutes. Samples were removed simultaneously and allowed to drip for 1 minute. Each was then weighed every 5 minutes. The test was run outside in the shade with temperatures of approximately 65 degrees F. Relative humidity was approximately 40 percent. Fabric swatches were laid flat on concrete while drying.

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 8
Figure 1. Drying time summary. We verified that synthetics dry faster. The two synthetic fabrics were dry after 65 minutes (Patagonia Lightweight Capilene) or 75 minutes (GoLite DriMove). The wool fabrics took 90 to 100 minutes to reach their dry weight.

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 9
Figure 2. Weight of fabric after soaking. When soaked, synthetic fabrics are lighter than their wool counterparts, even for a very absorbent synthetic like DriMove.

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 10
Figure 3. Percentage of water absorption in comparison to dry weight (X times dry weight) for selected synthetic and wool fabrics. This figure shows that there is not as much difference in percentage of water absorption between synthetics and wool as we initially believed. Taking out the anomaly of the very absorbent DriMove, the values ranged between 3 and 4x for all fabrics. GoLite DriMove, while the lightest fabric, absorbed the greatest percentage of its dry weight in water (10 g to 48 g is 4.8x) but was still the second fastest drying fabric. This underscores the ability of synthetics to dry quickly even with large amounts of water.

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 11
Figure 4. Detailed drying rates (base data) for selected synthetic and wool fabrics.

An Alternate Test - Drying Rate When Worn (Will Rietveld goes a step further)

All of us observed that in the field the synthetic side dried faster. But exactly how much faster? Swatches of fabric can tell us only so much. Obviously, the drying rate of a whole garment on a warm body would be a better indicator of field performance.

Will went a step further to get a more precise measurement of the drying rate of the clown shirts while on the body. He devised a simple but ingenious test to measure the drying rate of both sides of the wet shirt while on his body. Will measured the temperature of a wet clown shirt (on his body) with an infrared thermometer. He did this for both the synthetic and wool side at 5 minute intervals.

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 12
Test procedure: Clown shirt soaked for 4 hours, then run through the spin cycle in a washing machine to remove excess water. Will then put it on the shirt and measured the temperature of the wool and polyester sides with an infrared thermometer at 5 minute intervals. Ambient conditions 70 F and 15-20 percent relative humidity. Results: Synthetic (Caplilene) dries at 30 min. Wool (Smartwool Microweight) dries at 45 min.

Note that the shirt quickly warmed up from Will’s body heat after he put it on. It then stayed around a constant 80 degrees (because of evaporative cooling) for about 20 minutes. Finally, the fabric warmed up to a steady state of 88 °F indicating the fabric had dried (no moisture left for evaporative cooling).

It is always nice when lab tests and field tests agree. Don’s soak the shirt in the field and then wear it, with a subjective assessment of dryness was 20 minutes to dry for the synthetic side and 35 minutes to dry for the wool side. This is within reasonable agreement with Will’s on body tests. Don’s faster drying times are likely due to his exercising in the field (more heat generation and warmer skin temperature versus Will’s at rest skin temperature and heat generation) and that fact that Don was moving and therefore creating some wind aided drying. Don has long legs, is in excellent shape and hikes a good clip. Don’s subjective measurement in dryness versus Will’s temperature based measurement of “dryness” may also account for slight differences in perceived drying time. That is, the shirt may feel dry even though it has residual moisture.

Will also used the same infrared thermometer test method to see how much faster a shirt dried when worn versus on a hanger. Surprisingly, wearing the garment only decreased drying time by 15% for both fabrics (at least at an ambient temperature of 70 °F and low humidity of 15-20%). As temperatures drop and relative humidity increases, we would assume that the difference in drying time would increase between a shirt on a hanger and on a warm body.

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers - 13
Test procedure: Clown shirt soaked for 4 hours, then run through the spin cycle in a washing machine to remove excess water. Shirt then put on a clothes hanger and the temperature of the wool and polyester sides measured with an infrared thermometer at 5 minute intervals. Ambient conditions 70 *F and 15-20 percent relative humidity. Results: Synthetic (Caplilene) dries at 35 min. Wool (Smartwool Microweight) dries at 55 min.

In Will’s whole garment tests and the lab fabric swatch tests, Smartwool took 57% and 53% longer to dry respectively. That is, the percentage difference in drying rate between Capilene and Smartwool was the same even thought they used very different test methods - Will’s a full garment, a washing machine spin cycle which removed more excess water and dried on a hanger (both sides exposed) versus the lab tests with fabric swatches that were not wrung at all but only allowed to drip for 1 minute and laid on concrete (single side exposed).


General Note: Differences between wool and synthetics for both water absorption and drying times were not as earth shattering as we initially thought.

Choices we make for base layers should be based on conditions we hike in, and properties of fabrics. But even under similar conditions personal preference plays a big role. Each of us has different comfort levels for each fabric. Some people can’t wear even the softest wool fabrics without breaking out in a rash, while others cannot tolerate the stench of wearing a synthetic garment for a week, and still others hate the clammy feeling of synthetics. Whatever your personal preferences, we hope the information provided here will help you make more informed choices about your base layer.

Tester’s Personal Conclusions

The following are our personal conclusions from this series of tests and field testing.

  • Wool is significantly better at resisting buildup of body odors than most synthetics - even the improved ones
  • Wool feels warmer and less clammy on the skin when damp
  • Wool takes approximately 50 percent longer to dry than synthetics (range 40 to 60 percent). This was consistent across the board under a large range of conditions - in lab tests of fabric swatches, whole shirts on hangers, shirts on warm bodies, and actual in-field performance of shirts wet with sweat and shirts intentionally saturated with water.
  • In our soak test, the difference in water absorption (x increase over dry weight) between wool and synthetics was less than we initially believed. Approximately 3x dry weight for synthetics versus 4x dry weight for wool.
  • Synthetic fabrics wick moisture out faster over a larger surface area. This may in part contribute to their faster drying times but also to a chilling effect in cool and especially windy conditions.
  • Synthetic fabrics are lighter than wool for comparable warmth.
  • Many synthetic garments are significantly less expensive than wool.

Wool and synthetic base layers preferences of Backpacking Light Staff


  • A wool top serves as a combination shirt and base layer for most trips. Wool’s lack of stench and next to skin comfort (less clammy and feels warmer when wet) are major reasons for his selection. That wool is a minimally processed natural fiber also appeals to him.
  • In torrentially wet environments like the rain forests of the Pacific Northwest or New Zealand’s South Island, the lightest synthetic base layer like Capilene would be the preferred choice. Low water absorption and quick drying times are the major reasons.
  • In serious cold weather mountaineering a synthetic top like a Patagonia R1 hoodie. Warmth, flexibility for varying conditions, minimal water absorption and faster drying are reasons. Even in a cramped mountaineering tent, stench and next to skin feel are distant considerations.
  • On shorter trips in very warm conditions, a thin synthetic base layer may be selected. In desert conditions a thin nylon top like a RailRiders Adventure Top could be the most desirable.


  • In cool or cold, dry weather wool is the preferred choice.
  • About 80 °F synthetic or wool base layers (knit fabrics). Like Alan he is more apt to wear a thin woven nylon shirt (conventional fabric) like a RailRiders Adventure Top.


  • Will did not have a significant preference between wool and synthetic base layers. This may underscore the fact that under many conditions there are only minor performance differences between wool and synthetic base layers. It may also be that Will is an easy going guy with few idiosyncrasies about his trail clothing. Or it may be that Will actually washed his clown shirt a few times during testing. (Will eventually sided with the Smartwool. Not for its performance but for its better fit - longer sleeves and torso length.)


"Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers," by . (ISSN 1537-0364)., 2006-07-25 00:00:00-06.


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Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers
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Ken-Admin Knight
(ken-admin) - F
Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers on 07/25/2006 22:45:11 MDT Print View

Companion forum thread to:

Comfort and Moisture Transport in Lightweight Wool and Synthetic Base Layers

Steven Hanlon
(asciibaron) - F

Locale: Mid Atlantic
wool or synthetic? on 07/26/2006 06:42:43 MDT Print View

not sur what to make of the results. it appears that the increased cost of a wool base layer yields a garment that does not offend the olfactories.

i thought there was going to be a slam dunk result, but it appears that in the end, synthetics have the advantage in high humidity and in warm tempuratures.

with that said, i might get a wool base layer shirt just to avoid smelling like funk. you know it's bad when you can still smell yourself in 10mph winds.


Ken Walsh
(kwbackpack) - F
Color is important!!! on 07/26/2006 13:32:54 MDT Print View

First off, great test. I have one comment, especially about the SmartWool products. They do not make shirts in a light enough color, the albedo of a base layer plays a *huge* role in warm weather comfort. The fact that SmartWool makes a "microweight" crew that only comes in relatively dark colors is really silly. If you are hiking in the sun white, yellow or very light tan should be the only color you wear, but SmartWool refuses to make anything like this. The "clown" shirt at least appears to have realtively similarly toned halves, but it is worth noting that small changes in albedo can have a huge impact it hot weather comfort!

Great job!

Joseph Rothstein
(joe_r) - F
Re: Color is important!!! on 07/26/2006 13:53:28 MDT Print View

The fact that SmartWool makes a "microweight" crew that only comes in relatively dark colors is really silly. If you are hiking in the sun white, yellow or very light tan should be the only color you wear, but SmartWool refuses to make anything like this.

I avoided buying a Smartwoool shirt for a long time for exactly this reason. A few weeks ago I was in a discount store and found a Smartwool Moto Tee in "ash" (very light, off-white color). As soon as I tried it on I realized why Smartwool doesn't make more shirts in very light colors. It was one of the most transparent t-shirts I've ever worn.

I recently ordered the Smartwool Swoop Tee from Campmor ($40 right now) in two reasonably light colors. Both colors are lighter than the Microweight tee colors that I've seen, but there's barely any translucency, especially for the gray one.

Dale Wambaugh
(dwambaugh) - MLife

Locale: Pacific Northwest
Weather? on 07/26/2006 13:59:13 MDT Print View

"In torrentially wet environments like the rain forests of the Pacific Northwest or New Zealand’s South Island, the lightest synthetic base layer like Capilene would be the preferred choice. Low water absorption and quick drying times are the major reasons."

I don't know about New Zealand, but the Pacific Northwest rarely gets "torrential" rains. The significant weather is the cloud cover and that has a real impact on wilderness travel. It does rain-- it can drizzle non-stop for weeks, but large thundershowers where inches of rain fall in a short period of time are rare. So, with the dampness and less sunlight, if you get wet, drying out is a challenge. You won't find me using down west of the Cascades. I do prefer the lighter synthetics for a base layer-- Patagonia Capilene and GoLite C-thru work well for me. Having base layers that will dry while wearing them can be a life-saver.

Eric Noble
(ericnoble) - MLife

Locale: Colorado Rockies
Wool vs Synthetic on 07/26/2006 15:17:30 MDT Print View

Wow, what a great article! I agree with Ken's comment on color. I wonder if the bias toward dark colors by Smartwool is a reflection of the stereotype that wool is only good in the cold. Despite the fact that they state otherwise on their site. They apparently are not immune to the stereotype. Ibex does a little better job of this, but their base layers are also predominately dark. Retailers are particularly bad at this, to my benefit. Wool always goes on sale before spring. My new favorite garment for hiking, the Shadow's Hoody, comes in a lighter color. I've used it comfortably up to 78° F with the zipper and hood down, and the sleeves up. This article reinforces my experience that wool is great in warm weather. Stretching out the benefits of evaporative cooling can be a good thing.

I suspect the lanolin content of wool has dropped as wool is processed more. Washing wool in the washing machine with detergent doesn't help either. Part of what makes wool work for sheep is lanolin. I wonder what effect adding lanolin back into the wool would have? Would it make it more resistant to moisture, or would that just mess with it's ability to wick? Would it be beneficial to add lanolin in the winter and wash it out in the summer?

One further comment on stink. Not only do synthetics get smelly faster but at some point it is almost impossible to get rid of the smell. After years of wearing Smartwool socks every day, they have not had this problem. My Ibex wool polo shirts, that I wear every day in the summer, don't seem to get pit stains or smell either.

Do you guys have any comments on the durability of wool vs synthetic? Did both sides of the clown shirts wear the same? My wool briefs will definitely outlast my cotton briefs.

Edited by ericnoble on 07/26/2006 15:22:17 MDT.

kevin davidson
(kdesign) - F

Locale: Mythical State of Jefferson
Wool colour and durability on 07/26/2006 16:23:53 MDT Print View

One of the things I like about many of Ibex's wool garments are the range of unusual colours(sometimes light) they come in. My favorite all around wool shirt is an Ibex Pacifico L/S (disc.) ----deep zip front and thumbholes. It's in yellow---light and bright. But, I'm eyeing a Shadow hoody.

My only real complaint about Merino Wool is it's durability. When wet, wool is very fragile. I once had a wool baselayer drying on the rocks on a mtneering trip. A gust of wind took it, stripping off the weigh-down rocks and dragged along a rocky bench. It's now an ex-baselayer----totally shreaded. A capilene shirt that went on a similar trip was totally unscathed.

Edited by kdesign on 07/26/2006 16:25:18 MDT.

Tim Cheek
(hikerfan4sure) - MLife
Permethrin on 07/26/2006 18:30:44 MDT Print View

What effect is there on the two fabrics when permethrin is applied?

Antonio Abad
(tonyabad) - F
RE: I Want a Shadow Hoody on 07/26/2006 22:39:55 MDT Print View

Ooooooo....the shadow hoody! I've been wanting to get my grubby hands on one for some time. It seems like the perfect base layer for the cooler months in my neck of the woods but the price is pretty steep. I'm hoping to find it on sale at some point so that I can team it up with my micropuff vest.

kevin davidson
(kdesign) - F

Locale: Mythical State of Jefferson
re. Permethrin on 07/26/2006 23:09:54 MDT Print View

I've used Permethrin multiple times on both my Merino Wool and Capilene baselayers, outer shirts, and socks w/ no apparent deteriation of the fabrics.

two thumbs way up.

Ryan Jordan
(ryan) - BPL Staff - MLife

Locale: Greater Yellowstone
Shadow Hoody on 07/26/2006 23:22:12 MDT Print View

Two of my partners on a recent trek to the Arctic took a Shadow hoody and loved it. I took took the lighter (microweight) Spectrum Hoody, which is for girls.

As expected, my XL Spectrum Hoody (I'm usually an "M" in men's!) was, how shall we say, fashionable enough to show my hairy navel, so I hacked the bottom off of another merino shirt and sewed it to the hoody to extend its length. I resewed the hood so it had a more balaclava like feel, and I added (to the existing thumbcuffs) fold-over merino pajama mitts. The result was a hacked together garment that you'll have to pull off my dead body! I love it. I didn't take hat or gloves to the Arctic, neither did one of the other guys who wore a Shadow.

Just a great place to start for an all-purpose base layer.

Here's a picture of Roman and Jason, my partners, sporting their Shadows.

Roman Dial and Jason Geck in Arctic Alaska - Smartwool Shadow Hoody (photo: Ryan Jordan)

Edited by ryan on 07/26/2006 23:23:09 MDT.

Summit CO
(Summit) - F

Locale: 9300ft
Dark Colors on 07/27/2006 00:19:31 MDT Print View

speaking of permithren...

doesn't darkly colored clothing attract the mosquitos moreso than light colors?

Eric Noble
(ericnoble) - MLife

Locale: Colorado Rockies
Re: Shadow Hoody on 07/27/2006 08:11:42 MDT Print View

The one modification I made to my Shadow was to remove the stripes on the sleeves. They serve no purpose other than fashion. They only weighed 0.2 oz, but they would just be something extra to dry if they got wet. I plan on writing a review as soon as I have been out in it more.

Alan Amaya
(archeopteryx) - F
Smell after washing? on 07/27/2006 17:10:03 MDT Print View

How does the new Capilene fare in the odor department after being washed? I usually wash my base layers after a few uses and I've noticed that some (especially an older Capilene shirt and a very old polypropylene shirt) still smell after coming out of the wash, or will begin smelling during use much sooner than when they were new. Have the newer fabrics improved in this regard?

Tom Kirchner
(ouzel) - MLife

Locale: Pacific Northwest/Sierra
Shadow hoody on 07/27/2006 17:35:07 MDT Print View

Who manufactures the Shadow hoody? I checked Patagonia and came up empty handed.

Eric Noble
(ericnoble) - MLife

Locale: Colorado Rockies
Re: Shadow hoody on 07/27/2006 17:37:06 MDT Print View


Tom Kirchner
(ouzel) - MLife

Locale: Pacific Northwest/Sierra
smartwool hoody on 07/27/2006 17:55:28 MDT Print View

Thanks Eric!

Scott Ashdown
(waterloggedwellies) - F

Locale: United Kingdom
Paramo Mens Cambia Short Sleeve Tshirt on 07/30/2006 02:36:57 MDT Print View

A really good article and one that also took time and effort to put together, thanks!

I was looking at the Cambia Short Sleeve Tshirt by Paramo, link below. They are touting this as a 'Two in One' shirt. Wear it one way and it appranetly holds water against the skin to aid rapid cooling in hot weather with the smooth face of the fabric reducing the pooling of perspiration to eliminate cold spots after exercise.- yet still wicks. Turn it inside out and the honeycombed face of the fabric provides ‘dry’ comfort in cooler conditions.

Is this a practical shirt which 'Works' when worn either way or is it just manufacturers hype? After all, I could turn my underwear inside out and halve the underwear I take on the trail and claim they were a two in one garment! :-) (For the record - I don't do that!!!)

I was wondering if anyone had any experience of this using this top. I searched the site but could'nt come up with anything.

Tim Cheek
(hikerfan4sure) - MLife
Permethrin on 07/31/2006 17:30:51 MDT Print View

Kevin, I was wondering whether the Permethrin changed the performance of the fabrics in any discernable way. For example, does the merino wool dry quicker, breath less, etc.?

James Loy
(jimbluz) - M

Locale: Pacific NW
Smartwool Liner Socks on 08/01/2006 16:34:55 MDT Print View

I have had poor results from this product having had the first pair develop a hole in the toe after 27 miles of hiking, and the second pair experienced the same thing after only 10 miles. I am using them with Salomon Tech Amphibians and my toenails are properly trimmed. I emailed Smartwool about this but have as yet received no reply. Has anyone else tried these socks?

kevin davidson
(kdesign) - F

Locale: Mythical State of Jefferson
SW Shadow Hoody on 08/01/2006 18:43:02 MDT Print View

Just got my hoody in the mail direct from Smartwool (fast). Driftwood (a light gray) w/ red trim. Men's large is 12.5 oz. on the nose before cutting off tags and such. I like the fit----hood is snug but comfy. It is in a lighter, mediumweight wool---I think SW calls it versawear.

Ideally, I wish that Smartwool offered this w/ a microweight body and arms and a versawear weight hood--- that would be absolutely perfect for a primary 3 season layer. As it stands this is perfect for shoulder seasons in the Cascades and Sierra but a bit warm (for me) for most high Summer use.

Ryan Jordan
(ryan) - BPL Staff - MLife

Locale: Greater Yellowstone
Re: SW Shadow Hoody on 08/02/2006 00:19:32 MDT Print View

Kevin ya gotta switch hit if you want a microweight hoody.

Smartwool is making a microweight hoody for women, the Spectrum. See OR dispatches from the last show. I took a prototype to Alaska with me, but I had to sew a longer waist hem on it to make it man-belly-compatible. They may end up doing it in lightweight when it finally hits the market.


kevin davidson
(kdesign) - F

Locale: Mythical State of Jefferson
shadow hoody on 08/02/2006 01:22:33 MDT Print View

Your hack of the femme hoody was clever but my very long torso would require an awful lot of add'l material! Plus SW's women's sizing would have to go up to XL if not XXL for me to do something similar. You more gracile folk have it good---smaller clothing, less weight, etc., etc.

What I'm going to do in the LW vein is sew a hood to my Ibex Pacifica, using material from an old merino shirt of mine. The color combo is guarenteed to be atrocious.

Edited by kdesign on 08/02/2006 01:23:51 MDT.

Eric Noble
(ericnoble) - MLife

Locale: Colorado Rockies
Re: Smartwool Liner Socks on 08/02/2006 14:09:12 MDT Print View

James, I have not used the liners, but it do wear the Smartwool ultralight cycling socks every day during the spring, summer, and fall, along with Salomon Tech Amphibians. Over the course of 2 years they have yet to show any wear. My cycling sock are very thin. If yours are wearing out that fast they must be remarkably thin or defective. Is it the seam or the fabric that is wearing through?

Edited by ericnoble on 08/02/2006 14:14:34 MDT.

Stuart Bilby
(StuBilby) - MLife

Locale: New Zealand
Permethrin and merino durability on 08/04/2006 21:12:57 MDT Print View

Great article, nice to see solid measured numerical results.

The permethrin works well on merino. I haven't noticed any change in performance or feel of the fabric.

The icebreaker skin weight merino is quite a lot more fragile than any of the synthetics. I manage to put holes in it after a few trips, but sometimes it is still worthwhile for its non-stink properties.

Aaron Teasdale
(digaaron) - MLife
general insights on 08/08/2006 18:32:08 MDT Print View

Excellent review. I do wish you'd used Marmot silkwieght instead of Capilene however -- it seems to resist odor better. While it's quite durable, the latest iteration of Capilene gets very stinky very fast. The Marmot silkweight is also remarkably durable -- It held up amazingly well during an extended bushwacking session through savage alders -- not a single mark, and it was getting grabbed and pulled at every step. After a decade of using Capilene and wool, I was very impressed.

Wool is a wonderful base layer, especially in winter. It offers much more range than synthetics. But, over the years, I've had serious durability problems with it -- both socks and baselayer tops. It just hasn't held up for me. A major bummer given its cost.

Edited by digaaron on 10/11/2006 12:47:06 MDT.

Douglas Hus
(Hustler) - F

Locale: Ontario, Canada
Wool and Synthetic on 01/16/2007 12:57:42 MST Print View

Great article, I am now not in as big a rush to buy wool. But still; I fore see a need for a new base layer.

Most of my 3 season trips I have had the opportunity to do laundry, socks & base layers. Zip-lock bag, soap and a shake. Helps with the odour thing. Length of dry time varies.

Weight is still "King".



Adam Rothermich
(aroth87) - F

Locale: Missouri Ozarks
Re: Wool and Synthetic on 01/16/2007 13:15:13 MST Print View

Wow Doug, I'd never thought about doing laundry in a Ziploc before. That's a great idea, I'm surprised I've never heard of it elsewhere. Its such a simple to solution to smelly clothes. I'm definitely going to keep it in mind when I do some longer trips this spring and summer.
To stay somewhat on-topic, the only wool I own are some socks and a Smartwool beanie. The beanie is great in a wide range of temps and the socks are great too. I've been eyeing the SW Lightweight 1/4 zip too. SAC had them for $35 one day and I decided I didn't really need one. Now they haven't come back up on SAC and I can't bring myself to spend $70 on one shirt and I kick myself every time I think about how I let it slip away.


Einstein X
(EinsteinX) - F

Locale: The Netherlands
Why didn't BPL do this: on 01/19/2007 08:34:27 MST Print View

I think BPL missed out on calculating a ratio, which seems to me a more objective measure in reviewing the drying characteristics of the materials:

The weights of the samples before wetting [1]:
Pata 12 g
GL 10 g
Ibex 14 g
IB 18 g
SW 14 g

The weights of the samples after wetting [2]:
Pata 47 g
GL 54 g
Ibex 59 g
IB 63 g
SW 68 g

Absorbed mass of water [3] = [2] - [1]:
Pata 35 g
GL 44 g
Ibex 45 g
IB 45 g
SW 54 g

Time of drying [4]:
Pata 65 min
GL 75 min
Ibex 90 min
IB 90 min
SW 100 min

Now the amount of time it requires to remove 1 gram of water from the fabric is calculated by deviding [4] over [3]:
Pata 1:52 min
GL 1:42 min
Ibex 2:00 min
IB 2:00 min
SW 1:51 min

I think these values are very interesting. They show that each fabric takes almost the same time to loose a gram of water in the fabric. With an average time of 1:53 min and a 9,4% spread. Most interesting is that the smartwool shirt dries actually 1 second faster than the patagonia shirt. This result is also experienced in the field test.

The only right conclusion that can be made on this experiment is that each fabric is, within significance, equal in drying performance. The only difference is that wool can hold more water per whatever unit than a synthetic shirt. Thus in absolute time it will take the wool shirt longer to dry.

An interesting experiment would now be to let each fabric absorb exactly the same amount of water and perform a new drying test. My take on it is that they will dry in exactly the same time (with the wool shirt feeling dry earlier than the synth shirt)


eric levine
(ericl) - F

Locale: Northern Colorado
Permethrin - use it only when life and limb is at risk on 09/06/2007 02:29:23 MDT Print View

I’m amazed yet again at the cavalier way this presumed “nature” community treats these powerful, destructive, and not fully studied neurotoxins. It’s completely out of keeping with the natural ethos.

A few studies are in order, and yes, I fully expect to get slammed by some pretty analytical but not very convincing justifications. It’s happened before…..
Permethrin undergoes its first official testing for endocrine disruption this year, after years of evidence which suggests endocrine effects.

Melissa Kaplan's
Herp Care Collection
Last updated August 11, 2002

Pyrethrin and Pyrethroid Exposure Causes Adverse Reactions

Compiled by Melissa Kaplan

Mechanisms of pyrethroid neurotoxicity: implications for cumulative risk assessment.
Soderlund DM, Clark JM, Sheets LP, Mullin LS, Piccirillo VJ, Sargent D, Stevens JT, Weiner ML. Department of Entomology, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456, USA.
Toxicology 2002 Feb 1;171(1):3-59

The Food Quality Protection Act (FQPA) of 1996 requires the United States Environmental Protection Agency to consider the cumulative effects of exposure to pesticides having a 'common mechanism of toxicity.' This paper reviews the information available on the acute neurotoxicity and mechanisms of toxic action of pyrethroid insecticides in mammals from the perspective of the 'common mechanism' statute of the FQPA.

The principal effects of pyrethroids as a class are various signs of excitatory neurotoxicity. Historically, pyrethroids were grouped into two subclasses (Types I and II) based on chemical structure and the production of either the T (tremor) or CS (choreoathetosis with salivation) intoxication syndrome following intravenous or intracerebral administration to rodents. Although this classification system is widely employed, it has several shortcomings for the identification of common toxic effects. In particular, it does not reflect the diversity of intoxication signs found following oral administration of various pyrethroids.

Pyrethroids act in vitro on a variety of putative biochemical and physiological target sites, four of which merit consideration as sites of toxic action. Voltage-sensitive sodium channels, the sites of insecticidal action, are also important target sites in mammals. Unlike insects, mammals have multiple sodium channel isoforms that vary in their biophysical and pharmacological properties, including their differential sensitivity to pyrethroids.

Pyrethroids also act on some isoforms of voltage-sensitive calcium and chloride channels, and these effects may contribute to the toxicity of some compounds. Effects on peripheral-type benzodiazepine receptors are unlikely to be a principal cause of pyrethroid intoxication but may contribute to or enhance convulsions caused by actions at other target sites. In contrast, other putative target sites that have been identified in vitro do not appear to play a major role in pyrethroid intoxication. The diverse toxic actions and pharmacological effects of pyrethroids suggest that simple additivity models based on combined actions at a single target are not appropriate to assess the risks of cumulative exposure to multiple pyrethroids.

[Pyrethroid exposure following indoor treatments with a dog flea powder]
[Article in German] Schulze M, Helber B, Hardt J, Ehret W. Umweltmedizinisches Zentrum (Direktor: Prof. Dr. Dr. W. Ehret), Klinikum, Augsburg. Dtsch Med Wochenschr 2002 Mar 22;127(12):616-8

HISTORY: A 42 year old woman reported hair loss, gastrointestinal and non-specific symptoms. The patient has lived in a council flat and kept a dog who had been regularly treated with pyrethroid containing flea powder.

INVESTIGATIONS: The biological monitoring of pyrethroid meta-bolites in urine was performed using gas chromatography-mass spectrometry. The values at admission and follow-up after 4 weeks were highly elevated. Inspection of the flat revealed a humid and cramped dwelling.

TREATMENT: We recommended redevelopment and cleaning of the dwelling and the avoidance of ectoparasiticide use.

CONCLUSION: To our knowledge this is the first documented case of high indoor pyrethroid exposure following the use of ectoparasiticides with domestic animals. Pyrethroids can cause neurotoxic symptoms and skin irritation. There are few data concerning chronic effects. The causal connection between pyrethroid exposure and symptoms remains unclear and poses a great problem in environmental medicine.

Striatal dopaminergic pathways as a target for the insecticides permethrin and chlorpyrifos.
Karen DJ, Li W, Harp PR, Gillette JS, Bloomquis JR. Department of Entomology, Virginia Polytechnic Institute and State University, Blacksburg 24061, USA. Neurotoxicology 2001 Dec;22(6):811-7

Because insecticide exposure has been linked to both Parkinsons disease and Gulf War illness, the neurotoxic actions of pyrethroid and organophosphate insecticides on behavior and striatal dopaminergic pathways were investigated in C57BL/6 mice treated with permethrin (three i.p. doses at 0.2-200 mg/kg) or chlorpyrifos (three s.c. doses at 25-100 mg/kg) over a 2-week period. Permethrin altered maximal [3H]dopamine uptake in striatal synaptosomes from treated mice, with changes in Vmax displaying a bell-shaped curve. Uptake was increased to 134% of control at a dose of 1.5 mg/kg. At higher doses of PM (25 mg/kg), dopamine uptake declined to a level significantly below that of control (50% of control at 200 mg/kg, P < 0.01). We also observed a small, but statistically significant decrease in [3H]dopamine uptake by chlorpyrifos, when given at a dose of 100 mg/kg. There was no significant effect on the Km for dopamine transport. Evidence of cell stress was observed in measures of mitochondrialfunction, which were reduced in mice given high-end doses of chlorpyrifos and permethrin. Although cytotoxicity was not reflected in decreased levels of striatal dopamine in either 200 mg/kg PM or 100 mg/kg CPF treatment groups, an increase in dopamine turnover at 100 mg/kg CPF was indicated by a significant increase in titers of the dopamine metabolite, 3,4-dihydroxyphenylacetic acid. Both permethrin and chlorpyrifos caused a decrease in open field behavior at the highest doses tested. Although frank Parkinsonism was not observed, these findings confirm that dopaminergic neurotransmission is affected by exposure to pyrethroid and organophosphorus insecticides, and may contribute to the overall spectrum of neurotoxicity caused by these compounds.


Thursday, May 1, 2003
WASHINGTON (Reuters) - Farmers who use certain pesticides seem to have a higher-than-average risk of prostate cancer, U.S. government researchers said on Thursday.
The researchers, who published their study in the American Journal of Epidemiology, confirmed other findings that show farmers have an unusually high risk of prostate cancer.
"Associations between pesticide use and prostate cancer risk among the farm population have been seen in previous studies; farming is the most consistent occupational risk factor for prostate cancer," Michael Alavanja of the National Cancer Institute (NCI), who helped lead the study, said in a statement.
Researchers at NCI and at the National Institute of Environmental Health Sciences and the Environmental Protection Agency studied 55,332 farmers and nursery workers who worked with pesticides.
Between 1993 and 1999, 566 new prostate cancers developed among the men, compared to 495 that would normally be expected in Iowa and North Carolina, the two states studied.
The risk of developing prostate cancer was 14 percent greater for the pesticide applicators compared to the general population.
One pesticide, methyl bromide, increased the risk of prostate cancer in all men.
Six others raised the risk in men with a family history of prostate cancer. They are chlorpyrifos, coumaphos, fonofos, phorate, permethrin and butylate.
More than 220,000 U.S. men will be diagnosed with prostate cancer this year, according to the American Cancer Society, and 30,000 will die of it.
The biggest risk factors for prostate cancer are age and family history. African-American men have higher rates of prostate cancer, and some evidence suggests that men who eat lots of red meat and animal fat have a higher risk.

The insecticide permethrin (in the synthetic pyrethroid family) is widely
used on cotton, wheat, corn, alfalfa, and other crops. In addition, over 100
million applications are made annually in and around U.S. homes.
Permethrin, like all synthetic pyrethroids, is a neurotoxin. Symptoms
include tremors, incoordination, elevated body temperature, increased
aggressive behavior, and disruption of learning. Laboratory tests suggest
that permethrin is more acutely toxic to children than to adults.
The U.S. Environmental Protection Agency has classified permethrin as a
carcinogen because it causes lung tumors in female mice and liver tumors
in mice of both sexes. Permethrin inhibits the activity of the immune
system in laboratory tests, and also binds to the receptors for a male sex
hormone. It causes chromosome aberrations in human and hamster cells.
Permethrin is toxic to honey bees and other beneficial insects, fish,
aquatic insects, crayfish, and shrimp. For many species, concentrations of
less than one part per billion are lethal. Permethrin causes deformities and
other developmental problems in tadpoles, and reduces the number of
oxygen-carrying cells in the blood of birds.
Permethrin has been found in streams and rivers throughout the United
States. It is also routinely found on produce, particularly spinach,
tomatoes, celery, lettuce, and peaches.
A wide variety of insects have developed resistance to permethrin. High
levels of resistance have been documented in cockroaches, head lice, and
tobacco budworm.
Caroline Cox is JPR’s editor.
Permethrin is used to kill pest in-sects in agriculture, home pest control, forestry, and
in public health programs, including head lice control. It was first marketed in 1973.
Worldwide, the dominant use of permethrin is on cotton, accounting for about 60 percent
(by weight) of the permethrin used.1 In the U.S., al-most 70 percent of the permethrin
used in agriculture is used on corn, wheat, and alfalfa.2 Over 100 million applications of
permethrin are made each year in U.S. homes, and over 18 million applications are made
in yards and gardens.3
Permethrin is a synthetic pyrethroid. Like most members of this family of insecticides,
it has four isomers, molecules made up of the same atoms with different threedimensional
structures. (See Figure 1)
Mode of Action
Permethrin, like all synthetic pyrethroids, kills insects by strongly exciting their
nervous systems. Permethrin makes the nervous system hypersensitive to stimuli from
sense organs. Rather than sending a single impulse in response to a stimulus, permethrinexposed
nerves send a train of impulses. This excitation occurs because permethrin
blocks the movement of sodium ions from outside to inside of the nerve cells.
Permethrin’s mode of action is similar to that of the organochlorine insecticide DDT.5
Acute Lethal Dose
Permethrin’s LD50 (the amount of permethrin that kills 50 percent of a population of
test animals) is variable. In a summary of nine oral LD50 tests using rats, the LD50 varied
from 430 milligrams per kilogram of body weight (mg/kg) to over 4,000 mg/kg. Some of
this variability occurs because the proportions of isomers in the test materials vary. The
cis isomers are about ten times more toxic than the transisomers.6
In mammals, permethrin has complex effects on the nervous system. As in insects, it
causes repetitive nerve impulses. It also inhibits a variety of nervous system enzymes:
ATPase, whose inhibition results in increased release of the neurotransmitter
acetylcholine 7; monoamine oxidase-A, the enzyme which maintains normal levels of
three other neurotransmitters 8; and acetylcholinesterase, the enzyme that breaks down
acetylcholine.9 (Two large families of insecticides, the organophosphates and the
carbamates, are acetylcholinesterase inhibitors.) In addition, permethrin inhibits a
nervous system receptor, the GABAA receptor, producing excitability and
convulsions.10 Finally, permethrin inhibits respiration (the process by which cells use
sugars as an energy source) in a manner similar to other neurotoxic drugs.11 It is
therefore not surprising that permethrin causes a wide variety of neurotoxic symptoms.
At relatively high doses, these neuro-toxic symptoms of permethrin include tremors,
incoordination, hyperactivity, paralysis, and an increase in body temperature. These
symptoms can persist up to three days.12 Other behavioral effects have been observed at
lower doses. For example, sublethal exposure of mice to the permethrin-containing
insecticide Ambush increased activities like chewing 13 ; sublethal exposure of rats to
permethrin increased aggressive behavior, agitation, and resistance to being captured 14 ;
and permethrin disrupted a learned feeding behavior in rats at doses of about 20 percent
of the LD50. 15
Eye and Skin Irritation
Permethrin-containing products can be irritating to both eyes and skin. For example,
the agricultural insecticide Pounce 3.2 EC “causes moderate eye irritation.”16 Ortho Total
Flea Control 2 and Solaris Flea-B-Gon Total Flea Killer Indoor Fogger both cause
“tearing, swelling, and blurred vision.”17,18 They also cause “redness, swelling, and
possibly blistering” of the skin.17,18 Adams 14 Day Flea Dip “causes eye injury”19 and
“may cause al-allergic reactions”19 on skin.
Effects on the Immune System
Experiments with laboratory animals indicate that the immune system (used by living
things to defend themselves from disease) “appears to be a sensitive target for permethrin
activity.” Ingestion of permethrin reduces the ability of immune system cells called Tlymphocytes
to recognize and respond to foreign proteins. Doses equivalent to 1/100 of
the LD50 , inhibited T-lymphocytes over 40 percent. Permethrin ingestion also reduced
the activity of a second type of immune system cell, natural killer cells, by about 40
percent.20(See Figure 2.) In tests using mouse cell cultures, permethrin had similar effects
on the immune system, inhibition of two kinds of lymphocytes.21 Researchers concluded
that “the immune system is exquisitely sensitive … at exposure levels that cause no overt
Effects on Reproduction
Permethrin affects both male and female reproductive systems. It binds to receptors
for androgen, a male sex hormone, in skin cells from human males, causing researchers
to “advise protection from any form of contact or ingestion of the pyrethroids.” 22
Permethrin also binds to a different receptor, called the peripheral benzodiazepine
receptor, that stimulates production of the male sex hormone testosterone.23 In addition,
permethrin caused reduced testes weights in a long-term feeding study of mice.24 In
females, permethrin exposure has caused embryo loss in pregnant rabbits24 and in
pregnant rats.25
Permethrin was mutagenic (damaging to genetic material) in three tests with human
cell cultures, one with hamster cells, and one with fruit fly larvae. In cultures of human
lymphocytes (white blood cells), permethrin exposure caused an increase in chromosome
aberrations, chromosome fragments,26 and DNA lesions.27 In hamster ovary cell
cultures, permethrin exposure caused chromosome aberrations.28 Exposure to Ambush
(a permethrin-containing insecticide) during larval development increased sex-linked
lethal mutations in fruit flies.29
According to the U.S. Environmental Protection Agency (EPA), permethrin is a
possible human carcinogen (chemical that causes cancer).30 EPA found that permethrin
increased the frequency of lung tumors in female mice, and increased the frequency of
liver tumors in male and female mice.24 The World Health Organization reports that
permethrin increased the frequency of lung tumors in females in two out of the three
mouse studies it reviewed. (See Figure 3.) Lung tumors increased with increasing
permethrin exposure in the third study, but the increase was not statistically significant.31
There are no publicly available studies of the carcinogenicity of permethrin-containing
insecticide products.
There are two molecular mechanisms which could explain permethrin’s
carcinogenicity. First, permethrin reduces the activity of an enzyme involved in the
breakdown of the amino acid tryptophan. This can lead to the buildup of carcinogenic
tryptophan breakdown products.32 Second, permethrin inhibits what is called “gap
junctional intercellular communication” (GJIC), chemical communication between cells.
GJIC plays an important role in the growth of cells, and some cancer promoting
chemicals inhibit GJIC.33
Other Chronic Effects
The liver is a sensitive target for permethrin effects. When EPA summarized 17 mediumand
long-term laboratory studies that exposed rats, mice, and dogs to permethrin, effects
on the liver were noted at the “lowest effect level” in all of them.24 Other chronic effects
in laboratory tests include enlarged adrenal glands at all doses tested in a rabbit feeding
study, and increased kidney weights at all doses tested in a rat feeding study.24
Synergy occurs between two or more chemicals when their combined exposure causes
more adverse effects than the sum of their individual effects. A possible cause of the
health problems reported by 30,000 veterans who served in the Persian Gulf War is
exposure to a combination of chemicals, including permethrin. The combination of
permethrin, the anti-nerve gas drug pyridostigmine bromide, and the insect repellent
DEET has been tested in laboratory animals. Neurotoxic symptoms, including decreased
activity, diarrhea, shortness of breath, tremors, inability to walk, and damage to nerves,
were observed in hens exposed to all three chemicals, but not in hens exposed to
permethrin alone. Permethrin with just pyridostigmine bromide or just DEET also caused
tremors and inability to walk, but symptoms were not as severe.35
Other pesticides interact synergistically with permethrin with in other species.
Permethrin and the herbicide atrazine synergistically induce growth of the soil fungus
Pythium ultimum, 36 and permethrin and the insecticide amitraz are synergistically toxic
to the bollworm.37
Individual Susceptibility
Individuals vary in their susceptibility to permethrin, as has been illustrated by the
following research:
l Based on tests with laboratory animals, it appears children may be more sensitive to
permethrin than adults. Permethrin is almost 5 times more acutely toxic to 8-day-old
rats than it is to adult rats.38(See Figure 4.)
l Since sulfates are involved in one of the major pathways by which permethrin is
broken down in humans, individuals with defects in sulfate-related enzymes may be
unable to easily break down permethrin, leading to increased susceptibility to motor
neuron disease.39,40
l Individuals with genetic variants of the enzyme pseudocholinesterase that have
reduced activity are at higher risk of adverse effects from exposure to certain
chemicals, including the permethrin combination implicated in symptoms seen in
Gulf War veterans.35
“ It appears children may be more
sensitive to permethrin than adults.
Permethrin is almost 5 times more
acutely toxic to 8-day-old rats than it is to
adult rats.”
Effects on Nontarget Animals
Beneficial Arthropods: As a broad spectrum insecticide, it is not surprising that
permethrin impacts beneficial arthropods, those that are useful in agriculture. Examples
include the following:
l Permethrin is acutely toxic to honey bees; the median lethal dose is 0.008
micrograms per bee.41 Sublethal exposures cause increased abnormal behavior
(trembling, etc.), decreased foraging,42 and impairment of bees’ learning.43
l The International Organization for Biological Control tested the acute toxicity of
permethrin to 13 species of beneficial arthropods and found that permethrin caused
99 percent mortality of 12 of the species, and over 80 percent mortality of the other.
Effects were persistent, lasting over 30 days.44 Sublethal doses also impact
beneficial arthropods: permethrin inhibited the emergence of a parasitoid wasp from
eggs of the rice moth Corcyra cephalonica 45 and disrupted the foraging pattern of
another parasitoid wasp as it searched for its aphid prey.46 (Parasitoids are insects
that lay their eggs in, on, or near their prey. The eggs hatch and the larvae consume
the prey as they develop. They often keep populations of agricultural pests at low
Aquatic Insects: Because it is a broad spectrum insecticide, permethrin has severe
impacts on aquatic insects. Permethrin applications to forest streams caused “a major
increase in the density of drifting invertebrates” described as “catastrophic.” (Drifting
animals are those that are sufficiently poisoned by the insecticide that they are washed
down-stream.) Most applications were also followed by “rapid depletion of bottom
fauna,” insects that live in the stream bed. Recovery required between 1 and 18
months.47 Mayflies and damselflies are the most sensitive species.49 Permethrin also
bioconcentrates in aquatic insects; bioconcentration factors in stoneflies ranged from 43
to 570. 49
Birds: While permethrin’s acute toxicity to birds is low,50 it causes other ad-verse
effects. Three-week dietary exposure of chickens reduced hemoglobin (oxygen carrying
protein) levels, and red blood cell counts, while increasing the number of white blood
cells.51 The reduction in hemoglobin occurred at the lowest dose tested, 33 mg/kg.51
Permethrin also caused decreased immune responses in chicks,52 and damaged mallard
Fish: Permethrin is highly toxic to fish. This toxicity is due, in part, to the sensitivity of
their nervous system.54 Fish also lack the enzymes that break down permethrin in other
The LC
(the concentration that kills 50 percent of a population of test animals) is less
than 1 part per million (ppm) for almost all fish species tested, and for some fish is less
than 1 part per billion (ppb). Agricultural permethrin products called emulsifiable
concentrates are about twice as toxic to fish as permethrin alone. Small fish are less
tolerant of permethrin than large fish, and it is more toxic in cold water than in warm
water.56 Fish also have a particular developmental stage when they are most susceptible.57
Sublethal effects on fish include abnormal swimming, a reduced startle response, and
loss of equilibrium.58
Permethrin bioconcentrates in fish, so that concentrations in fish are higher than the
concentration in the water in which the fish live. Bioconcentration factors (the ratio
between the concentration in the fish and the concentration in the water) up to 113 have
been measured in brook trout,59 up to 613 in Atlantic salmon,59 and up to 631 in rainbow
Complex effects of permethrin on fish have been documented by the Canadian Forest
Service in field studies. They found that diets of trout and salmon were altered when
permethrin killed the insects these fish use as food. In some cases, diets were altered for
a year following treatment. Reductions in fish growth rates, and migration to untreated
areas followed; recovery required four months. The researchers concluded that
permethrin is “not an acceptable treatment for large-scale use in forest areas containing
Amphibians: Permethrin disrupts the growth and development of tadpoles. Exposure
slowed growth for two to three weeks, and increased the frequency of a tail abnormality.
(See Figure 5.) The increase in this deformity occurred at the lowest concentration of
permethrin tested, 0.1 ppm. At this concentration tadpoles also responded to prodding in
a jerky and disorganized way, making them vulnerable to predation. Tadpoles exposed to
an even lower concentration (0.05 ppm) reduced their feeding for several weeks after
Permethrin also effects brain function in tadpoles. Concentrations of 0.25 ppm
decreased the amounts of two specific proteins in the brain, while increasing the total
amount of protein. One of the proteins is associated with learning. Activity of several
nervous system enzymes, including acetylcholinesterase, decreased.63
Other Aquatic Animals: Permethrin is very highly toxic to lobster; the LC
is less than
1 ppb.64 It is highly toxic to oyster larvae, with an EC
(the concentration causing
abnormal development in half of the larvae) of less than 1 ppm.65 Permethrin
bioconcentrates in oysters, with a bioconcentration factor of 1900. 66 Water fleas are also
very sensitive to permethrin exposure; LC
s of several species are about 1 ppb.67
Permethrin also caused “severe mortalities” of two kinds of zooplankton, cladocerans
and copepods with recovery taking about 3 months.68
Mysid shrimp are killed by permethrin at concentrations so low that they cannot be
detected in water (the LC 50 is 0.02 ppb). This means that “any detection of these
insecticides in estuarine waters would likely be associated with adverse effects.”66
Another animal that is very sensitive to permethrin is crayfish; LC
s for the red swamp
crayfish vary from 0.4 to 1.2 ppb. Researchers concluded that “even the lowest
operational treatment level used for insect management would seriously impact crayfish
Residues on Food
The Food and Drug Administration’s (FDA’s) monitoring program routinely finds
permethrin on food. In 1996, it was the 13th most commonly detected pesticide.68 Similar
results were found in monitoring of 14 fruits and vegetables by the U.S. Department of
Agriculture; permethrin was the 10th most frequently detected pesticide and was often
found on spinach (60 percent of the samples tested) and tomatoes (11 percent of the
samples tested).69 Permethrin was also frequently found on celery and lettuce.70
Permethrin has also been found in baby food: FDA’s 1996 monitoring found it in 12
percent of the samples tested. The Environmental Working Group found permethrin was
the most commonly detected pesticide in peach baby food (44 percent of the samples
tested) and was also found in plums (11 percent of the samples tested).71
Contamination of Water
Permethrin has been found in ground and surface water. The U.S. Geological Survey
has found permethrin in streams and rivers in the Mississippi River Basin, 72 the Central
Columbia Plateau (Washington and Idaho),73 the Georgia-Florida Coastal Plain,74 the
San Joaquin-Tulare Basin (California),75 and the Ozark Plateau (Arkansas and nearby
states).76 Permethrin has also been found in groundwater in Virginia.76
Drift, pesticide movement during application away from the target area, has been
measured for two types of permethrin applications: aerial and back pack mistblower.
Aerially applied permethrin drifted 180-240 meters (590- 790 feet) under conditions
“highly conducive” to drift.78 These researchers sug-gested using buffers of 150 meters
(490 feet). Back pack mistblower applications of permethrin drifted 150 meters.79
According to EPA, permethrin’s half-life (the amount of time required for half of the
original amount of a chemical to break down or move away from the study site) was 17
days in a North Carolina agricultural soil and 43 days in Illinois.80 When used as a
termiticide, permethrin persists longer; soil concentrations did not decline during the first
year.81 Permethrin also persists longer in tree needles, foliage, and bark, up to 363 days.82
The ability of permethrin to persist in the environment was graphically illustrated by a
study of an application of permethrin ear tags to cattle. Permethrin was found on all
surfaces analyzed, not only on the cattle, but also on the bark of trees in their pasture, on
a fence pole, and in grass. Some residues were found three months after the ear tags were
Resistance (the evolution of a strain of insect that is able to tolerate a particular
insecticide) to permethrin has been documented in a wide variety of insects. These
species include pear psylla,84fall army-worm, 85 German cockroach,86 spotted tentiform
leafminer,87 diamondback moth,88 house fly,89 stable fly,90 head lice,91-93 and tobacco
budworm.94 Many of these species are resistant to other synthetic pyrethroids as well as
permethrin. The level of resistance is less than tenfold in some of the species but high
levels of resistance have been observed in cockroaches (45-fold),86lice (up to 385-fold)91
and budworm (1400-fold).94
Inert Ingredients
Like most pesticide products, permethrin insecticides contain ingredients that are
typically claimed as trade secrets by pesticide manufacturers. Limited information about
“inerts” in permethrin products is available. Examples include:
l Xylenes are in the agricultural insecticides Pounce 3.2 EC,16 Ambush 2E,95 and
Ambush 50. 96 Xylenes cause eye and skin irritation, headaches, nausea, confusion,
tremors, and anxiety in exposed humans. In laboratory tests, xylenes have caused
kidney damage, fetal loss, and skeletal anomalies in offspring.97
l Methyl paraben is in the head lice cream rinse Nix,98 regulated as a drug not as a
pesticide. Methyl paraben is a skin sensitizer, and causes eye, skin, digestive, and
respiratory irritation.99
l Dimethyl ether is in the household insecticides Flea-B-Gon Total Flea Killer
Indoor Fogger 17 and Ortho Total Flea Control 2. 18 It causes respiratory, skin, and
eye irritation and depresses the cen-tral nervous system. It is also a severe fire
l Butane is in the household insecticides Raid Yard Guard Outdoor Fogger V and
Off Yard and Deck Area Repellant 1. 101,102 It is “extremely flammable” and shortterm
exposure causes irritation, nausea, drowsiness, convulsions, and coma.103
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87. Marshall, D.B. and D.J. Pree. 1986. Effects of pyrethroid insecticides on eggs and larvae of
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88. Tabashnik, B.E., N.J. Cushing, and M.W. Johnson. Diamondback moth (Lepidoptera: Plutellidae)
resistance to insecticides in Hawaii: Intra-island variation and cross-resistance. J. Econ. Entomol.
89. Shen, J. and F.W. Plapp. 1990. Cyromazine resistance in the house fly (Diptera: Muscidae):
Genetics and cross-resistance to diflubenzuron. J. Econ. Entomol. 83:1689-1697.
90. Cilek, J.E. and G.L. Greene. 1994. Stable fly (Diptera: Muscidae) insecticide resistance in Kansas
cattle feedlots. J. Econ. Entomol. 87:275-279.
91. Rupes, V. et al. 1995. A resistance of head lice ( Pediculus capitis) to permethrin in the Czech
Republic. Cent. Eur. J. Public Health 3:30-32. (Abstr.)
92. Mumcuoglu, K.Y. et al. 1995. Permethrin resistance in the head louse Pediculus capitis from
Israel. Med. Vet. Entomol. 9:427-432, 447. (Abstract.)
93. Burgess, I.F. et al. 1995. Head lice resistant to pyrethroid insecticides in Britain. Brit. Med. J.
94. Wolfenbarger, A. and J. Vargas-Camplis. 1997. Tobacco budworm response to pyrethroid
insecticides in the Winter Garden area and in the Lower Rio Grande Valley. Resist. Pest Manage.
95. ICI Americas Inc. 1990. Material safety data sheet: Ambush 2E. Wilmington, DE, Aug. 9.
96. Ortiz, D. et al. 1995. Acute toxicological effects in rats treated with a mixture of commercially
formulated products containing methyl parathion and permethrin. Ecotoxicol. Environ. Safety 32:154-
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97. U.S. Dept. of Health and Human Services. Public Health Service. Agency for Toxic Substances
and Disease Registry. 1995. Toxicological pro-file for total xylenes. Atlanta, GA, Aug.
98. Warner-Lambert Co. Undated. Nix ® permethrin/lice treatment.
99. Acros Organics. 1996. Material safety data sheet: methyl p-hydroxybenzoate. Fairlawn, NJ, Feb.
100. MG Industries. 1997. Material safety data sheet dimethyl ether. Malvern, PA, Dec. 9.
101. S.C. Johnson & Son, Inc. 1997. Material safety data sheet: Raid ® Yard Guard Formula V.
Racine, WI, Feb. 25.
102. S.C. Johnson & Son, Inc. 1996. Material safety data sheet: OFF! Yard & Deck Area Repellent I.
Racine, WI, Oct. 2.
103. Messer-MG Industries. 1997. Material safety data sheet: n-Butane. Malvern, PA, Dec. 9.
chemicalWATCH Factsheet
Pesticide products containing pyrethroids
are often described by pest control operators
and community mosquito management
bureaus as “safe as chrysanthemum flowers.”
While pyrethroids are a synthetic version
of an extract from the chyrsanthemum,
they were chemically designed to be more
toxic with longer breakdown times, and are
often formulated with synergists, increasing
potency and compromising the human
body’s ability to detoxify the pesticide.
What are Synthetic Pyrethroids?
Synthetic pyrethroids are synthesized derivatives
of naturally occurring pyrethrins,
which are taken from pyrethrum, the oleoresin
extract of dried chrysanthemum flowers.
The insecticidal properties of pyrethrins
are derived from ketoalcoholic esters of
chrysanthemic and pyrethroic acids. These
acids are strongly lipophilic and rapidly penetrate
many insects and paralyze their nervous
system (Reigart et al., 1999). Both pyrethrins
and synthetic pyrethroids are sold as
commercial pesticides used to control pest
insects in agriculture, homes, communities,
restaurants, hospitals, schools, and as a topical
head lice treatment. Various formulations
of these pesticides are often combined with
other chemicals, known as synergists, to increase
potency and persistence in the environment.
While chemically and toxicologically similar,
pyrethrins are extremely sensitive to light,
heat and moisture. In direct sunlight, halflives
that can be measured in hours. However,
the pyrethroids, the synthetic analogues
of naturally occurring pesticides,
were developed to capture the effective insecticidal
activity of this botanical insecticide,
with increased stability in light, yielding
longer residence times (Gosselin et al.,
Pyrethroids and Health Effects
Pyrethroids have irritant and/or sensitizing
properties. They are not easily absorbed
through the skin, but are absorbed through
the gut and pulmonary membrane. Tests of
some pyrethroids on laboratory animals reveal
striking neurotoxicity when administered
by injection or orally. Systemic toxicity by
inhalation and dermal absorption is low. The
acute toxicity, calculated by LD50’s, ranges
from low to high, depending on the specific
formulation. Low toxicity is attributed to two
factors: limited absorption of some pyrethroids,
and rapid biodegradation by mammalian
liver enzymes (ester hydrolysis and
oxidation). Insects, without this liver function,
exhibit greater susceptibility to the
chemicals (Reigart et al., 1999).
Pyrethroids interfere with the ionic conductance
of nerve membranes by prolonging the
sodium current. This stimulates nerves to
discharge repeatedly causing hyper-excitability
in poisoned animals. The World Health
Organization explains that synthetic pyrethroids
are neuropoisons acting on the axons
in the peripheral and central nervous systems
by interacting with sodium channels in
mammals and/or insects. The main systems
for metabolism include breakage of the ester
bond by esterase action and oxidation at
various parts of the molecule. Induction of
liver microsomal enzymes has also been observed
(WHO, 1999).
Signs and symptoms of poisoning by pyrethroids
may take several forms. Because of
the similarities to crude pyrethrum, pyrethroids
may act as dermal and respiratory
allergens. Exposure to pyrethroids has resulted
in contact dermatitis and asthma-like
reactions. Persons, especially children, with
a history of allergies or asthma are particularly
sensitive, and a strong cross-reactivity
with ragweed pollen has been recognized.
Severe anaphylactic (allergic) reactions with
peripheral vascular collapse and respiratory
difficulty are rare. Other symptoms of acute
toxicity due to inhalation include sneezing,
nasal stuffiness, headache, nausea, incoordination,
tremors, convulsions, facial flushing
and swelling, and burning and itching
sensations. The most severe poisonings
have been reported in infants, who are not
able to efficiently break down pyrethroids
(ETN, Pyrethroids, 1994). With orally ingested
doses, nervous symptoms may occur,
which include excitation and convulsions
leading to paralysis, accompanied by muscular
fibrillation and diarrhea (ETN, Pyrethroids,
1994). Death in these cases is due to
respiratory failure. Symptoms of acute exposure
last about 2 days.
Endocrine Disruption and Breast Cancer
Many pyrethroids have also been linked to
disruption of the endocrine system, which
can adversely affect reproduction and sexual
development, interfere with the immune system
and increase chances of breast cancer.
Pyrethroids contain human-made, or
xenoestrogens, which can increase the
amount of estrogen in the body (Garey et al.,
1998). When tested, certain pyrethroids demonstrate
significant estrogenicity and increase
the levels of estrogen in breast cancer
cells (Go et al., 1999). Because increased
cell division enhances the chances for the
formation of a malignant tumor in the breast,
artificial hormones, like those found in pyrethroids,
may increase breast cancer risk
(PCBR, 1996). Some pyrethroids are classified
by EPA as possible human carcinogens.
Pyrethroids and the Environment
While the development of the synthetic pyrethroids
was heralded with claims of selective
toxicity to insects, both pyrethroids and
pyrethrins are extremely toxic to aquatic organisms,
including fish such as the bluegill
and lake trout, with LC50 values less than 1.0
parts per billion. These levels are similar to
those for mosquito, blackfly and tsetse fly
larvae, often the actual target of the pyrethroid
application. Lobster, shrimp, mayfly
nymphs and zooplankton are the most
Beyond Pesticides
National Coalition Against the Misuse of Pesticides
701 E Street, S.E., Suite 200 • Washington DC 20003 • 202-543-5450 (v) • 202-543-4791 (f) • •
susceptible non-target aquatic organisms
(Mueller-Beilschmidt, 1990). The nonlethal
effects of pyrethroids on fish include damage
to the gills and behavioral changes.
Pyrethroids are moderately toxic to birds,
with most LD50 values greater than 1000 mg/
kg. Birds can also be indirectly affected by
pyrethroids, because of the threat to their
food supply. Waterfowl and small insectivorous
birds are the most susceptible (Mueller-
Beilschmidt, 1990). Because pyrethroids are
toxic to all insects, both beneficial insects
and pests are affected by pyrethroid applications.
In some cases, predator insects may
be susceptible to a lower dose than the pest,
disrupting the predator-prey relationship.
Pyrethroids Residues / Persistence
As mentioned before, pyrethroids are designed
to breakdown more slowly than the
naturally occurring pyrethrins. While pyrethrins,
extremely sensitive to light, heat and
moisture, break down in a few hours, the synthetic
pyrethroids are stable and persist in
the environment much longer. As a general
rule, pyrethroids break down most quickly
in direct sunlight, usually just a few days
after application, with a few exceptions.
However, in areas with limited sunlight, such
as grain silos and subway tunnels, pyrethroids
can persist for months. For more specific
breakdown times see the sections below
on resmethrin, permethrin and sumithrin.
Both pyrethroids and pyrethrins are often
formulated with oils or petroleum distillates
and packaged in combination with synergists,
such as piperonyl butoxide (PBO) and
n-octyl bicycloheptene dicarboximide
(Gosselin et al., 1984). Synergists are added
to increase the potency of the pesticide. A
range of products from repellants to foggers
to pediculicides (lice killers) to garden sprays
contain synergists. Many formulations of
permethrin, resmethrin and sumithrin, including
ScourgeTM and AnvilTM, used along the
East Coast for mosquito control to combat
the West Nile Virus, contain the synergist
PBO inhibits important liver enzymes responsible
for breakdown of some toxins, in-
Beyond Pesticides
National Coalition Against the Misuse of Pesticides
701 E Street, S.E., Suite 200 • Washington DC 20003 • 202-543-5450 (v) • 202-543-4791 (f) • •
cluding the active ingredients of pesticides.
Specifically, it has been shown to inhibit hepatic
microsomal oxidase enzymes in laboratory
rodents and interfere in humans. Because
these enzymes act to detoxify many
drugs and other chemicals, a heavy exposure
to an insecticidal synergist may make a
person temporarily vulnerable to a variety
of toxic insults that would normally be easily
tolerated. Symptoms of PBO poisoning
include anorexia, vomiting, diarrhea, intestinal
inflammation, pulmonary hemorrhage and
perhaps mild central nervous system depression.
Repeated contact may cause slight skin
irritation. Chronic toxicity studies have
shown increased liver weights, even at the
lowest doses, 30 mg/kg/day. While not considered
a carcinogen by EPA, animal studies
have shown hepatocellular carcinomas, even
treatments as low as 1.2% (Takahashi et al.,
(PounceTM, TorpedoTM, DragnetTM)
Prior to 1978, permethrin was registered for
use on cotton crops only. During the early
1980’s registration was expanded to include
use on livestock and poultry, eggs, vegetables
and fruit. Today uses also include
lice treatments and urban/suburban pest
control. Permethrin resembles pyrethrins
chemically, but is chlorinated to increase its
stability. There are four isomeric forms, two
cis and two trans of technical permethrin.
Although the acute toxicity of the mixture
(oral rat LD50 > 5000 mg/kg, oral mouse LD50
= 500) is less than that of natural pyrethrins,
the cis-isomer is considerably more toxic (oral
mouse LD50 = 100), and in rats, the metabolites
of the cis-isomer are more persistent biologically.
(The cis and trans isomers differ in
the spatial arrangement of the atoms.) Formulations
of permethrin can vary greatly in
isomeric content. Compared to other pyrethroids,
permethrin is very stable, even when
exposed to ultraviolet light. Permethrin is
strongly absorbed to soil and other organic
particles, with half-lives in soil of up to 43
days. When used as a termiticide, permethrin
can persist up to 5 years.
Permethrin receives an EPA toxicity class
rating of II or III, (I = most toxic, IV = least
toxic) and carries either the word WARNING
or CAUTION on its label, depending on
the formulation. While it is not extremely
toxic to humans, there are numerous reports
of transient skin, eye and respiratory irritation.
Like all pyrethroids, permethrin is a central
nervous system poison. Workers and researchers
report tingling in face and hands,
and some report allergic reactions. Based on
studies demonstrating carcinogenicity, EPA
ranks permethrin as a class C, or possible
human carcinogen (U.S. EPA, 1997). Other
studies have shown effects on the immune
system, enlarged livers and at high doses,
decreased female fertility. Permethrin is extremely
toxic to aquatic life, bees and other
wildlife. It should not be applied in crops or
weeds where foraging may occur (ETN,
Permethrin, 1996).
(ScourgeTM, Raid Flying Insect KillerTM)
Resmethrin is used for control of flying and
crawling insects in homes, greenhouses,
processing plants, commercial kitchens, airplanes
and for public mosquito control.
Resmethrin is considered slightly toxic to
humans and is rated EPA toxicity class III,
bearing the word CAUTION on its label. The
oral rat LD50 is about 2500 mg/kg. Although
resmethrin has a very short half-life (under
an hour in direct sunlight), it persists much
longer in soil with a half-life of 30 days (ETN,
Resmethrin, 1996). Resmethrin breaks down
into a smelly byproduct, phenylacetic acid,
which binds strongly to textiles and dissipates
slowly, smelling of urine.
Resmethrin is absorbed rapidly and distributed
to all tissues including the brain. Skin
absorption is low, although it should be noted
that some individuals manifest allergic responses
including dermatitis, asthma, runny
nose and watery eyes after initial contact. In
laboratory animals, chronic toxicity studies
have shown hypertrophy of the liver, proliferative
hyperplasia and benign and cancerous
liver tumors. EPA reviewers noted slight,
but significant, increases in the number of
offspring born dead and decreased viability,
which they thought might be secondary to
trans placental toxicity. Tests for neurotoxicity
have been negative. Resmethrin is extremely
toxic to fish, other aquatic life and
bees. The domestic manufacturer of
Synthetic Pyrethroids chemicalWATCH Factsheet References
Beyond Pesticides
National Coalition Against the Misuse of Pesticides
701 E Street, S.E., Suite 200 • Washington DC 20003 • 202-543-5450 (v) • 202-543-4791 (f) • •
resmethrin, Penick Company, will not identify
the inert ingredients in its product, but
recommends that it is not sprayed on paint,
plastic or varnished surfaces, and that treatment
of living areas or areas with large
amounts of textiles be avoided.
(AnvilTM, d-Phenothrin)
Sumithrin has been registered for use since
1975. It is used to control adult mosquitoes
and as an insecticide in transport vehicles,
commercial, industrial and institutional nonfood
areas, in homes, gardens, greenhouses
and on pets. Chemically, it is an ester of
chrysanthemic acid and alcohol. It is a combination
of two cis and two trans isomers.
Sumithrin is slightly toxic and is rated EPA
toxicity class IV bearing the word CAUTION
on its label. The oral rat LD50 is greater than
5,000 mg/kg, and the LC50 for inhalation is
greater than 1210 mg/m3. Sumithrin degrades
rapidly, with a half-life of 1-2 days under dry,
sunny conditions. Under flooded conditions,
the half-life increases to 2-4 weeks for the
trans isomer and 1-2 months for the cis isomer.
In grain silos, with no sunlight and little
air circulation, most of the product still remains
after one year (WHO, 1990).
Symptoms of acute sumithrin poisoning include
hyperexcitability, prostration, slow respiration,
salivation, tremor, ataxia and paralysis.
Chronic feeding studies resulted in increased
liver weights in both males and females.
In rat studies, sumithrin was completely
excreted in 3-7 days (WHO, 1990).
Studies have shown that sumithrin demonstrates
significant estrogenicity and increases
the level of estrogen in breast cancer
cell, suggesting that sumithrin may increase
the risk of breast cancer (Go et al.,
Cassagrande, R.A. 1989. “Considerations
for state label for PermanoneTM.” RI
Pesticide Relief Advisory Board.
Providence, RI.
“EPA’s Recent Bets.” Science, vol. 218,
December 3, 1981.
Extension Toxicology Network (ETN).
1996. Permethrin.” Pesticide
Information Profiles. .
Extension Toxicology Network (ETN).
1994. Pyrethroids.” Pesticide
Information Profiles. .
Extension Toxicology Network (ETN).
1996. Resmethrin.” Pesticide
Information Profiles. .
Garey, J. and M. Wolff. 1998. “Estrogenic
and Antiprogestagenic Activities
of Pyrethroid Insecticides.” Biochem
Biophys Res Commun. 251 (3): 855-9
Go, V. et al. 1999. “Estrogenic Potential of
Certain Pyrethroid Comounds in the
MCF-7 Human Breast Carcinoma
Cell Line.” Environmental Health
Perspectives. 107:3
Gosselin, R.E. 1984. Clinical Toxicology of
Commercial Products. Williams
and Wilkins. Baltimore, MD.
Hallenbeck, W.H. and K.M. Cunningham-
Burns. Pesticides and Human Health.
Springer-Verlag, New York, NY.
Hayes, W.H., Pesticides Studied in Man,
Williams & Wilkins. Baltimore,
MD. 1982.
“Hormonal and Environmental Factors
Affecting Cell proliferation and
Neoplasia in the Mammary Gland.”
Progress in Clinical and Biological
Research (PCBR). 394:211-53, 1996.
Kaloyanova, F. and S. Tarkowski, eds,
Toxicology of Pesticides - Interm
Document 9, World Health
Organization, Copenhagen, 1982.
Klaassen, C.D. et al., eds, Casarett and
Doull’s Toxicology, Macmillan
Publishing Co., New York, NY.
Kolmodin-Hedman, B., et al. 1982.
“Occupational exposure to some
synthetic pyrethroids (permethrin
and fenvalerate).” Arch. Toxicol.
Mueller-Beilschmidt, Doria. 1990.
“Toxicology and Environmental Fate
of Sythetic Pyrethrois.” Journal of
Pesticide Reform. 10 (3):32-37.
National Research Council. 1987.
Regulating Pesticides in Food: The
Delaney Paradox. National Academy
Press, Washington, DC.
Olkowski, W. 1989. “Natural and synthetic
pyrethrum insecticides: Finding your
way through the maze.” Common
Sense Pest Quarterly. 5(1):8-12.
Reigart, J., M.D. et al., Recognition and
Management of Pesticide Poisonings,
EPA, 1999.
Scourge Insecticide Product Label with
SBP-1382/Piperonyl Butoxide 18% +
54% MF. U.S. EPA Reg. No. 432-667.
AgrEvo, montvale, NJ.
Takahashi, O., et al., 1994. “Chronic toxicity
studies of piperonyl butoxide in F344
rats: induction of hepatocellular
carcinoma.” Fund. Appl. Toxicol. 22:
Thomson, W.T. 1984. Agricultural
Chemicals: Insecticides. Thompson
Publications, Fresno, CA.
U.S. EPA. 1979. Environmental Fate Review
of Permethrin (activated sludge
metabolism study). November 29.
Office of Pesticide Programs.
Washington, DC.
U.S. EPA. 2000. For Your Information,
Synthetic Pyrethroids for Mosquito
Control. Washington, DC. May.
U.S. EPA. 1997. Office of Pesticide
Programs list of chemicals evaluated
for carcinogenic potential. Memo from
W.L. Burnman, HED, to HED branch
chiefs. Washington, DC. February 19.
World Health Organization (WHO). 1990.
d-Phenothrin. Environmental Health
Criteria. Geneva.
Wright, C., et al. 1981. “Insecticides in the
ambient air of rooms following their
application of control of pests.” Bull.
Environ. Contam. Toxicol. 26:548-553.

The 18th century Venetian adventurer, romancer, and alchemist Giovanni Casanova
reportedly said, "In the hands of the wise, poison is medicine. In the hands of a fool,
medicine is poison."
And in the hands of a pet owner who isn’t careful, the wrong kind of medicine can have
disastrous consequences.
Advancements in medicine over the last several decades have given us a cornucopia of
drugs that can help cure or alleviate many ills, both in humans and in animals. But
what’s good for the goose is definitely not always good for the German shepherd. A
medication that works for you may not work for your dog, and what works for your dog
may not work for your cat, rabbit, or ferret.
"People think that cats are like little dogs, and they’re all like little people. They don’t
think about it, and they don’t read the [medication’s] label," said Dr. Petra Volmer,
assistant professor of toxicology at the University of Illinois College of Veterinary
Medicine. "Someone thinks, ‘I use naproxen and it works great for my back. My dog has
a sore leg, so let’s give him some naproxen.’"
Animals of different species process drugs in widely varying ways. Dogs, for example,
are very sensitive to non-steroidal anti-inflammatory drugs (NSAIDs) that are common
in their humans’ medicine cabinets. This class of drugs includes pain medications like
acetylsalicylic acid (aspirin), ibuprofen (Advil and Nuprin), acetaminophen (Tylenol),
phenylbutazone ("bute," a common painkiller for horses) and naproxen.
According to pet poisoning guidelines issued by the American Veterinary Medical
Association, as little as two tablets of regular strength aspirin or Tylenol can cause
significant tissue damage in dogs, and repeated doses significantly increase the risk of
Cats are even more sensitive to NSAIDs because they have less of the enzyme that is
required to detoxify and utilize the drugs: just two extra-strength Tylenol can be deadly.
This page is a special feature of Working Dogs
Bad Medicine
Written by: Erin Harty, Staff Writer -
NSAID metabolites can impair the function of a cat’s red blood cells, which carry
oxygen from the lungs to all the cells of the body. If the red blood cells aren’t performing
their job properly, a cat can suffocate from lack of oxygen.
Well-meaning owners often assume that these common, over-the-counter drugs are safe
for their pets in smaller doses. "It has nothing to do with size; it’s the way they
metabolize it. With everything, we assume that cats are twice as sensitive," said Jill
Richardson, DVM, veterinary poison information specialist at the American Society for
the Prevention of Cruelty to Animals’ National Animal Poison Control Center.
The problems aren’t limited to human medications—a drug that may be safe and useful
for one animal can be deadly for another. Many of the calls to the National Animal
Poison Control Center are regarding accidental poisonings from topical flea medications.
Products that are designed for dogs often contain a drug called permethrin, which has a
wide safety range when used on Fido. But just a few drops of the product may cause
Fluffy the cat to suffer potentially fatal seizures.
Rabbits are also extraordinarily sensitive to flea medications intended for both dogs and
cats. The Frontline brand in particular has been blamed for several rabbit deaths,
although the manufacturer has never suggested that the product be used on rabbits.
"Drugs in rabbits are very different from dogs and cats. Extremely different, in some
cases," said George Flentke, a biochemist in the toxicology department at the University
of Wisconsin. "I always recommend that people contact a bunny-savvy vet [before using
any medication.] They keep up on this stuff."
Although the packaging for any such products will explicitly say that they’re only for use
on the species for which they’re intended, owners sometimes mix up medications that are
meant for other pets. Or they simply won’t read the label, and will (erroneously) assume
that a product can be used on any animal.
"Think about what you’re doing. Animals are different from people, and different from
each other. All different species metabolize things differently," said Dr. Volmer. "If
you’re not sure [if a medication is safe], call your vet or ask an animal poison control
In some cases, pets get themselves into poisoning trouble. Dogs may dig discarded
medication out of the trash, or chew up an entire box of cold medicine, contents and all.
Cats may lick the sugar coating off tablets of Advil that are left on the side of the sink. A
ferret with run of the house might get into a kitchen cabinet and help itself to some
Large breeds of dogs are the most frequent mass-consumers of medications. "They’re
less finicky. They don’t think, they just eat," said Dr. Volmer. "And they don’t stop at
one [tablet]. They have to eat the whole package."
Ferrets also seem prone to overdose accidents because they’re extremely curious and
have the appetites of dogs, according to Dr. Richardson. Cats, on the other hand, won’t
usually eat something unless it tastes good; their overdoses are most often humancaused.
According to guidelines issued by the National Animal Poison Control Center, all
prescription and over-the-counter drugs should be kept safely out of pets’ reach,
preferably in closed cabinets. Pain killers, cold medicines, anti-cancer drugs,
antidepressants, vitamins and diet pills can all be lethal to animals.
Also beware of herbal medications, Dr. Richardson warns. Just because something is
billed as "natural" doesn’t mean it won’t have adverse affects on a pet. One particular
herbal diet aid has caused numerous accidental poisonings.
"It contains mahuang, which is like herbal ephedrine, and guarana, which is like caffeine.
We’ve had so many horrible cases on that," said Dr. Richardson. She recently treated a
miniature pincher who’d eaten 30 to 50 of the pills. A lethal dose of this drug is 6
milligrams per kilogram of body weight; the pinscher consumed 7 times that, and
eventually died.
In most cases, animals have a good chance of survival if their owners seek medical
attention immediately. A veterinarian can pump the animal’s stomach, administer
activated charcoal and perform diuresis, all of which will help minimize the dangerous
effects of the drug. The sooner a veterinarian sees a pet, the more likely the case will
have a happy ending.
"We do have some ‘miracle cases.’ But not very many of them would have made it if
someone hesitated or waited a day or two," said Dr. Richardson. "I think in most cases
we get here, if we get the information early, the animals are treated promptly and
Article republished here with permission from
Copyright(c) 2000 by
All site contents and design Copyright 1996-2002 Working Dogs
3 : Is Spraying For West Nile Pointless? Page 1 08/16/03 12:33:07 AM
August 15, 2003
A truck sprays pesticides into the air in an area
threatened by West Nile. (
Useless Spraying?
West Nile Deterrent May Not Be Best
By Erin Hayes
Aug. 8 —If you live in one of the 34 states
where West Nile virus has been discovered,
should you draw comfort from the clouds of
pesticides being sprayed into the air to kill off
Print This Page
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Across U.S.
Maybe not.
"The chemicals have not been adequately tested for their
human health effects," cautioned Dr. Sheldon Krimsky, a
pesticide-risk expert at Tufts University. "There is a lot of
circumstantial evidence that they cause cancer in animal
studies, that they are hormone disruptors. Remember, these
are neurotoxins," Krimsy said, adding that most studies done
on the effects of spraying focused on agricultural spraying —
not spraying in populated areas.
"We simply don't know what effects it's going to have, the
indiscriminate spraying on human populations," he added.
So far this year, officials have reported 112 human cases of
West Nile, more than half in the past week, according to the
Centers for Disease Control and Prevention (CDC). Most of
these infections are in Louisiana. Mississippi and Illinois also
have reported human infections of the virus, which began
showing up in the United States in New York three years
Residents Protest Dangerous Pesticides
In Hays County, Texas, residents effectively stopped
spraying in many of their neighborhoods. They protested,
carrying posters decrying the "chemical warfare" on their
families. County commissioners agreed to severely limit
mosquito spraying near those homes.
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• Study: DEET Is Best
Bug Repellent
In Jefferson County, Ala., health officials are trying to decide
what to do. Many cities, including Birmingham, have been
spraying for weeks. County officials, though, haven't started
their own spraying efforts because they aren't convinced
spraying is all that effective against mosquitoes.
But after a case of West Nile infection was recently discovered in Alabama, some
residents began demanding that the county spray.
Brian Debrow, the environmental health program manager at the Jefferson County Health
Department, says the county may start spraying in order to ease anxiety. "The public likes
seeing the trucks going down the road and spraying for mosquitoes," he said. "It gives
them a sense of security they feel like something is being done."
CDC: Least Effective Deterrent
Scientists at the CDC, which is leading the surveillance effort on West Nile virus, do not
take a position on whether local health departments should spray for mosquitoes. But they
do point out that spraying is the least effective method of slowing the spread of the insects.
Since adult mosquitoes only live for about two weeks at the most— whether they're
sprayed or not— the most effective way to limit mosquitoes is to keep them from
breeding. And that means emptying out pools of water where they lay their eggs.
"These would include things like old tires, cans, flower pots and so forth, just do an
inspection and see what's out there that might serve as a breeding ground for the
mosquitoes," points out Dr. Julie Gerberding, the director of the CDC.
And that's exactly what Washington, D.C. health officials are doing. They've decided that
spraying pesticides into the air is too risky, especially since the incidence of asthma in the
city is already high.
Instead, they are targeting small amounts of pesticides directly into pools of standing water
and ditches where mosquitoes are breeding. And they're going door-to-door with
pamphlets that explain to residents how best to avoid mosquitoes altogether.
Even so, in the states where West Nile virus has been discovered, there is pressure on
health officials to appear to be fighting the virus on every front and many feel the most
convincing approach is the most visible one—spraying chemicals out where everyone can
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Duke Pharmacologist Says Animal
Studies on DEET's Brain Effects
Warrant Further Testing and Caution
in Human Use
DURHAM, N.C. -- A Duke University Medical
Center pharmacologist is recommending caution
when using the insecticide DEET, after his
animal studies last year found the chemical
causes diffuse brain cell death and behavioral
changes in rats after frequent and prolonged use.
Mohamed Abou-Donia, Ph.D. has also called
for further government testing of the chemical's
safety in short-term and occasional use,
especially in view of Health Canada's recent
decision to ban products with more than 30
percent of the chemical. Every year,
approximately one-third of the U.S. population
uses insect repellents containing DEET,
available in more than 230 products with
concentrations up to 100 percent.
While the chemical's risks to humans are still
being intensely debated, Abou-Donia says his
30 years of research on pesticides' brain effects
subject keywords : toxicology, chemical
exposure, insecticide
date : 5/1/2002
contact : Becky Levine , (919)
660-1308 or (919) 684-
Mohamed B. Abou-Donia, Ph.D.
More Info
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Extinguish Teen Smoking
Duke Health Briefs: Protecting
Your Skin From the Sun
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Image' in the Brain
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'Parkinson's Patch'
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Brain Aging in Healthy Adults
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Scientists Working to Develop
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clearly indicate the need for caution among the
general public.
His numerous studies in rats, two of them
published last year, clearly demonstrate that
frequent and prolonged applications of DEET
cause neurons to die in regions of the brain that
control muscle movement, learning, memory
and concentration. Moreover, rats treated with
an average human dose of DEET (40 mg/kg
body weight) performed far worse than control
rats when challenged with physical tasks
requiring muscle control, strength and
coordination. Such effects are consistent with
physical symptoms in humans reported in the
medical literature, especially by Persian Gulf
War veterans, said Abou-Donia.
"If used sparingly, infrequently and by itself,
DEET may not have negative effects – the
literature here isn't clear," he said. "But frequent
and heavy use of DEET, especially in
combination with other chemicals or
medications, could cause brain deficits in
vulnerable populations."
Children in particular are at risk for subtle brain
changes caused by chemicals in the
environment, because their skin more readily
absorbs them, and chemicals more potently
affect their developing nervous systems, said
Abou-Donia. Commonly used preparations like
insecticide-based lice-killing shampoos and
insect repellents are assumed to be safe because
severe consequences are rare in the medical
literature. Yet subtle symptoms -- such as
muscle weakness, fatigue or memory lapses --
might be attributed erroneously to other causes,
he said.
With heavy exposure to DEET and other
insecticides, humans may experience memory
loss, headache, weakness, fatigue, muscle and
joint pain, tremors and shortness of breath, said
Abou-Donia. His earlier research, examining the
brain effects of three chemicals used during the
Persian Gulf War, clearly demonstrated that
chickens exhibited similar signs that the Gulf
Duke Study: High Mutation
Rates in Cancer Cells Can Be
Caused By Non-DNADamaging
The Dangers of DEET 06/21/02
If you're headed for the beach
this summer, you might want to
forget the sunscreen, and pack
the sunblock instead.
War veterans complained of upon returning
from service. (Journal of Toxicology and
Experimental Health, May, 1996, Volume 48, p.
35 - 56).
Such overt symptoms are not seen immediately
after use but may manifest themselves months
or years after exposure, making a cause-andeffect
relationship difficult to establish , said
Abou-Donia. By studying animals such as
chickens and rats, however, researchers are able
to compress the time between exposure and the
onset of symptoms: 10 months of a rat's life is
several years in a human's life. Moreover,
researchers can study layers of the rats' brains at
various stages after exposure to measure the
chemical's effects on the brain.
Indeed, Abou-Donia's two most recent studies
demonstrate the severe brain and behavioral
deficits that rats experience after two months of
daily skin applications with DEET and
permethrin, another common insecticide,
(Experimental Neurology, 2001, volume 172 ,
p.153- 171); and following 60 days of exposure
to DEET and permethrin, and 15 days of
pyridostigmine bromide, an anti-nerve gas agent
(Journal of Toxicology and Environmental
Health, 2001, volume 64, p. 373-384). Both
studies examined the effects of each drug alone
and in combination.
In each study, the treated animals initially
appeared to be normal, just like the control
group, said Abou-Donia. But when challenged
with neurobehavioral tasks that required muscle
control, strength and coordination, the rats
demonstrated serious impairments. Moreover, a
detailed analysis of their brains clearly showed
that large numbers of brain cells were dying
within three critical brain structures: the cerebral
cortex, which controls muscles and movement;
the hippocampal formation, which controls
memory, learning and concentration; and the
cerebellum, which synchronizes body
In addition, many of the surviving brain cells
showed signs of degeneration and damage
consistent with the presence of harmful
byproducts called oxygen free radicals (also
known as reactive oxygen species), which can
damage DNA and cell membranes in the brain
and the nervous system.
The most severe brain cell changes and
sensorimotor deficits were seen among rats
exposed to combinations of DEET, permethrin
and the anti-nerve gas agent pyridostigmine
bromide, which reduces the body's normal
ability to inactivate pesticides. Such findings
confirmed Abou-Donia's 1996 and 2001 animal
studies demonstrating that harmless doses of
these three chemicals proved highly toxic to the
brain and nervous system when used in
"The take home message is to be safe and
cautious when using insecticides," said Abou-
Donia. "Never use insect repellents on infants,
and be wary of using them on children in
general. Never combine insecticides with each
other or use them with other medications. Even
so simple a drug as an antihistamine could
interact with DEET to cause toxic side effects.
Don't spray your yard for bugs and then take
medications. Until we have more data on
potential interactions in humans, safe is better
than sorry."
sources :
Mohamed Abou-Donia Ph.D. ,
(919) 684-2221
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Shawn Basil
(Bearpaw) - F

Locale: Southeast
Permthrine - use when in mosquito and tick country on 09/06/2007 06:41:14 MDT Print View

In an age of West Nile virus in mosquitoes and Lyme Disease amongst ticks, I'll gladly accept the risk posed by permethrine. Any wilderness endeavor is based upon a foundation of risk management, and I find the risks involved in using permethrine acceptable versus the other options available, such as being comsumed by insects or annointing my body in the more abrasive DEET option.

Now can we please get back to the topic of comfort and moisture control in merino wool products?

Is that cavalier enough for you?

Edited by Bearpaw on 09/06/2007 06:43:06 MDT.

Thomas Knighton
(Tomcat1066) - F

Locale: Southwest GA
Re: Permthrine - use when in mosquito and tick country on 09/06/2007 07:06:32 MDT Print View

>Now can we please get back to the topic of comfort and >moisture control in merino wool products?
>Is that cavalier enough for you?

Here here!

I know how to deal with bugs in multiple ways...moisture control is still an area I have a lot more to learn about ;)


Phil Barton
(flyfast) - MLife

Locale: Oklahoma
Re: permethrin on 09/06/2007 07:16:48 MDT Print View

Shawn expresses my thoughts very well. In light of the risk of insect borne disease, permethrin is a reasonable choice. There is no such thing as a free lunch.

eric levine
(ericl) - F

Locale: Northern Colorado
off topic on 09/06/2007 23:05:21 MDT Print View

Now now,

The off topic was started by folks asking which material was most compatible laced with Permethrin, a class C carcinogen. It was not started by me, who believes in giving people at least a clue, then cheerfully letting them choose their own poison. Since 80% of west nile virus has no effect save longterm future immunity, things could be worse.

Adulticides harm our environment and damage our planet's ecosystems (remember Rachel Carson?), so I usually assume outdoors people would be humble enough to show it some respect.

For the past 5 months I've been using Picaridin, a non-deet presumably much safer repellent said to be just as effective. Only Cutter Advanced outdoorsman has enough concentration (15%) to be truly useful. The standard in Europe is ~19.2%

Friends of mine have acid tested it with VERY good results in the Colorado's Rawah's this year.

My results have about equaled Sawyer controlled release deet, which is 20%. Equal to 20% deet was not enough for several really infested trails I hiked, but it did save my rear.

Edited by ericl on 09/07/2007 10:07:40 MDT.

donald buckner

Locale: Southeast U.S.
wool,synthetics,deet, permethrin on 10/02/2007 21:43:02 MDT Print View

As a hunter first, and a backpacker second, I am a fan of both synthetic and wool, but only recently used the really thin wool base layers. I prefer the wool to the synthetic for most applications. This is after a week of hunting in the Northwest Territories, Canada. I live in the Southeast, and redbugs (chiggers) and ticks and really bad in early hunting season. The permethrin works well for ticks, but I have found deet works best for chiggers and mosquitos. The device called thermacell is a lifesaver where mosquitos are a serious problem. (Walmart,$25). I don't know the weight, but it is not very heavy, and might be con sidered even for backpacking. I don't know how the native americans made it in some of the rotten insect conditions in the southeast. In the Northwest T. Canada, I underestimated the temps, so I layered icebreaker wool then Patagonia zip T, then Golite hooded windshirt, then a soft shell camo zip T, wool fleece blend vest, and finally a packable gortex rain jacket to withstand temps around 40F with 15-20mph winds,(and a good bit of 35mph boat ride around a big lake). I was very pleased with this setup and by layering the wool under the Patagonia, I did not have the stink issues. Of course, when running around the tundra, multiple layers were shucked to keep from overheating.

Forrest G McCarthy
(forrestmccarthy) - MLife

Locale: Planet Earth
Merino wool, Icebreaker, and New Zealnd on 11/28/2007 10:16:19 MST Print View

Great article. The objective and scientific testing procedure produced credible and highly informative data. I would love to have one of the clown shirts! Maybe an all wool replica?

The resonance of wool in outdoor apparel should be credited to New Zealand. In the late 90’s New Zealand companies, such as Icebreaker (, refined the processing of merino wool and produced an attractive line of highly functional outdoor garments. A regular visitor to New Zealand during that time I was an early convert. My wardrobe of Icebreaker products includes briefs, t-shirts, long sleeve shirts, leggings, and a hoodie. Marino base and insulating layers are my first chose on any outing with the exception of the hottest summer or desert adventures. On those hottest days I prefer a light nylon (Exoficio) shirt and pants. But that is another story.

I would like to comment on wool verses synthetics in extremely wet environments. The articles research did a wonderful job examining drying times. However, experiments that investigated insulating values while fabrics were wet maybe equally revealing. I prefer wool for both base and insulating layers when subjected to indomitable moisture. White water packrafting or “tramping” in New Zealand are prime examples of these moist and cold activities. It has been my observation that wool is a far greater insulter when saturated then any synthetic.

The challenging maritime weather of New Zealand Zealand’s South is legendary. My Kiwi accomplices have long advocated merino wools superior performance in cold, wet, and variable weather. Merino wools superior performance, odiferous appeal, and environmental benefits make it my first chose when packing for any outing.

Edited by forrestmccarthy on 11/28/2007 19:51:06 MST.

John Haley
(Quoddy) - F

Locale: New York/Vermont Border
Re: Merino wool, Icebreaker, and New Zealnd on 11/28/2007 10:41:52 MST Print View

For this winter season I have now accumulated an Icebreaker Exp 320 Conquest Hoodie, Icebreaker Bodyfit Oasis 200 Crewe top and Icebreaker Bodyfit 200 leggings, an Icebreaker 320 weight cap, and also Icebreaker glove liners. So far I've been able to layer under shell outerwear and be as warm as I've ever been without the usual buildup of moisture happening. I've been using Smartwool socks again this year since that's the one item that Icebreaker doesn't seem to make.

Michael Davis
(mad777) - F

Locale: South Florida
Re: Re: Merino wool, Icebreaker, and New Zealnd on 11/28/2007 11:09:51 MST Print View


Let us know how you like the Icebreaker Exp 320 Conquest Hoodie. I am a "dyed in the wool" merino fan and have been looking (read, drooling) at this piece for a while. I have every other weight of merino garments from the "big 3", Smartwool, Icebreaker and Ibex.

Of all of them, I think that Icebreaker makes the most comfortable clothes; "snuggly" is how I describe them. Ibex makes extra durable material but it's somewhat "hard" and their stupid little logos that are embroidered are down-right scratchy. I have no complaints with Smartwool.

Because at this thickness, 320 gm/m2, wool gets pretty heavy, I'm also considering a PowerStretch hoodie instead (with thin wool baselayer) to save weight. I'm not sure if this approach would actually save the weight and I'm also worried that the PowerStretch hoodie will take up more room in my pack when not wearing it, compared to the Conquest Hoodie.

John Haley
(Quoddy) - F

Locale: New York/Vermont Border
Re: Re: Re: Merino wool, Icebreaker, and New Zealnd on 11/28/2007 12:25:17 MST Print View

Hi Michael...

I got the Conquest first and was so pleased with it that I went for all the rest. I couldn't believe that a hoodie of that weight would be so comfortable against bare skin. The Conquest hoodie is heavy at 22 ounces (large), but being the EXP weave it is extremely warm and wind resistant. I have worn it alone with no base layer on 35F days, albeit with no strong wind.

I wrote to Icebreaker about weight recommendations on the base layer and was told that the 200 would probably fit my needs better than the very warm 260, particularly for the bottoms. One weight would be just as comfortable as the other against the skin.

Edited by Quoddy on 11/28/2007 12:26:47 MST.

Forrest G McCarthy
(forrestmccarthy) - MLife

Locale: Planet Earth
Re: Re: Re: Re: Merino wool, Icebreaker, and New Zealnd on 11/28/2007 19:42:45 MST Print View

My wife has the Conquest Hoodie and is very happy with it. She wears it mainly as a wintertime insulating layer. I have a Nomad Hoodie and wear it year round.

Kavin Davidson raised the question of the durability of wool, especially when wet. I have developed holes in several of my Icebreaker garments. They all occurred while wearing merino as an outer layer when bushwhacking. To my memory the wool was not particularly wet. I doubt other base/insulating fabrics would have faired better. The holes did not continue to grow as they often do with Polypro or Capiline. I was able to easily stitch the holes closed.

Michael Davis
(mad777) - F

Locale: South Florida
Re: Re: Re: Re: Re: Merino wool, Icebreaker, and New Zealnd on 11/29/2007 11:09:49 MST Print View

John or Forrest,

Can you post the weight (and size) of your Conquest Hoodie?


John Haley
(Quoddy) - F

Locale: New York/Vermont Border
Re: Re: Re: Re: Re: Re: Merino wool, Icebreaker, and New Zealnd on 11/29/2007 12:59:40 MST Print View


I answered that question in my original post. The Conquest hoodie, in size Large, weighs 22oz. BTW, the hoodie is quite a bit larger than the Bodyfit 200 base layer. The Bodyfit 200 in a large fits both my top and legs like skin (I'm 6'1" and 186#), the hoodie in 320 EXP is more like a normal sweater fit. It does fit easily over the base layer.

Michael Davis
(mad777) - F

Locale: South Florida
Re: Re: Re: Re: Re: Re: Re: Merino wool, Icebreaker, and New Zealnd on 11/29/2007 13:43:41 MST Print View

Thanks John,

Sorry about asking to repeat the weight.

It's getting bad when I can't remember back just 2 posts! I'm glad they display my name when logged into this website so I don't forget who I am. :-(

BTW: I'm exactly the same height & weight as you so fit should be the same.

Thanks again,

john griffith

Locale: Southeast U.S.
correct way to measure baselayer performance on 07/03/2008 18:21:24 MDT Print View

I can usually expect a plethora of keen insights from the forum comments after the articles. For once I'm disappointed at the lack of depth. All you guys are talking about is weight, warmth, drying time. I'm a total capilene guy. There is a certain amount of joy to be had when being out in the dead of winter with a hiking partner, and after a few days of putting in hard miles with windshirt/raingear on top of your baselayer, being the one that clearly smells the worst inside the tiny little tent that you don't dare ventillate because it's ten degrees outside. Does nobody else get this? Capilene rules!

LeRoy Schmeling
(leroys) - F
Wool - water absorption on 01/30/2009 15:26:04 MST Print View

I camp in the dampness of the Boundary Waters area of Northern Minnesota. Wool has become my BL of choice. I've gotten good deals on Terramar products.

Re water: I started treating my garments with the wash in water based DWR products and really like the result. There is less water absorption, they shed light showers and dry quicker.


James Moughan
Re: Why didn't BPL do this: on 02/01/2009 22:11:37 MST Print View

I think these values are very interesting. They show that each fabric takes almost the same time to loose a gram of water in the fabric. With an average time of 1:53 min and a 9,4% spread. Most interesting is that the smartwool shirt dries actually 1 second faster than the patagonia shirt.

2 years late ッ but this is a really good catch. When hiking, drenching your base layer should be rare if you control venting and such, so merino seems clearly superior.

David Shepherdson
(shepherdsond) - M
Re: Why didn't BPL do this: on 02/25/2009 13:05:14 MST Print View

Yes, great catch. This is a key variable IMO.

I like the feel and performance of wool baselayers and the sustainbility but my concerns are cost and durability. I have polyester baslayers that are 15 years old and still going strong whereas I just had to retire a Smartwoll microlight base layer after 4 years of average use (too ma ny rips and tears). My wife suggested that it might last longer if it was not dried in a dryer so I am trying that for my icebreaker replacement. I also have another durability question. I find that most of my wool sweaters end up with moth holes in them long before they wear out, is the same true for merino baselayers? I assume it is...

Mitchell Keil
(mitchellkeil) - F

Locale: Deep in the OC
Wool and the Sun on 08/27/2009 14:06:11 MDT Print View

Has anyone, including the testers and staff anything to say about the SPF of Wool vs Synthetics. I have worn both during many hikes. I prefer the wool and have used an Icebreaker LS 200 crew as my go to shirt/baselayer in everything from 30degrees to 90degrees in the Sierras and the San Gs near LA. I am very sun sensitive and from my observations whether I am wearing a high SPF rated synthetic or my wool I seem to get the same result: no over exposure to harmful solar radiation. But I can't really find anything online or from the manufacturers of wool apparel that indicates wool's sun protective qualities.

Any comments or citations?

Einstein X
(EinsteinX) - F

Locale: The Netherlands
Re: Comfort and Moisture Transport.... that's odd. on 08/27/2009 14:42:46 MDT Print View

Now this is an odd article! I read it already three years ago, as you can also see in the post date of the "Companion forum thread to" post. Even though being an old (but good) article it popped up in my iGoogle's BPL RSS reader as a fresh and new article. Or it least I was of the opinion that the 5 newest articles that BPL publishes will appear in my iGoogle page. So how come this one popped up??


John Davis
(Bukidnon) - F
Old article showing up as new on 11/01/2009 03:23:33 MST Print View

Same here, Eins.

But I'm glad because this was a good read. Your 2007 post is interesting and possibly calls for an answer from those who tested the swatches. Strangely, the conclusion drawn from your calculation is completely at odds with my experience. After a run I feel an urge to get out of a merino top within 30 seconds of getting back home because the heavy weight of cold sweat feels horrible against my skin. In the only synthetic top I have left, after ending the run, I can keep it on long enough to prepare a recovery drink and run a bath.

No one has mentioned getting a hot, sweaty back under a rucksack. I was one shouldering long before reading about it in Beyond Backpacking because I can sweat enough over a couple of days of backpacking to cause nappy rash. I find merino particularly unpleasant against the skin of my back when a day's walking has turned uphill and aerobic. It really itches when warmed up a bit.

Merino has numerous, clear disadvantages so why do I prefer it? On my last Pyrenean hike I had a Capilene T-shirt and a Smartwool long sleeved top. I had thought that the T would be the garment I walked in and the merino top would be for evenings. That is not what happened. The T-shirt became so stinky within half a day that I could not stand it. (One of the 2006 posts mentions this problem. I wish I'd read it back then.) The merino went on and stayed on day after day without becoming unpleasant.

There must be something physical and measurable, apart from lack of stench, which accounts for my preference for merino. Perhaps it's to do with the drape. Merino feels more classy, somehow.

By the way, synthetics do wear away. Mine became thinner and thinner till all that was left was a net of fine fibres. The seams on synthetic tops have always had a tendency to carve away at my delicate skin, particularly round the armpit, and this becomes much worse once the top is a bit worn.

Best wishes.

david plantenga
(dplant) - F
Synthetics & Wool on 02/03/2010 05:19:06 MST Print View

Interesting detailed article.

Did I miss "cost"?
Synthetic "T" shirt about $8 to $10
Wool "T" shirt about --------?

Off subject comment: First thing I thought of with Trail Runner Don in the Desert ... Rattlesnake Magnet ...?

Happy Trailz

Steve Arlowe
(sarlowe) - F
High Altitude Reflected Sun - Compression Shirt or Merino Baselayer? on 06/21/2010 15:06:39 MDT Print View

Great info here! Anyone have additional comments about using a long sleeve merino baselayer to block sun and keep cool, especially in more or less low-humidity conditions? I took a white Zensah compression shirt on a Kilimanjaro trek in 2007, and just last week to Muir on Rainier. The compression shirt is fantastic at blocking/reflecting the sun and acting almost as an air conditioner. Any sweat evaporates immediately, keeping me nice and cool. And since it's a compression shirt, this is the case for every square inch. No baggie sleeves to trap air - which seems to me would allow heat build up and impair wicking. So the only drawback on the white compression shirt in that setting is, of course, the stench. Zensah uses an antimicrobial treatment. But while the stink is reduced (and kind of a different, slighlty less pungant odor) than your typical untreated wicking synthetic, it's still there, still pretty objectionable. Meanwhile my Smartwool midweight that I also wore off and on all week has no discernable odor. So now I'm looking at Rapha's white merino baselayer or Smartwool's microweight in silver. I gather merino's wicking is better, but evaporation is not as good. Is the evaporation enough poorer that I'll go from feeling air-conditioner-like properties to feeling like I'm wearing a thin but soggy insulating layer? Anyone have any experience with the newer generation UnderArmour heatgear? It claims antimicrobial properties - does it fight stench as well as merino? Thanks!

Edited by sarlowe on 06/21/2010 15:07:31 MDT.

Paul McLaughlin
(paul) - MLife
Re: High Altitude Reflected Sun - Compression Shirt or Merino Baselayer? on 06/21/2010 21:35:49 MDT Print View

I have used lightweight wool long johns as a baselayer/sunscreen, up to about 12,500 in the sierra in early May on a ski trip - worked great. Have not used a wool top in the same way, as I don't have one in a light enough color. I found the wool bottoms to be nearly as cool as wearing nothing, and they definitely wicked, evaporated and kept the (very intense) sun off just fine.

Steve Arlowe
(sarlowe) - F
Re: Re: High Altitude Reflected Sun - Compression Shirt or Merino Baselayer? on 06/22/2010 09:53:30 MDT Print View

Thanks Paul! Good to know. I suppose it's a question of finding something that works in a certain range of conditions. A merino baselayer might fail in hot/humid conditions (while a synthetic compression shirt is able to keep up evaporation-wise). But I suppose on a glacier with ambient air temps of 20-40 degrees yet brutal sun, the merino's evaporation performance will be adequate to give me that effect.

Thomas Rayl
(trayl) - MLife

Locale: SE Tx
Drying times on 11/22/2013 10:07:34 MST Print View

I don't know if anyone will see this as it's 3 years+ since the last post.

I noticed the discussion points which seem to indicate that differences in drying time are apparently most closely related to difference in water-holding capacity of the different materials. In the tests, the swatches were soaked. So... If my shirt (or whatever) is soaked, I'm not just going to throw it over a bush as is...I'll WRING IT OUT FIRST. It would be interesting to see a follow-on which takes the soaked test swatches, wrings them out to see how much water they will "mechanically" release, then compares drying times.