Permethrin acts as a dye on many articles of clothing. Cotton, nylon are good examples. Polyester is not so good. Soo, it is hard to say without knowing the actual composition involved. Basically it forms a loose hydrogen bond. Water alone will leach a bit out with each washing. How much depends on the material used in the clothing. People, have a high resistance to it, but, you really shouldn't use it on underclothes or socks. More of a caution, than anything else. Like anything else, some people can be allergic to it. Depending on the exact chemicals in the water (alkaline and acid) water solubility can change a bit as does the degree of leaching. As with a lot of dyes, a stronger dye can replace a weaker dye. Soo, it may not be effective on some cloths, simply not bonding enough to be very effective and much more easily washed out. Blaze orange is one that does not pick up permethrin well.
Generally, dipping a shirt in the lake is fine. But, it should NOT be allowed to drain back into the lake. Walk back into the woods before squeezing any excess water out. Scatter the water a bit, do not bury it.
It degrades in sunlight, not sure which wavelength's, but I suspect it is the UV ranges, the higher energy ones. After washing, most water treatment plants will churn water allowing sunlight to get to the water...soo, most if not all, of the permethrin is degraded. However, untreated leftover wash water should be cared for responsibly. As others have pointed out, it bonds with many organics in soils fairly solidly, but, use it with caution. They said the same for DDT. It CAN poison soils for a long time if not exposed to sunlight. It is naturally occurring in some flowers. See: http://en.wikipedia.org/wiki/Chrysanthemum.
It is deadly to fish and bugs. I use stuff sold as "ant killer." It is primarily oil soluble, soo, higher concentrations are often found in solutions of kerosene or other petroleum based product.
It takes very little to treat items. Wash and spin dry. Mix a desired concentration (I usually toss in about 1/4 cup of dry mix at ~.5%) and a half to one gallon of water in a large
ziplock...2 gallon size if you can get them...or a bucket. Then add a pair of pants and shirt. Preferably, in a larger, open area where you do not want ants or other bugs (grubs, etc.) Let them sit in the solution about 1/2 hour and remove them, wringing them out as you go. (You ARE wearing gloves, of course.) Hang them and let them dry thoroughly. Then wash them again without detergent, drying normally. You are relying on the waste treatment plant to destroy the remainder in the wash water. The excess in the bucket can be used for treating anything that you like. Your hat, shoes, pack, etc. It will keep bugs down. The rest should be scattered on the open area you were working in. Let the sunlight get at it. Or leave it in the bucket in the sun for a few days, then scatter it on the driveway. Either way the sun will destroy the remaining permthy in the solution. DO NOT let it run off into a storm drain if you live in a city.
Basically, the concentration reduces with each application to cloths. It has an affinity to fabrics, so the initial concentration doesn't much matter. If you have enough, the rest is excess. It will reduce a lot the first time, quite a bit the second time, gradually reducing, but never quite reaching 0%. I like to think of an exponential scale, but this is not 100% accurate. But, trying to retreat clothing that was freshly treated MAY reduce the amount of permthy in the cloths, since you also wash out some. This is equilibrium. Depends on the concentration and the type of clothing. I won't pretend to be able to calculate these numbers, my chemistry is very old...though not as old as I am getting... Mostly, I just "eyeball" things these days...
The concentration on the cloths will be enough to kill most mosquitoes within 12 hours. And it will deter them from biting, but not from swarming. It does NOT have a very high vapour pressure, hence, doesn't form a "cloud" of resistance around you. Blackflies and Mosquitoes are my primary blood suckers in the ADK's. These are both attracted to IR and CO2. Black flies want open skin to chew on and they swarm. Very annoying. Permthy doesn't help much, 'cept to know I got revenge. Mosquitoes are a bit different and will bite through clothing. Permthy helps a lot. I use it in conjunction with DEET. This seems to keep the worst of the pests under control. Often, in May, I have been in the ADK's on a canoe trip and only gotten one or two bites in a week. Really Excellent coverage. It does NOT help with swarming and buzzing...but I think that may be in my head from the permthy.
I do NOT recommend this to anyone. As with any highly toxic substance, you have to know more than just a smattering of info. To me, it outweighs the risks of all sorts of insect born illnesses and the annoyance and possible infections of bites. You should study up on it and decide for yourself if it is worth it. It can cause nervous disorders, and may be related to an increased incidence of Parkinson's.