I would imagine that the FDA is talking about a minimum threshold for Crypto. This would be the most conservative route, considering that the lowest level of infectivity discovered even in the University study was 9 cysts – right on track with the FDA’s statement of less than 10 organisms representing an infectious dose.
The municipal watershed results previously mentioned were from a different study, conducted by America Water, that Backpacker magazine is referencing. In the actual Backpacker magazine study, they utilized Bio Vir Laboratories to check for the presence of Giardia and Crypto in “a variety of wilderness water sources around the country”, rather than municipal watersheds. The sources were sampled three different times between April and July. Here are the sources:
1. Greenwater River (Norse Peak Wilderness, WA) – 0 positive hits
2. Renard Lake (Rainbow Lakes Wilderness, WI) – 0 positive hits
3. White Pine Lake (Wasatch-Cache National Forest, UT) – 1 positive for Crypto, but not viable (unable to cause illness)
4. Neversink River (East Branch, Catskills, NY) – 1 positive for Giardia, but not viable
5. Wet Beaver Creek (Wet Beaver Wilderness, AZ) – 1 positive for Giardia, but not viable
6. Merced River (Yosemite National Park, CA) – 2 positive for Giardia, but not viable
7. Chattooga River (N. of Ellicotts Rock Wilderness, NC) – 2 positive for Giardia – the only sample still VIABLE. (The concentration of cysts here was 1.5 per liter, far below the infectious limit of 10 organisms).
As you can see, occurrences in the backcountry do tend to be at very low concentrations. The only viable sample occurred in N.C., and rather than the 100,000 cysts utilized in the EPA Guide Standard & Protocol for Microbiological Purifiers, there was only 1.5 cysts per liter! Thus, a 4-log removal would be overkill, literally, for the typical level of cysts encountered in the wilderness.
Basically, in the EPA Microbiological Purifier Protocol, they investigate “worst-case” water, represented by a more challenging high pH and low temperature, and then spiked with humic acid and high turbidity before a very large number of microorganisms are added. This water is actually far more contaminated than most waters you would find in the backcountry.
You may also be interested to know of a study conducted by the U.S. military before the purifier was commercialized. Here is a link to a photo of one of the water sources they treated: http://www.miox.com/images/H2O_Source_A_02_small.jpg
As you can see, it’s pretty filthy water, yet the purifier was still effective at inactivating the microorganisms. Essentially, the MSR MIOX Purifier was designed to handle worst-case waters. Most wilderness waters are fairly innocuous compared to the rigorous tests that the purifier underwent.
Please let me know if you have any more questions!