Almost 60% of thru-hikers suffer from diarrhea which can last as long as a week and is typically a monthly event during a thru-hike. If this is severe enough it can lead to a string of zeros of the unenjoyable and unrestful variety. A recent study conducted on water sources in the Great Smoky Mountains National Park attempted to evaluate the risk posed by various shelters’ water sources. During the summer, ten shelters’ water sources were sampled and tested. Seven of these ten shelters were found to contain coliform bacteria. The U.S. Environmental Protection Agency (USEPA) has a maximum contaminant level goal for coliform bacteria of zero per 100mL. The water sources which were positive for coliform were also tested for E. Coli and six of these also tested positive for E. Coli. Whether particularly virulent stains were included in these results was not an observation made by this study, so the risk of becoming ill from these colonies could not be assessed. The number of contaminated water sources was only three in seven in the fall, but a risk still existed. These results can’t be generalized to the overall trail since they were all within the park and not distributed along the trail’s route.

Personal hygiene and cleaning methods for cookware are partial contributors to this illness, especially if cookware is rinsed in unfiltered water. A summary study by Backer found that halogen disinfection (like chlorine or iodine) are not effective against Cryptosporidium oocyst or Giardia cysts.

The USEPA standard for water treatment states that they must reduce protozoa like Giardia lamblia by 3-Log or 99.9% and viruses by 4-log or 99.99% removal. The Sawyer Point ZeroTWO Purifier performs at >5.5-log (99.9997%) rate with its 0.02um filter, exceeding the USEPA's requirement. In protected watersheds where human pollution is minimal, standard mechanical filtration alone is adequate according to Backer, i.e.: 0.1-0.4um filter pore size. This means products like the Sawyer Squeeze with its 0.1um filter, 99.99999%, is adequate for this environment in removing bacteria like E.coli, and its 99.9999% removal of protozoa like Giardia and Cryptosporidium. The most full-proof one step treatment method is boiling water, but this isn't very practical while underway hiking. UV-light purifiers like the SteriPen Classic 3 work at 99.9% efficiency on bacteria and viruses, and two AA batteries will purify 150 liters. These work on water with no floating particulate. If particulate is present the water must be pre-filtered first. 

The best option as far as size and reliability is the sub-micron portable filters as they require no batteries and can be back flushed if clogged. They can freeze and burst, so in cold weather they should be kept in your sleeping bag at night or close to your body during the day.

Making the effort to use only filtered or treated water for drinking or cookware rinsing is indicated to avoid this inconvenient and possibly serious malady, especially with the high efficiency small portable filters now available.

Backer, H. (2002). Water disinfection for international and wilderness travelers. Clinical Infectious Diseases, 34(3), 355–364.

Reed BC, and Rasnake MS. 2016. "An Assessment of Coliform Bacteria in Water Sources near Appalachian Trail Shelters Within the Great Smoky Mountains National Park." Wilderness & Environmental Medicine 27 (1): 107-10.

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