Unless water quality is assured, do not drink tap water unless it has been boiled, filtered, or chemically disinfected. Vigorous boiling for one minute is the most effective means of water purification. At altitudes greater than 6500 feet (2 km), boil for three minutes.
Chemical disinfection with iodine is also effective. Add 2% tincture of iodine to one quart or liter of water (5 drops to clear water, 10 drops to cloudy water) and let stand for 30 minutes. If the water is cold, longer times may be required. (To kill cryptosporidia, which are an important cause of illness in those infected with HIV, let stand for 15 hours.) Alternatively, tetraglycine hydroperiodide tablets, such as Globaline, Potable-Aqua, and Coghlan's, may be purchased at most pharmacies. Instructions are enclosed and should be carefully followed. If the water is cloudy, twice the number of tablets should be used. If the water is extremely cold, it should be warmed, if possible, and the duration of contact time with the tablets should be increased. The taste of iodinated water may be improved by adding vitamin C (ascorbic acid). Iodinated water should not be consumed for more than a few weeks. Pregnant women, those with a history of thyroid disease, and those allergic to iodine should not drink iodinated water.
A number of water filters are available. Those with smaller pores (reverse osmosis filters) provide the broadest protection, but they are relatively large and are readily plugged by debris. Also, some filters may be damaged by chlorine, if present, in the water. Those with somewhat larger pores (microstrainer filters) are ineffective against viruses but remove other organisms. To kill viruses, those using microstrainer filters should disinfect the water after filtration with iodine.
Two important parasites, Cryptosporidia and Giardia, may be relatively resistant to disinfection by iodine. Filters that remove both these organisms will bear one of the following messages (verbatim) on their label:
- Reverse osmosis
- Absolute pore size of 1 micron or smaller
- Tested and certified by NSF Standard 53 or NSF Standard 58 for cyst removal
- Tested and certified by NSF Standard 53 or NSF Standard 58 for cyst reduction
Objective data comparing different filters is limited.
Be sure to change filter cartridges as recommended. When changing cartridges, always wear gloves and wash your hands afterwards, since the filters are typically contaminated by microorganisms.
From the Centers for Disease Control (CDC)
Treatment of Water to Make it Safe for Drinking
Preventing Cryptosporidiosis: A Guide to Water Filters and Bottled Water