How Purifiers Differ From Filters

To be identified as a water purifier, a device must conform to a US Environmental Protection Agency protocol (last revised in 1987). Purifiers are required to "remove, kill or inactivate all types of disease-causing microorganisms from the water, including bacteria, viruses and protozoan cysts so as to render the processed water safe for drinking." A device must inactivate 99.99 percent of viruses to be labeled as a purifier.

Does this mean purifiers are superior to filters? Not necessarily. Depending on conditions (when water is very cold or obviously contaminated, for example), manufacturers of iodine-based purifiers may recommend that users double-filter their water or significantly reduce the rate at which water is pumped through the unit. (The slower flow exposes water to the iodine or iodine resins for a longer period of time.) Sometimes resins must be allowed time to "recharge" after treating a few quarts of water.

Filter-makers, meanwhile, contend that quality filters routinely capture 99 to 99.9 percent of viruses on the first pass since viruses (and bacteria) often become clumped with organic or mineral particles in water. These clumps are easy for filters to trap. Still, when these clumps are "smashed" into a the wall of a filtering element as you pump the water, it's possible a virus could separate from its clump and still slip through.

The purifier-vs.-filter question stirs spirited debate among manufacturers. REI endorses a movement within the industry to work toward a common filter and purifier standard through the American Society for Testing and Materials. In addition, REI requires independent testing whenever a filter or purifier undergoes a substantial change. This is done to ensure the dependability of water-treatment devices found in REI's product mix.

Note:Pregnant women and people with thyroid conditions often have adverse reactions to iodine. Consult a physician before selecting a purifier.