Ever been tempted to drink from a stream but didn't because you weren't sure what was in the water? I know the feeling. I've always wondered whether filters remove all of the protozoans and bacteria found in ponds and streams, but I haven't had the guts to test them with my stomach. So I found a better way.

To determine how well four filters remove these stomach churners, I pumped water just downstream of a sewage treatment plant through each of them. Then Ken Crisp, a water specialist in Missoula, Mont., tested the samples against a sample of untreated water from the stream.

Filters work by straining protozoans, which are hard-shelled cysts ranging from 2 to 15 microns in diameter, and bacteria, which measure just 0.2 to 0.5 microns in diameter.

When Crisp tested the unfiltered river water, he found it contained coliform and E. coli, but no viruses. (E. coli is a potentially deadly bacteria and coliform is used by the EPA as an indicator bacteria; if it's present, there's a good chance that other bad things are too.) The filters removed the E. coli, but two of them failed to remove all of the coliform. The other two filters got the job done.—John Haviland