Our water was long gone as we stared glumly at what the folks who organized the 1995 Utah Eco-Challenge adventure race defined as a spring--an oil-slicked, mosquito-infested sludge pond containing a few odd tires, tons of cattle excrement, and one dead and bloated rat ... yummy. Armed with filters and iodine tablets, we managed to procure one gallon of potable water.

Although the lack of purity of this water source was obvious, determining what is safe to drink isn't always so easy. Microvermin such as Giardia and cryptosporidium potentially lurk under the surface of the most crystal-clear, seemingly pure water. To drink directly from any natural water source is to risk a bout of nausea, diarrhea, vomiting or worse. Which is why water filters, which continue to get lighter, smaller, and more convenient to use, are essential to have.

With a filter, you just pump and drink. Filtering strains out microscopic contaminants, rendering the water clear and somewhat pure. How pure? That depends on the size of the filter's pores-what manufacturers call pore size efficiency. It's a fairly technical matter, but the upshot is that any filter with a pore size efficiency of two microns or smaller will remove such protozoa as Giardia and cryptosporidium, as well as parasitic eggs and larva, but it takes a pore size efficiency of less than 0.4 microns to remove bacteria. All but one of the filters here take care of bacteria.

As for viruses-a concern in Third World countries or places where water may be contaminated by sewage- there is only one field device I am aware of that relies exclusively on filtration while claiming 100-percent virus removal, the General Ecology First Need. Since zapping viruses requires chemical assistance in most cases, many filters turn to iodine disinfection, either as an integral part of the system or by suggesting the addition of Potable Aqua tablets to the purification mix. Iodine tastes awful and it a possible health risk to some people, so a number of filters also employ a carbon-element (activated charcoal) to remove the iodine when its job is done and to get rid of pesticides, herbicides, heavy metals and chlorine. Be warned though: Once the carbon element has reached its limit for absorbing a particular chemical, the bad stuff can then sneak through undetected. You must replace the filter according to the manufacturer's recommended schedule.

One additional caveat: The carbon that removes the iodine may, in fact, be removing additional protection you need to ensure a good "kill" of little microbial nasties. Some studies have shown that the quick pass-through of heavily contaminated water through an iodine-resin impregnated filter may not achieve a 100% neutralization of viruses and that it is only because of the trace amounts of iodine that remain in the filtered water that microbes get completely nuked. What's a person to do? I would recommend that if you are in areas where the risk of pesticides, herbicides or heavy metals in the water source are greater than the risk of encountering sewage and viruses, you opt for the carbon element, otherwise, leave the carbon at home. If you really can't handle the taste of iodine, then add a little Vitamin C (ascorbic acid) to the water 15 to 20 minutes after you filter.

A good all-around filter should be compact, lightweight (forget anything over 20 ounces), simple to use, and easy to clean and maintain. For the best protection, you'll want to buy a filter that will remove both protozoa and bacteria. A number of value-oriented, pocket-size filters remove only Giardia and cryptosporidium--and that, in my book, is risking your health to save money. Consider the flow rate too: A liter per minute is OK; less, and your lips will pucker waiting for the water.

All filters will eventually clog--it's a sign that they've been doing their job. If you force water through a filter that's becoming difficult to pump, you risk injecting a load of microbial nasties into your bottle. Some models can be backwashed, brushed or, as with ceramic cartridges, scrubbed to extend their useful life. Because ceramic can be cleaned with a brush or scrubber, they appear to be more expensive but in reality are far more cost effective for the long haul if you're planning on pumping loads of water. If the filter has a prefilter to screen out the big stuff, use it; it will extend the filter's life dramatically (although I've yet to find any pre-filter that will work on the ultra-fine sediment common in southwestern U.S. rivers. For that, you'll need a bucket to hold the water to allow the sediment to settle before filtering.)

Typically, filters have a useful life of up to 100 gallons if they are not ceramic and hundreds more gallons if they are. The bottom line is that no matter what the manufacturer claims on the package, they are dreaming of best-case scenario water which I have never been able to find from any backwoods water source. Expect to realistically achieve only 75-percent of what a manufacturer claims, and then only if you are very careful.