(MONEY Magazine) – Clear, cool, fresh water--it's the supreme thirst quencher and the ultimate health elixir of the '90s. But most of us occasionally wonder whether our water is as pure and healthy as it should be. Rather than switching to expensive bottled water--which may be no better than the tap variety (see page 158)--you can get the protection you want from a filter.

Water anxiety is widespread: According to a 1999 survey conducted for the Water Quality Association, a trade group for filter manufacturers, 56% of consumers are concerned about their tap water's taste, odor or appearance; 48% also worry about contaminants ranging from byproducts of the chlorine used to disinfect public water supplies to pesticides and toxic chemicals that can leach into wells and aquifers. To alleviate their fears, Americans spent a record-breaking $1.75 billion last year on water filters, from simple $25 pitchers to treatment units costing $1,000 or more.

The good news is that many of us can rest easy with a pitcher, which filters out chlorine and reduces lead, a potential problem in older homes. Local utilities are now required to test water for more than 80 contaminants in order to meet federal and state drinking-water standards, and by October most public water bills will include reports on test results. (If you have questions about your water report, call the Environmental Protection Agency's Safe Drinking Water Hotline at 800-426-4791.)

But if you have infants at home, take immune-suppressing drugs or have health problems that compromise your immune system, such as cancer or HIV, you may want cleaner water than federal standards require.

And if your water comes from a private well--as is the case for 30 million Americans--monitoring its safety is up to you. The most efficient way to do that, says Charlene Shaw, program coordinator for the EPA's Office of Groundwater and Drinking Water, is to have it tested each spring by an independent lab. Consult your health department or consider an EPA-certified mail-order source such as National Testing Labs (800-426-8378). Costs typically range from $20 to $200, depending on the tests you request. At the least, suggests Shaw, check for coliform bacteria, which signals the presence of disease-causing organisms, and for nitrates.

If you decide you want a filter, here are the options:

Pitchers for better taste. Pour-through pitcher filters from Brita, PUR, Rubbermaid and others use granulated carbon, which removes chlorine and usually reduces lead. Similar granulated carbon filters are also available for refrigerator icemakers and water dispensers.

Faucet and countertop filters for microbe protection. These models typically use carbon block filters, which screen out some pesticide residues and microbes that may be present even in water that meets federal safety standards. The most serious microbial threat is from cryptosporidium. In healthy people, the bug may cause gastrointestinal illness for a week or so, but for people with compromised immune systems, cryptosporidium can be fatal. It can also result in hospitalization for infants and anyone with multiple health complications, such as heart disease and diabetes, says Dennis Juranek, parasitic disease specialist at the Centers for Disease Control.

Make sure to look for a filter that's certified for microbial cyst removal by NSF International, a nonprofit organization that verifies which contaminants a filter actually removes. Click on NSF's website (www .nsf.org) or call 800-673-8010.

Under-sink systems for the utmost security. These filters can usually be customized to zero in on specific contaminants. The top of the line is a reverse-osmosis filter, which employs a membrane similar to those used to purify water for kidney dialysis. It does have a major drawback, however: Reverse osmosis uses about three gallons of water to produce a single, scrupulously clean gallon. "Most people won't need the degree of protection reverse osmosis provides, but if you want to remove a wide range of contaminants such as nitrates, herbicides, cysts and lead, this type of filter would be the best," says Tom Bruursema, NSF's general manager of drinking water treatment. It also removes the fluoride that protects teeth from decay, so if you opt for reverse osmosis, tell your dentist.

How to make the most cost-effective choice. First you'll need to know how much water you use. Here's why: A filter's actual cleansing element must be replaced after you've run a specific number of gallons through it. Most pitcher filters, for example, require a new cartridge after 40 gallons, about two months' use for the average household. But if you are a heavy user--going through, say, 1.5 gallons a day--you'll need to replace your filters monthly. At that rate, if you used a Brita, for instance, you'd pay $85 a year for replacement cartridges vs. the average $35. You might find that a faucet filter, though more expensive up front, is cheaper in the end. To track your usage, pour your water from gallon jugs for a few days.

E-mail your health questions to health_column@moneymail.com.