The Environmental Protection Agency sets standards for 22 contaminants in drinking water, and has regulations for up to 25 more on the way. But many people think those limits are a drop in the bucket, considering that there are 60,000 different chemicals produced in the United States.

As a result, many consumers are considering home treatment of drinking water, and there is a plethora of water filters now on the market. One thing to remember is that these filters may remove all types of chemicals in the water, including some that can be helpful, like fluoride and calcium.

There are three main types of filters:

*Activated carbon filters. These units usually either attach to the faucet or fit underneath the sink. A few models also can be placed at the point of entry from the public water system.

Although they can cost as much as $500, most activated carbon filters run $60 to $70. Some are as low as $15.

Effectiveness, however, depends on the model, degree of contamination of the water and how long the water stays in contact with the filter. The less time spent in contact with the filter, the less effectively the filter can remove some chemicals.

In many flow-through models, the water is in contact with the filter for only a few seconds. It takes about 30 minutes of contact to remove a potentially harmful group of chemicals called trihalomethanes, which includes chloroform and dichlorobromomethane, said engineer David Burmaster, a former White House adviser on water.

It's also important that filters be changed regularly, experts say. Chemicals build up inside the filters and can be re-released in a big dose. And the filters can be a breeding ground for bacteria.

*Reverse osmosis purifiers. These filters can cost $450 to $600, but they can take out undesirable levels of contaminants, such as lead, mercury or sodium, as well as some organic chemicals. These purifiers usually are placed under the sink, where they are attached to the cold water line. They often use multiple filters: one for sediment and dirt, another main filter to trap more contaminants, and an activated carbon filter to get what's left. The drawback: many models also have bulky storage tanks because of the slow rate at which water is processed, sometimes just three gallons a day.

*Electric distillers are the Cadillacs of home water treatment systems. Units that attach to the faucet cost at least $250; large, counter models cost up to $800. Like reverse osmosis systems, electric distillers are slow. They operate by boiling water, then trapping the steam, which is usually free of contaminants. The cooled steam condenses into purified water, which is stored in a tank. Regardless of the system, home filtration devices also require a certain amount of vigilance. Filters must be changed frequently. Otherwise, they trap chemicals that then can be released as a megadose of contaminants from the tap.

Some people, like engineer Burmaster, think for that reason that the home water filters may give consumers a false sense of security. The only way to guarantee that the filter is operating properly, Burmaster said, is to do a water test, which costs about $150. "How often can you afford to test the water for that price?" he said.

But others, including environmental planner Jon Hutchison, believe the filters are a "good idea." Hutchison installed his own granular-activated carbon filter system in his Charlottesville, Va., home when he started tasting chlorine and smelling a musty leaf odor in his ice cubes.

One simple safeguard for people who don't want to install home filtration: let the water run for a few minutes before using it, particularly if it's been sitting overnight.

"I never drink first-draw water," said Harry Ways, Chief of the Army Corps of Engingeers Baltimore-Washington Aqueduct Division, which supplies water to the District. "Let it run for a few minutes," he advised, so that possible overnight build-up of lead,