When we think of air pollution, we usually associate it with outdoor air. But with the growing epidemic of asthma in the United States in the last 20 years, especially among infants and children who spend most of their time inside, much attention has been given to indoor air. In fact, in 1990 the United States Environmental Protection Agency (EPA) ranked indoor air pollution as "a high priority public health risk."

The American Lung Association and the EPA both recommend three strategies for reducing indoor air pollution:

• Controlling sources of pollution,
• Ventilating adequately and
• Cleaning indoor air.

The following are answers to some frequently asked questions about cleaning indoor air.

Although the American Lung Association and the EPA recommend air filtration, they say that controlling the sources of allergy-causing pollution and ventilation are more important. Air filters are worth considering, but not as a solution to your allergy problems by themselves. In fact, research studies disagree on whether or not filters give much added relief from asthma in a clean and well-ventilated home.

While many allergenic particles are suspended in household air, there are far more resting on surfaces like rugs, furniture and countertops. Keeping these areas clean is the most important step in controlling the cause of your asthma.

No. The Food and Drug Administration (FDA) has twice asked groups of experts to recommend national standards, but neither effort succeeded. Both groups concluded that there isn't enough research data on the relationship between air filtration and actual health improvement to recommend national standards.

When you shop for air filters, you will find several rating systems that claim to compare filters. But these are not health-related rating systems. They are standards used by manufacturers or manufacturers' organizations, and provide little or no guidance for the health-conscious shopper.

Although the FDA has no health-related standards, it does consider some portable air filtration systems to be Class II medical devices. In the United States, nothing can claim this status without FDA approval. To get approval, a manufacturer must show two things: (1) that the device is safe, usually indicated by the Underwriters Laboratory (UL) seal, and (2) that it has a medical benefit. Look for both the UL seal and a statement of the FDA's Class II approval. If no FDA statement is available with the device, check the FDA's medical device listing before buying. Ask your doctor for guidance.

Yes. There are five basis types.

Mechanical filters. These force air through a special mesh that traps particles including allergens like pollen, pet dander and dust mites. They also capture irritant particles like tobacco smoke.

Electronic filters, of which the most efficient are electrostatic filters. These use electrical charges to attract and deposit allergens and irritants. If the device contains collecting plates, the particles are captured within the system. Otherwise they stick to room surfaces and have to be cleaned away.

Hybrid filters. These contain the elements of both mechanical and electrostatic filters.

Gas phase filters. These remove odors and non-particulate pollution like cooking gas, gasses given off by paint or building materials, and perfume. They cannot remove allergenic particles.

Ozone generators. Although ozone cleans air, the EPA and the American Lung Association do not recommend these because ozone is harmful to lungs. When ozone concentration rises outdoors, people with asthma are warned to stay inside. Generating ozone in the house makes no sense.

Yes. When shopping for an air filter system, look for one that can recirculate 8 or 10 room volumes per hour. This doesn't guarantee completely clean air, but it will be much cleaner than with systems that recirculate less.

You should also ask about efficiency. You need a system that removes more than 90 percent of all particles larger than 0.3 microns in diameter. Most indoor allergens are larger than this, so this efficiency standard will handle them easily.

Yes. The most effective is a kind of mechanical filter called the high-efficiency particulate air (or HEPA) filter. (Note: This is a type of filter, not a product name.) HEPA was invented during World War II to prevent the escape of radioactive particles from laboratories. To qualify as a "true HEPA" filter, a device must be able to capture at least 90 percent of all particles 0.3 microns or more in diameter that enter it.

Be aware, however, that there are filters on the market that claim to be HEPAs, but may be only half as efficient. Insist on a system that meets "true HEPA" filtration standards. This way you will be certain to get a system that removes at least 90 percent of the indoor allergen particles.

The recommends that if you decide to clean indoor air, you choose a "true HEPA" filtration system.

No. Up to a point, filters improve with age, since trapped particles fill spaces that later particles might get through. Eventually, though, they become clogged and costly to operate. It is easy to replace the mesh once a year for greater efficiency.

Be sure to ask questions about cost, difficulty and useful life of filters in your conversation with salespeople so that you won't be surprised later with unexpected costs or difficulties.

If your home is heated or air conditioned through ducts, it may be possible to build filters into your air handling system. This has the advantage of the great force with which air will pass through the filter. And it eliminates a space-consuming appliance and an additional sound in your home. On the other hand, the filters may be more expensive and more difficult to handle; and they may need to be changed more often. Consult your doctor and your heating service on this alternative to a portable system.

Questions to Ask Before Purchasing an Air Filter

• What substances will the cleaner remove from the air in my home? What substances will it not?
• What is the efficiency rating of the cleaner in relation to the "true HEPA" standard?
• Will the unit clean the air in a room the size of my bedroom every four to six minutes?
• What is the device's clean air delivery rate (CADR)?
• How difficult is it to change the filter? (Ask for demonstration.) How often does it have to be changed? How much do filters cost? Are they readily available throughout the year?
• How much noise does the unit make? Is it quiet enough to run while I sleep? (Turn it on and try it, even though you will probably be in a noisy place.)