People buy an air purifier or air filter to improve the indoor air quality in their home or office by cleaning out pollutants like dust, smoke and pollen. In some cases, however, an air purifier may actually be bad for your health. What should you look for in an air purifier -- or should you buy one at all?

The first and most important step is to try to improve your indoor air quality without buying one more expensive, energy-consuming product.

If the issue is smoke or odors, for example, stop the problem at the source -- ban smoking indoors, open a few windows, or try cleaning and vacuuming a little more often. (And if the issue is your hypochondriac tendencies, consult a psychologist.) Common sense is often the best and cheapest way to clear the air.

If, however, you're determined to spend money on an appliance that might do little more than run up your electric bill, you should first determine what kind of pollutants you have in the air you breathe -- an air purifier that might be good at filtering out dust, for instance, could be useless at filtering out mold. Once you've decided what you need to filter out, buy a model designed to filter that specific pollutant.

Air Purifiers: Filtering Out the Nonsense

There are 5 main types of air purifiers:

Air filters use a paper, fiber or mesh filter to capture particle like dust and pet dander. The filters will need to be cleaned or replaced periodically. Highly efficient particulate air (HEPA) filters, and other filters that are about as good as HEPA filters, are included in this category.

Electrostatic air cleaners use an electrically charged panel or screen to capture particulates. Another type, called an ionizer or ion-generating air cleaner, attaches an electrically-charged ion particle to dust and other pollutants, making the particles heavier and causing them to drop from the air (and land on your carpet, furniture or walls, creating a mess for you to clean up).

Gas-phase air filters remove gases and odors with a product like activated carbon. They're not typically used in home air purifiers, may not remove many gases (like carbon monoxide), and often have a short effective lifespan.

UV filters claim to remove and destroy biological impurities like mold and bacteria with ultraviolet light. They may or may not work, and should probably be used in combination with a filter-type device to remove particulates as well as biological pollutants.

Ozone generators use either UV light or an electrical discharge to create ozone, a gas that can cause serious health problems like permanent lung damage and can aggravate asthma. As a result, these are not recommended by the EPA, Consumer Reports, or anyone with a brain (see "common sense," above).

Before You Buy an Air Purifier

Because most air purifiers have different fan speeds, remember that the product info may indicate the filter's effectiveness only at the highest setting, which will also be the noisiest and most energy-intensive setting. You'll want to find an air purifier with a clean-air delivery rate (CADR) of at least 300; any air purifier with a CADR below 100 is really just an overpriced fan.

Filters can be expensive to replace; a removable filter that can be cleaned may be more practical. And some purifiers with "odor-removal systems" were deemed a waste of money by Consumer Reports, which also encourages folks to consider a whole-house air filtration system for people who are seriously concerned with indoor air quality.

Finally, look for air purifiers that are Energy Star-certified to conserve electricity. This is a particular concern since the unit will be on for many hours at a time.