Air purifiers: What they could, can, and can't accomplish

In the quest to feel healthy, hold off on buying that air purifier and do your homework. Namely, read our latest report on air purifiers . Some so-called "air cleaners" do basically nothing to clean the air, while some claim to protect you against everything from H1N1 (swine flu) to secondhand smoke. Others, most notably the ones we single out as the Best Reviewed air cleaners , do a fantastic job filtering out dust and pollen in independent tests.

Here's a look at what air purifiers could, can and can't do:

Can ease allergies and asthma...if conditions are right

"Air filters are worth considering, but not as a solution to your asthma or allergy problems by themselves," says the Asthma and Allergy Foundation of America . "In fact, research studies disagree on whether or not filters give much added relief in a clean and well-ventilated home."

The problem is that allergens are heavy. Some float in the air, but most -- pollen, dust, pet dander, cockroach and dust mite droppings and some molds -- quickly settle onto your floors and furniture, so they never make it into your air filter at all.

It's actually more important to keep your home clean and well ventilated, experts say, than it is to have an air cleaner. Vacuum and dust often, and wash your sheets every week, the U.S. Environmental Protection Agency advises. Also, keep pets out of your bedroom, replace carpet with hard flooring, don't burn candles or wood fires, and ask smokers to step outside, ConsumerReports.org adds. If you've done all that and you're still suffering, both sources say then a good air filter might help.

Could remove some toxic gases

Some air filters, such as the top-rated IQAir HealthPro Plus (*Est. $900) and Austin Air HealthMate (*Est. $490), include an extra layer of carbon filter to strain out harmful chemical gases -- such as the volatile organic compounds (VOCs) like formaldehyde that can leak out of new carpet or paint. But they can't remove every toxic gas, the EPA says: For example, your fireplace pumps dangerous carbon monoxide into the air, and carbon filters can't capture it.

Cannot scrub out secondhand smoke

Some air filters do a great job scrubbing cigarette smoke particles out of the air in expert tests, while others are lousy. But even if you get the best-performing air filter, don't expect it to protect you from all of the toxic chemicals floating around in that smoke. "No air filtration or air purification system can completely eliminate all the harmful constituents of secondhand smoke," says the Asthma and Allergy Foundation of America. There's really only effective strategy: "Get smokers in your family to quit smoking.

HEPA: Help or Hype? | Allergy and Asthma Network Mothers of Asthmatics

First, there is no such thing as an “air purifier” or as the name implies, an air filter that purifies the air. Some units fitted with a ultraviolet (UV) light kill viruses and bacteria but even this does not purify all the air you breathe. The best any air cleaner can do is remove small particles that pass through the filter.

The term HEPA (high efficiency particulate air) describes filters designed to capture 99.7 percent of all particles 0.3 microns or smaller (too small to see, but the perfect size to penetrate your lungs). This size covers all common allergens, from mold and animal dander to dust mites and pollen, along with some small particles from smoke and pollution. 

HEPA filters are most commonly used in freestanding air cleaners and vacuum cleaners. If you see “HEPA-like” on the label, buyers beware. It could mean anything, but it is not a HEPA filtered unit.

Before You Buy Reduce indoor air particles from the outside in. Replace old and energy-inefficient windows and caulk window frames to keep moisture, mold, pollen and insects from finding a way inside. Seal foundation cracks, insulate your home.

Inside, choose smooth surface flooring and furniture that is easily damp-mopped or dusted. Most particles that trigger allergy and asthma symptoms, like dust mites and mold spores, are airborne for a short time and then settle as dust. Homes with carpets are a reservoir of allergens; each time someone walks across or vacuums the carpet, particles fill the air. Using a HEPA vacuum will trap some particles, leaving others that are stirred up into the breathing space to be removed by room or whole house air filtration. 

Whole-House Systems If you have a central HVAC system, consult with a reputable contractor about replacing the manufacturers’ filter with an approved (sized appropriately for your unit and the size of your air ducts) furnace filter with a Minimum Efficiency Rating Value (MERV) of 11 or 12 and then set the fan to run continuously. This will pull the air in your home through the filter and remove particles down to about 2 microns.  Remember to change the filter regularly, usually every 3 months. A whole house HEPA filter must be installed by a professional HVAC company and sized appropriately for your air handling unit, to protect the life of the equipment and to ensure air passes through and not around the HEPA filter.

This is a case where size matters or you are wasting money and could be creating indoor air problems of a different type. Improperly sized air filtration systems can cause your unit to ice up or burn out and in some cases void the manufacturer’s warranty.

Popular Mechanics

Building Systems for Interior Designers

Popular Science

Industrial ventilation design guidebook

Popular Science