Asthma sufferers are always interested in finding new ways to remove the particles in the air that aggravate their condition, and air purifiers certainly seem like a good solution to the problem and are often recommended by doctors. However, many studies by the EPA, Indoor Air Research facilities at major universities, and the American Lung Association have shown that this is not necessarily the case, although many air purification products advertise that they are effective in reducing asthma causing airborne contaminants. While this is true to a degree, studies show that residential air purification units -- especially portable ones -- have a limited effect.
Research reported in the New England Journal of Medicine in 2004 showed that adding an air purifier to an urban home with children who suffer from asthma did create an overall better environment, but did not totally alleviate their asthma symptoms entirely since many of the irritating factors-- especially dust mites and pet dander -- are stirred up by sitting on furniture, walking across carpet or laying down on a pillow, and air purifiers cannot remove them from the room until they reach the filtration mechanism.
It is recommended that the source of allergens needs to be eliminated from indoor environments before an air filter can make a substantial difference. Obvious suspects like shedding pets, outdoor dust infiltration, cigarette smoke and mold and mildew buildup must be addressed directly and it is not realistic to expect residential air purification products to thoroughly overcome such contaminants to a degree that it would eliminate the cause of asthma attacks.
Environmental Protection Agency (EPA) studies have found that many of the airborne particulates that cause asthma are not filtered out because they are too small -- under 0.3 microns -- and that although residential air purifiers consistently decrease indoor concentrations of airborne irritants by 80%, which is an improvement, but should not be considered a definitive solution.
The EPA actually warns consumers -- especially asthma sufferers -- about the dangers of air purifiers that emit ozone, claiming that short term or extended exposure to ozone can actually increase asthma symptoms because it is has toxicological properties that restrict and hamper lung function, even in healthy people. They do point out that there are no definitive studies on gas-phase filtration systems, photo catalytic oxidation cleaners (PCO) systems or UVGI (ultraviolet) systems, but express doubt that they are any more effective in reducing the tiny airborne particulates that most irritate asthma symptoms.
Another important observation is discussed on the WebMD website that examines the effectiveness of indoor air cleaners. The article points out that portable room cleaners only work in the room in which they are running, so they should be located in strategic areas where the asthma sufferer spends most of the time, and especially in the bedroom at night. It recommends HEPA filters as the most effective for removing asthma causing particulates from the air, but admits that their effectiveness is limited.
Bottom line - Air purifiers can help clear the air considerably of many (not all) asthma inducing particles and irritants but are only a partial solution for asthma sufferers. Surfaces that contain irritants need to be cleaned regularly and sources of irritants should be removed wherever possible.