The EPA says that portable air cleaners can "possibly" reduce symptoms. Portable air cleaners are generally good at trapping coarse particles including dust, pollen, animal dander and other allergens, but these particles are large enough to settle out of the air quickly, which means that air filters in portable air cleaners or central HVAC systems are unlikely to remove them effectively from the environment, according to the EPA. There are, however, some allergens from cats, dogs and dust mites that are carried on small particles and are more likely to be removed by air filters, and some studies have found that portable air cleaner use is correlated with reduced allergy and asthma symptoms. For more information see the Indoor Air Quality Scientific Findings Resource Bank page, "Particle Air Cleaning and Allergies and Asthma."