If you are purchasing an air purifier to help relieve allergy and asthma symptoms, a True HEPA air purifier is your best bet. They provide the highest air cleaning efficiency, capturing 99.97% of all airborne particles as small as 0.3 microns.
HEPA-type air purifiers
HEPA-type air purifiers remove up to 99% of all airborne particles. They function the same as True HEPA air purifiers and require replacement filters every 6 months to a year, just like True HEPA models.
Electronic filtration air purifier
Electronic filtration air purifiers function at the same level as HEPA-type models, but they have permanent filters that can be washed rather than disposable filters. While they generally cost more upfront, you save money in the long run by not having to buy replacement filters. They run much more quietly than True HEPA and HEPA-type air purifiers.
Each air purifier has a CADR number, which stands for Clean Air Delivery Rate. Most units will offer between 40 to 300 CADR. Multiply the CADR by 1.55 to calculate the optimal number of square feet that CADR can handle. For example, an air purifier with a CADR of 100 can effectively clean the air of a room that is 155 square feet. A unit with a lower CADR will also work in a room of that size, but not as efficiently or as effectively.