Air cleaning effectiveness is measured by how many and what kind of particles are captured and the amount of air that is purified in a period of time. Filters are tested and given American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) ratings.
There are two ratings: 1) Initial Staining Dirt Efficiency, measures how well a filter removes microscopic amounts of staining particles including grease, allergens and tobacco tar. 2) Average Arrestance Test measures the percentage of dust, hair, lint and other large particles in the air are removed.
The Association of Home Appliance Manufacturers or AHAM gives another measurement of efficiency called Clean Air Delivery Rate (CADR). This measures how many cubic feet per minute of air are rid of three different sized particles: minute (smoke), medium (dust), and large (pollen). The higher the rating, the less times that it takes the unit to remove these particles. The only units marked with this CADR rating will be those approved by the AHAM.