One of the challenges, if not the challenge in buying an air filter is that there is no one way to judge performance. A dozen manufacturers include the AHAM (Association of Home Appliance Manufacturers) test sticker and/or logo, which often a measurement of airflow in cubic-feet per minute. The faster the flow, the more rapidly room air is cleaned. How well the air is cleaned is another matter entirely. AHAM advises buying a unit with a CADR (clean air delivery rate) that is two-thirds the square foot measurement of the room you would like cleaned, provided that the ceiling is not exceedingly high. A 20' x 10' room, then, would need a CADR of about 140. Also, frequently seen on both portable and whole-house filters and inserts is information from ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers). ASHRAE certification offers a more informative measure of cleaning, in terms of "efficiency," or the percentage of air contaminants effectively filtered by the appliance.