For air filters that have air flowing through them, it is the fraction of particles (of a particular size distribution) that have been removed from the air times the CFM air flow rate through the device. More precisely, it is the CFM of air in a 1,008-cubic-foot (28.5 m) room that has had all the particles of a given size distribution removed from the air, over and above the rate at which the particles are naturally falling out of the air. Different filters have different abilities to remove different particle distributions, so three CADR's for a given device are typically measured: smoke, pollen, and dust. By combining the amount of airflow and particle removal efficiency, consumers are less likely to be misled by a high efficiency filter that is filtering a small amount of air, or by a high volume of air that is not being filtered very well.
The CADR ratings were developed by Association of Home Appliance Manufacturers (AHAM) and are measured according to a procedure specified by ANSI/AHAM AC-1. The ratings are recognized by retailers, manufacturers, standards organizations, and government bodies such as the EPAand the Federal Trade Commission. Whole house air cleaners are not covered by the CADR specification because the measurement is performed in a standard 1,008-cubic-foot (28.5 m) room, the size of a typical house room, which has different airflow patterns than whole-house filters. Measurements are made with the filter running and not running, so particles that naturally fall out of the air are not being counted as part of filter's operation. The measurement only applies to particulate matter, not to gases.
The AHAM seal (usually found on the back of an air cleaner's box) lists three CADR numbers, one each for smoke, pollen, and dust. This order is from the smallest to largest particles and corresponds to the most dangerous to the least dangerous particles. The higher the CADR number, the more air it filters per minute for that particle size range. Consumers can use these ratings to compare air cleaners from the various manufacturers.
In the early 1980s, AHAM developed a method for measuring the clean air delivery rate for portable household electric room air cleaners. The resulting standard became an American National Standard in 1988 and was last revised in 2006. Known as ANSI/AHAM AC-1, it measures the air cleaner's ability to reduce tobacco smoke, dust and pollen particles in a room. It also includes a method for calculating the suggested room size.
