Clean-air Delivery Rate - Understanding The Rating

Understanding The Rating

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.

The defined particle size ranges are 0.09 to 1.0 microns for smoke, 0.5 to 3 microns for pollen, and 5 to 11 microns for dust.

AHAM recommends that filters be sized for rooms so that the smoke CADR times 12 is equal to or greater than the cubic feet volume of the room. This recommendation is based on the assumption that the room will have air exchanged with other rooms at a rate of less than 1 room volume per hour, and that the customer desires at least 80% of the smoke particles removed from the air. For 8 foot ceilings, this means the area of the room (in square feet) should be less than 1.5 times the CADR rating for smoke. Much larger rooms can be effectively filtered if there is no air coming from the outside, and if there is no significant continuing source of particulates in the room.

MERV 14 filters are capable of reducing smoke particles by approximately 80% when operating at the filter's design velocity, so a CADR smoke rating on a simple filtering unit that uses a MERV 14 filter will be approximately 0.80 times the fan flow rate in CFM. If the filtering unit does not mix the test room's air very well, it may receive a lower CADR measurement because it does not operate as efficiently as it should. If a filtering unit uses a MERV 12 filter that removes roughly 40% of the smoke particles, then it may still obtain a smoke CADR of 80 by filtering 200 cubic feet per minute instead of 100 CFM. Conversely, a 99.97% HEPA filter (MERV 17) that removes over 99.9% of the smoke particles needs to filter 80 cubic feet per minute to get a CADR of 80. This shows the CFM airflow of a unit is always equal to or greater than the CADR rating.

Large particles naturally fall out of the air faster than small particles, but the CADR rating is based on how well the filter works over and above this effect. So CADR ratings for dust and pollen come out lower than would be expected by looking only at the filter's efficiency at removing large particles. This "bias" against the filter's efficiency at removing large particles is a relative bias in favor of the filter's ability to remove small (smoke) articles. Since smoke particles are the most difficult to filter (lower filter efficiency relative to large particles), the two effects largely cancel so that CADR ratings are usually similar for both small and large particles. A filter that is very good at removing smoke particles by using a slow fan or electrostatic effects will not get as good CADR numbers for pollen and dust because those particles will fall down and deposit on room surfaces during the test, before the filter has had a chance to collect them.

For smoke-sized particles, a MERV 12 filter may function as well as a MERV 14 filter at half its rated air velocity (for smoke particles), and a MERV 14 may function like a MERV 12 at double its rated velocity. This is because smoke-sized particles depend on diffusion (Brownian motion) onto fibers as much as impaction, rather than completely on impaction like dust. A slower air speed gives diffusion more time for the particle to stick to the fiber or previously attached particles. Conversely, a higher filter speed may increase the collection of larger particles because impaction depends on the inertia of the particles. As a filter gets clogged from use, the fan air speed drops so that the effective CADR for smoke may actually rise rather than decrease, while the CADR for dust will be lower from the decrease in fan speed, especially because the particles fall out before they are filtered.

If half the room's volume of air is exchanged with other rooms every hour, then HEPA filters are not more effective than dust spot 85% efficiency filters (roughly, MERV 13 or 3M's MPR 1900). The unit's fan speed will be the dominant factor if air from outside the room is coming in too quickly.