How to Choose an Air Cleaner (Air Purifier) for Allergies

What HEPA means and doesn't mean.

The filter is one of the key components of an air cleaner. Filter types are defined by the size and number of microscopic particles that they remove from the air passing through them. Microscopic particles are measured in microns. The period at the end of this sentence is approximately 400 microns in size, pollen grains are 30 microns, dust mite waste particles are about 20 microns, and cat allergen particles vary from about 1 to 20 microns in size. Bacteria, and exhaled droplets containing the Covid-19 virus, are about 5-10 microns, while dangerous air pollution particles are 1 to 2.5 microns. A HEPA (High Efficiency Particulate Air) filter removes 99.97% of all 0.3 micron particles from the air that passes through it. But note that these percentages refer to the air that passes through the filter, not necessarily to all of the air in the room. Many advertisements give the misleading impression that any HEPA filter will clean the air in a room of more than 99% of its particles. The reality is that the amount of cleaning of the air in the room depends not just on the filter, but also on the amount of air moving through the filter. This is expressed as the "Clean Air Delivery Rate", or CADR. The EPA and the American Lung Association recognize CADR as the appropriate test of air cleaner effectiveness.

What Clean Air Delivery Rate (CADR) means.

The Clean Air Delivery Rate is the volume (measured in cubic feet) of particle-free air being delivered into the room each minute. It is the best measure of the true cleaning capacity of the air cleaner, since it reflects both the effectiveness of the filter and the amount of air going through the filter. The higher the clean air delivery rate, the larger the size room that can be effectively cleaned of airborne particles. Clean air delivery rates are measured using procedures standardized by the Association of Home Appliance Manufactures (AHAM), which allows the capacities of various air cleaners to be compared to each other. Note that most air cleaners have more than one fan speed, and that the advertised clean air delivery rate applies to the highest speed.

How to choose the right size unit.

Air cleaners are usually listed with the room size for which they are recommended. This recommendation is based on the unit's ability to provide a certain number of air changes per hour in a room of that size. AHAM recommends that the total air of the room be replaced with filtered air 4.8 times per hour. The Center for Disease Control (CDC) recommends 5 air exchanges per hour, which is the standard that Mission: Allergy uses to calculate the room sizes listed here. Note that the listed CADRs (and therefore the room sizes) are based on the highest fan speed, so if you want to use a quieter lower speed you should choose a unit with a higher CADR. For very large spaces it is often more cost-effective to have multiple medium sized units rather than a single very large unit; simply add the CADRs of the units together. Be wary of advertisements for air cleaners claiming suitability for large rooms that are based on only 2 air-exchanges per hour, rather than the recommended 5.

CADR Calculator

To choose the air cleaner with the proper Clean Air Delivery Rate for any size room (to achieve the recommended 5 air-exchanges per hour)


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/ 12 =

Cubic feet per minute

For additional information, see

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