Mission Allergy: Air Cleaners

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.

A HEPA (High Efficiency Particulate Air) filter removes 99.97% of all particles as small as 0.3 microns 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 on the amount of air moving through the filter. This is expressed as the "clean air delivery rate", or CADR.

Both the EPA and the American Lung Association recognize CADR as the appropriate test of air cleaner effectiveness.

What CADR means
The Clean Air Delivery Rate is a measure of the amount of particle-free air being delivered into the room. 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). This allows the capacities of various air cleaners to be compared. Please note that most air cleaners have more than one fan speed, and that the advertised clean air delivery rates applies to the highest speed.

How to choose the right size unit
Air cleaners are usually advertised with the room size for which they are recommended. Keep in mind however, that this recommendation is based on the the unit's ability to provide five or six air changes per hour in a room of that size. That rate is sufficient to decrease particle levels in the room by about 70%. A larger unit in the same size room can produce eight or more air changes per hour, decreasing levels by 90% or more. This is the correct target in medical situations. Remember also that these figures are based on using the machine at its highest speed. For less disturbance (and longer motor life) you may want to run the unit at less than its maximum blower speed, especially when you are in the room.

Recognizing good air cleaner design
A final point to consider is that many popular air cleaners are designed such that the clean air is blown downwards onto the floor. This has actually been shown to blow allergens from the floor (especially if it is carpeted) into the air, thus negating the effect of the filter! More effectively designed air cleaners, including all of those featured here, do not exhaust the clean air downwards.