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An Air Cleaner (Air Purifier) can be helpful for allergy sufferers in situations where the allergy-causing substance (allergen) is on a particle that remains airborne long enough for it to be captured by the air cleaner. In addition, the air cleaner itself must be capable of two things: moving a volume of air large enough that most of the particles in the room’s air eventually pass through the cleaner; and having a filtering mechanism able to remove the particles from the air passing through it.
The filter is one of the key components of an air cleaner. Filter types are defined by the size and number of microscopic particles they remove from the air passing through the filter. 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 also on the amount of air moving through the filter. This is expressed as the "Clean Air Delivery Rate", or CADR. Both the Environmental Protection Agency and the American Lung Association recognize CADR as the appropriate test of air cleaner effectiveness.
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 an air cleaner, as 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. Note that most air cleaners have more than one fan speed, and that the advertised clean air delivery rate applies to the highest speed.
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 unit's ability to provide five or six air-changes per hour in a room of that size, a rate that 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, thus decreasing particle levels by 90% or more—the correct target in medical situations. Remember also that these figures are based on using the machine at its highest speed, which is also its noisiest speed. For less sound (and longer motor life) you may want to have a unit capable of effective cleaning when running at less than its maximum blower speed, especially when you are in the room.
Another point to consider is that some popular air cleaners blow the cleaned air downwards onto the floor. This has been shown to blow allergens from the floor (especially if it is carpeted) into the air, thus negating the benefit of the filter. More effectively designed air cleaners do not exhaust the clean air downwards. Good design is also reflected in the attractiveness of the unit.
Ionizer type air purifiers work primarily by generating charged particles (ions), which in turn cause other airborne particles, including allergens, to stick to solid surfaces. However, such ionizers also release into the air ozone, a form of oxygen that can worsen allergy and asthma symptoms. Although ionizer type air purifiers are best avoided in cases of allergy, HEPA type air cleaners that generate a minute amount of ozone confined to the machine increase the effectiveness of the HEPA filter, do not release ozone into the air of the room, and are safe.
The allergic person should look for a HEPA type air cleaner with a high Clean Air Delivery Rate. The CADR should be appropriate for the size room that needs to be treated, and capable of generating 8 or more air changes per hour in a room of that size.