In the selection of a portable air cleaner, sometimes called an air purifier, you should consider 1) the air cleaner’s clean air delivery rate (CADR) for particles; 2) whether the air cleaner removes gaseous pollutants as well as particles; 3) the level of noise produced by the air cleaner; 4) electricity cost, initial cost, filter replacement cost; 5) whether the air cleaner produces ozone or other undesirable pollutants.
The CADR is a measure of the rate at which the air cleaner removes particles, usually expressed in cubic feet per minute of (CFM). The Association of Home Appliance Manufacturers (AHAM) recommends different minimum CADRs depending on the size of the room.
Some air cleaners only remove particles from the air, other air cleaners remove both particles and gaseous air pollutants (some of these gases are odorous), and some air cleaners are designed to only remove gaseous pollutants. In most buildings, particles pose a greater health risks than gaseous pollutants; thus, in general, particle removal should be the primary consideration. Many people purchase air cleaners to remove allergens and these allergens are particles. However, in some situations, for example when odors are objectionable and odor sources cannot be removed, removal of gaseous pollutants by an air cleaner will be desirable. Air cleaners with media to remove gaseous pollutants will generally be more expensive to purchase, and the cost of periodically replacing their filtration media will be higher than the cost of just replacing particle filters. The effectiveness of portable air cleaners in removing gaseous pollutants, for example, how long they remain effective, is not well understood. Effectiveness will vary among air cleaners and independent data on effectiveness for gaseous pollutants are not available for most air cleaners. Some air cleaners marketed for gaseous pollutant removal, both with and without particle removal filters, cost hundreds of dollars and remove gaseous pollutants at a rate too low to have any benefit. Some similarly expensive particle-removal air cleaners remove particles at too low a rate to provide any significant benefits.
The fans and moving air of air cleaners create noise causing people to sometimes turn off air cleaners because they are too noisy. Technical literature for some air cleaners includes a noise rating, usually in A-weighted decibels or dBA. Lower dBA values correspond to a quieter air cleaner. The noise produced by an air cleaner will increase with fan speed and air flow rate. One of the quieter portable air cleaners with good particle removal has noise rating ranging from about 21 dBA at the lowest fan speed to 46 dBA at the highest fan speed. The noise level considered acceptable varies among people. Also, when background noise is low, such as at night in a quiet bedroom, air cleaner noise will be more noticeable. It would be best to listen to an air cleaner operating in a quiet space before making a purchase, but this step may not be practical.
There is a large variability in electricity use among air cleaners, even for devices with similar CADRs. Electricity use will increase with fan speed or air flow rate. The cost of electricity used by air cleaners can be significant. For example, continuous operation of a 200 Watt air cleaner for a full year will cost $200 at a typical residential electricity price of 11.3 cents per kilowatt hour. By choosing an energy efficient air cleaner, long term operation costs can be reduced. In addition to energy cost and first cost, the cost of replacing filtration media and the required frequency of replacement should be considered when contemplating a purchase.
Some air cleaners intentionally release ozone because ozone can react with and break down some gaseous air pollutants, often these devices are called ozone generators. The amount of destruction of gaseous pollutants by ozone released to indoor air is generally insignificant and the ozone produced poses respiratory health risks. Some air cleaners can produce ozone, sometimes only a small amount, because of their design; however, these units do not intentionally release ozone to the indoor air for purposes of air cleaning. California certifies air cleaners that produce only a small amount of ozone, see the following web site. Some air cleaners do not produce significant ozone; however, they incompletely break down gaseous pollutants yielding new air pollutants that may pose greater health risks than the original pollutants. Some ultraviolet photocatalytic air cleaners and plasma air cleaners have this problem; however, available data are very limited.
For more information, see:
b) the U.S. Environmental Protection Agency site
c) a position document on air cleaning from the American Society of Heating, Refrigerating, and Air Conditioning Engineers