1. Airflow, the amount of air being moved through the unit, measured in cubic feet per minute (CFM). If CFM is not provided, choose a machine designated for your room's size.
2. The types of pollutants the purifier extracts. Some filters do a better job than others on certain particles
3. The Clean Air Delivery Rate (CADR)—the amount of filtered air the machine adds to a space as determined by the Association of Home Appliance Manufacturers. The unit's CADR score should equal at least two-thirds of the room's square footage; for a 180-square-foot area, a CADR of 120 is acceptable.
Other considerations? Filter maintenance and noise levels. On the following page, we compare High Efficiency Particulate Air (HEPA), ultraviolet, electrostatic precipitator, and combination purifiers. Although ionizers are a popular choice, studies have shown that some models do not significantly improve air quality and can produce unhealthy levels of ozone, a lung irritant. If you're looking to clean multiple rooms simultaneously, consider whole-house systems, which use existing ductwork to catch particles and cycle purified air through your entire home. These systems cost three to 10 times as much as the freestanding models featured here.
HEPA filters were developed by the U.S. military in the 1940s to trap radioactive dust. A matrix of randomly overlapping layers of glass fibers snares particles while allowing air to pass through.
Pros: Ideal for capturing animal dander, mold spores, dust, and pollen. Extremely effective; snags at least 99.97 percent of airborne particles.
Cons: Filters are expensive, often costing half the price of the purifier itself. And washable HEPA filters have not yet come of age. (That could change. The Department of Energy recently created a washable metal HEPA filter that looks promising.) Units do not remove chemical fumes or gases.
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These devices use UV lamps to sterilize air by altering the DNA of bacteria, mold, and viruses.
Pros: Since UV purifiers kill airborne pathogens, they're ideal for those with weak immune systems—it's not unusual to find them in hospitals and clinics. And with no moving parts, these machines don't make a racket.
Cons: Units do not remove particulates, gases, or chemical fumes. Some models can produce small amounts of airborne ozone, which can trigger asthma.
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Because household members can be sensitive to different types of dust or chemical compounds, many new purifiers include several filters, each aimed at a specific type of airborne irritant.
Pros: Mixed-media purifiers meet multiple health needs. The presence of inexpensive (often washable) fiber prefilters tends to prolong the life of the expensive HEPA filter by removing large particles before they reach it. Many units contain odor-absorbing carbon filters.
Cons: Any non-washable filter will eventually need replacement, so the long-term maintenance costs may be higher for a multifilter model than for
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