The science of air cleaning can be confusing and the way manufacturers present scientific information is often misleading. The truth is, unless you're an excellent mathematician, it's difficult to determine which air cleaner is right for your specific needs, because you must weigh initial costs, frequency and cost of maintenance, power requirements, level and sources of pollution, room size and unit efficiency. On top of all this, there's no mandate from any independent scientific source that says what works best.

With all that in mind, reviews say to look for the following in an air cleaner:

  • A clean air delivery rate (CADR) that matches the size of the room to be cleaned. CADR is a standard developed by the independent Association of Home Appliance Manufacturers to measure the removal of dust, smoke and pollen from the air. CADR measures the amount of clean air (in cubic feet) that an air cleaner delivers to a room per minute. A CADR of 50 is appropriate for a room of 80 square feet; a CADR of 80 for 120 square feet; a CADR of 100 for 144 square feet; and a CADR of 200 for 320 square feet. However, not all manufacturers have submitted their models for CADR testing.
  • A High Efficiency Particulate Arresting (HEPA) filter is the best way to filter air. Experts generally agree that true HEPA filters, which trap 99.97 percent of particles, are the most efficient pollutant removers on the market. Some filters advertise themselves as HEPA or "HEPA-type" filters, but unless the word "true" appears in front, they may be lower-grade filters that don't trap as many pollutants.
  • Electrostatic ionizers charge particles so that they fall to the floor or cling to furnishings. Electrostatic ionizers charge particles in order to accelerate their fall to the floor (and hence remove them from the air). You then need to dust or vacuum to remove the allergens from your living space. Consumers should be aware that most electrostatic devices produce a small amount of toxic ozone as a byproduct. Some models use ionizers in conjunction with other filters, using collection plates for the charged particles. Some of these combination machines, like the Friedrich C-90B have been found effective, but you have to weigh that benefit against whether you're comfortable breathing small amounts of byproduct ozone.
  • Air purifiers with fans work better in tests. Without a fan, air is not pulled through the unit to either charge particles and/or filter them through collection media like a HEPA filter.
  • A filter monitor alerts you when filters need to be changed. Proper maintenance is crucial for air filter effectiveness. Once filters become clogged with particles, they can actually turn into air polluters, churning contaminants back into circulation. Some monitors have visual or audio cues that prompt you to change the filter. This feature is not essential; since most HEPA filters need to be changed about once a year, it's easy enough to mark the date on your calendar.
  • A noise level you can accept. Look for models with different fan speeds, so you can control the noise level. However, according to Mercia Tapping, president of AllergyBuyersClub.com, "Vendors routinely overstate the effective cleaning area of a machine because the area is calibrated at the machine's high fan speed. Now, try to sleep with that fan on high -- it sounds like a train going through your bedroom." Some reviewers, including Consumer Reports, suggest buying a larger unit than you need for a bedroom, then running it on low speed to cut down on noise.
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