Air Purifier CADR and Room Size

Those three numbers come from the ANSI/AHAM AC-1 test, which is why they can be compared from one maker to another. EPA's home air cleaner guide adds two limits worth knowing. The rating is typically measured at the cleaner's highest speed, and the CADR system covers particles only, so it says nothing about smells or chemical vapors.

Two other labels are easy to confuse with CADR. An ENERGY STAR mark on an air cleaner is about energy efficiency, and EPA says it "does not refer to its air-cleaning effectiveness in any way." The ENERGY STAR room air cleaner specification does set a floor of a smoke CADR of 30 for a model to qualify, but that is a minimum, not a recommendation. And EPA itself neither certifies nor recommends brands or models. A third mark matters only for electronic units: the California Air Resources Board's certified list covers ozone emissions and electrical safety, and CARB says certification does not reflect how well a unit cleans.

Adjust for the room you actually have. AHAM says ceilings higher than 8 feet call for a cleaner rated for a larger room. EPA says an open floor plan should be counted as the whole space the cleaner serves, and that an area bigger than any single product can handle may need more than one unit.

A unit sized by its smoke number handles the bigger dust and pollen too, since those are easier to trap. Sizing by the pollen or dust number instead can leave a room short of fine-particle cleaning.

None of the three numbers covers gases. For odors and volatile organic compounds, EPA suggests a cleaner with an activated carbon or other sorbent filter, and notes that a larger amount of filter material (thicker) tends to work better. There is no widely used rating for gas removal. EPA also says research has not shown that plasma, photocatalytic oxidation or UV light remove gases effectively in portable home units. The page on ozone air purifiers and ionizers explains why some of those technologies can add pollutants instead.

HEPA is still a good sign. EPA reports that several studies of portable HEPA cleaners found small improvements in heart and breathing measures, and some improvement in allergy or asthma symptoms. It is also careful to add that "the improvements are typically small and not always noticeable to the individual." Expect cleaner air, not a cure.

A cleaner also can't fix what causes the problem. EPA says filtration does not replace controlling pollutant sources and ventilating, and that air cleaners don't solve mold, which comes from a moisture problem. Run the kitchen hood when cooking and keep indoor humidity in range first; then add filtration.

Speed is where the rating meets real life. Because CADR is measured at the highest speed, a cleaner on its quiet setting delivers less than its label, sometimes much less. EPA's rule of thumb is simple: the longer the cleaner runs, the more air it filters.

Noise decides whether you'll actually leave it on. Some boxes show a noise rating; EPA says about 50 dB is roughly the hum of a modern refrigerator. If the top speed is too loud for the bedroom at night, size up so that a lower, quieter speed still meets the 2/3 rule. Check the filter on the maker's schedule, because a clogged filter cuts the clean air delivered. For the whole house, a better central-system filter by MERV rating does a different, complementary job.