CADR: the one number to learn.

In an independent lab, a purifier is sealed in a test chamber with a measured concentration of pollutant. CADR is the rate at which it removes that pollutant, in cubic feet per minute (CFM). It's tested separately against three particles:

Smoke — the smallest particles, the hardest to capture.Dust — mid-size particles. Pollen — the largest, the easiest. A purifier's three numbers are usually close together; if you only check one, check smoke.

Smoke particles sit around 0.1–1 microns — small enough to slip through weak filters and deep into lungs. If a purifier handles smoke, dust and pollen are easy. That's why every sizing rule on this site — and our own scoring — keys off smoke CADR, not the bigger, friendlier pollen number manufacturers prefer to advertise.

The rule of thumb: smoke CADR should be at least two-thirds of your room's square footage. A 300 sq ft bedroom needs 200 CFM. That delivers the 4–5 air changes per hour allergists recommend.

Where the models in our index land on smoke CADR. The middle of the market — 180 to 250 CFM — covers most bedrooms and dens. Big open-plan rooms shrink the field fast.