Shopping for an air purifier means wading through a swamp of marketing: HEPA this, ionizer that, "cleans up to X square feet," "removes 99.97% of particles." It is designed to overwhelm you into trusting the brand. Cut through it with one real spec and one healthy suspicion. The spec that actually tells you whether a purifier cleans your air at a useful rate is its CADR, and the word to be most suspicious of is the one plastered everywhere: HEPA.
Why CADR is the number that matters
CADR — Clean Air Delivery Rate — measures how much clean air a purifier actually delivers, combining how well it filters with how much air it moves. That combination is the whole point: a great filter that barely moves air cleans a room slowly, and lots of airflow through a poor filter cleans nothing. CADR captures both in one number, and a higher CADR (appropriate to your room size) means the unit actually clears the air at a meaningful rate. It is the spec that lets you compare units honestly and match a purifier to a room, which the vague "cleans up to X square feet" claims do not, because those often assume unrealistic conditions. Judge a purifier by its CADR relative to your room; most of the other claims are noise.
The ionizer footnote
Many purifiers tout ionizers or other add-on technologies as bonus cleaning power. Be cautious: these are marketing features more than reliable air-cleaning, some can produce ozone as a byproduct, and the real work is done by a good filter moving enough air — i.e., by CADR. Do not pay extra or compromise on the fundamentals for add-on gimmicks; a strong filter with good airflow is what actually cleans the room.
The takeaway
Ignore the marketing swamp and judge an air purifier by its CADR relative to your room size — that single spec captures both filtration and airflow, which is what actually cleans your air. Treat the word "HEPA" with suspicion until you confirm it's true HEPA and not a diluted "HEPA-type" cousin, and dismiss ionizer and gimmick claims as the noise they usually are. Match CADR to your room, insist on a real filter, and you'll buy a purifier that works — while everyone else pays for words.