| Quick Facts | |
|---|---|
| CADR sizing rule | clean air delivery rate at least two-thirds of room square footage (AHAM) |
| EPA CADR chart range | about 65 CFM for a 100 sq ft room up to 390 CFM for a 600 sq ft room |
| True HEPA standard | captures at least 99.97% of particles at 0.3 microns (US DOE definition) |
| Reference noise level | roughly 50 dB, about as loud as a refrigerator (EPA) |
| Wildfire smoke CADR rule | match CADR 1:1 to room square footage instead of the usual two-thirds ratio (AHAM) |
| Typical filter replacement interval | every 6 to 12 months depending on run time and air quality |
Size the CADR to your room, not the box
A purifier's clean air delivery rate needs to be at least two-thirds of your room's square footage to actually clear the air at a reasonable pace.
Shopping for a specific model? See the best air purifier we tested — 10 models compared on the specs that decide it.
Clean air delivery rate, or CADR, predicts real-world performance better than anything else printed on the box. The EPA's own sizing chart runs from about 65 CFM for a 100 sq ft room up to 390 CFM for a 600 sq ft space, and AHAM's rule of thumb backs it up: pick a unit whose smoke CADR is at least two-thirds of the room's square footage. A 120 sq ft bedroom, in AHAM's own example, needs a smoke CADR of roughly 80 or higher to qualify.
Manufacturers love to print a maximum-coverage number that assumes the highest, loudest fan speed running nonstop, which is not how anyone actually lives with these machines. If you're shopping specifically for a bigger space, our large-room air purifier guide breaks down which models keep their CADR high enough at speeds you'd tolerate for hours, not just on the spec sheet. Undersizing is the single most common purifier mistake, and it's an easy one to avoid once you've done the square-footage math first.
Confirm the filter is actually true HEPA
A true HEPA filter captures at least 99.97% of particles at 0.3 microns; "HEPA-type" is a lesser marketing term, not the certified standard.
The word HEPA gets stamped on a lot of packaging that doesn't meet the actual bar. The US Department of Energy's standard defines true HEPA as removing at least 99.97% of particles at 0.3 microns, the particle size researchers consider hardest to trap. Filters labeled "HEPA-type" or "HEPA-like" skip that certification and can sit well below it, even though the shelf tag looks nearly identical to the real thing.
This distinction matters most for allergy and asthma triggers, since the fine particles you're trying to remove sit right in that 0.3-micron range. Our HEPA air purifier guide only ranks models that meet the true-HEPA bar, which saves you from parsing a spec sheet line by line in the aisle. Pair a genuine HEPA layer with a washable pre-filter, since that's what keeps replacement costs from creeping up over the first year of ownership.
Check the noise rating at the speed you'll actually use
Most purifiers only stay quiet on their lowest fan speed; check the decibel rating at medium or high before buying one for a bedroom.
A purifier rated at 25 dB on its lowest setting can climb past 55 dB once it's working hard enough to hit its stated CADR, and that gap rarely makes it into the headline spec. The EPA notes that a noise rating around 50 dB is roughly what a modern refrigerator sounds like from the next room, which is a useful gut-check when a listing throws decibel numbers at you with no context attached.
For a bedroom, ask what fan speed the unit needs to run at overnight to keep pace with the room's CADR requirement, not the lowest number on the box. A machine that has to run on high all night to cover a room it's technically "rated" for will be louder than the spec sheet suggests, and that's worth knowing before you buy rather than after the first restless night with it running next to you.
How to choose an air purifier: match filtration type to your actual trigger
Allergy and pet households need true HEPA plus a washable pre-filter for hair; smoke and odors call for activated carbon layered on top of HEPA.
Not every air-quality problem needs the same filter stack. Pet dander and pollen respond well to a true HEPA filter paired with a coarse pre-filter that catches hair before it clogs the fine media underneath. Smoke and cooking odors need a layer of activated carbon in addition to HEPA, since particulate filters alone don't touch gas-phase odors no matter how fine their mesh is. For households juggling more than one of these at once, our air purifier guide for allergies sorts models by which combination of filter stages they actually carry, rather than by CADR alone.
Skip anything marketed as an ozone generator or a standalone ionizer sold in place of real filtration. The EPA is blunt about it: ozone is a lung irritant, and no federal agency has approved ozone-generating devices for use in occupied rooms. Some purifiers include an ionizer as a secondary, switchable feature alongside genuine HEPA filtration, which is a different and more defensible setup than a device that leans on ozone as its main cleaning method.
Weigh filter replacement cost against the sticker price
A cheaper purifier with frequent filter swaps can cost more over a year than a pricier unit with a longer filter life.
The upfront price tag is only part of the math. Filters typically need replacing every six to twelve months depending on run time and air quality, and that recurring cost varies a lot between brands, sometimes by more than the machine itself. Budget-tier units often use smaller, cheaper filters that need swapping more often, while mid-range and premium models tend to stretch that interval out, which evens out total ownership cost over a year or two even when the sticker price looks higher upfront.
Think in value tiers rather than exact figures when comparing options: budget units deliver baseline HEPA filtration and little else, mid-range models add real-world extras like washable pre-filters and a usable auto mode, and premium tiers bring lower noise at high CADR plus longer filter intervals. Our full air purifier hub rounds up picks across all three tiers if you want a shortlist built around the criteria above, and our best air purifier guide narrows that further to the handful worth comparing side by side.