Choose an air purifier by its CADR (clean air delivery rate) for smoke, which AHAM rates in cubic feet per minute. A simple rule: the smoke CADR should be at least two-thirds of the room's floor area in square feet for 8-foot ceilings. A 300 sq ft room needs a smoke CADR of 200 or more. Look for true HEPA, check filter prices and noise, and avoid purifiers that produce ozone.
An undersized purifier runs loud on high and still does little. CADR is the number that tells you if a purifier fits a room.
What CADR measures
The Association of Home Appliance Manufacturers (AHAM) tests purifiers for smoke, dust and pollen and publishes CADR in cubic feet per minute. Smoke particles are the smallest and hardest to remove, so smoke CADR is the one to compare.
Size by room
| Room area | Minimum smoke CADR (two-thirds rule) |
|---|---|
| 150 sq ft | 100 |
| 300 sq ft | 200 |
| 450 sq ft | 300 |
| 600 sq ft | 400 |
For allergies, asthma or wildfire smoke, size up. With higher ceilings, scale the area up proportionally.
Air changes per hour
ACH = CADR × 60 ÷ room volume in cubic feet. Four to five air changes per hour is a common target for allergy relief. Example: CADR 200 in a 300 sq ft room with 8 ft ceilings (2,400 cu ft) gives 5 ACH.
Beyond CADR
| Check | Why |
|---|---|
| True HEPA | Captures fine particles |
| Activated carbon weight | Helps with odors and some gases; thin carbon sheets do little |
| Filter price and life | The running cost |
| Noise at the setting you will use | Bedroom use needs quiet low settings |
| CARB certification | Confirms low ozone emissions |
| ENERGY STAR | Efficiency for always-on use |
Pair a purifier with a good HVAC filter and ventilation; see thermostat compatibility if you are upgrading controls. Browse climate and air.
Common questions
What is a good CADR for an air purifier?
At least two-thirds of the room's square footage for smoke CADR, assuming 8-foot ceilings. Higher is better for allergies or wildfire smoke.
What does true HEPA mean?
A filter that captures at least 99.97 percent of particles at 0.3 microns, the US Department of Energy definition.
Are ionizers safe?
Some produce ozone, a lung irritant. In California, air cleaners must meet CARB ozone limits; look for that certification.
How often do I change the filter?
Usually every 6 to 12 months for HEPA filters, sooner during smoke events. Check the cost before buying.