Most purifier questions come down to one thing: is it big enough for the room? The brand matters less than the number on the box, and a small unit running in a big room does very little.
The number to look for: CADR
The Clean Air Delivery Rate (CADR) is a standard test run by the appliance makers' association (AHAM). It measures how many cubic feet of clean air a purifier delivers per minute for three particle types: smoke, dust and pollen. Smoke is the smallest particle and the hardest test, so use the smoke CADR when comparing.
The rule of thumb
AHAM's guideline is a smoke CADR of at least two thirds of the room's floor area in square feet, for a room with 8-foot ceilings. That works out to about five full air changes an hour.
- 150 sq ft bedroom: smoke CADR of about 100.
- 300 sq ft bedroom: about 200.
- 460 sq ft living room: about 310.
For higher ceilings, scale up in proportion. For allergies, asthma or smoke, buy above the rule rather than right at it.
Size up so you can run it quietly
CADR is measured on the highest fan speed, which is also the loudest. On a quiet or sleep setting most purifiers move a fraction of that air. If you plan to run it on low in a bedroom overnight, pick one rated well above the rule so the low setting still does the job.
One large purifier or several small ones?
- Open floor plan: one larger unit in the middle of the space works.
- Closed rooms: a purifier cleans the room it is in. Air does not mix well through closed doors, so one big unit in the hallway will not clean three bedrooms. Put a correctly sized unit in each room that matters, starting with the bedroom where you spend the most hours.
- Placement: keep a foot or two of space around the intake and outlet, away from curtains and corners.
The real cost is filters and power, not the sticker
A cheap purifier with expensive filters can cost more over three years than a pricier one with cheap filters. Before you buy, look up:
- Replacement filter price and interval. Most makers say every 6 to 12 months for the main filter.
- Watts on the speed you will actually use. Example: a purifier drawing 50 watts around the clock uses about 438 kWh a year. At 17 cents per kWh that is about $74 a year. Check your own electric bill for your rate.
- ENERGY STAR certification, which means the model meets a minimum of clean air per watt. Worth having if it will run all day.
Skip these
- No CADR listed. If the maker will not publish one, assume it is low.
- Ozone generators and purifiers that advertise ozone. California certifies air cleaners for low ozone output, and its list is a good check anywhere.
Not sure you need one? An air quality monitor shows whether particles in your rooms are high to begin with. On smoke days, see the wildfire smoke guide.