How to Choose an Air Purifier: Seven Checks Before You Buy

1. Measure the room, including the ceiling

Measure length, width and ceiling height in feet and multiply them. A 15 by 20 ft room with 8 ft ceilings holds 2,400 cubic feet of air; with 10 ft ceilings it holds 3,000. A purifier cleans volume, so tall ceilings need more CADR. If the room opens onto another space with no door, count both.

2. Check the AHAM smoke CADR, not the box's square footage

The clean air delivery rate (CADR) is the volume of filtered air a purifier delivers per minute, measured on the whole machine under AHAM's standard. The AHAM Verifide directory lists three figures for each certified model: smoke, dust and pollen. Use smoke, because smoke particles are the smallest and hardest to remove.

The square footage printed on most boxes is the room size at one air change an hour, which is too slow for allergies or smoke. AHAM's own suggested room size, and the EPA's guidance, use a rule of thumb of a smoke CADR at least two thirds of the room's floor area for 8 ft ceilings, which works out to about 4.8 air changes an hour.

Smoke CADR for about 5 air changes an hour, 8 ft ceilings. CADR = area x 8 x 5 / 60. Our arithmetic.

Floor area	Smoke CADR needed	Typical room
120 sq ft	80 cfm	Small bedroom, nursery
200 sq ft	133 cfm	Bedroom, office
300 sq ft	200 cfm	Living room
450 sq ft	300 cfm	Large living room
650 sq ft	433 cfm	Open plan

Our air purifier sizing calculator does this for any room, and CADR explained goes deeper. Several big brands (Coway, Dyson, Alen, IQAir, Austin Air, Molekule, Rabbit Air) were not in AHAM's directory when we checked; their figures are their own, which makes comparison harder, not impossible.

3. Read the filter label carefully

The US Department of Energy's HEPA standard is 99.97% efficiency at 0.3 microns. "True HEPA", "HEPA-type" and "HEPA-grade" are marketing terms, and some makers state efficiency only at a specific fan speed. The whole-machine CADR is a better guide than the label. See True HEPA vs HEPA.

For odors, look at the carbon. A thin carbon-coated wrap handles light smells; a panel or bed of carbon granules handles cooking, smoke and chemicals for much longer.

4. Add up the running cost

Find the replacement filter on the maker's own store, note the price and the stated interval, and work out a year. Then look up AHAM's annual energy estimate and multiply by your electricity price; the 2025 US residential average was 17.2 cents per kWh, according to the EIA.

Yearly filter cost from maker list prices and stated intervals. Our arithmetic.

Purifier	Filter cost a year
Coway AP-1512HH	$59.79 (one set)
Levoit Core 300-P	$45 to $60
Austin Air HealthMate	$79 (5-year filter)
Medify MA-40, run 24/7	about $180
Molekule Air Pro	about $350

5. Check the noise at the speed you will use

Makers quote noise on the lowest speed. If the purifier needs its top speed to clean your room, you will hear its top-speed figure. Buying about 1.5 times the CADR your room needs lets it reach five air changes an hour on a quieter middle setting.

6. Decide how you feel about ionizers and other electronic stages

Ionizers, PlasmaWave-type features, photocatalytic stages, bipolar ionization and UV lamps are electronic stages. California requires electronic air cleaners sold there to be certified at or below 0.050 ppm of ozone, and its certified list shows each model's type. The EPA advises against ozone generators sold as air cleaners. Many purifiers let you switch the ionizer off; a filter-only model avoids the question. Our ionizer guide and ozone guide cover the evidence.

7. Look at sensors, warranty and the small print

• Sensors: a laser particle sensor runs the most accurate auto mode; a gas sensor follows odors, not dust. See smart features worth paying for.
• Warranty: two years is common; Honeywell gives five on the HPA300 and Coway five on the Airmega 400. Read lifetime guarantees closely: Alen's requires an active filter subscription.
• Filter availability: pick a model whose filters are sold widely, so you are not stuck if a retailer runs out.

A quick worked example

A 14 by 18 ft living room with 9 ft ceilings holds 2,268 cubic feet. Five air changes an hour need 2,268 x 5 / 60 = 189 cfm of smoke CADR. The Levoit Core 400S-P (231 to 234 cfm) clears it with room to spare on a middle speed; the Levoit Core 300-P (143 cfm) would give only 3.8 air changes an hour. Our best air purifiers list starts from there.

What a purifier will not do

A purifier removes particles and, with enough carbon, some gases from the room it is in. It does not ventilate, lower CO2, fix humidity, remove mold growing on a wall, or protect against carbon monoxide. For those, see our guides to CO2, humidity and mold prevention.