CADR Explained: What Clean Air Delivery Rate Measures and Why It Matters

How CADR is measured

Under AHAM's test method, a purifier is placed in a sealed room of a set size. The room is filled with a test contaminant, and particle counts are tracked over time, first with the purifier off to measure natural decay (particles settling and sticking to walls) and then with it running on its highest speed. CADR is the difference between the two decay rates multiplied by the room's volume: the extra clean air the purifier adds. AHAM's air filtration standards page describes the program, and the results go into the public AHAM Verifide directory.

Because CADR is measured on the whole machine, it accounts for the filter's efficiency, the fan's airflow and any air that leaks around the filter. A powerful fan with a poor filter and a fine filter with a weak fan can both score low.

The three CADRs: smoke, dust and pollen

AHAM's three test contaminants, and when each figure matters.

CADR	Particle size range	Use it for
Smoke	Smallest particles, about 0.09 to 1 micron	Smoke, wildfire, cooking particles, general sizing
Dust	Medium, about 0.5 to 3 microns	Household dust, workshop dust
Pollen	Largest, about 5 to 11 microns	Pollen, pet dander, mold spores

Smoke is usually the lowest of the three, because small particles are the hardest to catch, and AHAM's own sizing guidance uses it. Sizing on smoke CADR means the purifier is also big enough for the larger particles. Some purifiers list a PM2.5 CADR as well; AHAM's directory shows it where available.

From CADR to room size

CADR on its own is a flow. To know whether it is enough, compare it with the room's volume:

• Air changes per hour (ACH) = CADR x 60 / room volume in cubic feet.
• AHAM's suggested room size on the label is based on 4.8 ACH, according to AHAM's AC-1 standard.
• AHAM's rule of thumb is a smoke CADR of at least two thirds of the floor area for 8 ft ceilings, and for wildfire smoke, a smoke CADR equal to the floor area.

Examples from AHAM's directory, read November 3, 2026, with air changes in a 300 sq ft room with 8 ft ceilings (2,400 cu ft). Our arithmetic.

Purifier	Smoke CADR	AHAM room size	ACH in 300 sq ft
Levoit Core 300	143 cfm	222 sq ft	3.6
Levoit Core 400S-P	234 cfm	363 sq ft	5.9
Honeywell HPA300	300 cfm	465 sq ft	7.5
Blueair Blue Pure 211i Max	435 cfm	674 sq ft	10.9

Our sizing calculator runs this for any room.

The box number versus the CADR

Many boxes print a much larger room size than AHAM's. That figure usually assumes one air change an hour: a purifier rated 143 cfm can "cover" over 1,000 sq ft once an hour, which is true but too slow to matter for allergies or smoke. When the two numbers disagree, trust AHAM's room size. Our reviews show both.

What CADR does not tell you

• Gases and odors. CADR tests particles only. A purifier's carbon capacity is a separate question; see our VOC guide.
• Noise. CADR is measured on the top speed. A purifier may be much quieter, and much slower, on the setting you actually use.
• Running cost. Filter price and life, and energy use, are separate. AHAM's directory does list annual energy and an integrated energy factor for many models.
• Performance as the filter loads. CADR is tested with a clean filter. Airflow falls as the filter clogs.
• Ozone and byproducts. CADR does not test for them; California's certification of electronic air cleaners does.

Brands outside AHAM's program

When we searched AHAM's directory on November 3, 2026, Coway, Dyson, Shark, Alen, Austin Air, IQAir, Molekule and Rabbit Air had no listed models. Some publish their own CADR figures; IQAir says it does not consider CADR a valid method and does not take part. Their purifiers may perform well, but their figures are their own and not independently comparable. We say so on every page that uses them.