Efficiency Ratings
CADR Rating Explained: Smoke, Dust and Pollen Numbers by Room Size
CADR (Clean Air Delivery Rate) is the volume of particle-free air a portable air purifier delivers, in cubic feet per minute, measured separately for smoke, dust and pollen under the AHAM AC-1 test. For a room with 8-foot ceilings, choose a smoke CADR of at least two-thirds of the floor area in square feet; that gives about five air changes per hour.
What CADR measures
CADR combines two things into one number: how much air the purifier moves and how much of the target particle it removes from that air. A unit that moves 300 CFM but captures only half the smoke passing through behaves like one delivering about 150 CFM of perfectly clean air.
CADR ≈ Airflow (CFM) × Single-pass capture efficiency
In the lab, the test chamber is filled with a known particle load and the decay rate is measured with the purifier running, then subtracted from the natural decay rate without it. Because it is measured as a result rather than calculated from fan specs, CADR is the most useful single number for comparing portable purifiers.
Smoke, dust and pollen: the three CADR numbers
The AHAM standard reports three CADRs because filters treat particle sizes differently.
| CADR type | Approximate particle size | Maximum reportable CADR | What it tells you |
|---|---|---|---|
| Smoke | about 0.1–1 µm | 450 CFM | Performance on the finest particles: wildfire and tobacco smoke, cooking fumes, much of the PM2.5 in outdoor air |
| Dust | about 0.5–3 µm | 400 CFM | Household dust and many pet dander particles |
| Pollen | about 5–11 µm | 450 CFM | Large allergens that also settle out on their own fairly fast |
The smoke number is usually the lowest and the hardest to earn, so it is the one to size by. A purifier with a true HEPA filter typically has smoke, dust and pollen CADRs close together, because HEPA catches all three sizes well and the limit is mostly airflow.
Room-size rule of thumb
The common guideline is that the smoke CADR should be at least two-thirds of the room's floor area, assuming 8-foot ceilings. That works out to about five clean-air changes per hour.
Minimum smoke CADR (CFM) ≈ Floor area (sq ft) × 2 ÷ 3
Air changes per hour = CADR × 60 ÷ (Floor area × Ceiling height)
| Room floor area | Minimum smoke CADR (8 ft ceiling) | For smoke events or allergies (aim higher) |
|---|---|---|
| 100 sq ft | 67 CFM | 100+ CFM |
| 150 sq ft | 100 CFM | 150+ CFM |
| 200 sq ft | 133 CFM | 200+ CFM |
| 300 sq ft | 200 CFM | 300+ CFM |
| 400 sq ft | 267 CFM | 400 CFM, or two units |
| 500 sq ft | 333 CFM | two units |
Worked example: a 12 × 15 ft bedroom is 180 sq ft and 1,440 cubic feet with an 8-foot ceiling. A purifier with a smoke CADR of 120 CFM gives 120 × 60 ÷ 1,440 = 5.0 air changes per hour. For vaulted ceilings, scale the CADR up in proportion: a 12-foot ceiling needs 50% more. The air purifier sizing calculator and air changes per hour calculator do this for you.
Why the label room size can mislead
- CADR is measured on the highest fan speed. Many people run purifiers on low or sleep mode because high is loud, which can cut the delivered CADR by half or more. Buy enough headroom to get your target on a speed you will tolerate.
- Some brands quote room sizes at fewer air changes per hour. A box claiming 1,000 sq ft may be assuming one or two air changes per hour. Always go back to the smoke CADR and do the math.
- Placement matters. A purifier jammed behind a sofa or in a corner recirculates its own clean air. See where to place an air purifier.
- Doors and HVAC airflow mix air with the rest of the house, diluting the effect in one room.
CADR for wildfire smoke and allergy season
During smoke events, outdoor PM2.5 can climb to many times normal levels and seeps indoors through every gap. Public health guidance generally favors running a HEPA purifier in a closed room where people sleep or spend the most time, on the highest speed you can tolerate. Some practical points:
- Aim for at least the two-thirds rule, and preferably closer to 1 CFM of smoke CADR per square foot.
- Close windows and set the HVAC system to recirculate. If your system can take it, a MERV 13 filter helps the rest of the house.
- Expect filters to load faster; check them after a heavy smoke week.
For pollen allergies, CADR needs are usually lower because pollen grains are large and settle quickly, but running the purifier continuously in the bedroom during the season makes a noticeable difference for many people.
What CADR doesn't cover
CADR is a particle metric only. It says nothing about gases, odors or volatile organic compounds, which need a substantial bed of activated carbon or another sorbent rather than a thin carbon-coated sheet. It also does not measure noise, power use or filter cost, all of which decide whether you actually keep a purifier running.
Avoid devices that deliberately generate ozone, sold as ozone generators or with vague "activated oxygen" claims. Ozone irritates the lungs at levels these devices can produce indoors. California requires air cleaners sold there to meet an ozone emission limit, and that certification is a useful check anywhere.
Running cost and filter cost
Most room purifiers draw 20–60 W on high and a few watts on low. Running 24 hours a day at 40 W uses about 29 kWh a month, roughly $5 at $0.17/kWh; use the appliance electricity estimator to check yours. Replacement HEPA filters often cost more per year than the electricity, so compare filter prices and recommended change intervals before buying. Whole-house filtration through your HVAC system is rated differently, in MERV; see the MERV rating chart.
Frequently asked questions
What is a good CADR for a bedroom?
For a typical 150–200 sq ft bedroom with an 8-foot ceiling, a smoke CADR of 100–135 CFM meets the minimum. If you have allergies or deal with wildfire smoke, look for 150–200 CFM so you can run it on a quieter speed.
Which CADR number should I use?
Use the smoke CADR. It covers the smallest particles and is usually the lowest of the three, so a purifier sized on smoke will handle dust and pollen too.
Can one large purifier clean a whole house?
Not well. Walls and doors limit mixing, so a purifier mostly serves the room it sits in. Use one per main room, or upgrade the furnace filter to MERV 13 if your system can handle it.
Last reviewed 2026-10-28