CADR ratings explained: what the three numbers measure

CADR stands for clean air delivery rate. It is a laboratory measure of how much cleaned air a portable air purifier delivers, stated in cubic feet per minute (cfm), and it comes as three separate results: one for tobacco smoke, one for dust and one for pollen. A CADR of 200 for smoke means the unit removed smoke particles in the test as fast as if it had added 200 cubic feet of smoke-free air to the room each minute. The test belongs to the Association of Home Appliance Manufacturers (AHAM), under its AC-1 standard. It is the number to size a purifier by, provided you know three limits: it applies to particles only, it is measured at top speed, and it describes a test chamber and not your living room.

What a CADR number measures

EPA’s Technical Summary defines CADR as “a measure of a portable air cleaner’s delivery of relatively clean air, expressed in cubic feet per minute (cfm).” AHAM adds that it can also be given in cubic meters per hour, and it prints the conversion as 1 cfm = 1.699 m3/h.

The word that matters is clean. A fan’s airflow tells you how much air moves. AHAM describes CADR as “the volume of filtered air an air cleaner delivers,” so it depends on both the air moved and what the filter takes out of it. A unit that moves a lot of air through a weak filter and a unit that moves little air through a tight filter can land on the same CADR, and for the room the result is the same.

AHAM runs the test in a chamber 10 1/2 feet by 12 feet by 8 feet high, a volume of 1,008 cubic feet. AHAM reports a separate result for each of its three test particulates.

Smoke, dust and pollen: what each number covers

AHAM describes its three test particulates, engineering tobacco smoke, fine dust and paper mulberry pollen, as “representing fine, medium and large particles respectively.” The ranges below are as each body states them.

The two agree on smoke and dust. They disagree on the low end of the pollen range: EPA’s Technical Summary gives 5 micrometers and AHAM’s own FAQ gives 0.5 microns. For a buyer that gap changes nothing, because the pollen number is the one you are least likely to size by, but anyone quoting the range should say whose it is.

EPA’s Technical Summary also reports that on average pollen CADRs run about 5 percent greater than dust CADRs, and tobacco smoke CADRs about 4 percent lower than dust CADRs. On those averages, the three numbers sit close together. EPA says “tobacco smoke CADRs should be used as the most conservative estimate” for small particles such as PM2.5, and the sizing figures from both EPA and AHAM are built on the smoke number.

How to read the label

1. Find the smoke number first. Use it for sizing unless you have a specific reason to look at another one.
2. Check that it is a CADR and not an airflow figure. A number described only as airflow, or as a maximum coverage area, is a different claim.
3. Compare it with your room. AHAM’s rule of thumb is that “the CADR of your air cleaner should be equal to at least two-thirds of the room’s area.” A 10 by 12 foot room is 120 square feet, and AHAM says a smoke CADR of at least 80 would be best for it. The full method, with higher ceilings and open plans, is in the guide to matching a purifier to your room size.
4. Read the room size printed beside it with care. AHAM calculates it as Room Size (square feet) = CADR (Smoke) x 1.55, based on removing at least 80 percent of smoke particles in a steady-state room, assuming one air change per hour with complete mixing. A smoke CADR of 200 gives 310 square feet.
5. Check your ceiling. EPA says the CADRs are calculated based on an 8-foot ceiling. AHAM says that with a higher ceiling the coverage “will be less than the room size shown.”

By arithmetic, the multiplier and the two-thirds rule land close together: dividing a room’s area by 1.55 gives about 65 percent of it, which matches EPA’s chart (a 300-square-foot room gets a minimum CADR of 195).

The upper limit on CADR: why very high numbers need care

AHAM’s June 2020 FAQ says “the maximum allowable CADR that can be measured by the AC-1 method in the chamber is 450, so the maximum room size that the standard can confidently predict performance would be a room of 698” square feet. That is the 1.55 multiplier applied to 450.

The practical meaning is that, on AHAM’s June 2020 description, a claim above 450 cfm cannot have come from that method. It may be an honest figure from another test, but it is not comparable with a rating from this one, so find out how it was measured. The limit comes from a 2020 document, so ask for the current test report if a rating this high decides your purchase.

What CADR does not measure

CADR is a particle measure and nothing else. EPA’s Technical Summary says “a CADR can theoretically be generated for either gases or particles; however, current test standards only rate CADRs for the removal of particles.” EPA’s consumer guide puts it more bluntly: “The CADR rating system is for particles only.” AHAM confirms that “there is no universal test method yet accepted for measuring performance of air cleaners to clean gases or volatile organic compounds.”

Several other things also fall outside the number:

• Odors, VOCs and other gases. A high CADR says nothing about formaldehyde or cooking smells.
• Noise. EPA’s Technical Summary notes that ratings are taken at maximum airflow and “therefore typically maximum noise levels,” and that as of 2017 quantified noise was not a factor in U.S. performance ratings. The guide to air purifier noise and fan speeds covers what that leaves you to work out.
• Health outcomes. The test counts particles in a chamber. It does not measure symptoms, and a good score is not evidence that a unit will change how anyone feels. Anyone with a health condition should ask a clinician.
• Ozone. CADR is not an ozone test. A device can carry a respectable particle number and still emit ozone, so that needs a separate check.
• A filter after long use. The rating describes the unit as tested. EPA’s Guide says “If a filter is dirty and overloaded, it won’t work well.”

Why the top-speed result matters so much

AHAM’s FAQ says “the test is performed in the highest speed setting.” EPA says “The rating is typically measured at the air cleaner’s highest speed,” and adds: “An air cleaner will filter less air if it is set at a lower speed.” AHAM goes further: “If you run the air cleaner on a lower fan speed, it is understood that both efficacy and the room coverage will also decrease.”

The top setting is not always the one people will keep running. EPA’s Technical Summary warns that at lower airflow settings “the CADR may not be known (but it could be considerably lower than the highest advertised and thus significantly less effective at pollutant removal).” One study gives a sense of the size of the drop: Lu and colleagues, in a 2024 trial in 29 homes, used a unit with an advertised CADR of 330 cfm at its highest setting and measured 263 cfm on the quieter setting they chose for participants. That sets a measured figure beside an advertised one, for one model and one setting, so it is no general rule.

The sensible response is to buy with margin. If a room needs a smoke CADR of 120, a unit rated well above that has a better chance of delivering it at a setting you can tolerate. Check what the room needs first, then see how the other things to weigh when choosing an air purifier narrow the list.

What a chamber result can and cannot predict

A CADR is a controlled comparison, which is its strength: two rated units were put through the same procedure. It is not a forecast for your home. A chamber is a fixed, controlled space, while a real room has a door that opens, furniture in the way and, often, air arriving from outside. EPA says a portable air cleaner “will be much more effective for a specific room when any exterior doors and windows in a room are closed.”

Field results give some context. EPA’s Technical Summary reports that most of the studies it reviewed recorded particle exposure reductions with HEPA or other high-efficiency portable units “on the order of approximately 50 percent or higher.” Those are measured reductions in particles in the air. They do not show a health effect, and EPA describes most measured health improvements in this literature as “relatively small in magnitude.”

When the CADR is missing

Not every unit has one. CDC/NIOSH offers a fallback for units that publish only an airflow rate: for a room up to 8 feet tall, “choose an air cleaner with an air flow rate value (cfm) that is at least 2/3 of the floor area.” Airflow is a weaker basis, because it says nothing about what the filter removes as the air passes through, so treat it as a minimum and look for an independent test before relying on it.

CDC also explains why the smoke number is the one to use for fine airborne particles: “The smoke particles are the smallest, so that CADR number applies best to viral particles related to respiratory diseases.” That is a statement about particle size. It is not evidence that a purifier reduces infections.