CADR stands for clean air delivery rate, and it is the volume of filtered air a purifier delivers per minute, in cubic feet. It is the only number on the box that rates the machine as a whole system — fan and filter together — which is why it is worth more than every other specification combined.
There are three CADR figures, there is a hard ceiling on all of them, and there are several things the test deliberately does not measure. Each of those facts changes how you should read a spec sheet.
Why one number covers fan and filter together
Filtration efficiency on its own tells you nothing useful. A filter that removes 99.97% of particles from a tiny trickle of air cleans a room slowly. A powerful fan behind a loose filter moves a great deal of air and removes little from it. Either design can look good on a spec sheet that quotes only one half of the pair.
CADR closes that gap. Conceptually it is airflow multiplied by the fraction of particles removed on each pass, so a unit can only score well by doing both parts adequately. A machine with 200 cfm of airflow and a filter removing 50% of particles delivers about the same CADR as one with 100 cfm and a near-perfect filter.
That is also why you cannot improve a unit meaningfully by fitting a denser filter. A tighter filter raises efficiency and lowers airflow at the same time, and the two effects largely cancel. Manufacturers who push efficiency beyond a certain point are trading away the airflow that made it useful.
The three numbers
Certified units publish three CADR values. The EPA guide describes them as tobacco smoke, dust and pollen, used as examples representing small, medium and large particles respectively.
| CADR | Represents | What it tells you |
|---|---|---|
| Tobacco smoke | Small particles | The hardest test, and the one that predicts performance on combustion particles, cooking smoke and traffic pollution |
| Dust | Medium particles | General household dust performance |
| Pollen | Large particles | The easiest test; most competent units score well |
The smoke figure is the one to judge a unit by, for two reasons. It is the most demanding, so it separates machines that the pollen figure does not. And small particles are the ones that stay airborne longest, so they are the fraction a purifier is genuinely needed for — larger particles settle out on their own within minutes to hours.
If a product page quotes a single unlabeled CADR, assume it is the pollen figure. If it quotes a figure well above the others, it certainly is.
Why no certified unit lists a CADR above 450
This is the detail that explains most of the confusing marketing in the category. Under AHAM’s certification program, the maximum reportable values are 450 for tobacco smoke, 450 for pollen and 400 for dust. The limit comes from the test itself: the standardized chamber is 1,008 cubic feet, and AHAM states that CADR values outside the published ranges will not have the statistical data the method requires.
The reason is straightforward once you picture it. A unit delivering 450 cfm into a 1,008-cubic-foot chamber turns that volume over nearly 27 times an hour. The concentration crashes so fast that the measurement loses precision, and at that point the test is measuring its own chamber rather than the machine.
Three practical consequences:
- A certified CADR of 450 means “450 or more”. Two units both listed at 450 are not necessarily equal.
- Any CADR figure above 450 did not come from this certification program. It may be an in-house measurement, a different standard, or a calculation. It is not comparable with certified numbers.
- For a space genuinely needing more than about 450 cfm of clean air delivery — a large open-plan floor — two mid-sized units are the buyable answer, and they will cover the space more evenly than one large one.
What the test does not measure
The CADR test is a particle test in a sealed chamber at full fan speed. Everything outside that description is unmeasured.
Gases and odors. CADR is particles only. Cooking smells, paint fumes, volatile organic compounds and smoke odor are gas-phase, and a HEPA filter does not touch them. That work is done by the carbon stage, which CADR does not rate at all. A unit with an excellent smoke CADR can leave a room smelling exactly as it did.
Performance at usable fan speeds. CADR is typically measured at the highest speed. Since few people run a purifier at maximum in an occupied room, the certified figure is usually not the figure you experience.
Noise. Not part of the CADR test.
Filter life and cost. Not part of it either. A unit can hold its CADR for a month and then decline as the filter loads, and nothing in the rating captures that.
Ozone or byproduct emissions. CADR is silent on these. AHAM maintains a separate listing for low-ozone air cleaners, and the EPA advises avoiding units that intentionally produce ozone.
Real rooms. The chamber has no door gap, no furniture, no occupants and no ongoing pollutant source. All four reduce what you get in practice.
Using CADR to compare two units
Compare smoke CADR to smoke CADR, and only between certified figures. Then apply three corrections in your head.
First, CADR per dollar of running cost, not per dollar of purchase price. Work out the annual filter cost and add three years of it to the sticker price before dividing.
Second, CADR at the speed you will use. If one manufacturer publishes a per-speed table and another does not, the one that publishes it is easier to buy correctly.
Third, whether the figure is certified at all. An uncertified CADR, a CADR without a particle type, and a “coverage area” with no CADR behind it are three versions of the same problem.
One quick sanity check
Take the smoke CADR and divide it by 0.667. That is the floor area, in square feet, the unit genuinely suits at about five air changes an hour in an eight-foot room. Compare it with the coverage figure on the box.
If the box claims two or three times more, the manufacturer has assumed a weaker air change rate than the standard guidance uses. Nothing is being fabricated — it is just a different and much more generous assumption, and now you know which one you are being sold.
Related guides
- How to choose an air purifier: the full guide
- Sizing from room volume, with worked examples
- Activated carbon filters: the stage CADR ignores
- Ionizers, UV and ozone: what EPA says
Sources: AHAM, Scope of Air Cleaner Certification for the maximum reportable CADR values and the 1,008 cubic foot test chamber; AHAM Verifide air filtration standards; EPA, Guide to Air Cleaners in the Home (2nd edition, 2018). The air-change arithmetic is our own calculation.