One number decides whether an air purifier will do anything useful in your room, and it is not the “covers up to 1,500 sq ft” headline. It is the CADR — clean air delivery rate — measured against your actual floor area. Get that right and a cheap unit outperforms an expensive one. Get it wrong and the machine runs all night moving a trickle of clean air into a room that refills faster than it clears.
Everything else on the box is secondary, and some of it you should actively avoid paying for.
What CADR measures
CADR is the volume of filtered air a unit delivers, in cubic feet per minute. It is a combined score: a weak fan behind a perfect filter scores badly, and so does a strong fan behind a loose one. That makes it the only figure on the box that tells you about the machine as a system rather than one part of it.
There are normally three CADR numbers, not one. The EPA’s Guide to Air Cleaners in the Home describes them as tobacco smoke, dust and pollen, used to represent small, medium and large particles respectively. Smoke is the hardest test, so the smoke CADR is the one to judge a unit by. If a listing quotes a single CADR without saying which, assume it is the flattering one.
Two things about how the figure is produced change how you should read it. First, CADR is typically measured at the air cleaner’s highest speed. Second, it is measured in a sealed test chamber with no ongoing pollutant source. Your bedroom has a door, a gap under it, and a cat.
Sizing: the two-thirds rule
AHAM, which runs the certification program behind the CADR seal, gives a simple rule: the unit’s CADR should be at least two-thirds of the room’s floor area in square feet. A 10 by 12 foot bedroom is 120 square feet, so you want a smoke CADR of about 80 or better.
EPA’s own estimation chart lands in the same place from a different direction — a minimum of 65 cfm for 100 square feet, 130 cfm for 200, and 390 cfm for 600. That is a ratio of 0.65 throughout, which is the two-thirds rule with the arithmetic already done.
| Room floor area | Minimum smoke CADR | Typical room |
|---|---|---|
| 100 sq ft | 65 | Small bedroom, home office |
| 150 sq ft | 100 | Average bedroom |
| 250 sq ft | 165 | Large bedroom, small living room |
| 400 sq ft | 265 | Open living area |
| 600 sq ft | 390 | Large open-plan space |
Two adjustments matter. If your ceilings are higher than eight feet, AHAM advises choosing a unit rated for a larger room, because the rule is built on floor area as a proxy for volume and tall ceilings break that proxy. And for wildfire smoke, AHAM recommends a smoke CADR equal to the room’s square footage rather than two-thirds of it — so that 120-square-foot bedroom now wants 120, not 80.
Where the two-thirds rule comes from
It is worth seeing the arithmetic, because it tells you what the rule is actually promising. Take a room of area A with an eight-foot ceiling, so its volume is 8A cubic feet. A unit delivering 0.667A cubic feet per minute moves 40A cubic feet an hour, which is five times the room’s volume.
So the rule is buying you roughly five air changes per hour of filtered air — at the unit’s highest speed. That is the number to hold on to, because it is what lets you judge the marketing claim instead of the machine.
Why “covers up to 1,500 sq ft” is usually meaningless
A manufacturer can quote any coverage figure it likes by choosing a weaker air change assumption. Run the same arithmetic at two air changes an hour instead of five and the same machine suddenly “covers” two and a half times the floor area. Nothing about the hardware changed.
So when you see a large coverage claim, do the check yourself: divide the stated smoke CADR by 0.667. That is the room size the unit genuinely handles at about five air changes an hour. If the brand’s own claim is three times larger, you now know which assumption it used.
This is also why you should ignore coverage claims that come with no CADR at all. There is nothing to check them against, which is generally the point.
The filter stack, and what each layer is for
Most units stack three stages, and each has a different job and a different replacement interval.
The pre-filter is a coarse mesh or fabric screen that catches hair, lint and large dust. It exists to protect the expensive filter behind it. On better units it is washable, which saves money; on cheaper ones it is bonded to the main filter, which does not.
The particle filter does the work the CADR measures. If it is a true HEPA filter, it removes at least 99.97% of particles at 0.3 microns, which the EPA notes is the worst case — the most penetrating particle size. Particles both larger and smaller than that are captured at even higher efficiency, which is the opposite of what most people assume.
The carbon stage handles gases and odors, which a particle filter cannot touch at all. Its usefulness depends almost entirely on how much carbon is in there, and a thin carbon-impregnated mat is not the same thing as a packed carbon tray.
The HEPA wording trap
“True HEPA” and “HEPA” refer to the standard. “HEPA-type”, “HEPA-like”, “99% HEPA” and “HEPA-style” do not — they are marketing constructions that exist precisely because the filter does not meet the threshold. The gap is larger than it sounds: a filter at 99% rather than 99.97% lets through more than thirty times as many particles at the hardest size.
What not to pay for
Several add-ons raise the price and, in one case, the risk.
On ozone, the EPA’s position is blunt: ozone generators sold as air cleaners have not been approved by any federal agency for use in occupied spaces, and its guide advises avoiding air cleaners that intentionally produce ozone. This is not a close call.
Ionizers are a softer version of the same concern. EPA notes that some air cleaners may have the potential to emit ozone. If a unit has an ionizer, check that it can be switched off independently of the fan — many can, and the mechanical filtration is the part you bought.
UV lamps inside a consumer air purifier face a physics problem that has nothing to do with whether UV works in principle. Killing a microorganism requires a dose, which is intensity multiplied by exposure time, and air moving through a small chamber at hundreds of cubic feet per minute is not in there long.
Smart features are neutral. An automatic mode driven by a real particle sensor is useful; an app that shows you a number is not worth a price premium on its own.
Noise is the specification that decides whether you use it
CADR is measured at the highest fan speed, and on most units the highest speed is too loud to sleep beside. That creates a trap: you size the unit correctly, then run it two speeds down, and the clean air delivery you actually get is a fraction of what you paid for.
The fix is to buy for the speed you will really use. Oversize the unit relative to the room, then run it low — a larger machine at medium is quieter than a smaller one at maximum for the same clean air delivery. Where a manufacturer publishes CADR or airflow per speed setting, that table is more useful than the headline figure.
Published decibel figures are worth less than they look, because test conditions and distances vary between brands. Treat them as a rough ranking within one manufacturer’s range, not as a comparison across brands.
Running cost, which is where the real price is
Filters are consumables, and over several years they usually cost more than the unit did. Before buying, find the replacement filter’s actual price and its stated interval, then work out the annual cost. A cheap purifier with a proprietary filter at a high price and a six-month interval is often the expensive option.
Electricity matters less than people expect, but it is not nothing when a unit runs continuously. Check the wattage at the speed you plan to use, not the maximum.
One cost that is easy to miss: on units where the pre-filter is bonded to the HEPA layer, pet hair and lint shorten the life of the whole assembly. A washable pre-filter can halve the replacement frequency in a house with animals.
The order to decide in
- Measure the room. Floor area, and note the ceiling height if it is over eight feet.
- Set your minimum smoke CADR. Two-thirds of the floor area, or the full floor area if wildfire smoke is a real concern where you live.
- Filter the shortlist on that number alone. Discard anything that does not publish a smoke CADR.
- Oversize by a step so you can run it on a low, quiet setting.
- Confirm true HEPA, not a HEPA-adjacent phrase.
- Check carbon quantity if odors or gases are the reason you are buying.
- Price the filters for three years and add it to the purchase price.
- Switch off any ionizer, and skip ozone entirely.
Notice what is not on that list: brand, app, number of fan speeds, preset count, or the size of the number in the product title.
What an air purifier cannot do
Filtration is a supplement, not a fix. The EPA is clear that the most effective ways to improve indoor air are to reduce or remove the pollutant source and to ventilate with clean outdoor air; air cleaners and HVAC filters help, but they cannot remove all pollutants.
Practically, that means a purifier will not keep up with an active source — a smoker in the room, a gas burner with no extraction, visible mold. It will not reduce carbon dioxide, which is a ventilation problem. And it only treats the room it is in, with the door closed. A single unit in a hallway serving three bedrooms is doing very little for any of them.
Related guides
- CADR explained: what the three numbers on the box mean
- What size air purifier do you need?
- True HEPA vs HEPA-type: why the wording matters
- Activated carbon filters: what they do and don’t do
- Ionizers, UV and ozone: what EPA says about each
- MERV ratings explained, for whole-house filtration
Sources: EPA, Guide to Air Cleaners in the Home (2nd edition, 2018); EPA, What is a HEPA filter?; AHAM Verifide air filtration standards. Air change arithmetic is our own calculation from the stated figures.