Air purifiers: how to read CADR and size one for the room you actually have

Air purifier boxes are crowded with claims: five-stage filtration, “true HEPA”, ionisers, coverage “up to” some impressive number of square feet. Most of that is decoration. The number that tells you whether a portable unit can actually clean the air in your room is smaller and often tucked on the side of the box: the clean air delivery rate, or CADR.

What CADR measures

CADR is a measure of how much filtered air a unit delivers, in cubic feet per minute. It’s tested separately for smoke, dust and pollen, the three sizes of particle that cover most of what people buy a purifier for, so you’ll often see three figures. The U.S. Environmental Protection Agency’s guide puts the principle plainly: the higher the CADR, the more particles the cleaner can filter and the larger the area it can serve.

That’s why it’s more useful than the room size on the box. “Up to 1,000 sq ft” can mean the unit will change that room’s air slowly, over a long time, rather than the several times an hour you’d want if smoke or pollen is the reason you bought it.

The EPA’s sizing chart

The EPA gives a simple chart for removing particles, assuming an 8-foot ceiling:

The pattern is easy to remember: the CADR should be roughly two-thirds of the floor area in square feet. A 12-by-15-foot bedroom is 180 square feet, so look for a smoke CADR of at least about 120. If your ceilings are higher than 8 feet, or the room opens onto others, go bigger.

For smoke, use the smoke figure. It’s usually the lowest of the three, and smoke particles are the smallest and the hardest to catch.

Speed, noise and the honest trade-off

The published CADR is measured at the unit’s highest setting. The EPA notes that higher fan speeds and longer run times filter more air, and that a cleaner set to a low speed filters less.

That’s the real catch with portable purifiers. Many people run them on the quietest setting overnight, which can be a fraction of the rated CADR. A purifier that is too small for the room is not a slower version of the right one. Running quietly in the corner, it may be barely doing anything at all. If noise matters, buy a unit rated comfortably above what the chart asks for, so a middle setting still does the job.

Gases and odours are a different problem

CADR covers particles only. For cooking smells, some volatile chemicals or smoke odour, you need an activated carbon filter, and the EPA says these can be effective only with a substantial amount of carbon. A thin carbon-dusted sheet in a cheap unit won’t do much. There’s no widely used rating to compare carbon filters, so this part of the box is mostly on trust.

Skip anything that produces ozone on purpose. Ozone is a lung irritant, and the EPA advises against using ozone generators to clean indoor air.

The running costs

Filters need replacing, and a clogged one works poorly. Before buying, look up the price and the recommended replacement interval of the unit’s filters. A cheap purifier with expensive, frequently changed filters can cost more over three years than a better unit.

What we’d actually do

Measure the room. Multiply the floor area by about two-thirds and look for a smoke CADR at least that high. Better still, buy a size up so you can run it at a quieter setting. Choose a HEPA-type particle filter, add carbon only if smells are the problem, ignore ionisers and anything that makes ozone, and price the replacement filters before you pay. If you have asthma or another lung condition, an air cleaner is a sensible addition, but it doesn’t replace the advice of whoever treats you.