How to Choose an Air Purifier: HEPA, CADR, Room Size

Short answer: Buy a true HEPA purifier with a CADR number sized for your room, check what replacement filters cost before you buy, and skip ionizers and UV lights. Then run it on low all the time.

The three things that matter

Almost everything on an air purifier box is marketing. Three things are not:

The filter type. True HEPA catches the fine particles that cause the trouble; everything else is a compromise.
The CADR. The Clean Air Delivery Rate tells you how much clean air the unit actually produces, and so what size room it suits.
The filter cost. A cheap unit with expensive filters costs more over two years than a pricier unit with cheap ones.

Everything else, from app control to air quality lights, is a convenience.

Step 1: Get a true HEPA filter

A true HEPA filter removes almost all airborne particles down to a very fine size, including dust, pollen, mold spores, pet dander and most smoke particles. Look for the words “true HEPA” or “H13” on the box. Watch for “HEPA-type,” “HEPA-like” or “HEPA-style,” which means the filter looks similar but was not tested to the same standard and lets more through.

If smells or cooking fumes are the problem, you also want an activated carbon layer. The carbon does nothing for particles and the HEPA does nothing for odors, so a unit with both covers the most ground. The thicker the carbon layer, the longer it keeps working.

Step 2: Read the CADR

CADR is a number, usually three numbers for smoke, dust and pollen, that says how many cubic feet of clean air the purifier delivers per minute. Higher is better and it is the only honest way to compare two units. A unit with a big filter and a weak fan can have a lower CADR than a smaller unit that moves more air.

If the box does not show a CADR, treat the room-size claim with suspicion.

Step 3: Match it to your room

A rule of thumb used by the industry is that the smoke CADR should be at least two thirds of the room’s floor area in square feet. So a 12 by 15 foot bedroom, 180 square feet, wants a CADR of about 120 or more. A 300-square-foot living room wants 200 or more.

That gives you roughly five air changes an hour, which is enough for allergies and everyday dust. For wildfire smoke or someone with asthma, go higher, because the purifier will be doing more of the work.

Room	Floor area	Minimum smoke CADR
Small bedroom or office	Up to 150 sq ft	100
Large bedroom	150 to 250 sq ft	165
Living room	250 to 400 sq ft	265
Open-plan space	400 to 600 sq ft	400 or two units

Manufacturers often quote a room size based on one air change an hour, which makes a unit look suitable for twice the space it really handles. The CADR math above is the number to trust.

Step 4: Check the filter cost before you buy

Look up the replacement filter for any unit you are considering, and note two things: the price and how often the maker says to change it. A filter that costs a third of the unit’s price and needs changing twice a year makes a cheap purifier expensive. Units with a separate pre-filter you can vacuum off tend to stretch the main filter further.

Avoid units with “permanent” or washable HEPA filters. Real HEPA media cannot be washed without losing efficiency, so a washable filter is a sign it was never true HEPA.

Step 5: Think about noise

A purifier works best running all the time, so the sound level on low is the one that matters, not the maximum. Makers list noise in decibels; anything under about 30 on the lowest setting is quiet enough for a bedroom. The high setting on any unit is loud and is for clearing a room quickly, not for sleeping next to.

Features worth having

An auto mode with a particle sensor turns the fan up when it detects dust or smoke and down again when the air is clear. Useful, and increasingly standard.
A filter change indicator based on hours run, so you do not have to remember.
A sleep mode that dims the lights and holds the fan on low.
A timer, if you only want it running at night.

Features that are not worth paying for

Ionizers. They charge particles so they stick to surfaces, which does not remove them from the room, and many produce ozone as a byproduct. If a unit has one, make sure it can be switched off.
UV-C lights. The air passes too fast for the light to do much to anything living on the filter.
“Plasma” or “photocatalytic” technologies. Mostly marketing, sometimes with ozone.
Scent cartridges. They add things to the air; the purifier is meant to take things out.

Where to put it

Put the purifier in the room where you spend the most time, which for most people is the bedroom. Give it a foot of space on every side so air can reach the intake, keep it off the floor if it is a small unit, and close the door. One good unit in a closed bedroom does more than one large unit in a hallway trying to serve the whole house.

FAQ

Do air purifiers help with allergies?

They help with the airborne part: pollen, pet dander and dust that is floating. They do nothing for allergens that have already settled into carpet and bedding, so they work best alongside vacuuming and washing bedding in hot water.

Is a HEPA filter necessary?

For dust, pollen, smoke and pet dander, yes. True HEPA is the only widely available filter type that reliably catches the fine particles that cause the problems. A carbon layer is a useful addition for smells and cooking fumes, but it is not a substitute.

Should I leave an air purifier on all the time?

Yes, on a low setting. Particles come back as soon as the unit stops, so the best results come from continuous running. Most units are quiet on low and use very little power.

Are ionizers or UV lights worth paying for?

Usually not. Ionizers can produce ozone, which is itself a lung irritant, and UV lights in consumer units rarely get enough exposure time to do much. Spend the money on a bigger HEPA unit instead.