How to choose an air purifier: CADR, room size and filters

A portable air purifier can make a real difference to dust, pollen, pet dander and smoke indoors — but only if it is powerful enough for the room. Marketing terms make that hard to judge. This guide explains the one number that matters most, how to match it to your room and what else to check before you buy.

What an air purifier can and cannot do

Portable air purifiers (the US Environmental Protection Agency calls them air cleaners) mainly remove particles: dust, pollen, smoke and pet dander. Gases and odors are a different job that needs an activated carbon filter with a substantial amount of carbon. No purifier replaces ventilation or removing the source of the pollution, but a correctly sized unit lowers particle levels in the room where you use it.

CADR: the number to look for

The clean air delivery rate (CADR) tells you how much filtered air a purifier delivers, in cubic feet per minute. A higher CADR means more particles removed and a larger area served. Ratings are usually given separately for smoke, dust and pollen, and they are typically measured at the highest fan speed — so a purifier running on its quietest setting cleans much more slowly than its CADR suggests.

Independent verification helps. Programs such as AHAM Verifide have independent laboratories check the performance claims printed on the packaging, so the CADR on a verified unit has been tested by someone other than the manufacturer.

Matching CADR to room size

The EPA's guide gives a simple sizing chart for rooms with 8-foot ceilings: about 65 cfm for a 100 sq ft room, 130 cfm for 200 sq ft and up to 390 cfm for 600 sq ft. That works out to a CADR of roughly two-thirds of the room's floor area in square feet.

1. Multiply the length and width of the room to get its area.
2. Take about two-thirds of that number — that is the smoke CADR to look for.
3. If your ceilings are higher than 8 feet, choose a purifier rated for a larger area.

For example, a 12 × 15 ft bedroom is 180 sq ft, so look for a smoke CADR of about 120 or more.

Filters: HEPA, carbon and replacements

Many portable purifiers reach a high CADR by using a high-efficiency particulate air (HEPA) filter. Check what "HEPA" means on a specific product: some brands use terms such as "HEPA-type" or "HEPA-grade", so read the stated filtration efficiency.

All filters need regular replacement; a clogged filter works poorly. Before you buy, check the price and availability of replacement filters and how often the manufacturer recommends changing them — that is the real running cost.

Avoid ozone

The EPA advises avoiding air cleaners that intentionally produce ozone, which is a lung irritant, and notes that no federal agency has approved ozone generators for use in occupied spaces. Be cautious with products marketed with "activated oxygen" or similar wording, and check that ionizer features can be switched off.

Noise, energy and placement

• Noise: you will run a bedroom purifier for hours, so look for a quiet sleep mode — and remember it delivers less than the rated CADR on that setting.
• Energy: a purifier that runs all day adds to your electricity bill; compare stated power use.
• Placement: keep it away from walls and furniture that block the intake and outlet, and close doors and windows to the room you want to clean.

A quick checklist

1. Measure the room and aim for a smoke CADR of about two-thirds of its area.
2. Prefer independently verified ratings.
3. Choose a true HEPA filter for particles, and carbon if odors matter.
4. Check the cost and schedule of replacement filters.
5. Avoid devices that produce ozone.

A pick to start your search

The Levoit Core 300-P publishes AHAM Verifide CADR figures and suits a typical bedroom; our kitchen and home starters explain the rest of that list.