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How to choose an air purifier.

Marketing copy won't clean your air — physics will. This guide covers the four numbers that actually predict performance: CADR, room coverage, filter type, and independent certification. Ten minutes here saves you from an expensive paperweight.

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The 60-second version

Four rules before you buy

1. Match smoke CADR to your room: you want at least two-thirds of the room's square footage in CFM. 2. Insist on a True HEPA filter — "HEPA-type" is not HEPA. 3. Check for AHAM Verifide and CARB certification. 4. Price the replacement filters before you price the machine.

CADR: the only spec that matters first

Clean Air Delivery Rate (CADR) measures how many cubic feet of air a purifier can scrub per minute, tested independently against three pollutants: smoke, dust, and pollen. Smoke is the hardest to capture, so smoke CADR is the honest number — if a purifier handles smoke, dust and pollen are easy.

AHAM's rule of thumb: your purifier's smoke CADR should be at least two-thirds of your room's area in square feet. That delivers roughly 4–5 air changes per hour, which is what allergists recommend for meaningful symptom relief.

Room size	Minimum smoke CADR	What to look for
150 sq ft — small bedroom or office	100 CFM	Compact desktop units
300 sq ft — large bedroom or den	200 CFM	Most mid-size tower purifiers
500 sq ft — living room or open plan	330 CFM	Full-size consoles
800 sq ft — great room or basement	530 CFM	High-CADR flagships, or two mid-size units

From our index

Smoke CADR across 35 tracked models

Under 120 CFM6 models · 17%

120–179 CFM8 models · 23%

180–249 CFM9 models · 26%

250–349 CFM6 models · 17%

350+ CFM6 models · 17%

Sizing up is always safe — a purifier that's too big just runs quieter on a lower fan speed. Sizing down means it never catches up.

Filter types, decoded

The baselineTrue HEPACaptures 99.97% of particles at 0.3 microns — pollen, dander, dust mite debris, most smoke. This is the minimum acceptable standard for allergies and asthma.
The marketing trap"HEPA-type" / "HEPA-like"No standardized efficiency. These filters can capture as little as 50% of fine particles. If the box won't say "True HEPA," assume it isn't.
The odor specialistActivated carbonHEPA catches particles, not gases. For cooking smells, pet odor, VOCs, and wildfire smoke gases, you need a substantial activated carbon bed — measured in pounds, not a thin sprayed mesh.
The one to skipIonizers & ozoneIonizers and "plasma" features can emit ozone, a lung irritant. If a unit has one, make sure it can be switched off — and that the unit is CARB certified either way.

Certifications worth trusting

PerformanceAHAM VerifideIndependent lab verification of CADR and room-size claims. If the numbers on the box aren't AHAM verified, they're self-reported.
SafetyCARB certifiedCalifornia's air resources board certifies that a purifier emits ozone below harmful thresholds. Effectively a national standard — and a hard requirement.
Running costEnergy StarPurifiers run 24/7, so efficiency compounds. Energy Star models use roughly 40% less electricity than standard units over a year of continuous use.

The costs nobody puts on the box

Noise: check the decibel range, not just the minimum. A purifier that's silent on sleep mode but jet-engine loud on high will live on low — where its effective CADR is a fraction of the advertised number. Bedrooms do best under 30 dB on the low setting.

Filter replacements: most HEPA filters last 6–12 months and cost $30–$100. A $120 purifier with $80 filters is more expensive over three years than a $250 purifier with $40 filters. We list replacement cost and lifespan on every model page for exactly this reason.

Ready to shop?

Put the numbers to work.

Every model in our index lists CADR, coverage, noise, filter costs, and certifications side by side — plus what actual owners say about living with them.

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