Choosing an Air Purifier for Wildfire Smoke (the CADR Math That Matters)

The only math you need

Want the answer without doing the arithmetic? Use our free air purifier size calculator. It stores nothing and shows the sizing math.

CADR (Clean Air Delivery Rate) is a standardized third-party test — look for the smoke CADR in CFM, ideally AHAM-verified.

The AHAM “2/3 rule”: a purifier handles a room whose square footage is about 1.5× its smoke CADR (standard ceilings). Flip it around:

Needed smoke CADR ≈ room square feet × 2/3

• 12×12 bedroom (144 sq ft) → CADR ~100
• 15×20 living room (300 sq ft) → CADR ~200
• Open-plan space (500+ sq ft) → CADR 330+, or two units

That rule targets roughly 4–5 air changes per hour — the range public health guidance treats as meaningful during smoke events. During a bad smoke week, undersizing is the most common failure: the purifier hums reassuringly while the room’s PM2.5 barely moves. Round up.

What matters vs. what’s marketing

Matters

• True HEPA (or genuinely equivalent) main filter
• Honest, third-party smoke CADR
• Noise at the speed you’ll really use (check the dB at medium, not sleep mode)
• A particle sensor + auto mode — genuinely useful in smoke, when levels swing hour to hour
• Annual filter cost — a $150 unit with $60/yr filters costs more over three years than a $250 unit with $30/yr filters

Marketing

• Ionizers, “plasma,” PECO/PCO, UV lights: no smoke-relevant benefit beyond the filter they ride on. Where present (e.g., Plasmawave), turn them off.
• Anything advertising ozone output is a hard no.
• “Medical grade” and app dashboards move price, not particles.

Honest picks by room

• Bedroom / office (≤ ~200 sq ft): Levoit Core 300S — right-sized CADR, near-silent sleep mode, real sensor.
• Living room (~200–360 sq ft): Winix 5500-2 — strong verified CADR per dollar, washable pre-filter, Plasmawave defeatable with one button.
• Big spaces, multiple rooms, or ~$70 budget: build a CR box — EPA chamber testing puts the DIY build’s smoke performance in commercial territory. One purifier in the bedroom + a CR box in the living room is a very rational setup.

These are categories with interchangeable good options — if a comparable AHAM-verified unit is on deep sale, the math above matters more than the brand.

Run it right during smoke

Sizing is half; placement and habits are the rest — closed windows, recirculated HVAC with a MERV-13 filter, and airing out only when outdoor air improves. That whole playbook is in wildfire smoke indoors: what actually helps, and you can watch conditions and the 72-hour smoke forecast live on our free AirAware tool.