Air Purifier Sizing for Wildfire Smoke
For wildfire smoke, size stricter than everyday use: AHAM recommends a Smoke CADR at least equal to your room's square footage (a 1:1 ratio) rather than the standard 2/3 rule. A 200 sq ft room wants roughly 200 CFM Smoke CADR during an active smoke event, versus ~134 CFM under normal 2/3 sizing.
The one thing to know: CADR (Clean Air Delivery Rate) is an AHAM-administered test measured in CFM (cubic feet per minute) for three separate particle types — smoke, dust, and pollen — because smaller particles are harder to capture. AHAM's sizing guidance (the '2/3 rule') says the unit's Smoke CADR should be at least two-thirds of the room's square footage, e.g. a 300 sq ft room wants a Smoke CADR of at least ~200. Always size by the Smoke number, the smallest and hardest-to-catch particle class — a high Pollen CADR on the box can mask a mediocre Smoke CADR.
Normal sizing vs. wildfire sizing
| Room size | Normal (2/3 rule) Smoke CADR | Wildfire (1:1 rule) Smoke CADR |
|---|---|---|
| 150 sq ft | ~100 CFM | ~150 CFM |
| 250 sq ft | ~168 CFM | ~250 CFM |
| 400 sq ft | ~268 CFM | ~400 CFM |
Why True HEPA matters here specifically
Wildfire smoke particulate concentrates heavily in the fine PM2.5 range, close to the 0.3-micron most-penetrating particle size that True HEPA is verified against (≥99.97% capture). "HEPA-type" filters carry no verified efficiency threshold at all — a gap that matters more when outdoor particulate spikes far above normal baseline. See Is "HEPA-Type" Real HEPA?
Running the unit during an event
Continuous operation at a speed that sustains your target ACH matters more than intermittent max-speed bursts. Sizing up (per the table above) lets a purifier hold the wildfire-level 1:1 CADR target on a moderate, tolerable fan speed rather than forcing a loud maximum setting for the duration of a multi-day event. Pair with reduced outdoor-air infiltration (closed windows/doors) — a purifier cannot out-filter continuously incoming smoke-laden air.
This page is general sizing guidance based on AHAM standards, not a substitute for local air-quality-authority guidance during an active wildfire event.
Frequently Asked Questions
AHAM's standard 2/3 rule (Smoke CADR ≥ 0.67 × room sq ft) is meant for everyday use. During active wildfire smoke, AHAM recommends sizing up to a full 1:1 ratio — Smoke CADR at least equal to the room's square footage — because outdoor particulate levels can spike far above typical indoor baseline.
Using the 1:1 wildfire guidance, you'd want a Smoke CADR of at least ~300 CFM for that room during an active smoke event, versus ~200 CFM for the standard 2/3 rule in normal conditions.
Yes — wildfire smoke particulate is largely in the fine (PM2.5) range that True HEPA media is verified to capture at high efficiency. A 'HEPA-type' filter with no verified threshold is a riskier bet precisely when particulate levels are highest.
Running continuously at whatever speed maintains your target ACH matters more than always maxing out the fan — an oversized unit (per the sizing math above) can hit the wildfire-level ACH target on a moderate, sustainable speed rather than a loud maximum setting you won't tolerate running for days.
Reducing outdoor air infiltration (closing windows/doors, minimizing outside-air HVAC intake) works alongside a purifier rather than instead of one — a purifier can't out-filter air that's continuously being replaced with more smoke-laden outdoor air. This is general practice, not a substitute for local air-quality authority guidance during an active event.