Wildfire smoke field guide
CADR vs CFM vs ACH: what is the difference?
CFM describes airflow. CADR describes effective clean-air delivery. ACH describes how that clean-air flow relates to the size of a room.
Three quantities
Air moved, clean air delivered, and room turnover.
The three terms answer different questions and should not be used interchangeably.
Cubic feet per minute: a volumetric airflow rate. It tells you how much air moves, not how effectively particles are removed.
Clean Air Delivery Rate: an effective filtered-air delivery rate that combines airflow with particle-removal performance.
Air changes per hour: how many room-volume equivalents of air are moved or cleaned per hour, depending on how the term is being used.
The bridge between CADR and ACH. The same purifier produces a higher equivalent ACH in a smaller room.
The relationship
CADR must be compared with room volume.
For a simplified imperial calculation, equivalent clean-air changes per hour can be estimated from:
with CADR in cfm and room volume in cubic feet.
A 200 cfm cleaner in a 1,000 ft³ room corresponds to about 12 equivalent clean-air changes per hour in this simplified calculation. The same cleaner in a 2,000 ft³ room corresponds to about 6.
Why CADR beats raw airflow
A fan can move air without cleaning all of it.
EPA describes CADR as a measure of the flow of filtered air delivered by an air cleaner. A raw fan-airflow number does not include particle-removal efficiency, which is why CFM alone is not enough to compare smoke-cleaning performance.
Use the tools
Turn room size into a clean-air requirement.
Sources
EPA filtration guidance.
EPA — DIY Air Cleaners and CADR ↗
EPA — Guide to Air Cleaners in the Home ↗
Published 21 September 2026.