Sizing an air purifier
A portable air purifier pulls room air through a filter and returns it cleaner. Its effect depends on how much clean air it delivers compared with the size of the room. A purifier that is too small for its room barely changes the air quality; one that is right-sized can cut fine particles, pollen and pet dander substantially.
CADR and air changes per hour
The clean air delivery rate (CADR) is the flow of filtered air an appliance delivers, measured under the AHAM AC-1 standard for three pollutants: smoke, dust and pollen. Because the smoke particles are the smallest and hardest to catch, the smoke CADR is the most useful number.
Air changes per hour (ACH) express how many times per hour the purifier delivers a volume of clean air equal to the room’s volume:
CADR needed (cfm) = floor area × ceiling height × ACH ÷ 60
Recommended levels
| Use | Air changes per hour |
|---|---|
| general freshening | 2 to 3 |
| allergies, pets, reducing airborne viruses | 4 to 5 |
| wildfire smoke, asthma | 5 to 6 or more |
The Association of Home Appliance Manufacturers recommends a smoke CADR of at least two thirds of the room’s floor area in square feet, which corresponds to about 5 ACH with an 8-foot ceiling. The CDC also suggests aiming for 5 or more air changes per hour of clean air to reduce airborne viruses.
Example
A 300 square foot bedroom with an 8-foot ceiling has a volume of 2,400 cubic feet. For 5 air changes per hour, you need a CADR of 200 cfm. A purifier rated at 250 cfm would deliver about 6 air changes per hour.
Tips for real-world performance
- Run it continuously on a setting quiet enough to leave on; the rated CADR is usually at the highest speed.
- Close doors and windows in the room you want to clean.
- Place it where air can circulate freely, not behind furniture.
- Replace filters as recommended: a clogged filter cuts the flow.
- Use two smaller units in a large or oddly shaped room.
What purifiers do not do
HEPA purifiers remove particles, not most gases. They do not lower carbon dioxide or humidity. Activated carbon filters capture some odors and volatile organic compounds, but have limited capacity. Avoid ozone generators: the EPA warns that ozone irritates the lungs. Ventilation and controlling sources of pollution remain essential.