Wildfire smoke is mostly fine particles (PM2.5), small enough to get deep into the lungs and through gaps around doors and windows. Studies of homes during smoke events find indoor levels often reach a third to two thirds of outdoor levels when nothing is done. A purifier running in a closed room can bring that down a lot.
Size it by CADR
The Clean Air Delivery Rate (CADR) is a standard test of how much clean air a purifier delivers. Look for the smoke CADR on the box. The industry rule of thumb is a smoke CADR of at least two thirds of the room's floor area: a 300 square foot bedroom needs about 200. For smoke days, buy bigger than the rule and run it on high.
What to look for
- True HEPA filter for particles. A carbon layer helps with smoke odor.
- No ozone. Skip "ozone generators" and be careful with ionizers. California limits ozone from air cleaners and keeps a list of certified models; that list is a good check anywhere.
- Spare filters on hand. A long smoke event can clog a filter in weeks.
The backup: a box fan and a MERV 13 filter
When stores sell out, a 20-inch box fan with a MERV 13 furnace filter taped to the intake side works as a do-it-yourself air cleaner. EPA research has found these remove a meaningful share of smoke particles. Never leave one running unattended or while you sleep, and keep it away from water.
On a smoke day
- Close windows and doors. Pick one room to keep cleanest, usually a bedroom, and run the purifier there.
- If you have central air, set the fan to recirculate and use a MERV 13 filter if your system can take it.
- Avoid frying, candles and vacuuming without a HEPA vacuum, which all add particles.
- Check your local AQI, and wear a fitted N95 outdoors when it is unhealthy.
See how often smoke reaches your county on the air quality by state pages.