Why Wildfire Smoke Is Different
Normal household dust is mostly larger bits: dust, pet dander and pollen. Smoke is different.
The EPA says smoke is a mix of gases and fine particles, and that the biggest health threat from smoke is the fine particles. Those are the particles a HEPA filter is built to trap.
But particles are only half the problem. Smoke also has gases in it. A HEPA filter does not catch gases. For that you need activated carbon, and the EPA says it works when the filter holds a large amount of it. We cover gas removal in air purifiers for VOCs and odors.
The Bottom Line
For wildfire smoke, you need two things: a True HEPA filter for PM2.5 particles, AND a thick activated carbon filter for smoke gases and odors. A purifier with only HEPA removes particles but leaves the gases. A purifier with only carbon removes odors but leaves the particles.
HEPA + Carbon: Why You Need Both
Here is what each filter type does for smoke:
HEPA Filter
Catches: Smoke particles (PM2.5), ash, soot
Does NOT catch: Smoke odor, gases, VOCs, carbon monoxide
Activated Carbon Filter
Catches: Smoke odor, VOCs, formaldehyde, some gases
Does NOT catch: PM2.5 particles, soot, ash
The amount of carbon matters. The EPA says activated carbon filters can work if a large amount of material is used. A thin carbon sheet holds very little. Austin Air says its HealthMate holds over 780 cubic inches of activated carbon and mineral-based media.
Pre-Filtering: Protect Your Main Filter
Smoke is hard on filters. A pre-filter catches the larger particles before they reach the HEPA filter, so the HEPA filter clogs more slowly.
Many purifiers include a pre-filter, and some are washable. During smoke season, check it more often than usual and clean or replace it when it looks dirty.
If your purifier does not have a pre-filter, you can wrap a cut-to-size furnace filter around the intake. It is not pretty, but it works as a first line of defense during emergencies.
Seal Your Home First
An air purifier works best in a sealed room. If smoke pours in through open windows, gaps under doors, or a leaky HVAC system, even a high-CADR purifier cannot keep up. Before you turn on the purifier:
- Close all windows and doors. This sounds obvious, but many people leave bathroom exhaust fans running. These fans create negative pressure that pulls smoky air in through every crack.
- Turn off whole-house fans and attic fans. These pull outdoor air directly inside.
- Set your HVAC to recirculate. Switch from "fresh air" or "ventilate" mode to "recirculate" or "indoor only." If your system does not have this option, put your HVAC fan on "on" instead of "auto" to keep indoor air moving through the HVAC filter.
- Seal visible gaps. Wet towels under doors, painter's tape over drafty windows. For a permanent fix, add weatherstripping to doors and caulk around window frames.
- Upgrade your HVAC filter. The EPA says to consider a MERV 13 or higher filter if your system can take it. Check your furnace manual or ask an HVAC technician first.
When to Run on High vs Auto Mode
Auto mode is fine for daily use. But during a smoke event, auto mode has a problem: the sensor sits near the purifier's intake, where it reads already-filtered air. It may think the room is clean while the far corners are still smoky.
Here is a simple rule:
- Outdoor AQI below 100: Auto mode is fine. Let the purifier manage itself.
- Outdoor AQI 100-200: Set to high. Keep it running until a monitor shows the room has cleared.
- Outdoor AQI above 200:Max speed, all day, every day. Close off rooms you are not using to concentrate the purifier's power.
A standalone air quality monitoris the best way to know when it is safe to switch back to auto. The purifier's built-in sensor is not reliable during heavy smoke.
Best Purifiers for Smoke Season
These three purifiers have the right combination of HEPA filtration and activated carbon for wildfire smoke. Each one is reviewed in detail on our main air purifier guide. For the full list, see the smoke purifier picks.
Blueair Blue Pure 211i Max
$349.99 list priceBlueair calls its filter HEPASilent. It has 4 layers, with particle and activated carbon layers. Blueair rates it for 674 sq ft at 4.8 air changes an hour.
Emergency Option: The Box Fan Filter
If you cannot get a purifier in time, you can make one. The EPA says a do-it-yourself air cleaner made with a box fan and a MERV 13 filter is a cost-effective way to cut smoke indoors.
How to build one: attach a MERV 13 furnace filter to the back of a box fan with tape or a bungee cord. The EPA says to use only a newer box fan, made in 2012 or later, because those have added safety features. A thicker filter and a cardboard shroud make it work better.
This is a stopgap. There is no carbon filter for odors, and you should keep the fan clear of anything that blocks it. Use it until your real purifier arrives.
After the Smoke Clears
When the AQI drops back below 50, do not just turn off the purifier and open the windows. Smoke particles settle on every surface, carpets, furniture, curtains, bedding. They get kicked back into the air every time you walk across the room.
- Keep the purifier running for 2-3 days after AQI returns to normal.
- Vacuum carpets and upholstery with a HEPA-filter vacuum (a regular vacuum just blows fine particles back into the air).
- Wash bedding, curtains, and any fabric that was exposed during the smoke event.
- Wipe hard surfaces with a damp cloth, dry dusting sends particles airborne again.
- Check your purifier and HVAC filters. Replace them if they look gray or brown.
Common Questions
Can a HEPA air purifier handle wildfire smoke?
It can handle the particles. The EPA says portable air cleaners and high-efficiency HVAC filters can remove smoke particles from the air. A HEPA filter does not remove gases. For gases, the EPA says activated carbon can work when the filter holds a large amount of it.
Should I run my air purifier on high during wildfire smoke?
When smoke is heavy, a higher fan speed moves more air through the filter. Use a separate air quality monitor to see when the room has cleared, then turn the speed back down.
What size air purifier do I need for smoke?
The EPA says to choose a portable air cleaner that is the right size for the room you will use it in. Its sizing chart lists a minimum CADR of 130 cfm for 200 sq ft and 195 cfm for 300 sq ft, with an 8-foot ceiling.
Do I need to replace my filter after a wildfire smoke event?
Check it. If the filter has turned dark gray or brown, replace it even if it is early. Follow the maker's schedule the rest of the time.