How do I keep wildfire smoke out of my home?
EPA's guidance on HVAC settings, filters, portable air cleaners and a clean room, turned into steps you can take before and during a smoke event.
Published 5 min read
In short
Keep outdoor air out and clean the air inside. EPA advises setting a central system to recirculate, running the fan on "On" rather than "Auto", and using a MERV 13 filter or the highest rating your system can take.
In the room you use most, run a portable air cleaner sized for it, one that does not produce ozone. For a room with an 8-foot ceiling, EPA's minimum clean air delivery rate (CADR) is 65 cfm per 100 square feet.
Avoid adding particles indoors: no smoking, candles, frying or broiling, gas or wood stoves, aerosol sprays, or vacuuming without a HEPA filter. If you cannot stay cool with the windows closed, go somewhere else.
When wildfire smoke drifts over a region, closing the windows helps, but smoke still gets in through gaps, vents and every opening of the door. The practical goal is two-fold: bring in as little outdoor air as you can, and keep cleaning the air that is already inside. EPA’s guidance covers both, and most of it can be set up before the smoke arrives.
What should I do with my HVAC system?
If your home has central heating or air conditioning with ducts, it can act as a whole-house air cleaner. EPA’s advice:
| Setting | What to do | Why |
|---|---|---|
| Outdoor air | Set the system to recirculate, or close the outdoor intake damper | Stops the system from drawing smoky air inside |
| Fan | Set to “On” instead of “Auto” | Moves air through the filter continuously, not only when heating or cooling |
| Filter | MERV 13, or as high a rating as your system fan and filter slot can accommodate | Captures a large share of fine particles |
| Window air conditioner | Close its outdoor air damper; if it cannot be closed, do not use the unit | Otherwise it pulls outdoor air in |
Some central systems have no outdoor air intake, in which case they already recirculate. If you are unsure which kind you have, check the equipment documentation or ask a service technician.
Running the fan continuously uses more electricity, and a denser filter restricts airflow. EPA suggests consulting an HVAC technician about the highest-efficiency filter that works for your system. Check the filter more often during smoke events, since it loads up faster.
What does MERV 13 mean?
MERV, the Minimum Efficiency Reporting Value, reports how well an air filter captures particles between 0.3 and 10 microns, based on a test method developed by ASHRAE. The higher the number, the higher the capture efficiency for most particle sizes. EPA’s table for MERV 13:
| Particle size | Share captured by a MERV 13 filter |
|---|---|
| 0.3 to 1.0 µm | 50% or more |
| 1.0 to 3.0 µm | 85% or more |
| 3.0 to 10.0 µm | 90% or more |
For wildfire smoke, EPA recommends MERV 13 or the highest rating your system can accommodate.
Do I need a portable air cleaner?
A central system cleans the whole house slowly. A portable air cleaner cleans one room quickly, which makes it the core of a “clean room”, a room where the household can spend most of its time during a smoke event.
Size matters. The measure is the clean air delivery rate (CADR): the higher it is, the more particles the unit removes and the larger the space it can serve. EPA’s minimums for rooms with 8-foot ceilings:
| Room area | Minimum CADR |
|---|---|
| 100 sq ft (about 9 m²) | 65 cfm |
| 200 sq ft (about 19 m²) | 130 cfm |
| 300 sq ft (about 28 m²) | 195 cfm |
| 400 sq ft (about 37 m²) | 260 cfm |
| 500 sq ft (about 46 m²) | 325 cfm |
| 600 sq ft (about 56 m²) | 390 cfm |
For higher ceilings, choose a unit rated for a larger area. And avoid any air cleaner, or HVAC filter, that intentionally produces ozone. EPA calls ozone a lung irritant and notes that no federal agency has approved such devices for use in occupied spaces. The California Air Resources Board and the Association of Home Appliance Manufacturers keep lists of certified models.
EPA also describes do-it-yourself cleaners made by attaching one or more furnace filters to a box fan. It notes that commercial air cleaners with a known CADR are typically preferred over DIY units.
How do I set up a clean room?
Pick a room large enough for the household, ideally one with few windows and doors to the outside. Then:
- Close its windows and doors.
- Run a portable air cleaner sized for the room.
- Keep it cool: with windows shut, the room can overheat. Use air conditioning if you have it, in recirculate mode.
- Keep indoor sources of particles out of it (next section).
EPA is direct about heat: if you cannot stay cool, seek shelter elsewhere.
What should I avoid indoors?
Anything that adds particles defeats the filtering. EPA’s list of things to avoid during smoke events:
- smoking and vaping;
- using gas, propane or wood-burning stoves and furnaces;
- spraying aerosol products;
- frying or broiling food;
- burning candles or incense;
- vacuuming, unless the vacuum has a HEPA filter.
What can I do before fire season?
- Find out whether your central system has an outdoor air intake, and how to close it or set recirculation.
- Buy spare MERV 13 filters, or the highest rating your system accepts, in the right size.
- Choose and buy a portable air cleaner sized for your clean room, and a spare filter for it.
- Check that a window air conditioner’s outdoor air damper closes.
- Plan where you would go if the house gets too hot to stay in with the windows shut.
Questions
- Should I run my HVAC fan during wildfire smoke?
- EPA advises setting a central system to recirculate, or closing the outdoor air intake damper, and switching the fan from "Auto" to "On" so air passes through the filter continuously. Use a MERV 13 filter or the highest rating your system fan and filter slot can accommodate.
- What size air purifier do I need for wildfire smoke?
- Match the clean air delivery rate (CADR) to the room. EPA's guide gives minimums for rooms with 8-foot ceilings: 65 cfm for 100 square feet, 130 for 200, 195 for 300, 260 for 400, 325 for 500 and 390 for 600. Choose a larger rating for higher ceilings, and avoid models that produce ozone.
- Can I use my window air conditioner during wildfire smoke?
- Only if you can close its outdoor air damper. EPA says that if you cannot close the damper, do not use the window air conditioner.
- Does a box fan with a furnace filter work?
- EPA describes do-it-yourself air cleaners made by attaching one or more furnace filters to a box fan, but notes that commercial air cleaners with a known clean air delivery rate are typically preferred over DIY units.