Wildfire smoke and the air inside your home
Wildfire smoke is mostly fine particles, small enough to reach deep into your lungs, and at the same level it harms the lungs more than everyday haze. Indoors, what protects you is closing the house, filtering what gets in, and not adding particles of your own. A filter catches the particles, not the smoke's gases, and settled ash keeps mattering for months.
more respiratory hospital stays per 10 µg/m³ of wildfire fine particles, against 0.67% to 1.3% for ordinary ones
1.3% to 10%
At the same level, wildfire smoke harms the lungs more than everyday haze.
Wildfire smoke and the air inside your home: what is it, and where does it come from?
Wildfire smoke is the fine particles and gases a burning landscape gives off, carried downwind for miles.
The particles are the part that is measured, as PM2.5 in micrograms per cubic meter, and they are the same size class as the particles in the fine particles article. What differs is the mix: smoke carries more reactive organic compounds and metals than the regional haze most days bring.
Inside a house, smoke arrives three ways: through open windows and doors, through anything mechanical that pulls outdoor air in, and through the gaps around closed windows. Once the fire is out, the ash that settled on roofs, in attic insulation and in ductwork can be stirred back into the air for months.
Wildfire smoke and the air inside your home: why does it matter?
Fine particles are the most studied environmental exposure in modern epidemiology, and the research on smoke itself points the same way. In Southern California, fine particles from wildfires raised respiratory hospital stays more per unit of mass than ordinary fine particles do. The direction of that difference is consistent across studies.
How big it is remains uncertain, so we give no multiple. Over a decade, wildland fire smoke has been tied to more than 50,000 early deaths in California alone. A smoke event is also not one bad day: multi-day events recur across the West, and the ash left behind keeps adding fine particles to indoor air after the sky clears.
That is why the steps that work on a smoke day, sealing and filtering, are the same ones that matter in the weeks after.
In Southern California, wildfire-derived PM2.5 raised respiratory hospitalizations by 1.3 to 10% per 10 micrograms per cubic meter, against 0.67 to 1.3% for ordinary regional PM2.5. The direction is consistent across studies, while the size of the difference is uncertain.
Aguilera et al., 2021 (Nature Communications), wildfire vs ambient PM2.5
Fine particles from wildland fires were attributed with more than 50,000 premature deaths in California from 2008 to 2018.
The EPA's guidance for a smoky day is to close windows and doors to keep smoke from entering the room.
Wildfire smoke and the air inside your home: What do the air quality categories mean?
| Category | PM2.5 (µg/m³) | Source |
|---|---|---|
| Good | 0 to 9.0 | US EPA, Air Quality Index (2024 revision) |
| Moderate | 9.1 to 35.4 | US EPA, Air Quality Index (2024 revision) |
| Unhealthy for sensitive groups | 35.5 to 55.4 | US EPA, Air Quality Index (2024 revision) |
| Unhealthy | 55.5 to 125.4 | US EPA, Air Quality Index (2024 revision) |
| Very unhealthy | 125.5 to 225.4 | US EPA, Air Quality Index (2024 revision) |
| Hazardous | 225.5 and up | US EPA, Air Quality Index (2024 revision) |
These are the EPA's reporting categories for outdoor air, not a legal limit and not a line your home is graded against. They tell you how the outdoor air is described on a smoke day, so you know when to close the house; they do not say what the air inside is.
What helps against Wildfire smoke and the air inside your home?
The approaches below are the ones people use for Wildfire smoke and the air inside your home at home. Each entry says what it does and what it cannot do, and not every one helps.
Direct fixes
Close the house, including the machines that pull outdoor air in
Closing windows and doors can cut outdoor air entry by a third or more. In a study of 1,274 Bay Area buildings, the typical ratio of indoor fine particles from outdoors to outdoor fine particles was 0.23 on fire days, compared with 0.42 on other days. On fire days, the middle half of buildings ranged from 0.16 to 0.36. The researchers attributed the drop to people closing up and filtering.
A single-hose portable air conditioner or an evaporative cooler pulls outdoor air in, so use them sparingly, or not at all, during a smoke event.
A true HEPA purifier in the room where you sleep, sized to the room
HEPA captures at least 99.97% of the hardest-to-trap particle size in a single pass. In seven Seattle homes during wildfire smoke, a HEPA purifier cut indoor fine particles by 48 to 78 percent with windows closed. Across 83 field studies of portable air cleaners, the average reduction was 49 percent, with results varying widely.
A HEPA filter is a particle tool. It does not remove the gases in smoke, which is where the smell lives, and it does nothing about carbon monoxide.
Stop adding particles indoors
Frying, candles, incense, wood fires and aerosol sprays add fine particles to air that is already carrying smoke's. Leave them off until the outdoor number drops.
Bigger retrofits
MERV-13 filter on the central HVAC, fan switched to On
Removes at least 50 percent of the smallest particles tested in a single pass. Running the fan continuously recirculates air through the filter and extends coverage beyond one room.
Confirm the blower can handle a MERV-13 filter without strain on older or marginal systems. It filters particles only.
Free and behavioral
After the smoke, damp-wipe and wet-mop the ash
Ash on roofs, in attic insulation and in ductwork keeps re-entering indoor air for months. Damp-wiping and wet-mopping rather than dry-sweeping keeps it from becoming airborne, and filters should be changed early.
Surface cleaning of the attic, roof and ducts does more for post-fire indoor air than ventilation does, and it is the step this article cannot do for you.
Skip ionizers and ozone generators
These devices generate ozone, itself a lung irritant and a component of smog, and their byproducts can be worse than the fine particles they claim to address. Air-purifying plants, charcoal bags and salt lamps do not meaningfully remove fine particles.
We read the air around your home, not inside it, so we can tell you when smoke is up and what to do, but we can't see the smoke in your rooms. Your plan covers the usual sources: a sealed house, a filter sized to the room, and nothing added indoors.
Common questions about Wildfire smoke and the air inside your home
Is wildfire smoke worse than regular air pollution?
Per unit of mass, yes, for the lungs. Smoke carries more reactive organics and metals than ordinary regional haze, and in Southern California wildfire fine particles raised respiratory hospital stays more per microgram than ordinary fine particles did. How much more is uncertain, so we give no multiple.
Does a HEPA filter remove wildfire smoke?
It removes the particles in smoke, which are the part that does the most measured harm. It does not remove smoke's gases, so the smell can stay even when the particle count drops, and it does nothing about carbon monoxide.
How long does the ash matter after a fire?
For months. Ash that settled on roofs, in attic insulation and in ductwork keeps re-entering indoor air when it is stirred up by cleaning, foot traffic, the HVAC running or wind. Damp-wiping and wet-mopping beat dry-sweeping.
Should I open the windows to air the house out during smoke?
Not while the outdoor number is high. Open them whenever it drops, even briefly, because a sealed house builds up the carbon dioxide you breathe out.
Sources cited for Wildfire smoke and the air inside your home
Every number on this Wildfire smoke and the air inside your home page traces to one of the 10 sources below: 6 peer-reviewed, 4 government or regulatory.
Peer-reviewed
- Aguilera et al., 2021 (Nature Communications), wildfire vs ambient PM2.5
- Connolly et al., 2024 (Science Advances), mortality attributable to PM2.5 from wildland fires in California, 2008-2018
- Liang et al., 2021 (PNAS), wildfire smoke and indoor air in California
- Xiang et al., 2021 (Science of the Total Environment), air cleaners during wildfire smoke
- Ebrahimifakhar et al., 2024 (Science of the Total Environment), review of portable air cleaners
- Budaniya and Rai, 2022 (Building and Environment), plants and indoor particle removal