Staying indoors during wildfire smoke still means breathing some of it. In 36 Seattle-area homes, fine particles in the indoor air rose 73% on average during smoke events.
Some homes took in much more smoke than others. The way to know how your house handles it is to measure the air inside. The authors make the same point: households varied so much that they call for real-time, individualized air quality information and for guidance tailored to households rather than blanket recommendations.
The study
A University of Washington team working with Public Health, Seattle and King County published the study on May 14, 2026, in Environmental Research Communications. The authors are Katharine Teigen, Mariana Cortes Espinosa, Hannah McKinley, Anna Reed, Elizabeth Berrang, Julio Ramos-Vazquez, C. Addison Houston and Tania Busch Isaksen.
Over the summers of 2023 and 2024 they put an indoor PurpleAir sensor in each participating King County home and paired it with the three nearest outdoor PurpleAir sensors. 12 households completed the study in 2023 and 24 in 2024, 36 in all, for more than 60,000 hours of indoor and outdoor readings.
Both summers had smoke. In late August 2023, King County air reached “unhealthy.” In early September 2024 it reached “moderate.” A July 5, 2023, event mixed Fourth of July fireworks with smoke from British Columbia.
What they found
72.6% higher
Indoor PM2.5 during smoke events compared with other times (95% CI 54.5% to 92.9%), holding temperature, the house and the household steady.
The authors call that figure conservative. Sensors dropped out more often during smoke, 16.07% of hours missing against 9.07% at other times, so the worst hours are underrepresented in the data.
Heat mattered too. Each 1°F rise in outdoor temperature went with 1.27% more fine particles indoors, across all hours, smoke or not.
Year built, gas or electric stove, and heating type showed minimal effect on indoor levels overall. The authors caution that with 36 homes the study had limited power to test building characteristics, and it did not test whether any of them changed how much smoke got in. Homes without air conditioning ran 19.5% higher, but that difference was not statistically significant. Homes did differ in how strongly smoke raised their indoor levels, some much more and some much less, and the study did not pin down why.
Some of the particles started indoors. In both summers, just over a third of hours had more fine particles inside than outside: 36.54% in 2023 and 36.79% in 2024. The authors read that as a consistent share from indoor sources across both years. EPA’s list of indoor particle sources to avoid during smoke includes frying, candles and aerosol sprays.
The one building measure that did register, and one the authors could not fully explain: higher appraised home value went with slightly higher indoor particles, 6.58% per $100,000 of value (95% CI 0.72% to 12.80%). They suggest gas appliances might play a part, while noting their own gas stove result does not necessarily support that.
How smoke gets in
The US EPA lists three routes for outdoor air, including smoke, to enter a home: open windows and doors, fans and HVAC systems that pull in outdoor air, and infiltration through small openings, joints and cracks around closed windows and doors (EPA, Wildfires and Indoor Air Quality, page updated September 2, 2026).
Citing other studies, the authors suggest people open windows and run ventilation more on hot days. That part is their inference. This study did not record when windows were open.
The limits, stated plainly
- This is a pilot of 36 households who volunteered and kept the equipment afterward. The authors say the findings “are not generalizable to other geographic contexts and may not be generalizable to the greater King County area.” Portland and Vancouver are other geographic contexts.
- PurpleAir units are consumer sensors. The indoor units have a single channel, so they could not get the same quality checks as the outdoor units. The team applied EPA’s correction factor to both.
- No health outcome was measured. This study measured particles, not people.
- The 2024 smoke only reached “moderate,” and King County’s worst recent smoke years, 2020 to 2022, came before the study began.
What survives: smoke outside meant noticeably more particles inside, and homes differed in how much.
The intervention evidence
Households in both years got a DIY box fan fitted with a MERV-13 filter. In 2024 the 24 homes also got a HEPA portable air cleaner, and that year each device was used for its own week, turned on when the sensor showed poor air. The filter comparison comes from those 24 homes.
After adjusting for outdoor air, indoor particles were 12.7% lower in the HEPA week (95% CI 11.21% to 14.19%) and 7.54% lower in the box fan week (95% CI 5.96% to 9.09%) than in the baseline week. Without that adjustment the order flipped, with the box fan week 21.70% lower and the HEPA week 4.50% lower, partly because outdoor air quality changed from week to week. The authors ask readers to interpret these results with caution.
Two more caveats. Nobody logged when the machines actually ran, and no major smoke event fell in those weeks. Participants also reported that the box fan was noisy and could overheat.
What gets lost in the air purifier marketing
Air cleaner ads sell the machine as the answer to smoke. In this study, filtration explained 3.6% of the variation in indoor fine particles, and outdoor air explained much more.
That does not make the machines useless. EPA recommends portable air cleaners and high-efficiency HVAC filters during smoke, and says commercial units with a known clean air delivery rate (CADR) are “typically preferred” over DIY builds. EPA also says to choose a unit that does not produce ozone. A purifier helps. It does not tell you how much smoke your house lets in, or whether the unit is keeping up.
What the data supports
Before the next smoke event, find out whether your HVAC system has a fresh air intake and how to close it or set it to recirculate. Use a MERV 13 filter if your system can take one, and during smoke set the fan to “On” rather than “Auto,” as EPA advises.
Put a portable air cleaner sized for the room where people spend the most time, and run it on its highest speed during smoke. Choose one that makes no ozone.
Cut indoor sources during smoke. EPA lists frying or broiling, candles and incense, gas, propane or wood stoves, aerosol sprays, and smoking or vaping.
Measure indoors. Homes in this study responded to the same smoke very differently. A reading of your own indoor air tells you whether your steps are working.
Bottom line
Wildfire smoke gets into homes, and homes differ in how much. Measuring your own indoor air is how you find out.
First step: this week, check whether your HVAC has a fresh air intake and whether you know how to close it.
Talk to illumenair
illumenair measures PM2.5 room by room in Portland and Vancouver homes.
971-363-5626
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References
Teigen KA, Cortes Espinosa M, McKinley H, Reed A, Berrang E, Ramos-Vazquez J, Houston CA, Busch Isaksen T. “A two-year indoor-outdoor monitoring study of household clean air interventions during ambient and wildfire smoke periods in King County, Washington.” Environmental Research Communications, 2026;8:055027. doi:10.1088/2515-7620/ae69a6.
US Environmental Protection Agency. “Wildfires and Indoor Air Quality (IAQ).” Page last updated September 2, 2026. https://www.epa.gov/emergencies-iaq/wildfires-and-indoor-air-quality-iaq