Wildfire smoke is reaching schools more often, and researchers found indoor air is not always safer for children.
Keeping children indoors seems like a sensible response to wildfire smoke. But a new study found that some classrooms had dirtier air than the outdoors during wildfire smoke events.
Smoke reached US schools and childcare centers far more often between 2006 and 2023, with the sharpest rise after 2019.
Children spend much of their day in these buildings, including mid-morning, when smoke pollution tends to peak.
Stephanie Foster led the research at Oregon State University’s College of Health. Her team tracked smoke at 115,612 public schools and 131,372 licensed childcare centers across the lower 48 states.
Molly Kile, an environmental epidemiologist at the college, co-authored the study.
“Our children are collectively experiencing billions of days of smoky air each year,” Kile said.
The average public school went from 6.8 smoke days in 2006 to 54.4 in 2023, a nearly eightfold increase.
Childcare centers went from 9.4 to 112.2, with higher counts because the researchers treated them as open all year.
Smoke days at schools rose slowly for roughly the first decade, then climbed sharply.
Between the 2019-2020 and 2022-2023 school years, the average jumped from about 10 to 54 smoke days.
North America had an especially extreme fire year in 2023. Yet when the researchers left out that final school year, the upward trend looked much the same.
The team also multiplied enrollment by smoke days, with each “student-smoke-day” representing one child and one day with smoke in the area.
In the final year, the totals reached 2.64 billion at public schools and 1.02 billion at childcare centers.
These counts came from estimates, rather than air monitors at every school.
The researchers used satellite images of smoke plumes and computer simulations that followed air from known fires.
Each school or center received the daily estimate for its census tract, a small area used in the national census. The team also estimated how much fine particle pollution the smoke added to the air.
These particles, called PM2.5, measure 2.5 micrometers across or less.
Even the largest is about 30 times thinner than a human hair, and roughly 90% of wildfire smoke particles fall into this size range.
Fine particle pollution had been falling nationwide under the Clean Air Act.
Since 2016, that progress has slowed or slightly reversed, partly because of wildfire smoke, especially in the West and Midwest.
These tiny particles can reach deep into the lungs and cross into the bloodstream.
Exposure early in life has been linked to asthma, wheezing, and changes in lung growth.
Higher exposure has also been linked to poorer performance on tests of memory and learning. Children breathe more air for their size than adults do, and they spend more time outdoors.
The researchers grouped smoke days by how much PM2.5 the smoke added to each cubic meter of air.
They used two cutoffs drawn from Environmental Protection Agency standards: 9 micrograms for a yearly average and 35 micrograms for a single day.
A single day above either cutoff does not establish a federal violation, because the rules use three-year averages.
The U.S. has no separate wildfire smoke standards, so these numbers served as a rough guide to severity.
Around the middle of the study, days above 9 micrograms were increasing by about 5.4% a year, while days above 35 were rising by about 24.5%.
Smoke days overall increased by about 2.4% a year, suggesting the heaviest smoke days were growing fastest.
Western schools had the most heavy smoke days across the study. They averaged about 2.7 days a year above 9 micrograms and half a day above 35.
Every region saw smoke days jump after 2019. Across all 18 years, schools in the South averaged the most smoke days, about 20 a year, followed by the Midwest at about 18.
The researchers suspect that large prescribed burns and agricultural fires helped raise those counts. Prescribed burns are fires people set deliberately to manage land.
Canada’s record-breaking fire season raised smoke counts in the Midwest, South, and Northeast in the final year.
Many schools were still open in June, when those fires were at their height.
“Yes, there is more severe wildfire smoke in the West, but fine particulate matter travels hundreds and thousands of miles. And the prevailing wind blows from west to east, so it all goes downwind,” Kile said.
Schools in small towns and rural areas had nearly 20% more smoke days each year than schools in cities and suburbs.
Schools with more students receiving free or reduced-price lunch, a common sign of lower family income, also saw slightly more smoke.
Those in the top quarter for these students had about 4% more smoke days than those in the bottom quarter.
The indoor findings came from a much smaller sample: 71 buildings in Washington, Oregon, and California.
Low-cost PurpleAir sensors measured particles inside 65 schools and six childcare centers.
Each indoor sensor was paired with up to 10 outdoor sensors within about 3 miles (5 kilometers). The team compared weekday readings from 7 a.m. to 5 p.m. between 2018 and 2023.
On a typical day, indoor air at schools contained about 86% as much fine particle pollution as nearby outdoor air.
On smoke days, that dropped to about 80%, showing that the buildings kept some pollution out.
Results varied widely, with typical indoor levels at individual buildings ranging from about a third of outdoor levels to more than double them.
At 11 public schools, the air inside was dirtier than the air outside on a typical smoke day.
“We are often told to stay indoors when air pollution gets severe but being indoors does not guarantee clean air,” Kile said.
“Some schools had higher indoor PM2.5 concentrations than nearby outdoor measurements during smoke days,” Kile said.
“This is a particularly actionable finding and emphasizes the need for investment in improving indoor air quality in schools through HVAC upgrades, which can be expensive, or portable HEPA filters, which can be a more affordable approach.”
A 2020 federal report cited in the study found that about 36,000 US schools needed upgrades to their heating, ventilation, and air conditioning systems.
Portable filters may help. In one classroom study, HEPA filters reduced indoor particle levels to between 10% and 50% of outdoor levels.
The indoor sensors picked up all fine particles, so cooking, cleaning, and dust stirred up by children could have affected the readings.
The outdoor sensors also sat some distance from each building.
The researchers had no records of ventilation systems, filters, or open windows. That makes it hard to explain why some buildings offered more protection than others.
For childcare centers, location records came from 2022 alone, and one in five centers listed no enrollment. The smoke model also works less precisely in the South than in California and the northern states.
The team did not separate results by children’s ages, even though infants are especially sensitive to smoke.
Foster, the study’s corresponding author, now works in the research development office at Samaritan Health Services.
“We found that the buildings where children spend much of their day do not consistently protect them from air pollution,” Foster said.
“But educational facilities are places where coordinated preparedness plans, air filtration, indoor air quality monitoring and changes to activities during smoke events could reduce exposure.”
The researchers still need to learn which steps work best in different buildings. More indoor sensors, along with records of how schools run their ventilation and filters, could help answer that question.
The study is published in the journal Environmental Research.
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