Across the American West, managers of fire-prone landscapes are increasingly using a practice that seems counterintuitive: setting small fires to prevent larger, more destructive ones. Commonly called “prescribed burns,” these targeted, controlled fires keep forests healthy by reducing the buildup of grasses, leaves, branches, and other debris that can fuel larger wildfires and smoke out nearby communities.����

But smoke from prescribed burns also presents health risks. Today’s forest managers must ask themselves — how much prescribed burning is too much? When do the long-term benefits of fuel reduction no longer outweigh the short-term smoke costs? And how can nearby communities better prepare for a fire season?����

An international team led by researchers at the 91̽�� built a framework to help land managers assess the air quality implications of land management scenarios with different levels of prescribed burning. To apply the framework, researchers��linked together a series of models that estimate the smoke effects of various levels of prescribed burning on ecosystems and nearby communities.����

After using those models to estimate the smoke produced under six different levels of prescribed burning across California’s Central Sierra range, the researchers found that moderate amounts of burning would reduce overall smoke levels. All tested levels of prescribed fires led to less wildfire smoke overall. But greater amounts of prescribed fires could present notable health hazards of their own.����

The researchers reported their findings — specific to the Central Sierra landscape — in a pair of recently published papers. The first, , estimated how different levels of prescribed burning affected the total amount of smoke produced during an average wildfire season. The second, , analyzed the impacts on the region’s outdoor agricultural workers.��

“We haven’t had a good way to put numbers to that smoke exposure trade-off previously because of challenges in integrating data and methods across sectors,” said , a 91̽��doctoral alum of the Department of Environmental & Occupational Health Sciences and lead author of both papers. She is now a postdoctoral scholar at UCLA. “We know that if we can reduce fuel density, then wildfires may be less severe when they do come through. Emissions may also be lower, and thus subsequent smoke exposure and health impacts will be less. We also must consider that the location and timing of prescribed burns are planned, which is not the case for wildfires. That’s the concept. But I think communicating that has previously been difficult.����

“What’s cool about this work is we were finally able to quantify the trade-off between reducing wildfire risks and its impacts on human health through prescribed burning at a local scale.”��

Researchers focused on the Tahoe Central Sierra Initiative, a 2.4 million acre expanse covering public, private and commercial land. A consortium of land managers in the area developed six forest management scenarios with increasing levels of prescribed burning. They ranged from Minimal Management, with no prescribed burns and limited efforts to trim back excess fuels, to a scenario dubbed Fire++, with an estimated 30,000 acres of prescribed burning each year.����

Those scenarios were fed into a series of models that estimated the amount of smoke generated by wildfires and prescribed burns in each scenario, and the health impacts on nearby communities.����

Every scenario that included prescribed burning in the Tahoe Central Sierra Initiative resulted in a shorter wildfire smoke season, with less overall smoke, than those without prescribed burns. As a result, nearby communities and outdoor agricultural workers could be exposed to less smoke.����

The model predicted that overall smoke levels as measured by concentrations of fine particles (PM 2.5) were lowest with a moderate amount of prescribed burning — a scenario researchers called, simply, Fire. Scenarios that involved greater amounts of burning — Fire+ and Fire++ — produced slightly more total smoke than the moderate scenario. ��

Schollaert hopes forest managers across the country will replicate the methods, so they can better incorporate public health considerations into management planning on their specific landscapes.��

“The exact placement of that sweet spot of prescribed burning is going to vary. But when mitigating extreme wildfire risk, the more you can lower severity of fire, the lower your emissions are going to be, generally,” Schollaert said. “And baked into that sweet spot is also coordination with health agencies, because you can theoretically plan for smoke from prescribed burns. That’s the kind of planning I’m hoping can come from this.”��

Other authors on both papers include and of the 91̽��Department of Environmental & Occupational Health Sciences; of the 91̽��School of Environmental and Forest Sciences and of the 91̽��Department of Civil and Environmental Engineering, among others.����

Research for the Nature Sustainability paper was funded by Science for Nature and People Partnerships. Research for the Environmental Research Letters paper was funded by NASA and the U.S. Department of Energy.��

For more information, contact Schollaert at cschollaert@ucla.edu.��

Despite their invisibly small size, ultrafine particles have become a massive concern for air pollution experts. These tiny pollutants — typically spread through wildfire smoke, vehicle exhaust, industrial emissions and airplane fumes — can bypass some of the body’s built-in defenses, carrying toxins to every organ or burrowing deep in the lungs.����

New research from the 91̽�� found that those effects aren’t felt equitably in Seattle. The most comprehensive study yet of long-term ultrafine particle exposure found that concentrations of this tiny pollutant reflect the city’s decades-old racial and economic divides. ��

The study, ��in Environmental Health Perspectives, also found that racial and socioeconomic disparities in ultrafine particle exposure are larger than those observed in more commonly studied pollutants, like fine particles (PM 2.5) and nitrogen dioxide (NO2).��

The study used mobile monitoring — a car loaded with air pollution sensors driving around the city for the better part of a year — to examine long-term average levels of four pollutants: soot (or black carbon), fine particles (PM 2.5), nitrogen dioxide (NO2) and ultrafine particles. Researchers found the highest concentrations of all four pollutants on census blocks with median household incomes under $20,000 and those with proportionately larger Black populations. ��

Disparities in concentrations of ultrafine particles — which are less than 0.1 micron in diameter, or 700 times thinner than the width of a single human hair — were especially stark. Blocks with median incomes under $20,000 had long-term UFP concentrations 40% higher than average. Blocks where median incomes are over $110,000, meanwhile, saw UFP concentrations 16% lower than average.����

“We found greater disparities with this pollutant of emerging interest, a pollutant that hasn’t been well-characterized. That’s very interesting,” said senior author , a 91̽��professor in the Department of Environmental and Occupational Health Sciences. “Our work has shown the highest ultrafine particle concentrations are north of the airport and below common aircraft landing paths, downtown, and south of downtown where there are port and other industrial activities.”��

The study also found that modern-day air pollution disparities mirror Seattle’s history of redlining, the racist practice that denied racial minorities and low-income residents access to bank loans, homeownership and other wealth-building opportunities in more “desirable” areas. The practice shaped American cities throughout the early 20th century, building a foundation of segregation and environmental racism.��

Today, neighborhoods once classified as “hazardous” are still exposed to higher concentrations of pollution than those once labeled “desirable,” the study found. This was true for all sizes of particles. The spatial disparities were largest, however, in Seattle neighborhoods that received no label because they were once considered industrial areas.��

In those previously industrial areas, ultrafine particle concentrations were 49% above average.����

“These results are important because air pollution exposure has been shown to lead to detrimental health effects, and these health effects disproportionately impact racialized and low-income communities,” said , the study’s lead author, who graduated from the 91̽��in 2022 with a degree in industrial and systems engineering. “Notably, air pollution is just one factor, and there are plenty of other examples of how systemic racism is detrimental to people’s health and well-being.”��

Bramble said the results didn’t surprise her. She was raised in Tacoma, in a neighborhood near Interstate 5, where the constant crush of cars and diesel trucks spewed pollution into the air. And as a student journalist at the UW, she researched the relationship between redlining, green spaces, heat and air pollution.����

“In the case of air pollution exposures, these policies affect the health of real people. I think at a time where the teaching of systemic racism is a controversial topic in this country, being ignorant is not going to reduce the number of children who suffer from asthma due to air pollution,” Bramble said. “Instead, I hope we can have conversations about how past policies affect us today, to drive efforts toward a healthier, sustainable society.”��

Bramble proposed and carried out this study for the grant program, which provides National Institute of Environmental Health Sciences funding and mentorship to undergraduates from underrepresented backgrounds to pursue research. She joined the program in June 2020 under Sheppard’s mentorship.����

Other 91̽��authors are Magali Blanco, Annie Doubleday and Amanda Gassett of the Department of Environmental and Occupational Health Sciences, Anjum Hajat of the Department of Epidemiology and Julian Marshall of the Department of Civil and Environmental Engineering.����

For more information, contact Sheppard at sheppard@uw.edu.��