Articles on U.S. wildfires don’t often show a photo of someone gasping in a hospital bed or felled by a heart attack. Yet an increasing body of evidence suggests that the biggest societal impacts of increasing wildland fire are happening in our own bodies, the result of tiny particles spewed in vast amounts.
Research on wildfire smoke and health is advancing hand in hand with the threat itself. An expanding array of satellite imagery is helping pinpoint where and when smoke is being emitted and transported. That’s helping scientists determine the number of people hospitalized or killed in a given area as a consequence of smoke.
“I think one of the biggest developments of the last three years has been the intense interest on the part of government, health organizations, media, and the public on the whole topic of fire smoke and health,” says Wayne Cascio, who directs EPA’s Center for Public Health and Environmental Assessment. “It’s been raised to such a high level nationally and even globally that it’s motivating a lot of action to support science and to answer key questions.”
Among other relevant issues, smoke appears more likely than the fires themselves to affect communities already struggling with socioeconomic and race-based health disparities.
“Nearly all the media attention during wildfires focuses on the lives and property directly in harm’s way. These are important and tragic impacts, but are likely only a very small portion of the overall societal impacts of wildfire,” says Marshall Burke, an associate professor of Earth system science at Stanford University. Burke is lead author of a 2021 paper in the Proceedings of the National Academy of Sciences (PNAS) on the evolving, multipronged threat posed by increasing U.S. wildfires.
How wildfire smoke affects health
In an October 2020 policy brief, Burke and two Stanford colleagues noted that wildfire smoke likely is responsible for 5,000 to 15,000 U.S. deaths in a typical year, and that especially smoky years like 2018 or 2020 will have a much higher death toll.
“Our research suggests that many more people likely perish from smoke exposure during large fire events than perish directly in the fire, and many more people are made sick,” Burke says.
The most concerning byproducts of wildland fire are the smallest particulates routinely tracked by EPA: PM2.5, those no larger than 2.5 microns in diameter. These have long been linked to increased risk of illness and death, as they’re small enough to enter lungs and also the bloodstream, thus affecting both cardiovascular and respiratory systems.
Globally, more than 4 million deaths per year are estimated to be triggered by outdoor air pollution. The actual toll could be twice that, if one recent study is correct. A large share of those fatalities can be chalked up to PM2.5.
Although PM2.5 from pollution has decreased by more than 40% in the U.S. since 2000, wildfire-related PM2.5 is on the increase. Burke and colleagues found that the fraction of total nationwide PM2.5 that originates from wildfire has jumped from around 10% in 2010 to around 25% today. In parts of the West, they estimate, fires now produce up to 50% of all PM2.5 — in other words, as much as all pollution sources together.
Startlingly, the immense amounts of smoke thrown into the air by wildland fire aren’t covered at all under the federal Clean Air Act: As their very name suggests, wildfires are uncontrollable and thus not subject to regulation.
At the same time, the periodic controlled burning of fire-prone areas in order to forestall much bigger fires — a practice used by Indigenous people for centuries, and more recently taken up by federal, state, local, and private land managers — does fall under Clean Air Act regulation of both PM2.5 and ozone.
The paradoxical upshot: clean-air law is limiting our ability to quell wildfires that are potentially riskier than controlled burns to air quality and collective health. This conflict is especially pronounced in regions already struggling to meet national ambient air quality attainment levels because of pollution unrelated to fire.
It’s a conundrum that forest managers and air quality regulators are increasingly pondering, according to Stanford-based research scholar and environmental law expert Michael Wara, who coauthored the PNAS paper.
“Doing prescribed fire on a site in a way that prevents a catastrophic wildfire dramatically reduces emissions of PM per acre. It’s an order of magnitude difference,” says Wara. “You burn so much less material when you just come in and burn the forest floor and [not] the large trees.”
Currently, a land manager — whether a private owner, a government entity, or an Indigenous tribe — can apply for permission to exceed 24-hour guidelines for PM2.5 and ozone with a single prescribed burn. However, a season’s worth of controlled burning could still run up against annual PM2.5 and ozone limits, even if these burns help avoid an eventual wildfire that would be truly catastrophic.
Wara is part of an embryonic effort to examine how the Clean Air Act could be reinterpreted to balance the relative emission harms from controlled burns versus massive wildfires. “We need to bring realistic versions of land management into the models used to evaluate these processes,” Wara says.
The ultimate air-quality goal, he adds, is to reduce exposure to harmful smoke in all the ways such exposures can occur. “Nobody wants to change the Clean Air Act,” says Wara, but “everybody recognizes we need to change how we manage this.”
Smoke, health, and the environmental equity implications
Wildfires are seldom viewed through the lens of environmental justice. The reason, in part, is that most immediate impacts of U.S. wildfire, including injuries and deaths as well as structural damage, tend to affect exurban and rural communities in the West that are largely white. For example, the Camp Fire of November 2018 — California’s deadliest (85 deaths) and most destructive modern-day fire (18,804 structures lost) — hit hardest in the town of Paradise, which was 92% white as of the 2010 census.
The impacts of smoke, which can extend hundreds of miles from a major fire, are another matter. Easterly winds pushed smoke from the Camp Fire into the Bay Area for two weeks on end, giving San Francisco six of its 10 worst PM2.5 days up to that point in data going back to 1999. In the East Bay city of Vallejo, as reported by KQED public radio, the levels of PM2.5 soared to 247 micrograms per cubic meter (μg/m3 ) — far above the federal 24-hour standard of 35, and closer to levels observed on a bad day in pollution-choked cities such as Beijing or Delhi. Vallejo is one of the state’s most diverse cities: 33% white as of the 2010 Census.
Equity comes into play even more when smoke worsens health conditions that are already more prevalent in lower-income locations, including some communities of color. One 2016 study in northern California found that people in lower-income Zip code areas were disproportionately likely on wildfire smoke days to visit emergency rooms for asthma complications.
“Health insurance may be a key factor,” says Colleen Reid of the University of Colorado, lead author of that study and of a 2016 research review on wildfire smoke and health that she expects soon to update. Reid points out that one study from Canada, where universal health care access is the norm, found no differential impact from wildfire smoke based on socioeconomic status, whereas several U.S. studies have found such effects.
People in lower-income areas may also be more vulnerable to smoke impacts simply because of their housing. Especially when prolonged, wildfire smoke can easily infiltrate homes and compromise indoor air quality.
“At least in California, older housing is much less airtight than newer housing stock,” says Wara. Moreover, he adds, “Wealthy people tend to have the disposable income to drop a couple hundred dollars on HEPA filters. I know other people who suffer terribly through wildfire season. People I know are taping plastic sheeting over their windows. Even if you put a box fan together with a filter, if your house isn’t tight, the smoke just gets in.”
There’s much yet to learn about wildfire smoke and health disparities. A study published in Science Advances on April 28 found that nearly all U.S. sources of PM2.5 emissions disproportionately affect people of color. However, the study did not cover emissions from wildfires.
Zeroing in on longevity and health impacts
Back in 1994, the landmark “Six Cities” study from the Harvard School of Public Health revealed pollution’s terrible toll: long-term exposure to high levels of PM2.5, even in cities that met existing air quality standards, could shorten life expectancy by up to three years. Reducing PM2.5 gives the most health benefit for each dollar of pollution control.
Although PM2.5’s biggest threat is to the cardiovascular system, such effects have been difficult to nail down for wildfire smoke, according to Reid. “A few more recent studies have found more significant relationships,” Reid says. “There’s a lot of statistical methodology differences among different groups, so we need to do some more work.”
There are also emerging signs that PM2.5 from wildland fires may be more health-hazardous than other PM2.5, at least in some cases. A recent Nature Communications study led by Rosana Aguilera, a postdoctoral researcher at the Scripps Institution of Oceanography, examined 14 years of Southern California fires. The authors found up to a 10% increase in respiratory hospitalizations for every 10 μg/m3 increase in wildfire-specific PM2.5, compared to a 1.3% increase for non-wildfire PM2.5.
Similarly, a 2020 study led by Daniel Kiser of the Desert Research Institute found that asthma-related visits to urgent care centers and emergency rooms in the Reno area were boosted by an additional 6.1% for every 5 μg/m3 increase in PM2.5 for cases when wildfire smoke was present compared to when it was not present.
Not all studies have found such differences, however, perhaps because of methodological and regional variations. Health impacts from wildfire smoke may also vary based on exactly what’s being consumed by a wildfire (oak, pine needles, eucalyptus, or peat, to name just a few), and on whether a fire is smoldering versus raging. Such nuances are “pretty challenging to investigate,” says the EPA’s Cascio, “but certainly the lab data suggest there may be differences.”
Along with its growing body of research on wildfire smoke and health, including new efforts focused on public communication and intervention, EPA launched its Smoke Sense app (available through Android and iOS systems) in 2017. More than 40,000 users have downloaded Smoke Sense, which provides current fire and air-quality data and hourly forecasts of smoke and ozone. People can also gauge their own vulnerability to smoke impacts and anonymously report any symptoms via the app.
Bracing for more fire and smoke
The accumulating research on wildfires and health could lead to a transformation in how we view the infernos that are becoming more widespread across ever-more-intense fire seasons. Human-produced global heating is not only raising temperatures — it’s also raising the stakes for wildfire risk. In a warming climate, landscapes can dry out more readily even where precipitation trends aren’t changing.
“Among the many processes important to California’s diverse fire regimes, warming‐driven fuel drying is the clearest link between anthropogenic climate change and increased California wildfire activity to date,” concluded Park Williams of Columbia University in a 2019 paper.
Thanks to Dr. Jeff Masters for contributing to this post.
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