Legacy of fires’ foul air
Research from Stanford, UC San Diego says blazes threaten decades of progress toward cleaner skies
Wildfires in the western United States have exacerbated pollution enough to threaten decades of progress toward sustaining cleaner skies, according to new research underscoring one of the alarming ways that climate change can harm public health.
The findings from researchers at Stanford University and UC San Diego show that wildfire smoke is now responsible for as much as half of the fine-particulate air pollution in western states. That’s about double the level that smoke accounted for in the mid2000s, according to the paper published Tuesday in the Proceedings of the National Academy of Sciences.
While the country took large strides to improve air quality after passage of the federal Clean Air Act decades ago, the latest research sug
“The gains in reduced illness and death that were created by the Clean Air Act are being undermined by wildfire.”
Michael Wara, director of Stanford’s climate and energ y policy program
gests that smoky skies are frustrating the trend, particularly in western states such as California.
Researchers developed a statistical model to examine how much wildfires contributed to PM 2.5, the name for inhalable particles 2.5 micrometers or smaller. Those particles can cause a range of serious health issues, including decreased lung function and heart attacks.
“The gains in reduced illness and death that were created by the Clean Air Act are being undermined by wildfire,” said Michael Wara, director of Stanford’s climate and energy policy program and one of the paper’s authors.
The authors examined data from 2006 through 2018, so the impacts of California’s 2020 wildfire season, when more land burned than any other year on record, were not included. But the paper still captured years of increasingly extreme fires.
For example, 2018 was the state’s secondmost severe fire season as measured by acres burned. That year, the air basins also recorded their largest total number of days since 2002 when PM 2.5 levels exceeded the national 24hour standard, according to the California Air Resources Board. The air board data did not separate sources of PM 2.5, which includes sources such as fossilfuelpowered cars and power plants in addition to smoke from fires.
“It should be a concern for everyone in the western U.S.,” Wara said of pollution caused by fires. “People living in downtown Oakland or in the heart of San Francisco, where there’s never going to be a wildfire, should be concerned about what happens in Lake County, because the smoke from Lake County is going to drift over their homes and cause sickness and death.”
In the paper, Wara and his colleagues noted that the country’s annual burned area has about quadrupled over the past 40 years. Climate change accounts for about half of the increase, and another doubling could occur as temperatures continue rising, the authors explained.
Accordingly, wildfire smoke “is going to be a lot of people’s first real personal experience with climate change,” said Marshall Burke, a Stanford associate professor of Earth system science and another author of the paper.
“It’s super widespread and it’s really hard to avoid,” Burke said. “So most in the U.S. are going to experience this much more readily than they are going to experience something like sea level rise or increasing hurricane intensity and that kind of stuff.”
As the threats from fires have soared, so has the number of people living in harm’s way: The number of homes on the edge of the fire danger zone has grown by about 350,000 annually over the past 20 years, the paper said.
And the researchers found that counties with whiter and wealthier populations tended to be more directly exposed to fineparticulate pollution from wildfire smoke, even though lowerincome communities of color are more exposed to PM 2.5 overall. But the paper also stresses that “infiltration of outdoor pollutants into homes is known to be higher on average for older, smaller homes and for lowerincome households.”
“What we measure in this paper is outdoor spread, but people spend most of their time indoors,” Burke said. “Poor communities could still be more exposed once you account for this indoor infiltration.”
While fires are an obvious air quality problem, more research is needed to determine whether fine particles in wildfire smoke are more or less harmful to human health than the fine particles found in other sources of air pollution, said Tony Wexler, a UC Davis professor who was not involved in the study.
“My hunch is it’s less damaging,” said Wexler, director of the university’s Air Quality Research Center. “We as humans have evolved over millions of years with wildfire smoke and we have always had campfires and we always have cooked over stoves with wood. My guess is that our body is better adapted to that stuff than it is to diesel exhaust.”
But not all wildfire smoke is the same. Catastrophes such as the 2018 Camp Fire that raze whole neighborhoods are likely much worse than fires that burn mostly through forests, Wexler said.
A Stanford research team that included Burke estimated in 2020 that the excess particulate matter from wildfires that year may have indirectly contributed to thousands of deaths.
Dr. Stephanie Christenson, a UCSF pulmonologist and assistant professor of medicine, has seen the health consequences of wildfire smoke firsthand. When air quality deteriorates as the state burns, she said her patients with chronic lung problems report having a harder time breathing.
And the effects aren’t limited to the days when fires are actively burning.
“I’ll hear about even months out where people will say, ‘This all started with the fires. My breathing hasn’t been as good,’ ” Christenson said. “Anecdotally, just when I talk to my patients, they’re all feeling it. Many Californians are feeling it in general on many days of the year — and even people without lung problems.”