Smoke from wildfires has worsened over the past decade, potentially reversing decades of improvements in Western air quality made under the Clean Air Act, according to research published Thursday from Stanford University.
The new analysis reveals a picture of daily exposure to wildfire smoke in better geographic detail than ever before. Researchers found a 27-fold increase over the past decade in the number of people experiencing an “extreme smoke day,” which is defined as air quality deemed unhealthy for all age groups. In 2020 alone, nearly 25 million people across the contiguous United States were affected by dangerous smoke.
Where Wildfire Smoke Pollution Increased Over the Past Decade
While the growing threats posed by fire have been explored in detail, particularly for populations in the most fire-prone regions, the risks that smoke pollution pose have been stymied by lack of precise data until now.
“People may be less likely to notice days with a modest increase in fine particulate matter from smoke, but those days can still have an impact on people’s health,” said Marissa Childs, who led the research while getting her Ph.D from Stanford. She also noted that the most extreme smoke days were rarely seen between 2006 to 2010. But in the more recent study years, from 2016 to 2020, she said the research shows that more than 1.5 million people, particularly in the Western United States, were routinely exposed to levels that carry immediate risks.
Daily average PM2.5 from smoke
Dr. Childs, currently a fellow at Harvard’s Center for the Environment and School of Public Health, had originally intended to focus on the health effects of fire-related air pollution. “When we started, we realized there were a lot of questions about how smoke affected people’s health that we didn’t have answers to,” she said, including basic questions around estimated mortality from smoke fine particulate matter, and how the health effects of smoke compare with other sources of pollution.
Filling in that gap was a long and arduous process. The analysis began with satellite data to map the geographic spread of particulate matter from above, and incorporated ground-level monitors to measure pollution where it matters most to human health. The research isolated wildfire smoke from background pollution from other sources, which has actually decreased in recent decades.
“We have been remarkably successful in cleaning up other sources of air pollution across the country, mainly due to regulation like the Clean Air Act,” said Marshall Burke, a co-author of the research and professor of earth system science at Stanford. “That success, especially in the West, has really stagnated. And in recent years this started to reverse.”
The research indicates that wildfire smoke could be a leading cause of that reversal, wiping out most of the progress. Some areas in the Western United States had increases in particulate pollution from smoke that were about the same amount as the improvements in air quality from regulating factories and other point-source pollution. As climate change intensifies fire risk across the country and smoke plumes can travel thousands of miles from their source, no one is safe from the effects.
Particulate pollution causes more than short-term irritation. It has been linked to chronic heart and lung conditions, as well other negative health effects like cognitive decline, depression and premature birth. But more work remains to be done on pollution specifically from wildfires.
“There is no safe concentration,” said Tarik Benmarhnia, an environmental epidemiologist at the University of California, San Diego, who contributed to an earlier study on hospitalizations showing that smoke from wildfires can be 10 times more harmful than other sources of air pollution.
And yet, much of the existing research sees wildfires as something “rare, exceptional and intense,” according to Dr. Benmarhnia. But in a changing climate, he sees the health impacts of chronic exposure to wildfire smoke as one of the biggest unknowns.
Subsequent research with this data will have important policy implications, both for local governments at the source of wildfires and for wider populations affected by smoke.
One solution, experts say, is to reduce the potential for wildfires to grow into long-lasting and destructive infernos. In recent years, California has recognized that decades of fire suppression have led to a build-up of fuel in forests where smaller, contained fires actually contribute to the health of the forest. The state has been increasing prescribed fires and other forest management techniques to help reduce the risk of out-of-control megafires.
The new research indicates that the health risk is rising as the hot and dry conditions for wildfires continue to worsen with climate change.
And existing warning systems won’t be enough in the future. “We can’t just ask people to stay inside half the year,” said Dr. Benmarhnia. “At the end of the day, the best type of policy is to proactively prevent these big fires in the first place.”