Wildfire smoke contributes to thousands of deaths each year in the U.S.

New research shows that the health consequences of wildfire smoke exposure stretch well beyond the smoky days themselves, contributing to nearly 16,000 deaths each year across the U.S., according to a National Bureau of Economic Research (NBER) analysis released in April. The analysis warns that number could grow to nearly 30,000 deaths a year by the middle of the century as human-driven climate change increases the likelihood of large, intense, smoke-spewing wildfires in the Western U.S. and beyond.

"This really points to the urgency of the problem," says Minhao Qiu, a researcher at Stanford University and the lead author. "Based on our results, this should be one of the policy priorities, or the climate policy priority, of the U.S., to figure out how to reduce this number."

Another analysis, led by researchers from Yale University, finds that the human death toll every year from wildfire smoke could already be near 30,000 people in the U.S. Deaths from cardiovascular disease, respiratory problems, kidney disease, and mental health issues all rise in the days and weeks after smoke exposure.

Together, the studies point to an underappreciated threat to public health, says Yiqun Ma, a researcher at Yale and an author of the second study.

"It's a call to action," she says—outlining the real, and significant, human stakes of failing to rein in further human-caused climate change.

How bad is smoke for health?

Wildfire smoke is rarely listed as a cause of death on people's death certificates. But research has shown that tiny particles present in smoke worsen many different health problems. These particles penetrate deep into people's lungs and can cross into the bloodstream or even into the brain. Repeated exposures, or high-concentration exposures, can supercharge other health problems, from heart and kidney disease to hastening the onset of dementia symptoms. In some cases, the stress from wildfire smoke is so great that some people die.

Because the harm from wildfire smoke can accumulate and isn't always immediately obvious, the long-term risks from wildfire smoke exposure have gone underappreciated.

"It's not obvious, necessarily, if you're looking at any individual case," says Sam Heft-Neal, an environmental economist at Stanford and an author of the NBER study. Stepping back and looking at the data statistically makes the picture much clearer, he says: smoke is a big problem that is contributing to thousands of deaths already in the U.S.

Physician and researcher Juan Aguilera, now at the University of Texas School of Public Health in El Paso, has studied the impacts of air pollution on his patients' health. He had just moved to the San Francisco Bay Area in 2020 when the smoke descended. "The 2020 wildfires brought a lot of perspective to people living in the Bay Area," he says.

Aguilera and colleagues sampled people's blood before and during the smoke event. They found markers of stressed immune systems and inflammation, signs that people were heavily impacted during the smoke. "As scientists, we do understand that things like chronic inflammation, chronic stress, lead to chronic conditions that are often related to mortality," he says.

The exact mechanisms by which smoke impacts people's health are still being unraveled. Some evidence suggests that wildfire smoke is more harmful than other tiny particles, like pollution from fossil fuel combustion or fine dust. It's likely more harmful smoke is produced when wildfires burn through urban areas, where everything from houses full of insulation to car batteries, and metal are torched.

Aguilera compares the risk of inhaling wildfire smoke to smoking cigarettes. "Being in a wildfire-prone area, it seems something equivalent to smoking like one pack a day, or 10 packs a week," says Aguilera.

Big problems, big impacts

Despite the growing understanding of the health risks from wildfire smoke, the costs have not been factored into most policy decisions, says Susan Anenberg, a public health and pollution expert at George Washington University.

The new research adds to a body of work "showing that wildfire smoke is one of the largest public health consequences of climate change," Anenberg says.

By 2050, the overall annual economic cost credited to lives lost from wildfire smoke could reach $240 billion, according to the NBER analysis. That is larger than previous estimates of all climate-related damages combined—including direct costs related to wildfire and tropical cyclone damages.

"Our estimates of the damages of climate change are undercounting the true effects," Anenberg says.

The NBER analysis used a suite of different computer models, trained on fire observations from 2000 to 2021, to figure out the relationship between fire activity and how much smoke was produced. The researchers then linked that smoke to weather patterns, letting them see how the smoke spread and drifted into different parts of the U.S. at various times. They linked those maps of smoke pollution to county-level death records across the country from 2006 to 2019 to see how deaths changed when the overall exposure to wildfire smoke went up or down.

In years like 2020, some northern California counties were exposed to double their normal pollution load for the year. In conditions like those, the total number of deaths increased by almost 6%. But even small increases in smoke exposure averaged out over the year, push mortality up. "Our findings are consistent with a host of recent work suggesting there is no safe level of air pollution exposure," the study authors write.

Those same models also let them look further into a climate-changed future. Even with aggressive climate action in coming decades, wildfire activity is forecast to grow—and with it, smoke exposure. By the middle of the century, models suggest people across the U.S. will likely experience two to three times as much smoke as they did before 2020. Smoke-related deaths could rise by at least 8,000 people every year. With less aggressive climate action, the number of deaths could be even higher.

"Not to be an alarmist, but the results are staggering," says Aguilera. "They paint a difficult picture for years to come."

The Yale study, led by Ma, uses a similar strategy to estimate the impact of smoke on deaths across the country. But the researchers also looked at the recorded causes of death. Even at very low concentrations, smoke was associated with a higher frequency of deaths related to heart disease. They also saw upticks in the number of deaths related to mental health, endocrine problems, and even diabetes.

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