If you are one of the more than 2 billion people on Earth who have inhaled wildfire smoke, then you know firsthand that it is nasty stuff. It makes your eyes sting and your throat sore and raw; breathe in smoke for long enough, and you might get a headache or start to wheeze. Maybe you’ll have an asthma attack and end up in the emergency room. Or maybe, in the days or weeks afterward, you’ll suffer from a stroke or heart attack that you wouldn’t have had otherwise.
Researchers are increasingly convinced that the tiny, inhalable particulate matter in wildfire smoke, known as PM2.5, contributes to thousands of excess deaths annually in the United States alone. But is it fair to link those deaths directly to climate change?
A new study published Monday in Nature Climate Change suggests that for a growing number of cases, the answer should be yes. Chae Yeon Park, a climate risk modeling researcher at Japan’s National Institute for Environmental Studies, looked with her colleagues at three fire-vegetation models to understand how hazardous emissions changed from 1960 to 2019, compared to a hypothetical control model that excluded historical climate change data. They found that while fewer than 669 deaths in the 1960s could be attributed to climate change globally, that number ballooned to 12,566 in the 2010s — roughly a 20-fold increase. The proportion of all global PM2.5 deaths attributable to climate change jumped 10-fold over the same period, from 1.2% in the 1960s to 12.8% in the 2010s.
“It’s a timely and meaningful study that informs the public and the government about the dangers of wildfire smoke and how climate change is contributing to that,” Yiqun Ma, who researches the intersection of climate change, air pollution, and human health at the Yale School of Medicine, and who was not involved in the Nature study, told me.
The study found the highest climate change-attributable fire mortality values in South America, Australia, and Europe, where increases in heat and decreases in humidity were also the greatest. In the southern hemisphere of South America, for example, the authors wrote that fire mortalities attributable to climate change increased from a model average of 35% to 71% between the 1960s and 2010s, “coinciding with decreased relative humidity,” which dries out fire fuels. For the same reason, an increase in relative humidity lowered fire mortality in other regions, such as South Asia. North America exhibited a less dramatic leap in climate-related smoke mortalities, with climate change’s contribution around 3.6% in the 1960s, “with a notable rise in the 2010s” to 18.8%, Park told me in an email.
While that’s alarming all on its own, Ma told me there was a possibility that Park’s findings might actually be too conservative. “They assume PM2.5 from wildfire sources and from other sources” — like from cars or power plants — “have the same toxicity,” she explained. “But in fact, in recent studies, people have found PM2.5 from fire sources can be more toxic than those from an urban background.” Another reason Ma suspected the study’s numbers might be an underestimate was because the researchers focused on only six diseases that have known links to PM2.5 exposure: chronic obstructive pulmonary disease, lung cancer, coronary heart disease, type 2 diabetes, stroke, and lower respiratory infection. “According to our previous findings [at the Yale School of Medicine], other diseases can also be influenced by wildfire smoke, such as mental disorders, depression, and anxiety, and they did not consider that part,” she told me.
Minghao Qiu, an assistant professor at Stony Brook University and one of the country’s leading researchers on wildfire smoke exposure and climate change, generally agreed with Park’s findings, but cautioned that there is “a lot of uncertainty in the underlying numbers” in part because, intrinsically, wildfire smoke exposure is such a complicated thing to try to put firm numbers to. “It’s so difficult to model how climate influences wildfire because wildfire is such an idiosyncratic process and it’s so random, ” he told me, adding, “In general, models are not great in terms of capturing wildfire.”
Despite their few reservations, both Qiu and Ma emphasized the importance of studies like Park’s. “There are no really good solutions” to reduce wildfire PM2.5 exposure. You can’t just “put a filter on a stack” as you (sort of) can with power plant emissions, Qiu pointed out.
Even prescribed fires, often touted as an important wildfire mitigation technique, still produce smoke. Park’s team acknowledged that a whole suite of options would be needed to minimize future wildfire deaths, ranging from fire-resilient forest and urban planning to PM2.5 treatment advances in hospitals. And, of course, there is addressing the root cause of the increased mortality to begin with: our warming climate.
“To respond to these long-term changes,” Park told me, “it is crucial to gradually modify our system.”
