July 28, 2022 1:51 PM EDT

Two climate-related health risks are converging with alarming frequency: record high temperatures, and air pollution from things like car exhaust and wildfire smoke. Separately, these conditions can make people acutely sick and exacerbate existing health problems. But what happens when they coincide?

Recently, researchers at the University of Southern California set out to answer that question. Their results, based on mortality data from California between 2014 and 2019 and published at the end of June in the American Journal of Respiratory and Critical Care Medicine, indicate that the combined mortality risk of extreme temperatures and thick pollution is significantly more than the sum of their individual effects.

As the chart below shows, a person’s odds of dying increased 6.1% on extreme temperature days and 5% on extreme pollution days compared with non-extreme days. But on days with both extreme conditions, the risk of death jumped by 21%.

Like vehicle emissions, wildfires release PM2.5, a type of very fine particulate matter that measures less than 2.5 micrometers across. (For comparison, the diameter of a hair is 30 times larger than the largest of these fine particles.) While the USC researchers analyzed PM2.5 pollution levels regardless of its source, they found that days with extremely high pollution happened to coincide with California wildfire events. “When you consider our top 1% of most polluted days, the pollution concentration is really very, very high… four times higher [than normal],” says Md Mostafijur Rahman, a postdoctoral researcher in the Department of Population and Public Health Sciences at USC’s Keck School of Medicine and one of the study’s co-authors. “That is definitely driven by another source. It’s not like the normal source from the traffic.”

Fine particulate matter can penetrate deep into the lungs and enter the bloodstream, says Francesca Dominici, a biostatistics professor at Harvard’s T.H. Chan School of Public Health who has studied the noxious stuff. But, while PM2.5 is known to cause cardiovascular diseases, respiratory problems, and cancers, some forms of it are worse than others. “Fine particulate matter during wildfires tends to be even more toxic,” Dominici says. “We have buildings burning, we have cars burning, we have all kinds of stuff that is burning. There is emerging research to show that the chemical composition is even more dangerous.”

What’s more, when those tiny particles react with high temperatures and sunlight, they can worsen ground-level ozone—smog—which can trigger respiratory effects like asthma attacks. One study from Washington State University published earlier this year found that periods of high ​​PM2.5 and ozone have “become significantly more frequent and persistent” across the western U.S. in the last 20 years, due to “simultaneous widespread heat and wildfire activity.” A notable 12-day stretch in the summer of 2020 included one August day where nearly 70% of that region—encompassing 43 million people—was affected by harmful levels of air pollution due to unprecedented wildfire activity around that time.

Make no mistake, though. The American West is certainly not the only place grappling with the double threats of heat and pollution. Extreme temperatures have touched just about every corner of the country this summer, and fires are searing through forests as far north as Alaska. Eastern Australia, known for its hot summers and dangerous bushfires, had an historically devastating 2019-2020 season. Russia experienced one of its largest wildfires on record last year in Siberia amid hot and dry conditions. In Europe, infernos ravaged Turkey and Greece last year; this year they’re sweeping through Spain and France, fueled by heat waves that smashed records for both how early in the year they appeared and how high the mercury rose.

This confluence of events during summer months, when temperatures soar to unbearable levels that our bodies cannot handle, are becoming more common: The heat waves make dry regions even drier—and ideal for wildfires which spew smoke plumes far and wide. Erika Garcia, assistant professor in the Department of Population and Public Health Sciences at USC’s Keck School of Medicine who co-authored the study with Rahman, warns that even though wildfires are episodic, their effects can last for weeks.

“With climate change progression, we will continue to experience more frequent, more intense, and longer extreme heat events, and extreme particulate pollution events,” she says. “We really need to have better interventions and adaptation policies so that we can save lives during these extreme heat and pollution days.”

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