As humans warm the planet, heat waves and wildfires are becoming more severe and lasting longer in the American West. This also means they are more likely to overlap. The researchers estimated that two-thirds of California’s land area simultaneously experienced searing heat and thick wildfire smoke at some point during the state’s record wildfire year, 2020.

Both hazards alone are harmful to health: heat stress increases heart pressure, and inhaling smoke from forest fires can worsen lung conditions. The new study, led by researchers at the Scripps Institution of Oceanography at the University of California, San Diego, examined the health effects when the two threats appear in tandem.

On unusually hot, smoky days, staying indoors doesn’t always help, and certainly not for people who don’t have air conditioners or air purifiers, said Tarik Benmarhnia, an environmental epidemiologist at Scripps and the one of the authors of the study. “Air pollution doesn’t stay politely outside,” he said. “It gets indoors, interacts with many indoor air pollutants and can cause many problems.”

Researchers took state data on unplanned hospitalizations between 2006 and 2019 and combined it with detailed readings of temperatures and wildfire smoke.

They found that combined exposure to both risks had a greater effect on hospitalizations than the sum of the effects of each separately. In other words, the health damage caused by simultaneous heat and smoke was greater than the sum of its parts.

California’s agricultural heartland, the Central Valley, and its forested far north experienced more hot, smoky days than other regions during the study period, the researchers found.

They also found that the magnitude of the cumulative effect of heat and smoke varied across communities with different demographics. Areas with lower socioeconomic indicators and higher proportions of non-white residents fared worse.

Currently, heat advisories in California come from local National Weather Service offices, while hazardous air warnings are issued by local air quality management districts.

The study results suggest that a joint heat and smoke warning would help keep more people safe, Dr. Benmarhnia said. To account for the added danger on cooler days, authorities could also consider issuing air quality alerts even when pollution has not reached the level that would trigger an alert on cooler days, a he declared.

A representative from California’s Air Resources Board said the agency is preparing new educational resources this year to help residents protect themselves from simultaneous heat and smoke.

“These types of joint events are going to happen more and more,” Dr. Benmarhnia said.

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