Heat and smoke from wildfires are even more harmful when combined, a study shows
Background: Global warming amplifies both threats.
As humans warm the planet, both heat waves and wildfires are becoming more severe and longer lasting in the American West. This also means that they are more likely to overlap. Researchers have estimated that two-thirds of California's land area simultaneously experienced scorching heat and heavy wildfire smoke at some point during the record fire year of 2020.
Both dangers are harmful to health in themselves: heat stress increases heart strain, and inhaling wildfire smoke can worsen lung disease. The new study, led by researchers at the Scripps Institution of Oceanography at the University of California, San Diego, looked at the health effects when the two threats appeared together.
On exceptionally hot and smoky days, staying indoors doesn't always help, especially for people who don't have air conditioners and purifiers, says Tarik Benmarhnia, an environmental epidemiologist at Scripps and one of the study's authors. “Air pollution is not politely left outside,” he said. “It gets in, interacts with a lot of indoor air pollutants and can lead to a lot of problems.”
The findings: Compound damage, and also dissimilar damage.
The researchers collected state data on unplanned hospitalizations between 2006 and 2019 and combined it with detailed measurements of temperatures and wildfire smoke.
They found that the combined exposure to the two hazards had a greater effect on hospital admissions than the sum of the effects of each alone. In other words, the damage to health from simultaneous heat and smoke was greater than the sum of its parts.
California's agricultural heartland, the Central Valley, and the forested Far North experienced more of these hot and smoky days than other regions during the study period, the researchers found.
They also found that the magnitude of the composite effect of heat and smoke varied between communities with different demographics. Areas with weaker socio-economic indicators and a higher share of non-white residents fared worse.
What could be next: Joint warnings for heat and smoke.
Currently, heat advisories in California come from local offices of the National Weather Service, while hazardous air warnings are issued by local air quality management districts.
The study's findings suggest that a joint heat and smoke warning would help keep more people safe, said Dr. Benmarhnia. To account for the added danger on sweltering days, officials might also consider issuing air quality warnings even if pollution has not yet reached levels that would trigger an alert on cooler days, he said.
A spokeswoman for the California Air Resources Board said the agency is preparing new educational tools this year to help residents protect themselves from simultaneous heat and smoke.
“These types of joint events will become increasingly common,” said Dr. Benmarhnia.