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Converging Climate Risks Interact to Cause More Harm, Hitting Disadvantaged Californians Hardest


“The future happens in California first” long referred to the state’s reputation as an environmental leader. It’s come to describe the Golden State’s front row seat to a rotating list of overlapping extreme events that are appearing with ever more frequency and ferocity as the climate warms.

Now new research suggests that being on the front lines of climate change is making Californians sick.

Extreme heat and wildfire smoke can each increase the risk of cardiac and respiratory disease and death. But recent studies show that exposure to heat and particle pollution  together can be far more dangerous than exposure to either alone. And emerging evidence suggests that the fine particulate matter, or PM2.5, from wildfires may be far more harmful than PM2.5 emissions from other sources like car exhaust. Now, it’s clear that PM2.5 from smoke, combined with heat, is a particularly harmful combination.

In the new study, published in the peer-reviewed journal Science Advances, University of California researchers from the San Diego and Los Angeles campuses studied hospital emissions for cardiac and respiratory diseases between 2006 and 2019 across California. The researchers found that simultaneous exposure to extreme heat and PM2.5 from wildfire smoke led to far more hospitalizations than exposure to either hazard alone.

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Those bearing the brunt of these compounded health risks were concentrated in low-income communities, communities of color and neighborhoods where many people lacked health insurance, didn’t own cars or attend college, or lived in densely populated areas without the benefit of cooling green spaces.

These results clearly show that “we should stop considering extreme heat and wildfire separately,” said Tarik Benmarhnia, an environmental epidemiologist at UC San Diego who co-led the research.

Right now, one agency delivers heat advisories and another issues air quality warnings. But these amplified health effects demonstrate why agencies need to coordinate and create joint warning systems, Benmarhnia said. “The first thing is to start a conversation between these two stakeholders.”

What’s more, he said, agencies should concentrate their efforts on the environmental justice communities that are most vulnerable.

“This study did a great job of thinking about the reality that people experience on the ground, and to what extent things are worse when you are experiencing both extreme heat and extreme wildfire smoke exposure,” said David González, an environmental epidemiologist at UC Berkeley who was not involved in the research.

Most studies have looked at only one or the other extreme event, González said. “This study shows the value of looking at cumulative exposure to multiple different hazards at the same time.” 

And one of the big takeaways is that if it’s hot and there’s wildlife smoke, he said,
“we should be more worried about lower concentrations of smoke than if there’s just smoke alone.”

Californians already breathe the worst air in the nation, as emissions from congested highways, oil and gas development, industrial ports and facilities, dairies and farms release fine particles and other pollutants that form ozone as they hang in warmer air. Yet it is the PM2.5 from wildfires that most concerns health experts.

Fine particles from wildfire smoke can travel hundreds of miles on air currents, get sucked into the lungs, enter the bloodstream and wreak havoc on most organ systems. And like extreme heat, these particles damage the heart and lungs, which is why both exposures increase the risk of fatal cardiovascular and respiratory attacks.

“We know living in California that there’s been an increase in extreme heat and an increase in wildfire smoke,” said Tracey Woodruff, who directs the Program on Reproductive Health and the Environment at UC San Francisco and was not involved in the research. “What’s interesting is that they saw a synergistic effect,” she said, meaning the combined effects exceed the sum of the two exposures. “That’s a really important finding.”

Studies show that co-exposures to toxic chemicals produce more harmful effects than single exposures combined, said Woodruff, a leading expert on the health impacts of toxic chemicals and pollution. “This is very consistent with that.”

And it makes physiological sense that heat and PM2.5 interact to cause more harm, Woodruff said. 

“You’re responding to high heat, you’re sweating more, you’re probably breathing harder because you’re trying to cool down, and then your blood vessels dilate,” she explained. “All those things increase susceptibility to particulate matter exposure, because as the heart beats faster it distributes whatever toxic chemicals are present in the bloodstream.”

It’s likely that the effects are incremental up to a certain point then start to exponentially increase at the highest levels of exposure, Woodruff said. “The breakdown of the body’s ability to handle these environmental insults probably increases more rapidly.”

These results should serve as a wakeup call, public health experts say.

“The big picture here is, we should be even more concerned about the health impacts of climate change,” González said. “As temperature extremes become more common and as wildfire smoke becomes more common, we could see even greater health impacts when these things happen at the same time in the same place.”

Rising Inequality 

The new paper builds on earlier work by Benmarhnia and his colleagues, which investigated how overlapping extreme events impact health and who’s most affected.

In one study, published in 2021, they analyzed hospital admissions for respiratory conditions in Southern California over 14 years and found that PM2.5 from wildfires is up to 10 times more harmful than particulate matter from other sources. Last year, Benmarhnia and others reported that ozone—formed as chemical reactions between pollutants increase on hot, sunny days—appeared to contribute to deaths during heat waves in France. But ozone’s role varied across urban areas, likely reflecting the fact that marginalized communities are particularly susceptible to its impacts.

For the new study, the team harnessed the method they developed to understand how ozone’s health effects vary geographically to study interactions of heat and smoke across California. 

“We really wanted to emphasize that when there is a heat wave, and when there is a wildfire smoke event, there is a synergistic effect,” Benmarhnia said. “But there is a lot of heterogeneity spatially speaking. Not all communities are equal.”

That is particularly true in California. It’s one of the most racially and ethnically diverse states in the country. But it also has one of the largest income gaps nationwide, with 20 percent of the state’s wealth concentrated in ZIP codes where just 2 percent of Californians live.

Top-earning families net 11 times more than low-income families, a gap that has risen over the past several decades, research from the nonprofit Public Policy Institute of California shows. And while Black and Latino families make up 43 percent of California families, they account for more than half of the state’s poorest families.

It’s these communities that are most likely to suffer the health effects of coinciding wildfires and heat waves, Benmarhnia and his teams show.

The results come as no surprise to environmental justice and policy experts. 

The data validate the experiences of people on the ground who are most impacted by these compounding disasters, said Michael Méndez, an environmental planning and policy expert at UC Irvine who was not involved in the research. And it’s no accident that some communities are disproportionately affected, he added.

Low-income, Black and Latino communities have historically been denied resources and investments in the type of infrastructure that would make them more resilient and protected from the heat and wildfires, Méndez explained. Instead of trees and other natural cooling systems in many of these communities, he said, there’s just concrete and asphalt, which makes these places 5 to as much as 20 degrees hotter than an area with abundant green space.

Plus, much of the housing in these communities is rundown, lacks proper insulation or air conditioning and has rickety windows that let in hot, smoky air. 

These stresses are compounded for individuals like farmworkers, “who are working all day in the heat, not getting any shade, very few water breaks, breathing in toxic air coming in from these wildfires,” Méndez said. “They go home, and because they live in poor housing conditions, their house is still hot even at night, so their bodies cannot cool down. And they don’t have air filters, they don’t have proper insulation or double pane windows that can keep out some of the smoke.”

All these factors combine to place “an amplified, disproportionate impact” on communities like farmworkers, he said.

Scores of California farmworkers died in the heat in counties with chronically unsafe air over just five years, Inside Climate News reported last year. As wildfire season returns to California, it will heighten the risks agricultural laborers already face working in the heat in a state with notoriously bad air quality.

Preparing for Compound Hazards

Much of California had a reprieve from historic wildfires and heat waves last summer, but no one expects that to last. 

Wet weather returned to California in December and heavy storms known as atmospheric rivers started dumping massive amounts of water on parts of the state Wednesday. All that precipitation spurs the growth of grass and other vegetation that could provide fuel for wildfires when soils dry out again.

California faces among the highest future climate risks in the United States within the next two decades, a 2021 study found. The study’s projections included extreme temperatures and precipitation but not wildfire, which the authors acknowledged could exacerbate the state’s risks as warmer, drier conditions lead to more fires.

It may just be a matter of time before Californians once again find themselves trying to get through punishing heat waves while navigating dense clouds of smoke from wildfires that drag on for weeks, as they did throughout the 14-years covered by the new study. 

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That prospect highlights the urgent need for better coordination of emergency responses, including warning systems for both wildfire smoke and heat, public health experts say.

“The state is already doing some type of new heat wave classification system, because a bill mandates them to do that,” said Paul English, an epidemiologist who heads the Public Health Institute’s Tracking California and was not involved in the research. The new system will be run by a newly created Office of Planning and Research starting in 2025. Since the study detected synergistic effects, English said, “a suggestion to OPR would be to add wildfire smoke into their heat-wave definitions for public health reasons.”

And when there’s extreme heat, English said, agencies may need to look at lower PM2.5 levels than they think are harmful by themselves. 

California could help the most vulnerable communities deal with these compound risks, Benmarhnia said, by equipping cooling centers with air filters so people have a place to escape both heat and toxic smoke.

For Méndez, it’s critical to determine who is most vulnerable before compounding heat and wildfire disasters strike. Because when they do, he said, those communities will be disproportionately affected.

Those most affected include low-income communities of color, people with limited English proficiency, the elderly and children, Méndez said. He’s seen schoolkids spend eight or more hours a day in schools where heat rises from asphalt playgrounds with little vegetation and ramshackle buildings leave classrooms filled with hot, smoky air. 

Also vulnerable are individuals like farmworkers who live paycheck to paycheck and often feel pressured to continue working in dangerous situations where there’s toxic smoke. 

Méndez has talked to workers who labored for weeks or even months under blankets of smoke from wildfires. “They would come home and literally have black saliva.”

“Yet our disaster and emergency response systems have largely made these communities invisible in terms of policy,”said Méndez, who showed how undocumented Latino and Indigenous immigrants’ needs were largely ignored during and after the destructive 2017 Thomas Fire.

“We’re going to see more extreme weather and more extreme wildfires,” said UCSF’s Woodruff. “And just like when we talk about how climate change is going to be nonlinear, that things could take off very rapidly once we pass a certain point of no return on global temperature changes,” she said, “we’re going to see pretty extreme responses in terms of impact on people’s health.”

The stark evidence of extreme events compounding health effects underscores the urgent need to address the underlying drivers of climate change while protecting those who are going to be hit hardest, González said. “I think this study is going to be foundational in helping us understand other ways the climate crisis is affecting the health of Californians and people across the world.”



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