The combination of extreme high temperatures and poor air quality from Canadian wildfires and other sources of air pollution may become a major contributor to illness and early death in the United States, experts warn.
"Individually, each one of these is hazardous," said Melissa Gonzales, chair of environmental health sciences at the Tulane University School of Public Health and Tropical Medicine in New Orleans. "Put them together, and they're compounding the effects of one another."
The warning comes as tens of millions of Americans struggle to catch their breath amid the smog of Canadian wildfires and record-setting temperatures.
Though older adults and people with weakened immune systems are most at risk for dire health outcomes related to extreme heat and air pollution, no one is immune, said Jun Wu, a professor in the department of environmental and occupational health at the University of California, Irvine.
"They may think it's not a big deal, that it's only weak people or homeless people who may be at risk," she said, but that's not true.
"The body has a limited capacity to handle stress," Wu said. And while extreme heat and air pollution impact the human body in different ways, the existence of one appears to worsen the impact of the other.
A 2022 study by the University of Southern California found that the risk of early death increased by 21% on days that were both unusually hot and polluted with high concentrations of fine particulate matter.
"Short-term exposure to extreme heat and air pollution alone were individually associated with increased risk of mortality," the study authors wrote, "but their co-exposure had larger effects beyond the sum of their individual effects."
How the heat dome, smoke damage the heart
On its own, extreme heat can dehydrate the body. Blood vessels get smaller, decreasing the flow of oxygen-rich blood to tissues and organs. The heart tries to compensate by beating faster, increasing blood pressure.
Adding poor air quality to the equation makes the situation worse by causing oxidative stress. That is, the body's natural ability to rid itself of toxins is hampered.
Oxidative stress is one of the biggest problems. It can cause long-term tissue and cell damage, increasing the risk of cancer later in life. While the DNA damage from exposure to wildfire smoke can be repaired in younger people, as the body ages, the repair mechanisms aren't as robust.
Cells that line the airways called respiratory mucosal cells, for example, normally band together to form a barrier and protect the body from inhaling toxins. During that process, many of those cells slough off and must be regenerated.
But as the air becomes more polluted, the body's ability to regenerate those cells can't keep up to offset the buildup of pollutants.
That fine particulate matter doesn't get filtered out, and "can actually lodge inside your lungs," said Dr. Farrah Kheradmand, a physician scientist and professor of medicine and immunology at Baylor College of Medicine in Houston.
People with chronic health problems, such as asthma or heart disease, are most at risk for developing dangerous and potentially deadly outcomes, said Dr. Ali Raja, deputy chair of the department of emergency medicine at Massachusetts General Hospital in Boston.
"We see a higher mortality and increased risk of death, not only during times of excessive heat, but also when you pair excessive heat with poor air quality," he said.
A government-run website called AirNow provides county-specific air quality indicators. And the Centers for Disease Control and Prevention says the best ways to protect against heat-related illness include drinking plenty of water, wearing lightweight, loose-fitting clothing outdoors and trying to avoid physical activity during the middle of the day.
CORRECTION (July 20, 12:40 p.m. ET): A previous version of this article misstated the location of Baylor College of Medicine. It is in Houston, not Waco, Texas.
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