How extreme heat makes air pollution worse

Heat waves are typically caused by high-pressure systems that trap warm, stagnant air.

"Basically, you've got this big cooking pot," said Jim McQuaid, an atmospheric chemist at the University of Leeds. Without any wind to blow away pollutants, they stay in one area and intermingle in a process McQuaid compares to stirring a sauce.

"You've got all of the ingredients going in, then you've got heat going in, and it just all cooks," McQuaid said. Like a sauce reducing in a pot, the mixture slowly gets stronger and more potent, McQuaid added.

There are two main types of air pollution that experts watch for their effects on health: ground-level ozone and particulate matter. Typically, summer days see higher concentrations of ground-level ozone, a pollutant that forms through chemical reactions near the ground.

Ozone can irritate the lungs and cause coughing and shortness of breath. Its formation accelerates under hot and sunny conditions, and a buildup of emissions from burning fossil fuels can also contribute.

Meanwhile, particulate matter refers to tiny pieces of solids and liquids in the air, all of which are smaller than a grain of sand or a strand of human hair. When these particles, known as PM 2.5, are inhaled, they can reach the deepest part of the lungs and ultimately pass into the bloodstream, causing harm throughout the body, including in the heart, lungs, and brain.

The 'double whammy' on public health

Experts have been increasingly concerned about the health hazards posed by the combination of extreme heat and pollution, Brown and Kelso report. As climate change has driven global temperatures up, the frequency of days when it is both very hot and polluted has also been increasing.

"We're in the middle of a bad air quality summer in a lot of the country," said Joseph Goffman, who led the Office of Air and Radiation at the Environmental Protection Agency under the Biden administration. "And now it's more likely than not that we'll see summers like this in the coming years."

Air pollution on its own contributes to millions of deaths worldwide. The World Health Organization has estimated that outdoor and indoor air pollution combined cause 6.7 million premature deaths a year, mostly in low- and middle-income countries.

And while heat can exacerbate air pollution, the combination of the two can have a significant impact on public health. Multiple cities during the current heat wave warned of air quality that is unhealthy for sensitive groups, including New York City; Philadelphia; Cincinnati; and Grand Rapids, Michigan.

One analysis from 2023 of more than 20 million deaths around the world found that hot days and days with bad air quality both resulted in higher-than-normal mortality rates, but periods where heat and pollution were combined were even more deadly.

The combination of heat and air pollution can be especially difficult for children, older people, and anyone with respiratory diseases like asthma.

Mary Rice, director of the Center for Climate, Health, and the Global Environment at the Harvard T.H. Chan School of Public Health, described the phenomenon as a "double whammy" that can result in increased hospital admissions.

"I think of heat as especially bad for the brain, for cardiovascular and mental health admissions," she said. "Air pollution is very harmful for respiratory and cardiovascular admissions and also stroke. Those together can further enhance cardiovascular and respiratory risk."