The heat dome that is scorching more than 90 million people across the United States this week also threatens to deliver polluted air, something that scientists say is a growing problem on a warming planet.

Researchers are increasingly worried about the twin health hazards of extreme heat and pollution, which can amplify each other. As climate change drives up global temperatures past record levels, the frequency of days when it is both 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 during the Biden administration. “And now it’s more likely than not that we’ll see summers like this in the coming years.”

Several factors are converging to amplify health risks during sweltering days. Extreme heat can trap polluted air in place, so it does not dissipate. Climate change is making wildfires more frequent and intense, pumping smoke into the atmosphere and contributing to the reversal of a decades-long trend of air quality improvement in some parts of the United States.

And the Trump administration is moving to weaken limits on emissions from power plants and cars, which could increase carbon dioxide, sulfur dioxide, particulate matter and other pollutants. It is also encouraging more mining and burning of coal, the dirtiest of the fossil fuels.

Even as heat can make air pollution worse, the combination of the two environmental hazards can have outsize effects on public health, experts say. Multiple cities warned of air quality this week that is unhealthy for sensitive groups, including New York City, Philadelphia, Cincinnati and Grand Rapids, Mich.