Soaring temperatures are expected to collide with wildfire smoke across parts of the United States this week, a combination that can pose health risks.

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.

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.

In addition, the Trump administration has issued sweeping rollbacks of environmental regulations. It has moved to weaken limits on emissions from power plants and vehicles, which could increase carbon dioxide, sulfur dioxide, particulate matter and other pollutants. It has also issued orders compelling several aging coal plants to stay open past planned retirement dates, extending pollution from those plants. Last year, coal emissions in the U.S. increased after years of decline.

Even as heat can make air pollution worse, the combination of the two environmental hazards can have outsize effects on public health, experts say.

A 2023 analysis of more than 20 million deaths across the world found that hot days and days with bad air quality both resulted in higher-than-normal mortality rates. But periods in which heat and pollution are combined were even more deadly.

The combination can be particularly stressful for children, older people or anyone with respiratory diseases like asthma. Dr. Mary Rice, director of the Center for Climate, Health and the Global Environment at the Harvard T.H. Chan School of Public Health, called the phenomenon 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.”

How heat worsens air quality

There are two main types of air pollution that experts track for their effects on human health: ground-level ozone and particulate matter.

Summer days tend to see higher concentrations of ground-level ozone, which forms when sunlight cooks pollutants from vehicles and industrial sources. Ozone irritates the lungs and can cause coughing and shortness of breath.

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

Extreme heat can also contribute to the conditions that make wildfires more severe. Wildfire smoke contains particulate matter, and can travel for thousands of miles. A 2024 study found that wildfire smoke may have killed as many as 12,000 Californians prematurely in 2018.

“The intensity of that exposure tends to be quite high,” said Dr. Meredith McCormack, director of pulmonary and critical care at Johns Hopkins University and a representative of the American Lung Association.

When buildings burn, the smoke contains metals and chemicals, Dr. McCormack noted. “You have a mixture of natural and man-made sources often during wildfire events at levels that are really extraordinary,” she said.

Regulatory rollbacks

In February, the Trump administration revoked the government’s legal authority to fight climate change, a move that eliminated limits on greenhouse gas emissions from motor vehicles and cleared the way for other regulatory rollbacks.

And last year, Republicans in Congress also eliminated tax breaks that supported the growth of renewable energy, while the Senate blocked California’s plans to implement clean air standards that would have phased out the sale of new gas-powered cars by 2035.

The Trump administration has been promoting the production and use of oil, gas and coal, the burning of which is the main driver of climate change. Mr. Trump has been especially focused on trying to revive the flagging coal industry.

“The continued operation of coal-fired power plants is harmful to the health of those communities,” Dr. Rice said. She added that there was “very clear evidence” that pollutants from coal emissions increased the risk of asthma and could be deadly.