Double threat: Heat and pollution amplify health risks in American cities

American cities are heating up in ways that go beyond sweaty afternoons or hazy skies. Scientists say something more dangerous is unfolding. When heat waves collide with polluted air, the effects are multiplied.

These synergistic incidents, known as “compound events,” are showing up more often in cities across the country.

Heat is the killer

According to the National Weather Service, heat kills more Americans every year than floods, hurricanes, or tornadoes. Cities make the danger worse. Buildings, roads, and traffic trap heat and churn out pollution.

Chenghao Wang, a professor at the University of Oklahoma, has been studying how these threats overlap. He leads the Sustainable Urban Futures Lab (SURF), and his research points to one conclusion: cities face a growing health crisis.

“Compound heat and air pollution episodes occur when extreme heat and high levels of pollution happen at the same time. They are an increasing threat to public health, especially for urban populations,” said Wang.

Cities face worse heat and pollution

Wang’s team compared what happens in cities and rural areas. Urban heat waves, they found, last longer and burn hotter. Rural places, however, often record higher ozone levels.

Both settings carry risk, but when heat and ozone spike together, cities take the bigger hit. In fact, almost 89 percent of U.S. cities recorded higher combined intensities than nearby rural areas.

The researchers didn’t just study a few years of data. They pulled together 23 years of records on heat and fine particulate matter, also known as PM2.5. That long view allowed them to track patterns that short studies might miss.

“Fine particulate matter, or PM2.5, is small enough in size that it can penetrate deep into the respiratory system and enter the bloodstream, and exposure is associated with adverse health outcomes such as respiratory infections and cardiovascular diseases,” said Jessica Leffel, a master’s student at OU and the lead author of this work.

When heat and pollution meet

Nearly every city in the dataset (about 98 percent) showed more frequent and intense overlaps of heat and PM2.5. More than half also endured longer episodes.

These findings make one thing clear: it is not only the heat or the pollution that matters, but the two together.

The study also revealed something striking about western cities. Wildfires are now a major driver of PM2.5 pollution. That smoke drifts into cities and makes compound events worse. Yet health assessments often leave wildfire smoke out of the picture.

“Wildfire-related PM2.5 is often excluded from air quality assessments under the EPA’s Exceptional Events Rule, which can hide the true health burden of these episodes,” said Leffel. “Integrating wildfire-related PM2.5 into air quality evaluations could better align policy with public health risks.”

Why cities stay hotter

The urban heat island effect explains much of the problem. Concrete and asphalt soak up heat during the day and release it at night. Rural areas cool off, but cities stay hot long after sunset.

Add tailpipes, factories, and energy demand, and you have a recipe for heat and pollution to reinforce each other.

Researchers say the solutions must be local. Cities may benefit from planting more trees, installing green roofs, or using reflective building materials.

Rural regions, where pavement plays a smaller role, may need different tools. There is no single answer, but without targeted action, the risks will keep climbing.

Heat and pollution crisis is worsening

Taken together, these studies send a clear signal. U.S. cities are not only hotter than before but also more polluted during those hot spells. The overlap makes life riskier for millions of people, especially the most vulnerable.

Without better planning, compound events will shape the future of urban living, straining health systems, increasing economic costs, and challenging city planners to rethink infrastructure, public health strategies, and long-term climate resilience measures.

If cities continue to grow without adapting to these dual threats, the consequences could include more hospital visits, reduced worker productivity, and even population shifts as people seek safer environments.

The researchers emphasize that solutions must come quickly and be tailored to each region, because the combined pressures of heat and air pollution will only intensify as the climate continues to change.

The study is published in the journal Environmental Research.

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