The researchers also found a connection between the pollution and patterns of atmospheric “ridging,” the development of stagnant zones of high-pressure air. These zones, sometimes called heat domes, lead to increasing heat and drying that can cause wildfires to ignite and spread more readily, and can also cause dangerous heat waves. The frequency of these ridging patterns has increased significantly since 2000, according to the new research.

The study analyzed data only through 2020. “But I think 2021 would show up pretty high on some of these metrics, too,” Mr. Kalashnikov said. There were several enormous wildfires in the West last summer that spread smoke across the West, and extreme heat and drought persisted throughout the region.

Colleen Reid, a health geographer at the University of Colorado who has studied the combined effects of wildfire smoke and ozone but was not involved in this research, said the findings showed that the seasonality and extent of PM2.5 pollution in the West is changing and now overlapping more with high-ozone days.

“What has been happening a lot lately is that we’ve been having really bad air quality as well as extreme heat,” she said.

Dr. Reid said the study also highlights how heat has to be taken into account when it comes to helping the public cope with air pollution, because the recommended public health measures for extreme heat and extreme air pollution can be completely opposite, especially for those who cannot afford air conditioning.

“When it’s hot, you want to open your windows so that you don’t overheat in your home,” Dr. Reid said. “But when the air quality is bad, you want to close your windows to keep as much of the air pollution out.”

Dr. Swain said that with wildfires, the short-term dangers to individuals and communities usually get the most attention. But this study looks at the longer-term risks to the broader public.

“Something may not necessarily have a high likelihood of killing you personally in the short term,” he said. “But if you impose that same risk on tens of millions of people over and over again, the societal burden is actually very high.”