Around 156 million Americans, nearly half of the US population, now live in areas with unhealthy levels of air pollution, part of a trend toward declining air quality as climate change-related extreme weather events like wildfires occur more frequently.
That’s according to the 2025 State of the Air report from the American Lung Association, which each year tracks air quality around the nation. The report, released Wednesday, focuses on two common and dangerous pollutants: ozone and fine particles. The number of Americans exposed to high levels of these has risen by about 25 million since last year’s report.
“Depending on where you live, you knew from looking out your window,” said Katherine Pruitt, the group’s national senior director for policy. “But, even expecting it, it was a surprise to us how much worse particularly ozone levels were in much of the country.”
The 2025 report is based on data from 2021 to 2023, years in which smoke from Canadian wildfires swept south to places like New York City and extreme temperatures scorched the southwestern US.
These kinds of extreme weather events can contribute to particle pollution, in which microscopic particles — formed when materials burn or from chemical reactions involving pollutants in the atmosphere — can enter the lungs and bloodstream and cause damage.
Ozone is a layer in the Earth’s atmosphere but can also form as a pollutant closer to the ground, where it’s often called smog. People can breathe in the corrosive gas, leading to inflammation in the lungs and other harmful reactions in the body. Both extreme heat and wildfires can promote the formation of ozone.
Nearly 43 million Americans live in areas with high levels of short-term and long-term particle pollution as well as ozone pollution, according to the report. Many more experience at least one of these hazards.
The geographic distribution of the problem has changed. Previously, high readings of ozone and particle pollution were concentrated in Western states, but in 2023, levels in the West eased. Meanwhile, the Midwest and Northeast saw greater daily spikes in particle pollution, and parts of the country including central states like Minnesota and Texas experienced unhealthy levels of ozone, the report authors found.
Still, the cities with the highest levels of ozone and particle pollution, respectively, are both in California: Los Angeles-Long Beach and Bakersfield-Delano.
Unhealthy air poses particular risks to vulnerable groups like children, seniors, people with asthma or other lung conditions and pregnant people and their unborn babies, Pruitt said. People of color also tend to live in areas with high levels of pollution, increasing their likelihood of exposure.
