Wildfires are reversing decades of ozone gains

A new study suggests that one of the nation’s fastest-growing air quality challenges is coming from increasingly intense wildfire seasons, rather than tailpipes or smokestacks.

Researchers found that wildfires have reversed years of progress in reducing ground-level ozone pollution across the U.S., potentially complicating efforts to meet federal air-quality standards and increasing health risks for millions of people.

The study, published in Science, analyzed ozone concentrations across the continental U.S. from 2003 through 2024. Using a combination of air-quality observations, satellite data, meteorological information and advanced modeling techniques, the researchers created a detailed record of how ozone levels have changed over the past two decades.

How fires are erasing progress made on ozone pollution

From 2003 to 2015, ozone pollution generally declined as emissions from vehicles, power plants and other human activities were reduced through regulations and cleaner technologies. National ozone concentrations fell by roughly 11% during that period.

However, after 2015, that progress began to stall and eventually reverse.

The researchers found that surface ozone concentrations increased by about 0.13 parts per billion per year between 2015 and 2024. According to the study, growing wildfire activity was the primary reason for the turnaround.

Without the influence of wildfire emissions, ozone levels likely would have continued their long-term decline, the authors concluded.

Wildfire smoke can affect air quality far beyond the fire

Wildfires release a mixture of gases, including nitrogen oxides, carbon monoxide and volatile organic compounds. When those pollutants interact with sunlight, they can produce ground-level ozone, a harmful air pollutant that can irritate the lungs, worsen asthma and contribute to other respiratory and cardiovascular health problems.

Unlike some forms of pollution that remain relatively close to their source, wildfire smoke can travel hundreds or even thousands of miles. That means fires in one region can contribute to ozone pollution in communities located far downwind.

The study found the strongest ozone increases across much of the western United States, though impacts extended into parts of the Midwest and other regions as smoke traveled across the country.

The researchers pointed to 2023 as a particularly notable example. Massive Canadian wildfires sent smoke across large portions of the United States, creating hazy skies and poor air quality from the Midwest to the East Coast.

According to the study, wildfire-related pollution exposed millions of additional Americans to ozone levels that exceeded air-quality standards during that year.

Health implications from wildfire smoke are significant

Researchers estimate that wildfire-driven increases in ozone pollution have contributed to more than 300 additional premature deaths annually in the United States since 2013. For 2023 alone, they calculated nearly 8,000 ozone-related premature deaths.

While decades of pollution-control efforts have successfully reduced emissions on many accounts, the researchers found that growing wildfire activity is eroding those gains, creating a new challenge for air-quality managers and public health officials across the country.