Wildfires are reversing America’s progress on ozone pollution

For decades, the United States achieved steady and measurable progress in reducing surface ozone pollution, the primary chemical ingredient in smog. This environmental improvement was driven by cleaner vehicles, modernized industries, and more efficient power sources. However, that hard-won progress is increasingly being overshadowed by a different and growing source of ozone pollution: wildfires. The gases released during wildfires are chemically altering the atmosphere in ways that negate previous regulatory successes.

A team of atmospheric and wildfire scientists analyzed the contribution of wildfires to surface ozone levels from 2003 to 2024 across the United States. The analysis revealed a critical turning point. The gases contained in wildfire smoke have effectively reversed the national ozone trend. Before 2015, ozone levels were declining. After 2015, driven by wildfire emissions, those levels began to increase. This shift has had severe health consequences. Since 2015, the number of ozone-related premature deaths attributed to wildfires has been increasing by approximately 300 deaths per year.

Most people are familiar with ozone as the protective layer of the atmosphere located high above the Earth. This stratospheric ozone shields the planet from harmful ultraviolet radiation. However, ozone has two very different faces depending on its location in the atmosphere. High in the atmosphere, it is beneficial. Near the ground, it is a harmful air pollutant that can irritate the lungs and worsen respiratory diseases such as asthma and chronic obstructive pulmonary disease.

Los Angeles made the dangers of ozone visible to the nation in the 1940s and 1950s. Thick, eye-stinging smog often blanketed the city, turning an invisible chemistry problem into a public-health crisis that people could see and feel. That visible crisis helped motivate decades of rigorous air pollution control efforts in California and, later, across the United States. After the passage of the Clean Air Act and its subsequent amendments in the 1970s, the U.S. made steady progress in cleaning up surface ozone. Regulations imposed on vehicles, power plants, and industrial sources successfully reduced emissions of nitrogen oxides and other ozone-forming chemicals.

To monitor this progress, the U.S. Environmental Protection Agency operates over 1,000 stations that measure ozone around the country. These stations cover many locations, but they are concentrated mostly in urban areas. They do not measure ground-level ozone everywhere at the neighborhood scale, leaving significant gaps in the data. Our research team was able to fill in these gaps by combining those monitoring station measurements with satellite-derived information about air pollution and human activity. We also incorporated weather and air quality model simulations. We then used artificial intelligence to estimate daily surface ozone levels everywhere in the contiguous United States, with data resolution at every square kilometer, over the past 22 years.

The results show that national progress in reducing surface ozone reversed around 2015 as North America began to face more severe wildfires. In many regions, ozone levels are now increasing, especially in the western United States and the Midwest. This increase occurs because smoke and gases from wildfires are becoming more common as they are transported through the air. Overall, surface ozone levels that had been falling by about 0.65 parts per billion per year from 2003 to 2015 have since increased by about 0.13 parts per billion per year. If wildfires had not been an influence, we found, the trend of falling surface ozone levels would have continued instead.

People often think of wildfires as a problem for the western United States, but smoke and gas pollutants from their emissions can travel thousands of miles, affecting communities far from the fires themselves.

The 2023 Canadian wildfires offered a vivid example of this long-range impact. In much of the Midwest, ozone reached unhealthy levels for more than a week. The impact of wildfire smoke reached as far as Georgia and New York. That year, an additional 43 million Americans lived in areas with ozone exceeding healthy standards compared to previous years because of increased wildfire emissions. This demonstrates that the source of pollution and the population affected are not always geographically aligned.

As the Earth and its atmosphere warm, wildfire seasons are becoming longer and more severe across many parts of North America. This trend is predicted to continue. In line with scientific projections, Canada experienced its most devastating wildfire seasons on record in 2023 and 2025. In January 2025, destructive fires burned more than 16,000 homes and businesses in and around Los Angeles during a time of year when such events have historically been uncommon. The shift toward more frequent and intense fires suggests that the rising ozone problem could become even greater in the future. This is a significant problem for human health.

People can reduce their exposure to ozone pollution by checking air quality forecasts and limiting outdoor activities when wildfires are sending smoke into the air. These individual actions provide temporary relief. However, protecting public health in the long run will require broader actions to reduce ground-level ozone itself. This requires addressing the root causes of both the pollution and the fires.

That includes efforts to mitigate fire risk by improving wildfire management. Strategies such as reducing brush and other dry undergrowth that can fuel fires are essential. It also requires scaling back the causes of rising global temperatures, such as the burning of fossil fuels. As temperatures rise, the ground loses moisture, creating conditions for more extreme fires. This creates a feedback loop where climate change intensifies wildfires, which in turn produce more ozone.

Protecting public health also means strengthening air quality forecasting systems to provide accurate early warnings. This allows people to take precautions when pollution levels are expected to spike. It also involves maintaining air pollution monitoring networks and investing in satellite sensors to continue measuring progress. These tools allow scientists to identify problems and fix them before they cause widespread harm. Without these comprehensive strategies, the gains made over the last half-century will continue to erode.

The reversal of ozone trends highlights the complex interplay between climate change and air quality. While regulatory successes have reduced industrial and vehicular emissions, the increasing frequency of wildfires has introduced a new variable that these regulations do not address. The data from 2003 to 2024 clearly shows that without addressing the underlying drivers of wildfire severity, such as climate change and land management practices, surface ozone levels will continue to rise. This rise poses a direct threat to public health, particularly in regions that were previously considered to have clean air. The scientific community emphasizes that the solution lies not only in tighter emissions standards for traditional sources but also in a holistic approach that integrates wildfire management with climate mitigation strategies. Only by tackling these issues together can the United States resume its trajectory toward cleaner air and better health outcomes for all citizens.