Wildfire smoke does not have to look thick for ozone levels to be elevated. Fires release nitrogen oxides and organic compounds that can react in sunlight and favorable atmospheric conditions to form ground-level ozone, a pollutant distinct from the smoke particles themselves. Ozone can build downwind, where the visible plume is less dense, so smoke’s appearance alone cannot show a community’s ozone concentration.
How can a fire lead to ozone far from the flames?
Ozone is a secondary pollutant: it forms in the atmosphere rather than being emitted directly from a fire. The Centers for Disease Control and Prevention identifies nitrogen oxides and various organic compounds in wildfire smoke as ozone precursors. Sunlight and atmospheric conditions influence whether and where those emissions produce ozone. Smoke and its precursors can also travel far from a fire, carrying the potential for air-quality effects beyond the visibly smoky area.
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The U.S. Environmental Protection Agency (EPA) cautions that ozone concentrations may be elevated downwind of fires even where smoke is less dense. A clear-looking sky, or a plume that appears to be moving away, therefore does not establish the ground-level ozone concentration. Ozone and particulate matter such as PM2.5 are separate pollutants; one cannot be used as a direct measure of the other. CDC: Wildfires and health; EPA: Health effects attributed to wildfire smoke
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What does the evidence say about wildfire-related ozone?
Observed regional conditions during the 2023 Canadian fires
A 2024 peer-reviewed study documented surface ozone enhancements associated with Canadian wildfire smoke across the U.S. Upper Midwest in May and June 2023. Within the monitoring coverage available for the study’s historical comparison, the region’s 50th-percentile ozone concentration was higher than in any year since 1995, and its 90th percentile was the highest since 2002. These are regional comparisons for that episode, not claims that every monitoring site or every fire season set a record. Cooper et al., Geophysical Research Letters (2024)
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U.S. trend and impact estimates in a 2026 study
A June 2026 study published in Science reports that the U.S. ozone trend shifted from a decline of 0.65 parts per billion per year during 2003–2015 to an increase of 0.13 parts per billion per year during 2015–2024. The authors attribute the reversal primarily to increasing wildfire emissions.
| Study estimate | What it refers to |
|---|---|
| 318 additional premature deaths per year | The authors’ estimate of the increase after 2013 in annual premature deaths associated with fire-sourced ozone. They estimate the post-2013 average was 46% higher than the pre-2013 average. |
| 43 million people | The authors’ estimate of the number of people in the United States exposed to ozone nonattainment conditions during 2022–2024 due to wildfire emissions. |
These are study-derived estimates, not a count of individually confirmed deaths or direct ozone measurements at every person’s location. They describe population-level modeled exposure and associated impacts; they do not predict an individual’s exposure or health outcome. The direct study record is available through PubMed, and NASA summarizes the work at NASA.
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How to interpret the wider international evidence
A separate 2024 multicountry analysis of wildfire-related ozone and mortality used overlapping data from 749 locations in 43 countries or areas for 2000–2016. Those figures describe the study’s geographic coverage; they are not a mortality result and should not be read as one. 2024 multicountry analysis
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EPA says elevated ozone can reduce lung function and cause airway inflammation, chest pain, coughing, wheezing, and shortness of breath, including in otherwise healthy people. Effects may be more serious for people with asthma or other lung disease. EPA also identifies children, older adults, and people who work outdoors as groups at higher risk. In the cited course material, EPA describes the evidence for ozone’s cardiovascular effects and premature mortality as more limited.
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Wildfire smoke itself has a broader mixture of pollutants and health effects. CDC lists people with chronic conditions, pregnant people, children, and emergency responders among groups at increased risk from smoke exposure. Smoke can cause coughing, breathing difficulty, wheezing, asthma attacks, eye or throat irritation, headache, fatigue, chest pain, and a fast heartbeat. Those symptoms can occur with smoke exposure, but they do not establish that ozone specifically caused them. CDC: People at increased risk from wildfire smoke
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why not treat smoke and ozone as the same health risk?
PM2.5 refers to fine particles in smoke; ozone is a gas formed through atmospheric chemistry. EPA says the evidence for short-term PM2.5 exposure includes respiratory and cardiovascular illness and premature mortality, with smoke-focused studies showing particularly consistent respiratory effects. Those findings should not be attributed automatically to ozone. Ozone may rise during some smoke episodes, but an association between smoke and a health outcome is not, by itself, an ozone-specific effect.
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For example, CDC found asthma-associated emergency department visits were 17% higher than expected during 19 wildfire-smoke days in its U.S. surveillance period from April 30 through August 4, 2023. That comparison concerns smoke days and asthma visits; it is not an estimate of ozone’s contribution. CDC MMWR (2023)
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What should people do when smoke is present but ozone is hard to see?
- Check current local air-quality information rather than judging exposure from how dark or dense the smoke looks. Smoke visibility does not establish the ozone concentration.
- Pay attention to guidance for both ozone and particle pollution: they are different pollutants, and a reading for one does not describe the other.
- If you have asthma or another lung condition, or are in another higher-risk group, follow your clinician’s or local public-health authority’s advice during smoke events. Smoke symptoms alone cannot identify ozone as the cause.
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