The Ozone Rebound: How a Warming Climate Is Undoing America's Clean-Air Progress

Research Desk — This is a study analysis built from a single published report: the American Lung Association's State of the Air 2026, the 27th edition of its annual assessment, based on quality-assured monitoring data for 2022, 2023 and 2024. It goes deep on one finding: the rebound of ground-level ozone and the warming climate driving it.

A hard-won trend line has bent the wrong way

There are a handful of environmental numbers in the United States that have, for decades, pointed reliably in the right direction. The concentration of ground-level ozone in the air most Americans breathe was one of them. Since the Clean Air Act was passed in 1970, the American Lung Association notes, the combined emissions of the six pollutants the U.S. Environmental Protection Agency regulates fell by 78 percent through 2020 — one of the more consequential public-health achievements in the country's history, accomplished quietly, county by county, monitor by monitor.

The Lung Association's State of the Air 2026, the 27th edition of a report first published in 2000, is the moment that trend line visibly bends. Drawing on the three most recent years of publicly available, quality-assured monitoring data — 2022, 2023 and 2024 — the analysis finds that 129.1 million people, some 38 percent of the U.S. population, were exposed to levels of ozone that put their health at risk. That is not a marginal uptick. It is the highest number the report has recorded in six years, and an increase of 3.9 million people over last year's edition alone.

These three goals map the causal chain in the report. SDG 13 (Climate Action) is the driver — the report attributes the ozone rebound to heat, drought and cross-border wildfire smoke. That worsening ozone concentrates in dense metro areas, the concern of SDG 11 (Sustainable Cities & Communities), where 77 metros contain a failing county and Los Angeles has ranked worst for 26 of 27 years. The ultimate stake is SDG 3 (Good Health & Well-being) — the asthma attacks, impaired lung growth and premature deaths that 129.1 million exposed Americans now face.

Ozone is measured against a national health standard of 70 parts per billion averaged over eight hours, the threshold the EPA has set under the Clean Air Act as the limit above which the air is considered unhealthy. Counties that repeatedly breach it earn a failing grade in the Lung Association's severity-weighted scoring system. In this year's report, failing ozone grades were spread across 219 counties in 36 states and the District of Columbia — the largest count of failing ozone counties since the 2016 report. Seventy-seven metropolitan areas contain at least one county that flunked. The geography of American smog, in other words, is not shrinking. It is widening.

What ozone actually does, and why the rebound matters

It is worth pausing on what this pollutant is, because ground-level ozone is uniquely a product of the conditions the report is worried about. Unlike soot or industrial particulate matter, ozone is not emitted directly from a tailpipe or a smokestack. It forms in the atmosphere when nitrogen oxides and volatile organic compounds — precursor emissions from vehicles, power plants, refineries and, increasingly, wildfire smoke — react in the presence of sunlight and heat. Ozone is, in effect, cooked. The hotter and stiller the air, the more efficiently the recipe works.

That chemistry is why the pollutant is so damaging to the human respiratory system and why a rebound carries real consequences. The Lung Association catalogues ozone's effects across the full arc of a life: it inflames and irritates the airways, triggers wheezing, coughing and shortness of breath, provokes asthma attacks, worsens chronic obstructive pulmonary disease, and is linked to premature death. In children, whose lungs are still developing, it can impede lung growth and contribute to new cases of asthma. The people most exposed are frequently those least able to absorb the harm. Counting across all pollutants, the report finds that nearly 2.4 million children and almost 11.9 million adults with asthma, roughly 6.7 million people with COPD, and close to 10 million people with cardiovascular disease live in counties that earned at least one failing grade — the populations for whom a bad-air day is not an inconvenience but a medical event.

The broader backdrop makes the ozone finding harder to dismiss as a statistical wobble. Across all the pollutants it tracks, State of the Air 2026 concludes that 44 percent of Americans — 152.3 million people — live in places that received failing grades for unhealthy levels of ozone or particle pollution. Nearly half of the nation's children, 46 percent or 33.5 million people under the age of 18, live in a county that failed at least one measure. Ozone is the single largest contributor to that failing-grade population, and it is the one moving most clearly in the wrong direction.

A single-screen snapshot of the ozone story — the headline 129.1M Americans (38% of the population) breathing unhealthy ozone, up 3.9M and the highest in six years, alongside the 219 failing counties across 36 states, 77 affected metros, L.A.'s 26-of-27-year record, and the even split of 7 states worsening versus 7 improving. The green panel names the climate mechanism in the report's own words, with full source attribution to State of the Air 2026 and its 2022–2024 monitoring data.

The climate fingerprint the report names out loud

What sets this year's analysis apart from a routine annual tally is the directness with which the American Lung Association attributes the ozone rebound to a changing climate. The report does not hedge. “A changing climate is making it harder to protect this hard-fought progress on air quality and human health,” it states, pointing to “increases in high ozone days and spikes in particle pollution related to extreme heat, drought and wildfires.”

The mechanism is laid out with unusual specificity. The report identifies two climate-linked drivers behind the worsening ozone numbers. The first is smoke that crossed a border. “Extensive wildfires in Canada in 2023 generated ozone-forming pollutants (also called precursor emissions) that blew across the border,” the analysis explains — a reminder that ozone precursors do not respect jurisdictions, and that a fire season hundreds of miles away can register on a monitor in the American Midwest or Northeast. The second driver is heat itself. “In both 2023 and 2024, high temperatures and other weather conditions were ideal for ozone formation, especially in the South,” the report finds.

The chemistry underneath is precisely what a warming atmosphere delivers. “Climate change contributed to worsening ozone pollution by increasing precursor emissions (found in worsening wildfire smoke) and by creating atmospheric conditions, including higher temperatures and lower wind speeds, that allow pollutants to accumulate and ozone to form,” the Lung Association writes. Higher temperatures accelerate the reactions that produce ozone; lower wind speeds let the pollutant pool over cities instead of dispersing. Each additional degree of warming and each stagnant-air episode is, functionally, more raw material and better cooking conditions for smog. The report's blunt framing — “A changing climate is making the job of cleaning up the air more difficult” — captures the structural problem. The country can keep tightening emissions from cars and industry, and still watch ozone rise, because the atmosphere is doing more of the manufacturing on its own.

A geography that is shifting under the old assumptions

For decades the mental map of American smog had a fixed center: Southern California. That has not changed at the top of the table — Los Angeles again ranks as the city with the worst ozone pollution in the nation, a distinction it has held in 26 of the 27 years the report has been published, and its ozone worsened relative to last year. California as a whole accounts for eight of the 25 most ozone-polluted cities in the country, a legacy of geography, traffic and the sun-drenched basins where ozone forms readily.

But the more revealing story in State of the Air 2026 is at the edges of the map, where the climate signal shows up as movement. Seven states saw their ozone pollution worsen in this year's data: Arizona, Connecticut, Louisiana, New Jersey, New Mexico, Oklahoma and Texas. The Southern and Gulf-state presence on that list is consistent with the report's own observation that heat made 2023 and 2024 ideal for ozone formation, especially in the South. Seven other states improved — Colorado, Michigan, Nebraska, Nevada, Rhode Island, Utah and Wyoming — a reminder that local emissions controls still work and that the trend is not uniformly negative. The picture is one of a country pulling in two directions at once: conventional pollution control grinding levels down in some places while climate-amplified conditions push them up in others.

This churn matters because it undercuts a comfortable assumption baked into how the nation manages air quality — that once a region has cleaned up, it stays clean. A metro area that spent years earning a passing grade can be pushed back into failure by a single hot, stagnant summer or a season of imported wildfire smoke, without any local increase in cars or factories. Ozone, uniquely among the criteria pollutants, is now partly hostage to the weather, and the weather is trending hotter.

The blind spots in the data

One of the quieter but more important contributions of State of the Air 2026 is what it says about the limits of the monitoring network itself. The 129.1 million figure for unhealthy ozone exposure is, if anything, likely an undercount. The report can only grade counties that have enough quality-assured monitoring data — 885 counties, home to more than 267 million people. But the United States has 2,295 counties or county-equivalent jurisdictions, about 73.5 million people's worth, where neither ozone nor particle pollution is being monitored at all. Those places are not necessarily clean; they are simply unmeasured. As the climate drivers the report identifies — heat, drought, wildfire smoke — spread ozone into rural and previously unaffected areas, a monitoring network built around historically polluted urban basins may be looking in the wrong places.

The report also flags an emerging pressure on the system that could complicate the ozone outlook further: the electricity appetite of data centers. U.S. data centers have consumed roughly 4.4 percent of the nation's total electricity, a figure the Lung Association notes could climb to as much as 12 percent of total U.S. electricity demand within the next decade. Where that power comes from — and the precursor emissions associated with generating it — will feed directly into the ozone equation. The report cites modeling suggesting the public-health harms of data-center-related pollution could reach 167 million to 266 million dollars in California alone. It is a signal that the sources of ozone precursors are not static, and that new demand can offset old gains.

There is also an equity dimension the report is careful not to bury, even in a year focused on climate drivers. People of color make up 42.1 percent of the U.S. population but represent 54.2 percent of the people living in a county with at least one failing grade. The burden of dirtier air, including the ozone the climate is now amplifying, does not fall evenly, and the communities carrying more of it are frequently those already living with the asthma, diabetes and heart disease that make ozone most dangerous.

A methodology built to reflect lived exposure

Part of what gives the ozone finding its weight is the Lung Association's deliberate choice about how to count bad-air days. The report uses monitoring data drawn from the EPA's own Air Quality System, characterized by Dr. Allen S. Lefohn of A.S.L. & Associates, with the health-impacts section reviewed by John Balmes, M.D. Crucially, its grades include every day a monitor recorded unhealthy air — even days the EPA later designates “exceptional events,” a category that frequently covers wildfire smoke. The Lung Association's position is explicit: “if an air pollution exceedance is ruled an exceptional event and is omitted for the purposes of regulatory compliance, it remains a threat to people's health.” In a year when so much of the ozone rebound is being driven by wildfire smoke, that methodological choice is the difference between a report that measures regulatory compliance and one that measures what people's lungs actually experience. It is why the 129.1 million figure reflects the air Americans breathed, not the air that counts on a compliance ledger.

Why It Matters

The ozone rebound in State of the Air 2026 lands at a moment when the machinery built to fight it is being loosened rather than tightened. The report itself calls attention to the many ways the current EPA is weakening, delaying and eliminating clean-air protections, and it urges the agency to value the health of America's children and to follow the science and the law. That is the accountability question this data forces. The Clean Air Act delivered a 78 percent cut in key pollutants over half a century precisely because health-based standards were set, defended and enforced — because, in the Lung Association's phrasing, our health should count. A warming climate is now adding ozone back into the air faster than local controls can remove it, which means the standards matter more than ever, not less. Rolling back protections while the atmosphere itself becomes a pollution source is a decision to accept more sick children, more asthma attacks and more premature deaths as the price of deregulation. The report's demand is straightforward: defend the Clean Air Act's progress, restore the EPA to its public-health mission, and hold state and local governments to their share of the work. Clean air is not a settled achievement to be spent down. It is a standard that has to be actively held — and this year's numbers are the cost of letting it slip.

This Research Desk analysis is based on figures published in the American Lung Association's State of the Air 2026. All statistics are attributed to that report and its underlying 2022-2024 monitoring data.