Ozone pollution is ground-level ozone, a harmful gas that forms when pollutants from cars, power plants, and industrial facilities react with sunlight. It’s the main ingredient in smog and one of the most widespread air quality problems in the world. Unlike the ozone layer high in the atmosphere that protects us from ultraviolet radiation, ozone at ground level damages lungs, harms crops, and worsens on hot, sunny days.
How Ground-Level Ozone Forms
Ozone isn’t emitted directly from a tailpipe or smokestack. It’s a secondary pollutant, meaning it forms in the atmosphere when two types of precursor chemicals, nitrogen oxides and volatile organic compounds, cook together under sunlight. Nitrogen oxides come primarily from burning fuel: car engines, power plants, and industrial boilers are the biggest contributors. Volatile organic compounds come from gasoline vapors, chemical solvents, paints, and even natural sources like trees and soil.
When sunlight hits these chemicals on a warm day, a chain of reactions produces ozone molecules at breathing level. This is why ozone pollution peaks in the afternoon during summer months. Studies of daily ozone cycles consistently show a single peak around midday to early afternoon, driven by intense solar radiation and high temperatures. Cooler, cloudier days produce far less ozone. The seasonal pattern is equally clear: summer brings the worst ozone levels, with concentrations climbing as temperatures and sunlight hours increase.
Natural sources also play a role. Soil microorganisms produce nitrogen oxides in quantities that may rival fossil fuel contributions, and lightning generates them as well. Ozone from the stratosphere occasionally descends to ground level. But human activity, particularly vehicle exhaust and industrial emissions, is the dominant driver of the elevated concentrations that cause health problems in cities and downwind areas.
“Good” Ozone vs. “Bad” Ozone
The same molecule behaves very differently depending on where it sits in the atmosphere. Stratospheric ozone, found roughly 10 to 30 miles above Earth’s surface, forms naturally and acts as a shield against the sun’s ultraviolet radiation. Without it, rates of skin cancer and cataracts would skyrocket. This is the ozone layer environmental campaigns have long worked to protect.
Ground-level ozone is chemically identical but biologically destructive. At breathing height, it irritates and inflames airways, damages vegetation, and degrades materials like rubber and certain plastics. When people talk about ozone pollution, smog, or ozone alerts on the news, they mean this lower-atmosphere version.
What Ozone Does to Your Lungs
Ozone is a powerful oxidizer. When you breathe it in, it triggers inflammation throughout the bronchial tubes and makes airways hyper-responsive, meaning they constrict more easily in response to other irritants. This combination of inflammation and tightening leads to measurable drops in lung function. You may feel it as chest tightness, coughing, shortness of breath, or pain when taking a deep breath. That pain comes from sensory nerves in the respiratory tract activating in response to the chemical damage.
Short-term exposure on a high-ozone day is enough to cause problems. Higher daily ozone concentrations are associated with increased asthma attacks, more hospital admissions, and higher daily mortality rates. For people with asthma or chronic obstructive pulmonary disease (COPD), even moderate ozone levels can trigger flare-ups that send them to the emergency room.
Long-term exposure carries its own risks. Large cohort studies have found that years of ozone exposure accelerates the decline of lung function and contributes to the progression of emphysema. One study using CT scans found that for every 10 parts per billion increase in long-term ozone exposure, emphysema severity increased measurably, both at baseline and over the follow-up period. Heavy smokers with or without COPD showed worsened lung function, more severe emphysema, and more frequent respiratory flare-ups with greater long-term ozone exposure.
Who Is Most at Risk
Ozone doesn’t affect everyone equally. Children are particularly vulnerable because they spend more time outdoors, breathe faster relative to their body size, and have developing lungs that may be more susceptible to repeated cycles of damage and repair. Animal studies and the pattern of injury ozone causes suggest that avoiding repeated exposures in young children is especially important.
Older adults face the highest absolute risk of ozone-related death. While the relative increase in mortality risk is only slightly higher for people over 65 compared to younger adults, their higher baseline death rates mean ozone pushes far more older people over the threshold. Anyone with underlying respiratory disease, particularly asthma or COPD, is at greater risk of serious respiratory problems on high-ozone days.
There’s also a body-size connection that surprises many people. Young adults (teens through their thirties) and those with a higher BMI show greater lung function responses to ozone than older or leaner individuals. The reasons aren’t fully understood, but it may relate to differences in breathing patterns and the total dose of ozone delivered deep into the lungs.
Damage to Crops and Ecosystems
Ozone doesn’t just harm people. It’s one of the most damaging air pollutants for agriculture. The gas enters plants through the tiny pores on their leaves, where it disrupts photosynthesis and accelerates aging. Global estimates of present-day crop losses from ozone pollution run at about 7.2% for wheat, 6.7% for soybeans, 3.6% for maize, and 2.6% for rice. Those percentages represent millions of tons of lost food production each year.
Sensitive crop varieties fare much worse. Modeling with high-sensitivity parameters puts soybean losses as high as 26 to 63% and maize losses between 9.5 and 41%, depending on the variety. Forests and wild vegetation are also affected: ozone weakens trees, reduces growth rates, and makes ecosystems more vulnerable to drought and disease.
Air Quality Standards and Guidelines
The U.S. EPA sets the national standard for ozone at 0.070 parts per million (70 parts per billion), measured as the fourth-highest daily maximum 8-hour concentration averaged over three consecutive years. This standard, established in 2015, applies to both the primary standard (protecting health) and the secondary standard (protecting the environment). Many counties across the United States still fail to meet it.
The World Health Organization sets a tighter target. Its 2021 air quality guidelines recommend that peak-season 8-hour ozone concentrations stay below 60 micrograms per cubic meter, roughly equivalent to 30 parts per billion. Very few urban areas worldwide meet that benchmark. The gap between the WHO guideline and the U.S. standard reflects the tension between what’s achievable with current emissions and what the health evidence suggests is safe.
Climate Change and Worsening Ozone
Rising temperatures are expected to make ozone pollution worse. Ozone formation depends heavily on weather: warm temperatures, light winds, cloudless skies, and stagnant high-pressure systems all boost production. These are exactly the conditions that climate change is making more frequent, particularly in regions already prone to heat waves.
Modeling for the southeastern United States, one of the areas projected to see the largest increases in extreme heat, estimated an average ozone increase of 0.43 parts per billion during the 2040s compared to 2000 from climate change alone, before accounting for any changes in emissions. That may sound small, but ozone health effects don’t have a clear safe threshold, and even modest increases translate into additional hospital admissions and deaths across large populations. Hotter summers with more stagnant air will likely mean more frequent ozone alerts, longer ozone seasons, and greater exposure for everyone who spends time outdoors.
How to Reduce Your Exposure
On days when your local air quality index shows elevated ozone, the simplest protective step is to shift outdoor exercise to the morning. Ozone concentrations are typically lowest in the early morning hours before sunlight has had time to drive the chemical reactions that produce it. By mid-afternoon, levels peak.
If you’re in a group with higher risk (young children, older adults, anyone with asthma or COPD), limiting prolonged outdoor exertion on high-ozone afternoons makes a meaningful difference. Air quality forecasts are available from most weather apps and from the EPA’s AirNow system, which color-codes days from green (good) to purple (very unhealthy). Staying indoors with windows closed reduces exposure substantially, since ozone breaks down on indoor surfaces and concentrations inside are typically much lower than outside.
On a broader scale, ozone pollution drops when its precursors drop. Regulations targeting vehicle emissions and industrial sources have significantly reduced ozone levels in many U.S. cities over the past several decades, even as population and vehicle miles have grown. Electrification of transportation and cleaner industrial processes continue to push precursor emissions downward, though rising temperatures work against that progress.