Tropospheric ozone is harmful because it is a powerful oxidizer sitting right where people breathe, plants grow, and ecosystems function. Unlike the ozone layer high in the stratosphere, which blocks ultraviolet radiation, ozone near the ground damages lung tissue, worsens heart disease, kills crops, and weakens forests. Globally, it was responsible for an estimated 423,100 deaths from chronic respiratory disease in 2019 alone.
Good Ozone vs. Bad Ozone
Ozone is the same molecule regardless of where it exists: three oxygen atoms bonded together. The difference is location. Stratospheric ozone sits roughly 6 to 31 miles above Earth’s surface, where it absorbs harmful UV radiation before it reaches the ground. That is the protective “ozone layer.” Tropospheric ozone exists in the lowest layer of the atmosphere, from the surface up to about 6 to 12 miles high depending on latitude. This is the air you actually inhale. Down here, ozone’s reactive chemistry has no protective role. It simply attacks biological tissue and organic material on contact.
How Ground-Level Ozone Forms
Ground-level ozone is not emitted directly. It forms when two families of pollutants, nitrogen oxides and volatile organic compounds, react in the presence of sunlight. Nitrogen oxides come primarily from vehicle exhaust and power plants. Volatile organic compounds are carbon-containing gases released by gasoline fumes, industrial solvents, and even some natural sources like trees. Carbon monoxide also plays a role in some of the key reactions.
The process is photochemical, meaning sunlight is required. No sunlight, no ozone. This is why ozone concentrations peak on hot, sunny afternoons and why ozone pollution is worst in summer months around urbanized areas. The specific ratio of nitrogen oxides to volatile organic compounds in the air determines how efficiently ozone is produced, which is why some cities have far worse ozone problems than others despite similar traffic levels.
One counterintuitive detail: reducing nitrogen oxide emissions has actually increased nighttime ozone in many parts of the United States. Nitrogen oxides normally break down ozone after dark through a process called titration. With fewer nitrogen oxides available at night, more ozone persists. Daytime summer peaks, however, have declined with cleaner emissions.
What Ozone Does to Your Lungs
When you breathe in ozone, it reacts almost immediately with the thin layer of fluid lining your airways. It breaks down the fats in that lining through a process called lipid peroxidation, generating a flood of reactive oxygen species, essentially aggressive molecules that damage cells on contact. Your lung tissue responds to this chemical injury the way it would to any threat: inflammation.
The immune system sends waves of white blood cells into the airways. Those cells release even more reactive oxygen species as they work, creating a feedback loop where the inflammatory response itself causes additional tissue damage. This cascade triggers the release of multiple inflammatory signaling molecules that spread the response deeper into the respiratory tissue. Over time, repeated exposure doesn’t just irritate your lungs temporarily. It degrades a key protective protein in the airway lining and can make the tissue less responsive to standard anti-inflammatory treatments like corticosteroids.
In practical terms, this means reduced lung function, chest tightness, coughing, and painful breathing, especially during physical activity when you’re pulling more air into your lungs. For healthy adults, these effects are typically reversible after short exposures. For people with existing lung conditions, the consequences are more serious.
Why Some People Are More Vulnerable
People with asthma face a compounded problem. Ozone doesn’t just trigger the same inflammation it causes in healthy lungs. It intensifies the underlying disease. Studies show that people with asthma exposed to ozone develop a more intense inflammatory response, with larger influxes of immune cells and greater changes in inflammatory markers compared to people without asthma. Ozone also increases airway reactivity, meaning the lungs become more sensitive to other triggers. In people with allergic asthma, ozone exposure makes the airways more responsive to common allergens like house dust mites.
Children are disproportionately affected for two overlapping reasons. Asthma prevalence among children is particularly high, and children spend more time exercising and playing outdoors, which increases both the volume of air they breathe and the amount of ozone reaching deep into their lungs. Adults who work outdoors, runners, cyclists, and anyone exercising during afternoon hours face similarly elevated exposure.
People with other chronic respiratory diseases like COPD have less lung capacity to spare. They cannot tolerate the reduction in lung function or the increase in symptoms that ozone causes, even at levels a healthy person might barely notice.
Damage Beyond the Lungs
Ozone’s effects extend past the respiratory system. EPA-funded research published in 2019 found a strong correlation between long-term ozone exposure and physical changes in the carotid arteries, the major blood vessels that supply the brain. Specifically, a 3 parts per billion increase in average annual ozone concentration was associated with a 5.6 micrometer increase in carotid artery wall thickness, along with greater plaque buildup. Thicker carotid walls and more plaque mean higher stroke risk. Interestingly, the same study did not find similar changes in the coronary arteries feeding the heart, suggesting ozone’s cardiovascular effects may be selective rather than uniform.
Crop Losses Worth Billions
Ozone enters plants through the same tiny pores, called stomata, that they use to absorb carbon dioxide. Once inside, it damages cell membranes and disrupts photosynthesis. The result is slower growth and lower yields. A 40-year analysis of U.S. agriculture from 1981 to 2021 found that ambient ozone reduced maize yields by 3.5% and soybean yields by 6.1%, translating to roughly $2.6 billion in annual economic losses.
Globally, the picture is worse. A risk assessment covering 2010 to 2012 estimated annual yield reductions of 4.4% for rice, 6.1% for maize, 7.1% for wheat, and 12.4% for soybeans, amounting to 227 million tons of lost crop production worldwide. Soybeans are particularly sensitive to ozone, losing more than double the yield percentage of rice. These are not projections for a distant future. They reflect damage already occurring at current pollution levels.
Forests Lose Carbon-Absorbing Power
Ozone also weakens the ability of forests to pull carbon dioxide out of the atmosphere, which makes it a quiet accelerant of climate change. A study of managed European forests estimated that if ozone levels dropped to pre-industrial conditions, forests would grow 9% faster in terms of stem volume. But the impact on net carbon storage is even larger: forests would accumulate 28% more standing wood biomass each year. The reason for the gap is that ozone doesn’t just slow growth; it also makes trees more susceptible to other stresses, so more of what grows is lost.
In carbon terms, European forests currently sequester about 343 million metric tons of CO₂ equivalent per year. Without ozone pollution, that figure would rise to 449 million metric tons, a 31% increase. That difference, roughly 106 million metric tons of CO₂ equivalent annually, represents a significant chunk of carbon that forests could be absorbing but aren’t.
Current Air Quality Standards
The U.S. National Ambient Air Quality Standard for ozone is 70 parts per billion, measured as the annual fourth-highest daily maximum 8-hour average concentration, averaged across three consecutive years. This standard was set in 2015 and retained without revision in 2020. Both the primary standard (protecting public health) and the secondary standard (protecting public welfare, including crops and ecosystems) are set at the same level.
Many metropolitan areas still exceed this threshold on summer afternoons. Because ozone formation depends on sunlight and heat, climate change is expected to make compliance harder over time, even as precursor emissions decline. The chemistry favors ozone production on exactly the kind of hot, stagnant days that are becoming more frequent.