Ground level ozone is a harmful air pollutant that forms near the Earth’s surface when emissions from cars, power plants, and industrial facilities react with sunlight. Unlike the protective ozone layer high in the atmosphere, ground level ozone irritates the lungs, damages crops, and is a primary ingredient in smog. It is not released directly from any single source, which makes it one of the trickier pollutants to control.
How Ground Level Ozone Forms
Ozone at ground level is a secondary pollutant. No tailpipe or smokestack emits it directly. Instead, it forms when two types of chemicals, nitrogen oxides (NOx) and volatile organic compounds (VOCs), react together in the presence of sunlight. Nitrogen oxides come mainly from vehicle exhaust, power plants, and industrial boilers. Volatile organic compounds are released by refineries, chemical plants, gasoline vapors, solvents, and even some natural sources like trees.
When these precursor chemicals mix in warm, sunny air, they trigger a photochemical reaction that produces ozone molecules. This is why ozone levels tend to spike on hot summer afternoons and in urban areas with heavy traffic. The peak season for ozone pollution in the United States runs roughly from May through September. That said, ozone can still reach unhealthy levels during colder months under the right conditions.
Good Ozone vs. Bad Ozone
The word “ozone” describes the same molecule, three oxygen atoms bonded together, but its effects depend entirely on where it sits in the atmosphere. High above the Earth, roughly 10 to 25 miles up in the stratosphere, ozone forms a protective layer that absorbs ultraviolet radiation from the sun. That layer shields us from the type of UV light linked to skin cancers, cataracts, and damage to crops and marine life.
Ground level ozone sits in the troposphere, the layer of air we actually breathe. Down here it provides no protective benefit. Instead, it acts as a powerful oxidant that damages living tissue on contact, from human lungs to plant leaves. The informal shorthand, “good up high, bad nearby,” captures the distinction well.
Health Effects of Breathing Ozone
Ozone irritates the airways in a way that has been compared to sunburn on the lining of the lungs. It can cause the muscles surrounding your airways to tighten, trapping air in the small sacs deep in the lungs. The result is wheezing, shortness of breath, and pain when you try to take a deep breath. Even at moderate concentrations, ozone exposure can trigger coughing, a sore or scratchy throat, and difficulty breathing vigorously.
Beyond the immediate discomfort, ozone inflames and damages airway tissue, making the lungs more vulnerable to infection. For people with existing lung conditions like asthma, emphysema, or chronic bronchitis, ozone aggravates symptoms and increases the frequency of asthma attacks. Long-term exposure is linked not only to worsening asthma but is likely one of the causes of asthma developing in the first place.
Who Is Most at Risk
Children face the greatest risk because their lungs are still developing and they tend to spend more time playing outdoors when ozone is at its highest. Older adults, people with asthma, and anyone who works or exercises outside for extended periods are also especially vulnerable. Research also points to genetic factors and nutritional status: people with lower intake of vitamins C and E appear to be more susceptible to ozone’s effects.
Damage to Crops and Vegetation
Ozone doesn’t just harm people. It is one of the most damaging air pollutants for agriculture. When ozone enters a plant’s leaves through tiny pores, it disrupts the enzyme responsible for photosynthesis, essentially slowing the plant’s ability to convert sunlight into energy. Less energy means less growth and lower yields.
Current ozone concentrations across many regions of the U.S. suppress yields of sensitive crops by 5 to 15 percent. In California, the losses are steeper for certain crops: cantaloupes, grapes, cotton, oranges, onions, and beans see projected yield reductions of 12 to 31 percent. Staples like wheat, corn, rice, and alfalfa fare somewhat better, with losses in the 3 to 9 percent range, but those losses still represent enormous economic and food supply costs at scale.
Soybeans offer a telling example. At current average summertime ozone levels in central Illinois (around 60 parts per billion), chamber studies show soybean yields drop by roughly 10 percent. If concentrations rise to levels projected for mid-century, around 72 parts per billion, yields fall by 15 percent or more. These are not hypothetical numbers. They reflect the ozone concentrations farmers are already dealing with in productive agricultural regions.
How Ozone Is Regulated
The EPA classifies ozone as one of six “criteria” air pollutants and sets a national standard for it. The current limit is 0.070 parts per million (70 parts per billion) measured as an 8-hour average. Specifically, an area meets the standard if its fourth-highest daily maximum 8-hour concentration, averaged over three years, stays at or below that threshold. Areas that exceed it are designated “nonattainment” zones and must develop plans to reduce precursor emissions.
Because ozone is not emitted directly, reducing it requires cutting the nitrogen oxides and volatile organic compounds that create it. That means tighter vehicle emission standards, controls on industrial facilities, and limits on the solvents and chemicals that release VOCs. Progress has been substantial over recent decades, but millions of Americans still live in areas that exceed the standard, particularly in sun-drenched metro areas with heavy traffic.
Checking Ozone Levels in Your Area
The easiest way to know whether ozone is a concern on any given day is the Air Quality Index (AQI), a color-coded scale the EPA uses to communicate outdoor air quality. For ozone and other pollutants, an AQI of 100 corresponds roughly to the national health-based standard. Here is how the scale breaks down:
- Green (0 to 50): Air quality is good, posing little or no risk.
- Yellow (51 to 100): Acceptable for most people, though unusually sensitive individuals may notice effects.
- Orange (101 to 150): Unhealthy for sensitive groups, including children, older adults, and people with asthma.
- Red (151 to 200): Unhealthy for the general public; sensitive groups face more serious effects.
- Purple (201 to 300): Very unhealthy, with increased risk for everyone.
- Maroon (301 and higher): Hazardous, representing emergency conditions.
You can check the current AQI for your zip code at AirNow.gov or through most weather apps. On days when the AQI for ozone climbs into the orange range or above, reducing time spent outdoors during the afternoon hours, when ozone peaks, makes a meaningful difference in how much you breathe in. If you run, bike, or do physical labor outside, shifting those activities to morning hours helps, since ozone levels are typically lowest before midday.