Ozone
Ozone is a gas that helps you high in the sky and harms you at ground level. Miles overhead, a layer of it shields the planet from ultraviolet radiation.
Down where you breathe, it becomes a pollutant that the EPA counts among the most widespread air quality problems in the country. More than 100 million Americans live in counties where ozone levels exceed the national health standard, according to the American Lung Association's 2024 State of the Air report.
Ozone is the pollution you cannot see forming. It builds through the afternoon while the sky stays blue, and it reaches into your lungs on the kind of warm, sunny day most people head outside.
What is ground level ozone?
A gas that forms in the air when sunlight reacts with pollution from cars, power plants, and industry. High in the atmosphere ozone protects the planet, but at ground level it is a pollutant that irritates the lungs.
How is ozone different from other air pollution?
Nothing releases ozone directly. It forms through a chemical reaction between nitrogen oxides and volatile organic compounds in sunlight, which is why levels rise on hot, sunny afternoons and fall after dark.
When are ozone levels highest?
On warm, sunny days, usually peaking in the late afternoon. Summer is the high season, and cities with heavy traffic and strong sunshine tend to see the worst counts.
Can ozone get inside my home?
Yes. The EPA reports indoor ozone typically runs between 10 and 50% of the outdoor level, drifting in through windows and doors. Some air cleaners also produce ozone indoors, which adds to the problem.
What does ozone do to my health?
It irritates the airways, causing coughing, throat soreness, chest tightness, and shortness of breath within hours. For people with asthma it can trigger attacks, and long term exposure can permanently reduce lung function.
Who is most at risk from ozone?
Children, older adults, outdoor workers, and people with asthma or other lung conditions. Anyone exercising outdoors during peak hours also breathes in more, since deeper breathing pulls ozone further into the lungs.
Do ozone generating air purifiers help clean the air?
No. The California Air Resources Board warns against them because they release the same gas you want to keep out. For indoor air, a HEPA filter handles particles without adding ozone to the room.
How do I know when ozone is high, and what should I do?
Check the Air Quality Index at AirNow.gov, which forecasts ozone daily. When levels rise, exercise in the morning, keep windows closed in the afternoon, and delay driving, refueling, and mowing until evening.
What ground level ozone is
Ground level ozone is not something a factory or a tailpipe releases directly. It forms in the air through a chemical reaction. Two ingredients start it: nitrogen oxides from cars, trucks, and power plants, and volatile organic compounds from fuels, paints, and industrial solvents. Sunlight cooks those two together and produces ozone.
The reaction needs heat and light, so ozone follows the sun. Levels climb through the afternoon on hot days, peak in the late afternoon, and fall off after dark. Summer is the high season, and cities with heavy traffic and strong sunshine, like Los Angeles and Houston, carry some of the highest counts in the nation.
How outdoor and indoor ozone connect
Ozone starts outside, and it drifts indoors through open windows, doorways, and the gaps in every building. The EPA reports that indoor ozone levels typically run between 10 and 50% of the outdoor level, so a high ozone day outside raises the count in your living room too.
Some homes add their own ozone. Certain air cleaners and "ionizing" purifiers generate ozone on purpose or as a byproduct, and the California Air Resources Board warns against these devices because they release the same gas you are trying to keep out. Once ozone is inside, it reacts with chemicals from cleaning products and air fresheners to form more fine particles and irritants, which means a device sold to clean the air can leave it worse than before.
What ozone does to the body
Ozone irritates the airways the way sunburn irritates skin. Breathing it can cause coughing, a sore or scratchy throat, chest tightness, and shortness of breath, the EPA reports. The effects show up within hours of exposure and hit harder during exercise, when you pull more air deep into your lungs.
For people with asthma, ozone is a known trigger. It inflames the airways, raises the odds of an attack, and drives up emergency room visits on high ozone days. Long term exposure does deeper damage. A study published in JAMA linked chronic ozone exposure to a higher risk of dying from respiratory disease, and the EPA notes that repeated exposure can permanently reduce lung function. Children, older adults, outdoor workers, and anyone with a lung condition carry the heaviest load, because their airways react fastest and recover slowest.
Why it matters more as the climate warms
Ozone is tied to heat, and heat is rising. Warmer temperatures speed up the chemical reaction that forms ozone, so the same pollution produces more of it on a hot day than a mild one. The EPA projects that climate change will increase ozone levels across large parts of the country unless emissions of the ingredients that form it keep falling.
Longer, hotter summers stretch the ozone season and widen the number of days a household has to think about it. What used to be a handful of bad air days each summer becomes a longer stretch, which turns checking the forecast into a regular habit rather than an occasional one.
What a household can do
Check the air before you plan your day. The Air Quality Index at AirNow.gov reports local ozone levels in real time and forecasts the next day. When the number climbs into the orange range or higher, move outdoor exercise to the morning, when ozone is lowest, and keep hard activity short in the late afternoon.
Cut the pollution that forms ozone. Combine errands into one trip, avoid idling, and hold off on gassing up the car or mowing the lawn until evening on high ozone days, since gasoline vapors and small engine exhaust feed the reaction. Keep windows closed during afternoon peaks and run the air conditioner instead. Skip any air cleaner that produces ozone, including ionizers and "ozone generators," and choose a HEPA filter for particles instead. On high ozone days, plan the day around the morning hours and give your lungs a break from the afternoon air.
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