Ozone is a gas made of three oxygen atoms (O₃) instead of the two atoms in the oxygen we breathe. It is highly reactive, meaning it readily interacts with other molecules, which makes it both essential in the upper atmosphere and harmful at ground level. That dual nature is what makes ozone one of the most important and misunderstood substances in environmental science.
How Ozone Forms
Ozone forms through two very different processes depending on where it is in the atmosphere. High up in the stratosphere, about 10 to 30 miles above Earth’s surface, intense solar radiation splits regular oxygen molecules (O₂) apart. The freed oxygen atoms then collide with other O₂ molecules, creating O₃. This is a natural, ongoing cycle driven by the sun’s shortest-wavelength ultraviolet rays.
At ground level, ozone forms as a byproduct of pollution. Volatile organic compounds (released by vehicles, industrial solvents, and gasoline vapors) react with nitrogen oxides (from car exhaust and power plants) in the presence of sunlight. Because sunlight is the critical ingredient, ground-level ozone concentrations typically peak in mid-afternoon during warm, sunny days. This is why smog tends to be worst in summer and in sun-drenched regions, though it can form year-round in some southern and mountain areas.
The Ozone Layer and UV Protection
The stratospheric ozone layer acts as Earth’s sunscreen. It absorbs most of the sun’s UV-B radiation, the wavelengths between 280 and 315 nanometers that cause sunburn, skin cancer, and cataracts. Without this layer, life on land would be exposed to far more damaging radiation.
Starting in the 1970s, synthetic chemicals called chlorofluorocarbons (CFCs), used in refrigerators, aerosol cans, and foam insulation, began destroying stratospheric ozone faster than it could replenish. The result was a thinning ozone layer, most dramatically over Antarctica, where a seasonal “ozone hole” forms each year. The 1987 Montreal Protocol banned these chemicals, and the layer has been slowly recovering since. The 2025 Antarctic ozone hole was the fifth smallest recorded since 1992. However, the banned chemicals still linger in old insulation, landfills, and the atmosphere, so scientists project the Antarctic ozone hole won’t fully recover until around the late 2060s.
Ground-Level Ozone and Smog
While stratospheric ozone protects us, ground-level ozone is a pollutant. It is the main ingredient in smog, the hazy, irritating air that hangs over cities on hot days. You can’t see ozone itself (it’s a colorless to faintly blue gas), but it contributes directly to the visible haze and reduced air quality that affects millions of people.
The U.S. EPA sets a national standard for ground-level ozone at 70 parts per billion, measured as an 8-hour average. To put that in perspective, 70 ppb is an extraordinarily small concentration, which speaks to how potent ozone is even in trace amounts. You can check your local ozone levels through the Air Quality Index (AQI), which color-codes conditions: green (0 to 50) means air quality is satisfactory, yellow (51 to 100) means acceptable but potentially risky for sensitive individuals, and red (151 to 200) means even healthy people may start noticing effects.
How Ozone Affects Your Health
Breathing ozone irritates and inflames the airways. Even short-term exposure at elevated concentrations can trigger coughing, chest tightness, and pain when taking a deep breath. This happens because ozone causes oxidative damage to the lining of the respiratory tract, essentially a chemical burn on lung tissue. The body responds with inflammation, which narrows the airways and reduces lung function.
People with asthma, chronic bronchitis, or other lung conditions are especially vulnerable, but healthy adults exercising outdoors on high-ozone days can also experience symptoms. Ozone activates sensory nerves in the airways, which reduces how deeply you can inhale and makes breathing feel uncomfortable or painful.
Long-term exposure carries more serious consequences. Large population studies have linked years of ozone exposure to accelerated decline in lung function and progression of emphysema, a condition where the tiny air sacs in the lungs break down irreversibly. Chronic exposure has also been associated with increased risk of death from both cardiovascular and respiratory disease. One particularly deceptive quality of ozone: your nose adjusts to the smell quickly, so you can’t rely on odor to tell you whether levels are still high. And the most serious lung effects, like fluid buildup in the lungs, can be delayed by hours after exposure, meaning you may not feel the worst of it until well after you’ve gone indoors.
Ozone in Water Treatment
Ozone’s aggressive reactivity, so harmful in the air we breathe, makes it a powerful disinfectant when applied deliberately to water. Municipal water treatment plants use ozone to kill bacteria, viruses, and parasites. It works by breaking apart the cell walls of microorganisms and damaging their genetic material, effectively destroying pathogens on contact. Ozone is more effective than chlorine at inactivating viruses and bacteria, and it works fast, requiring only about 10 to 30 minutes of contact time.
Because ozone is unstable and quickly breaks back down into regular oxygen, it has to be generated on-site at treatment plants rather than shipped in. This instability is actually an advantage: it means ozone doesn’t leave lingering chemical residues in treated water the way chlorine can. That said, at low doses ozone may not fully inactivate certain tougher organisms like bacterial spores and parasitic cysts, so treatment plants carefully calibrate their systems. Ozone disinfection is generally used at medium to large facilities after the water has already gone through earlier stages of treatment.
Ozone and Medical Claims
Some alternative health practitioners promote “ozone therapy” for conditions ranging from chronic pain to infections. Federal regulators have taken a clear position against this. The FDA classifies ozone as a toxic gas with no known useful medical application, whether as a primary treatment, a supplement to other therapy, or a preventive measure. The agency’s reasoning is straightforward: for ozone to be effective as a germicide, it must be present at concentrations far higher than the human body can safely tolerate.
Ozone’s primary effect on the body is irritation of mucous membranes, with potential impacts on the central nervous system, heart, and vision at higher exposures. Inhaling enough ozone can cause fluid to accumulate in the lungs, a condition called pulmonary edema. The FDA specifically prohibits ozone-generating devices from being marketed for any medical condition lacking proof of safety and effectiveness, and bans their use in hospitals or facilities occupied by sick or vulnerable people.
What Ozone Smells Like
Ozone has a distinctive sharp, clean smell that many people recognize from thunderstorms or the area around electrical equipment. The name itself comes from the Greek word “ozein,” meaning “to smell.” You may notice it after a lightning strike, when electrical discharge splits oxygen molecules in the air and some recombine as O₃. But this odor is unreliable as a safety indicator. Your sense of smell adapts to ozone rapidly, so you may stop noticing it even as concentrations remain high or continue to rise.