Ozone is a molecule made of three oxygen atoms bonded together. Unlike the ordinary oxygen you breathe, which contains two atoms, ozone’s extra atom makes it highly reactive, giving it a distinctive sharp smell and the ability to both protect life on Earth and harm it, depending on where it exists in the atmosphere.
The Molecule Itself
Ozone’s chemical formula is O₃. The three oxygen atoms form a bent, angled shape rather than a straight line. The pure gas has a pale blue color and a pungent, acrid smell often described as sharp or metallic. You’ve probably noticed it near electrical equipment or after a lightning storm, that clean, slightly sharp scent in the air. Human noses are remarkably sensitive to it: most people can detect ozone at concentrations as low as 7 parts per billion, and recognize the smell clearly at around 15 to 20 ppb.
What makes ozone chemically interesting is that extra oxygen atom. It doesn’t hold on tightly, which means ozone readily gives it up to other molecules. This reactivity is what makes ozone a powerful oxidizer, useful for disinfection but damaging to living tissue.
The Ozone Layer: Earth’s UV Shield
About 10 to 30 miles above Earth’s surface, in a region of the atmosphere called the stratosphere, ozone exists in a thin but critical layer. If you compressed all the ozone overhead into a single sheet at sea level, it would be roughly 3 millimeters thick, about the height of two pennies stacked together. Scientists measure this using Dobson Units, and the global average is around 300 DU.
That thin layer does enormous work. It completely absorbs the most dangerous type of ultraviolet radiation (UVC) and blocks most of the next most harmful type (UVB), the wavelengths responsible for sunburn, skin cancer, and cataracts. It does not filter UVA, the longer-wavelength UV light that still reaches the surface and contributes to skin aging. Without the ozone layer, UVB and UVC radiation would make the Earth’s surface far more hostile to life.
The Ozone Hole and Recovery
Starting in the 1980s, scientists discovered that manufactured chemicals, primarily chlorofluorocarbons (CFCs) used in refrigerants and aerosol sprays, were breaking apart ozone molecules in the stratosphere. The damage was most severe over Antarctica, where a seasonal “hole” in the ozone layer forms each spring. In 2025, that hole reached a peak size of about 8.8 million square miles, ranking as the fifth smallest since 1992, a sign that the damage is slowly healing.
The 1987 Montreal Protocol, an international agreement to phase out ozone-depleting chemicals, is widely considered one of the most successful environmental treaties ever enacted. Under current policies, the ozone layer is expected to recover to its pre-1980 levels by around 2040 for most of the world, by 2045 over the Arctic, and by roughly 2066 over the Antarctic, where the damage was deepest.
Ground-Level Ozone: A Different Story
Ozone high in the stratosphere protects life. Ozone at ground level is a pollutant. It forms when nitrogen oxides (from vehicle exhaust, power plants, and industrial emissions) and volatile organic compounds (from gasoline vapors, solvents, and even some natural sources) react in the presence of sunlight. Specifically, sunlight breaks apart nitrogen dioxide, releasing a single oxygen atom that then combines with an ordinary oxygen molecule to form ozone. Volatile organic compounds fuel this cycle by converting nitric oxide back into nitrogen dioxide without consuming ozone in the process, so the production keeps going.
This is why ozone pollution peaks on hot, sunny afternoons in urban areas and can drift downwind into suburbs and rural communities. The U.S. EPA sets the safe limit for ground-level ozone at 70 parts per billion, averaged over eight hours. Tens of millions of Americans live in areas that regularly exceed this threshold.
How Ground-Level Ozone Affects Your Health
Because ozone is so reactive, breathing it triggers oxidative damage to the lining of your airways. The lungs respond with inflammation, causing the airways to narrow and become overly sensitive. This leads to reduced lung function, a feeling of tightness in the chest, coughing, and pain when you take a deep breath. Ozone exposure also activates sensory nerves in the respiratory tract, which limits how fully you can inhale.
People with asthma and other respiratory conditions are especially vulnerable, but even healthy adults can experience these effects during high-ozone days, particularly during outdoor exercise. Children are at greater risk because they breathe faster and spend more time outside. Over time, repeated exposure can impair the lungs’ ability to fight off infections and may worsen allergic sensitivities.
Damage to Crops and Ecosystems
Ground-level ozone doesn’t just affect people. It damages plants by entering through the tiny pores on leaves and disrupting photosynthesis. A study analyzing 31 years of U.S. agricultural data found that ozone reduced maize yields by roughly 10% and soybean yields by about 5%, costing an estimated $9 billion annually. Air quality regulations have improved the situation since 2000, but losses still hover around 7% for maize and 4% for soybeans. Forests, grasslands, and other natural vegetation are similarly affected, with ozone reducing growth and making plants more vulnerable to drought and disease.
Ozone as a Disinfectant
The same reactivity that makes ozone dangerous to breathe also makes it one of the most powerful disinfectants available. Water treatment plants use ozone to kill bacteria, viruses, and parasites. It works far more efficiently than chlorine: to achieve the same level of parasite reduction (specifically against Giardia, a common waterborne pathogen), chlorine requires 50 to 200 times the concentration-times-contact that ozone does. At modest concentrations, ozone can reduce bacterial populations by 99.999% in treated water.
Because ozone breaks down into ordinary oxygen within minutes, it leaves no chemical residue in the water. This makes it appealing for drinking water treatment, food processing, and medical sterilization, though it also means it provides no lasting disinfection in a distribution system the way chlorine does.
Two Roles, One Molecule
Ozone’s dual nature is one of the clearest examples in environmental science of how location changes everything. The same molecule that shields the planet from ultraviolet radiation at high altitude becomes a health hazard and crop destroyer at the surface. Stratospheric ozone is thinning but recovering thanks to global cooperation. Ground-level ozone remains a persistent pollution problem tied to fossil fuel combustion and industrial activity, worst on the hottest, sunniest days in the most populated areas.