Ozone
Ozone is a reactive form of oxygen that protects life high in the stratosphere but can harm lungs and vegetation when it forms near the ground.
What ozone is
Ozone is a gas made of three oxygen atoms. It is much more reactive than the oxygen people breathe, which is made of two oxygen atoms. That reactivity gives ozone two very different public meanings: it is protective high in the atmosphere, but harmful when people breathe elevated amounts near the ground.
Ozone in the stratosphere
Most atmospheric ozone is in the stratosphere, above the weather layer where people live. There it absorbs much of the Sun's harmful ultraviolet radiation before it reaches the surface. This ozone layer is why ozone is often described as beneficial when it is high overhead.
Ground-level ozone
Near the ground, ozone is a pollutant and a major ingredient in photochemical smog. It is not usually emitted directly. Instead, it forms when nitrogen oxides and volatile organic compounds react in sunlight. Vehicle exhaust, industrial sources, power plants, solvents, fires, and some natural sources can supply the ingredients.
Weather and timing
Ground-level ozone often becomes a larger problem on hot, sunny, relatively stagnant days. Sunlight drives the chemistry, and weak winds or temperature inversions can allow pollution to build up. Ozone can also travel downwind, so rural and suburban areas may experience high ozone even when major emissions are elsewhere.
Health effects
Breathing ozone can irritate airways, reduce lung function, trigger coughing or chest discomfort, and worsen asthma or other lung disease. Children, older adults, people with respiratory conditions, outdoor workers, and people exercising outside are often more vulnerable because they may breathe more air or have more sensitive lungs.
Plants and ecosystems
Ozone near the ground can also damage vegetation. It can enter leaves through small openings used for gas exchange, interfere with photosynthesis, injure leaf tissue, and reduce plant growth. Crops, forests, and natural ecosystems can all be affected when exposure is frequent or high.
Ozone, AQI, and action
Air quality agencies monitor and forecast ozone, then translate health concern through the Air Quality Index. On higher-ozone days, people can reduce exposure by moving long or intense outdoor activity to the morning, taking more breaks, choosing indoor exercise, and following local health guidance for sensitive groups.
Why it matters
Ozone matters because location changes the story. The same molecule helps shield Earth from ultraviolet radiation high above us and contributes to unhealthy smog where people live. Understanding that contrast makes air quality alerts, ozone-layer protection, and pollution-control policy easier to interpret.