Ozone exists in two distinct layers of Earth’s atmosphere. About 90% of it sits in the stratosphere, roughly 15 to 35 kilometers (10 to 22 miles) above the surface, forming what we call the ozone layer. The remaining 10% is found at ground level in the troposphere, where it forms through pollution-driven chemical reactions and acts as a harmful air pollutant.
The Stratospheric Ozone Layer
The bulk of Earth’s ozone concentrates in the stratosphere, with peak levels occurring between 30 and 35 kilometers (19 to 22 miles) above the surface. At that altitude, ozone reaches concentrations of about 8 molecules per million air molecules, and can spike as high as 15 parts per million. That sounds tiny, and it is. If you compressed all the ozone in the atmosphere into a single sheet at sea level, it would be just 3 millimeters thick, roughly the height of two pennies stacked together. Scientists measure this total column of ozone in Dobson Units, and the global average hovers around 300.
Despite being so thin, this layer is remarkably effective at filtering ultraviolet radiation. It completely absorbs UV-C, the most dangerous type, and blocks most UV-B, the wavelength responsible for sunburn and skin cancer. UV-A passes through unaffected.
Stratospheric ozone forms naturally when intense UV radiation from the sun splits oxygen molecules into individual atoms, which then bond with intact oxygen molecules to create ozone (a molecule made of three oxygen atoms instead of the usual two). This process happens most actively over the tropics, where sunlight is strongest. But large-scale air currents in the stratosphere slowly carry that ozone toward the poles, where it accumulates. This is why total ozone is actually lowest at the equator and highest at middle and high latitudes, a pattern that holds across all seasons.
Ground-Level Ozone
The ozone you encounter near the Earth’s surface is not emitted directly by any source. It forms when two types of pollutants, nitrogen oxides and volatile organic compounds, react together in the presence of sunlight. Cars, power plants, industrial facilities, and even agriculture release these precursor chemicals. On hot, sunny days with stagnant air, ground-level ozone can build up to concentrations that affect breathing, irritate the lungs, and damage crops.
The U.S. EPA sets the safe limit for ground-level ozone at 0.070 parts per million over an 8-hour period. Cities with heavy traffic and warm climates tend to have the worst ozone pollution, particularly during summer afternoons when sunlight and heat are at their peak.
Ozone Indoors
Small amounts of ozone can also be found inside buildings. Older laser printers that used a corona wire to charge the drum were significant sources of indoor ozone, though newer models that use a charged roller produce very little. Some air purifiers marketed as “ionizers” or “ozone generators” deliberately produce ozone, which is worth knowing since even low indoor concentrations can irritate airways over time.
How Ozone Varies by Location and Season
Ozone distribution across the planet is far from uniform. In the tropics, between about 20°N and 20°S latitude, total ozone stays relatively stable year-round. Outside the tropics, it fluctuates much more on daily and seasonal timescales as ozone-rich stratospheric air gets transported poleward and mixed with ozone-poor air. Natural air currents constantly blend regions of high and low ozone, creating a dynamic, shifting map.
The most dramatic variation happens over Antarctica. Each spring (September through October), chemical reactions involving chlorine from human-made compounds destroy massive amounts of stratospheric ozone, creating the infamous ozone hole. In 2024, the hole averaged nearly 20 million square kilometers, almost three times the size of the contiguous United States. Its lowest ozone reading that year hit 109 Dobson Units on October 5, far below the 300 DU global average.
Is the Ozone Layer Recovering?
The Montreal Protocol, an international agreement that began taking effect in 1992, phased out the chlorine-based chemicals most responsible for ozone destruction. The results have been measurable. During the 2024 depletion season, the Antarctic ozone hole ranked as the seventh smallest since recovery tracking began. NASA and NOAA scientists project the ozone layer could fully recover to its 1980 levels by 2066, a timeline that reflects how slowly these long-lived chemicals break down in the upper atmosphere.