Good ozone is located in the stratosphere, roughly 15 to 40 kilometers (10 to 25 miles) above Earth’s surface. This high-altitude ozone layer contains about 90% of all the ozone in the atmosphere and acts as a shield against the sun’s most harmful ultraviolet radiation. The remaining 10% sits near ground level in the troposphere, where it functions as a pollutant and is known as “bad” ozone.
What the Ozone Layer Actually Does
The stratospheric ozone layer blocks nearly all of the sun’s most energetic ultraviolet radiation (UVC), most of the UVB radiation responsible for sunburns and skin cancer, and roughly half of UVA radiation. Without this shield, UV rays would be intense enough to sterilize Earth’s surface, making life as we know it impossible.
Despite its importance, the ozone layer is remarkably thin. If you compressed all the ozone in a column of atmosphere down to sea-level pressure, it would form a layer only about 3 millimeters thick, roughly the height of two pennies stacked together. Scientists measure this using Dobson Units, and a healthy global average sits around 300 DU.
How Ozone Forms in the Stratosphere
Stratospheric ozone is constantly being created and destroyed in a natural cycle driven by sunlight. UV radiation splits oxygen molecules into individual oxygen atoms. Those free atoms then collide with other oxygen molecules to form ozone. At the same time, UV light also breaks ozone back apart into regular oxygen and a free atom, and occasionally an oxygen atom recombines with ozone to produce two oxygen molecules. The balance between these creation and destruction reactions maintains a steady concentration of ozone in the stratosphere.
This cycle runs continuously, powered entirely by solar energy. The overall amount of ozone stays relatively stable as long as nothing disrupts the balance. Human-made chemicals like chlorofluorocarbons (CFCs) disrupted it for decades by releasing reactive chlorine into the stratosphere, which catalyzed the breakdown of ozone far faster than natural processes could replace it.
Ozone Thickness Varies by Location
The ozone layer is not uniform around the globe. Total ozone is generally thinnest over the tropics and thickest near the poles. Two factors drive this pattern. First, the troposphere (the lower atmosphere where weather happens) extends to a higher altitude in tropical regions, which compresses the ozone-rich stratosphere above it. Second, large-scale wind patterns in the stratosphere transport ozone-rich tropical air toward the poles, where it accumulates.
This means that if you’re standing at the equator, there’s less ozone overhead than if you’re in Canada or Scandinavia. The exception is the Antarctic during spring, when chemical reactions on polar stratospheric clouds cause the famous “ozone hole,” temporarily driving ozone levels far below normal.
Why Ground-Level Ozone Is Different
The same molecule that protects you in the stratosphere becomes harmful at ground level. Tropospheric ozone forms when pollutants from vehicle exhaust, industrial emissions, and chemical solvents react with sunlight. It’s the main ingredient in smog.
Because ozone doesn’t dissolve easily in the moisture lining your airways, inhaled ozone passes deep into the lungs. It irritates and inflames lung tissue, worsening asthma, emphysema, chronic obstructive pulmonary disease, and cystic fibrosis. When ground-level ozone concentrations exceed health standards, it’s classified as a pollutant that harms people, animals, plants, and materials. Rising anthropogenic emissions over the past century have steadily increased tropospheric ozone concentrations in many regions.
Recovery of the Ozone Layer
The 1987 Montreal Protocol phased out most ozone-depleting chemicals, and the stratospheric ozone layer has been slowly recovering since. The 2024 Antarctic ozone hole ranked as the seventh smallest since recovery began, a sign that the trend is heading in the right direction. NOAA and NASA scientists project the ozone layer could fully recover by 2066, with ozone holes over the poles disappearing in the second half of this century.
Recovery is slow because CFCs linger in the atmosphere for decades after they stop being produced. But the trajectory is clear: the good ozone layer is rebuilding, and each year a little more UV protection returns to the stratosphere where it belongs.