Information on Ozone and Ozone Depletion
Unlike the oxygen molecules present in the atmosphere, which are composed of two oxygen atoms bound together, ozone is made up of three oxygen atoms. This chemical structure grants ozone some unique properties which make it essential for the health and well-being of humans and ecosystems. Specifically, ozone absorbs harmful ultraviolet B (UV-B) radiation from the sun, which can cause cancer and cataracts in humans, and upsets biogeochemical cycles.
Ozone’s beneficial effects are especially impressive given how little ozone actually sits between the sun and Earth’s surface. Most of the ozone present in the atmosphere is concentrated in the stratospheric “ozone layer,” about 9 to 22 miles (15 – 35 km) above the surface (around twice the altitude at which commercial airplanes fly). Even in this layer, there are only a few thousand ozone molecules for every billion air molecules.
In the 1970s, scientists discovered that certain human-made chemicals containing chlorine and bromine can cause the depletion of stratospheric ozone. These chemicals have historically been used in a number of applications, from foam-blowing to refrigeration. Once released at Earth’s surface, they eventually migrate upwards to the stratosphere, where ultraviolet (UV) radiation from the sun converts them into reactive gases that destroy ozone. By the 1980s, it was clear that these ozone-depleting substances (ODS) were indeed having a significant impact on stratospheric ozone, leading to the creation of a progressively worsening “ozone hole” each springtime over Antarctica.
The international community reacted quickly, and the Montreal Protocol on Substances that Deplete the Ozone Layer was signed in 1987. The Montreal Protocol and its amendments provide for a complete phaseout of ODS, and the Protocol is the first treaty in the history of the United Nations to achieve universal ratification. To fulfill its international obligations, the United States passed Title VI of the Clean Air Act in 1990, empowering EPA to protect the stratospheric ozone layer. As a result of this coordinated action, the concentration of ODS in the atmosphere has dropped substantially over the past decades, and the ozone layer is expected to nearly completely recover to its pre-1980 levels by the middle of the 21st century.
Learn about the ozone layer and ozone depletion.
Health and Environmental Effects of Ozone Layer Depletion
Learn about the consequences of ozone depletion and UV-B for human health and the environment.
See a list of chemicals categorized as ozone-depleting substances (ODS) under the Montreal Protocol.
Current State of the Ozone Layer
Learn about the recovery of the ozone layer.
Learn about EPA’s model to estimate the health effects of ultraviolet radiation and the positive impacts of the Montreal Protocol.
EPA’s Vintaging Model of ODS Substitutes
Learn about EPA’s model to estimate the annual chemical emissions from industrial sectors that have used ODS in their products.