The ozone layer, also called the ozone shield, is a part of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. This layer has a higher amount of ozone (O₃) compared to other parts of the atmosphere, but ozone still makes up only a small portion of the gases in the stratosphere. The ozone layer reaches concentrations of 8 to 15 parts per million, while the average ozone level in Earth's entire atmosphere is about 0.3 parts per million. The ozone layer is mainly located in the lower part of the stratosphere, between 15 and 35 kilometers (9 to 22 miles) above Earth's surface. Its thickness changes depending on the season and location.

The ozone layer was discovered in 1913 by French scientists Charles Fabry and Henri Buisson. Studies of sunlight showed that the radiation reaching Earth's surface matched the expected spectrum of a black body at temperatures between 5,500–6,000 K (5,230–5,730 °C), except for a missing part of the spectrum below 310 nm in the ultraviolet range. Scientists concluded that something in the atmosphere absorbed this missing radiation. Later, the missing radiation was identified as ozone. British meteorologist G. M. B. Dobson studied ozone's properties and created a simple tool called the Dobsonmeter to measure ozone levels from Earth's surface. Between 1928 and 1958, Dobson set up a global network of ozone monitoring stations, many of which are still active today. The "Dobson unit" (DU), a way to measure ozone levels, is named after him.

The ozone layer absorbs 97 to 99 percent of the Sun's medium-frequency ultraviolet light (wavelengths from about 200 nm to 315 nm), which could otherwise harm living things near Earth's surface.

In 1985, research showed that industrial chemicals, mainly chlorofluorocarbons (CFCs), were causing the ozone layer to shrink. Concerns about increased UV radiation harming life, including higher rates of skin cancer in humans and other environmental issues, led to bans on these chemicals. Recent evidence suggests that ozone depletion has slowed or stopped. The United Nations General Assembly has set September 16 as the International Day for the Preservation of the Ozone Layer.

Venus also has a thin ozone layer located 100 kilometers above its surface.

Ultraviolet light

The ozone layer has a small amount of ozone, but it is very important for life because it absorbs harmful ultraviolet (UV) radiation from the Sun. Extremely short or vacuum UV (10–100 nm) is blocked by nitrogen. UV radiation that can pass through nitrogen is divided into three types based on its wavelength: UV-A (400–315 nm), UV-B (315–280 nm), and UV-C (280–100 nm).

UV-C is very harmful to all living things and is completely blocked by a combination of dioxygen (less than 200 nm) and ozone (more than about 200 nm) at around 35 kilometres (115,000 ft) altitude. UV-B radiation can harm the skin and is the main cause of sunburn. Too much exposure can also lead to cataracts, weakened immune systems, and genetic damage, which may cause skin cancer. The ozone layer is very effective at blocking UV-B (absorbing from about 200 nm to 310 nm, with the strongest absorption at about 250 nm). For UV-B radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at Earth's surface. However, some UV-B, especially at its longest wavelengths, still reaches Earth's surface and helps the skin produce vitamin D in mammals.

Ozone allows most UV-A radiation to pass through, so most of this longer-wavelength UV reaches Earth's surface. This type of UV makes up most of the UV that reaches Earth. Although UV-A is less harmful to DNA than UV-B, it can still cause physical damage, premature aging of the skin, indirect genetic damage, and skin cancer.

Distribution in the stratosphere

The thickness of the ozone layer changes around the world. It is usually thinner near the equator and thicker near the poles. Thickness means the amount of ozone in a column over a specific area, and it changes with the seasons. These changes happen because of air movement patterns in the atmosphere and the strength of sunlight.

The ozone layer gradually ends. Its top limit is where the air becomes too thin for UV light to create much ozone. Its bottom limit is where the ozone produced blocks enough UV light to stop most ozone formation.

In the homosphere, wind movement is more important than the weight of gases. Most ozone is made over the tropics and carried toward the poles by wind patterns in the stratosphere. In the northern hemisphere, these patterns, called the Brewer–Dobson circulation, make the ozone layer thickest in spring and thinnest in fall. When UV light from the sun creates ozone in the tropics, air that has little ozone is lifted from the troposphere into the stratosphere. There, sunlight breaks apart oxygen molecules to form ozone. This ozone-rich air then moves to higher latitudes and descends into lower parts of the atmosphere.

Studies show that ozone levels in the United States are highest in April and May and lowest in October. While the total amount of ozone increases as you move from the tropics to higher latitudes, concentrations are greater in northern high latitudes than in southern high latitudes. In northern high latitudes, spring ozone levels sometimes reach over 600 DU and average 450 DU. In the Antarctic, 400 DU was once the usual maximum before human activities caused ozone loss. This difference happened naturally because the northern hemisphere has weaker polar winds and stronger Brewer–Dobson circulation due to large mountain ranges and bigger temperature differences between land and oceans. The gap between northern and southern high latitudes has grown since the 1970s because of the ozone hole. The highest ozone levels are found over the Arctic in March and April, while the lowest levels are in the Antarctic during September and October.

Depletion

The ozone layer can be damaged by substances called free radical catalysts, such as nitric oxide (NO), nitrous oxide (N₂O), hydroxyl (OH), atomic chlorine (Cl), and atomic bromine (Br). These substances occur naturally, but the amounts of chlorine and bromine have increased greatly in recent decades because of human-made chemicals, especially chlorofluorocarbons (CFCs) and bromofluorocarbons. In the lower atmosphere, wind mixing spreads gases evenly, even though some are heavier than nitrogen or oxygen. Because of this, stable chemicals like CFCs rise into the stratosphere, where ultraviolet light releases chlorine and bromine radicals. Each of these radicals can break down over 100,000 ozone molecules through chain reactions. By 2009, nitrous oxide was the largest ozone-depleting substance (ODS) released by human activities.

When ozone in the stratosphere breaks down, less ultraviolet radiation is absorbed. This allows more harmful ultraviolet radiation to reach Earth’s surface. Since the late 1970s, global ozone levels have dropped by about 4%. In polar regions, such as near the North and South Poles, ozone levels have dropped much more during certain seasons, creating areas called "ozone holes." These are thin spots in the ozone layer, especially near Earth’s poles. The discovery of the annual ozone loss above Antarctica was first reported in a 1985 paper by Joe Farman, Brian Gardiner, and Jonathan Shanklin.

Efforts to protect the ozone layer include laws like the Clean Air Act in the United States, which requires air quality standards for pollutants, including ozone. This law has helped reduce ozone-depleting substances because regions must follow these rules, and the Environmental Protection Agency (EPA) supports them. Clear communication has also been important. A study by Sheldon Ungar showed that using simple comparisons, like "ozone shield" and "ozone hole," helped people understand the risks of ozone depletion. People were more concerned about ozone loss than climate change because it seemed more immediate.

When satellites burn up as they re-enter Earth’s atmosphere, they release tiny particles called aluminum oxide (Al₂O₃) that stay in the air for many years. In 2022 alone, about 17 metric tons of these particles were released from satellites. More satellites in space could increase ozone depletion over time.

Ozone that is harmful can cause breathing problems and worsen conditions like asthma, chronic obstructive pulmonary disease (COPD), and emphysema. Because of this, many countries have laws to reduce harmful ozone in cities and homes. The European Union has strict rules about which products can be sold or used. With proper regulations, the ozone layer is expected to heal over time.

In 1978, the United States, Canada, and Norway banned CFCs in aerosol sprays, but the European Community did not support a similar ban. In the U.S., CFCs were still used in refrigeration and cleaning until the Antarctic ozone hole was discovered in 1985. After the Montreal Protocol was signed, CFC production was limited to 1986 levels, with a plan to reduce it further. Developing countries had ten years to adjust. By 1996, only recycled or stored CFCs could be used in developed nations. This was possible because alternatives were created to replace ozone-depleting chemicals.

In 2003, scientists found that ozone loss might be slowing because of international efforts to control ozone-depleting substances. Studies using satellites and ground stations showed that ozone depletion in the upper atmosphere had decreased over the previous decade. However, some loss is expected to continue because of gases already in the stratosphere and substances not banned in some countries. CFCs stay in the atmosphere for 50 to over 100 years. Scientists predict the ozone layer will recover to 1980 levels by the middle of the 21st century. A 2016 report showed early signs of recovery.

Chemicals with C–H bonds, like hydrochlorofluorocarbons (HCFCs), were created to replace CFCs. These chemicals break down faster in the atmosphere and are less harmful to the ozone layer. However, HCFCs still damage ozone and are being phased out. They are now being replaced by hydrofluorocarbons (HFCs) and other chemicals that do not harm the ozone layer.

CFCs in the atmosphere create a difference in concentration between the air and the ocean. These chemicals dissolve into ocean water and act as tracers, helping scientists study ocean currents and their effects on the environment.

Implications for astronomy

Ozone in the atmosphere blocks most of the energetic ultraviolet radiation from reaching Earth's surface. Because of this, scientists must collect data in these wavelengths using satellites that orbit outside Earth's atmosphere and ozone layer. Young, hot stars emit a lot of ultraviolet light, so studying these wavelengths helps scientists learn about how galaxies form. The Galaxy Evolution Explorer, GALEX, is a space telescope that observes ultraviolet light. It was launched on April 28, 2003, and worked until 2012.

• This GALEX image of the Cygnus Loop nebula could not be taken from Earth because the ozone layer blocks the ultraviolet radiation coming from the nebula.