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Frequently Asked Questions about the Ozone Layer

What is the ozone layer?

The ozone layerHelpozone layerThe region of the stratosphere containing the bulk of atmospheric ozone. The ozone layer lies approximately 15-40 kilometers (10-25 miles) above the Earth's surface, in the stratosphere. Depletion of this layer by ozone depleting substances (ODS) will lead to higher UVB levels, which in turn will cause increased skin cancers and cataracts and potential damage to some marine organisms, plants, and plastics. The science page ( offers much more detail on the science of ozone depletion. is a concentration of ozone molecules in the stratosphereHelpstratosphereThe region of the atmosphere above the troposphere. The stratosphere extends from about 10km to about 50km in altitude. Commercial airlines fly in the lower stratosphere. The stratosphere gets warmer at higher altitudes. In fact, this warming is caused by ozone absorbing ultraviolet radiation. Warm air remains in the upper stratosphere, and cool air remains lower, so there is much less vertical mixing in this region than in the troposphere.. About 90 percent of the planet's ozone is in the ozone layer. The layer of the Earth's atmosphere that surrounds us is called the troposphereHelptroposphereThe region of the atmosphere closest to the Earth. The troposphere extends from the surface up to about 10 km in altitude, although this height varies with latitude. Almost all weather takes place in the troposphere. Mt. Everest, the highest mountain on Earth, is only 8.8 km high. Temperatures decrease with altitude in the troposphere. As warm air rises, it cools, falling back to Earth. This process, known as convection, means there are huge air movements that mix the troposphere very efficiently.. The stratosphere, the next higher layer, extends about 6 to 31 miles (or 10 to 50 kilometers) above the Earth's surface. Learn more about the ozone layer.

Why is the ozone layer important?

Stratospheric ozone is a naturally occurring gas that filters the sun's ultraviolet (UVHelpUVUltraviolet radiation is a portion of the electromagnetic spectrum with wavelengths shorter than visible light. The sun produces UV, which is commonly split into three bands: UVA, UVB, and UVC. UVA is not absorbed by ozone. UVB is mostly absorbed by ozone, although some reaches the Earth. UVC is completely absorbed by ozone and normal oxygen. NASA provides more information on their web site ( radiation. A diminished ozone layer allows more UV radiation to reach the Earth's surface. For people, overexposure to UV rays can lead to skin cancer, cataracts, and weakened immune systems. Increased UV can also lead to reduced crop yield and disruptions in the marine food chain. Learn about the health and environmental effects of ozone layer depletion.

What is ozone depletion, and how does it occur?

Ozone molecules in the stratosphere are constantly being produced and destroyed by different types of UV radiation from the sun. Normally, the production and destruction is balanced, so the amount of ozone in the stratosphere at any given time is stable. However, scientists have discovered that certain chemicals react with UV radiation in the stratosphere, which causes them to break apart and release chlorine or bromine atoms. These atoms, in turn, destroy ozone molecules.

Ozone-depleting substances (ODSHelpODSA compound that contributes to stratospheric ozone depletion. ODS include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, methyl bromide, carbon tetrachloride, hydrobromofluorocarbons, chlorobromomethane, and methyl chloroform. ODS are generally very stable in the troposphere and only degrade under intense ultraviolet light in the stratosphere. When they break down, they release chlorine or bromine atoms, which then deplete ozone. A detailed list ( of class I and class II substances with their ODPs, GWPs, and CAS numbers are available.), which include chlorofluorocarbonsHelpchlorofluorocarbonsGases covered under the 1987 Montreal Protocol and used for refrigeration, air conditioning, packaging, insulation, solvents, or aerosol propellants. Since they are not destroyed in the lower atmosphere, CFCs drift into the upper atmosphere where, given suitable conditions, they break down ozone. These gases are being replaced by other compounds: hydrochlorofluorocarbons, an interim replacement for CFCs that are also covered under the Montreal Protocol, and hydrofluorocarbons, which are covered under the Kyoto Protocol. All these substances are also greenhouse gases. See hydrochlorofluorocarbons, hydrofluorocarbons, perfluorocarbons, ozone depleting substance. (CFCs) and hydrofluorocarbonsHelphydrofluorocarbonsCompounds containing only hydrogen, fluorine, and carbon atoms. They were introduced as alternatives to ozone depleting substances in serving many industrial, commercial, and personal needs. HFCs are emitted as by-products of industrial processes and are also used in manufacturing. They do not significantly deplete the stratospheric ozone layer, but they are powerful greenhouse gases with global warming potentials ranging from 140 (HFC-152a) to 11,700 (HFC-23). (HCFCs), were once used widely in refrigerants, insulating foams, solvents, and other applications. These substances all release chlorine atoms into the stratosphere. A single chlorine atom can break apart more than 100,000 ozone molecules.

Other chemicals that damage the ozone layer include methyl bromideHelpmethyl bromideA compound consisting of carbon, hydrogen, and bromine. Methyl Bromide is an effective pesticide used to fumigate soil and many agricultural products. Because it contains bromine, it depletes stratospheric ozone and has an ozone depletion potential of 0.6. Production of methyl bromide was phased out on December 31, 2004, except for allowable exemptions. Much more information is available ( (used as a pesticide), halonsHelphalonsCompounds, also known as bromofluorocarbons, that contain bromine, fluorine, and carbon. They are generally used as fire extinguishing agents and cause ozone depletion. Bromine is many times more effective at destroying stratospheric ozone than chlorine. See ozone depleting substance. (used in fire extinguishers), and methyl chloroformHelpmethyl chloroformA compound consisting of carbon, hydrogen, and chlorine. Methyl chloroform is used as an industrial solvent. Its ozone depletion potential is 0.11. (used as a solvent in industrial processes). As methyl bromide and halons are broken apart, they release bromine atoms, which are 60 times more destructive to ozone molecules than chlorine atoms.

Atmospheric levels of these ODS rapidly increased before the implementation of the Montreal Protocol on Substances that Deplete the Ozone Layer and its subsequent revisions and amendments. However, the atmospheric levels of nearly all of these substances have declined substantially in the past two decades.

What is the ozone hole?

One example of ozone depletion is the annual ozone "hole" over Antarctica that has occurred during the Antarctic spring since the early 1980s. This is not really a hole through the ozone layer, but rather a large area of the stratosphere with extremely low amounts of ozone.

It is important to understand that ozone depletion is not limited to the area over the South Pole. Research has shown that ozone depletion occurs over the latitudes that include North America, Europe, Asia, and much of Africa, Australia, and South America.

What is the connection between ozone depletion and climate change?

ODSs and many of their non-ozone depleting substitutes are potent greenhouse gases that contribute to climate change. Some ODSs and ODS substitutes have global warming potentials that are several thousand times greater than that of carbon dioxide. Recently, ODS alternatives that have lower global warming potentials have become available. Learn more about EPA’s efforts to ensure a safe, smooth transition away from ODSs to substitutes that have reduced effects on climate change.

How do we know that natural sources are not responsible for ozone depletion?

Although it is true that volcanoes and oceans release large amounts of chlorine, the chlorine from these sources is easily dissolved in water and washes out of the atmosphere in rain. In contrast, CFCs do not break down in the lower atmosphere or dissolve in water. Although they are heavier than air, they are eventually carried into the stratosphere. Scientists use balloons, aircraft, and satellites to measure the composition of the stratosphere. These measurements show a noticeable increase in stratospheric chlorine since 1985. The timing of this increase corresponds with the increase in emissions of CFCs and other ODS caused by human activities.

What is being done to protect the ozone layer?

As required under Title VI of the Clean Air Act, EPA is responsible for developing and implementing programs that protect the ozone layer. EPA has established regulations to protect Learn more about EPA’s efforts to protect the ozone layer.

Is there general agreement among scientists on the science of ozone depletion?

Yes, an international consensus about the causes and effects of ozone depletion has emerged. Under the auspices of the UN Environment Programme (UNEP) Exit  and the World Meteorological Organization (WMO) Exit, the scientific community issues periodic reports on the science of ozone depletion. Over 300 scientists worldwide drafted and reviewed the most recent “state-of-the-science” analysis, WMO/UNEP Scientific Assessment of Ozone Depletion: 2014. Exit

Will the ozone layer recover?

The ozone layer is expected to return to normal levels by about 2050. But, it is very important that the world comply with the Montreal Protocol; delays in ending production and use of ozone-depleting substances could cause additional damage to the ozone layer and prolong its recovery. Learn more about the current status of the ozone layer.

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