Ozone Depletion Information Center

Ozone Depletion and UV Radiation

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  1. Summary: Two types of Ozone: Good ozone (high level) and Bad ozone (ground-level ozone )
  2. What is ozone (this page)
  3. The science of ozone depletion
  4. The effects of ozone depletion
  5. Ozone depletion graphics and images
  6. FAQs about ozone depletion
  7. Ozone Depletion Publications, References and Resources

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The ozone layer forms a thin shield in the stratosphere, protecting life on Earth from the sun's harmful UV rays. In the 1980s, scientists began accumulating evidence that the ozone layer was being depleted. This depletion is likely to allow an increased amount of UV radiation to reach Earth's surface, which, in turn, strengthens the chance of overexposure to UV radiation and the resulting health effects.

The UV Index, Weather and You - Includes a stunning NASA picture of the Ozone Hole

Stratospheric Ozone - the beneficial or good ozone
The stratosphere, or "good" ozone layer extends upward from about 6 to 30 miles and protects life on Earth from the sun's harmful ultraviolet (UV) rays. This natural shield has been gradually depleted by man-made chemicals like chlorofluorocarbons (CFCs). A depleted ozone shield allows more UV from the sun to reach the ground, leading to more cases of skin cancer, cataracts, and other health problems.

The Stratosphere extends around 31 miles (50 km) down to anywhere from 4 to 12 miles (6 to 20 km) above the Earth's surface. This layer holds 19 percent of the atmosphere's gases but very little water vapor. In this region the temperature increases with height. Heat is produced in the process of the formation of Ozone and this heat is responsible for temperature increases from an average -60°F (-51°C) at tropopause to a maximum of about 5°F (-15°C) at the top of the stratosphere.  This increase in temperature with height means warmer air is located above cooler air. This prevents "convection" as there is no upward vertical movement of the gases. As such the location of the bottom of this layer is readily seen by the 'anvil-shaped' tops of cumulonimbus clouds.

  • Basic information about stratospheric ozone
  • Ultraviolet radiation and your health
  • * The UV index
  • * Tips to reduce the effects of the sun and uv radiation
  • Ground-level Ozone - the bad form of ozone
    In the troposphere, the air closest to the Earth's surface, ground-level or "bad" ozone is a pollutant that is a significant health risk, especially for children with asthma. It also damages crops, trees and other vegetation. It is a main ingredient of urban smog.

  • Basic information about ground-level ozone
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    Ozone - What Is It?

    Ozone is a naturally occurring gas found in the Earth's atmosphere that absorbs certain wavelengths of the sun's UV radiation. Ozone is a gas that occurs both in the Earth's upper atmosphere and at ground level. Ozone is concentrated in a part of the atmosphere called the stratosphere. Stratospheric ozone is most concentrated between 6 and 30 miles above the Earth's surface. Ozone is formed when oxygen molecules in the atmosphere absorb UV radiation and split into two oxygen atoms (O), which combine with oxygen molecules (O 2 ), to form ozone molecules (O 3 ). Ozone is also broken apart as it absorbs UV radiation. In this way, UV radiation helps sustain the natural balance of ozone in the stratosphere, while ozone, in turn, absorbs it, protecting life on earth from harmful radiation.

    Ozone can be "good" or "bad" for your health and the environment, depending on its location in the atmosphere.

    How Can Ozone Be Both Good and Bad?

    Ozone occurs in two layers of the atmosphere. The layer closest to the Earth's surface is the troposphere. Here, ground-level or "bad" ozone is an air pollutant that is harmful to breathe and it damages crops, trees and other vegetation. It is a main ingredient of urban smog. The troposphere generally extends to a level about 6 miles up, where it meets the second layer, the stratosphere. The stratosphere or "good" ozone layer extends upward from about 6 to 30 miles and protects life on Earth from the sun's harmful ultraviolet (UV) rays.

    What is Happening to the "Good" Ozone Layer?

    Ozone is produced naturally in the stratosphere. But this "good" ozone is gradually being destroyed by man-made chemicals referred to as ozone-depleting substances (ODS), including chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, methyl bromide, carbon tetrachloride, and methyl chloroform. These substances were formerly used and sometimes still are used in coolants, foaming agents, fire extinguishers, solvents, pesticides, and aerosol propellants. Once released into the air these ozone-depleting substances degrade very slowly. In fact, they can remain intact for years as they move through the troposphere until they reach the stratosphere. There they are broken down by the intensity of the sun's UV rays and release chlorine and bromine molecules, which destroy the "good" ozone. Scientists estimate that one chlorine atom can destroy 100,000 "good" ozone molecules.

    Even though we have reduced or eliminated the use of many ODSs, their use in the past can still affect the protective ozone layer. Research indicates that depletion of the "good" ozone layer is being reduced worldwide. Thinning of the protective ozone layer can be observed using satellite measurements, particularly over the Polar Regions.

    What Causes "Bad" Ozone?

    Ground-level or "bad" ozone is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC.

    Sources of NOx and VOC

    At ground level, ozone is a harmful pollutant. Ozone pollution is a concern during the summer months because strong sunlight and hot weather result in harmful ozone concentrations in the air we breathe. Many urban and suburban areas throughout the United States have high levels of "bad" ozone. But many rural areas of the country are also subject to high ozone levels as winds carry emissions hundreds of miles away from their original sources.

    How Does "Bad" Ozone Affect Human Health and the Environment?

    Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. "Bad" ozone also can reduce lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue.

    Healthy people also experience difficulty breathing when exposed to ozone pollution. Because ozone forms in hot weather, anyone who spends time outdoors in the summer may be affected, particularly children, outdoor workers and people exercising. Millions of Americans live in areas where the national ozone health standards are exceeded.

    Ground-level or "bad" ozone also damages vegetation and ecosystems. It leads to reduced agricultural crop and commercial forest yields, reduced growth and survivability of tree seedlings, and increased susceptibility to diseases, pests and other stresses such as harsh weather. In the United States alone, ground-level ozone is responsible for an estimated $500 million in reduced crop production each year. Ground-level ozone also damages the foliage of trees and other plants, affecting the landscape of cities, national parks and forests, and recreation areas.

    What Is Being Done About "Bad" Ozone?

    Under the Clean Air Act, EPA has set protective health-based standards for ozone in the air we breathe. EPA, state, and cities have instituted a variety of multi-faceted programs to meet these health-based standards. Throughout the country, additional programs are being put into place to cut NOx and VOC emissions from vehicles, industrial facilities, and electric utilities. Programs are also aimed at reducing pollution by reformulating fuels and consumer/commercial products, such as paints and chemical solvents, that contain VOC. Voluntary programs also encourage communities to adopt practices, such as carpooling, to reduce harmful emissions.

    How Does the Depletion of "Good" Ozone Affect Human Health and the Environment?

    Ozone depletion can cause increased amounts of UV radiation to reach the Earth which can lead to more cases of skin cancer, cataracts, and impaired immune systems. Overexposure to UV is believed to be contributing to the increase in melanoma, the most fatal of all skin cancers. Since 1990, the risk of developing melanoma has more than doubled.

    UV can also damage sensitive crops, such as soybeans, and reduce crop yields. Some scientists suggest that marine phytoplankton, which are the base of the ocean food chain, are already under stress from UV radiation. This stress could have adverse consequences for human food supplies from the oceans.

    What is Being Done About the Depletion of "Good" Ozone?

    The United States, along with over 180 other countries, recognized the threats posed by ozone depletion and in 1987 adopted a treaty called the Montreal Protocol to phase out the production and use of ozone-depleting substances.

    EPA has established regulations to phase out ozone-depleting chemicals in the United States. Warning labels must be placed on all products containing CFCs or similar substances and nonessential uses of ozone-depleting products are prohibited. Releases into the air of refrigerants used in car and home air conditioning units and appliances are also prohibited. Some substitutes to ozone-depleting products have been produced and others are being developed. If the United States and other countries stop producing ozone-depleting substances, natural ozone production should return the ozone layer to normal levels by about 2050.

    As part of the United States' commitment to implementing the Montreal Protocol on Substances that Deplete the Ozone Layer, Congress amended the Clean Air Act to add provisions (under Title VI) for protecting the . As required under Title VI of the Clean Air Act, EPA is responsible for developing and implementing programs that protect the stratospheric ozone layer. Title 40, Part 82 of the Code of Federal Regulations contains EPA’s regulations to protect the ozone layer.


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    Repairing the Damage

    Countries around the world have recognized the threats posed by ozone depletion. The initial concern about the ozone layer in the 1970's led to a ban on the use of CFCs as aerosol propellants in several countries, including the U.S. However, production of CFCs and other ozone-depleting substances grew rapidly afterward as new uses were discovered.

    Through the 1980s, other uses expanded and the world's nations became increasingly concerned that these chemicals would further harm the ozone layer. In 1985, the Vienna Convention was adopted to formalize international cooperation on this issue. Additional efforts resulted in the signing of the Montreal Protocol in 1987. The original protocol would have reduced the production of CFCs by half by 1998.

    Parties to the Montreal Protocol treaty, including the United States, are phasing out the production of ozone-depleting substances. Scientists predict that CFC levels should peak by the turn of the century and fall back to pre-ozone damage levels between 2020 and 2050.

    After the original Protocol was signed, new measurements showed worse damage to the ozone layer than was originally expected. In 1992, reacting to the latest scientific assessment of ozone layer, the Parties decided to completely end production of halons by the beginning of 1994 and of CFCs by the beginning of 1996 in developed countries.

    As international control measures reduce the release of CFCs and other ozone-depleting substances, the natural atmospheric process will repair the ozone layer. Until that time, it is likely that increased levels of UV radiation will reach the Earth's surface. These higher levels can lead to a greater chance of overexposure and the consequent health effects

    The Earth's ozone layer protects all life from the sun's harmful radiation, but human activities have damaged this shield. Less protection from ultraviolet light will, over time, lead to higher skin cancer and cataract rates and crop damage. The U.S., in cooperation with over 140 other countries, is phasing out the production of ozone-depleting substances in an effort to safeguard the ozone layer.

    Assuming continued compliance, stratospheric chlorine levels will peak in a few years and then slowly return to normal. The good news is that the natural ozone production process will heal the ozone layer in about 50 years.

    Dispelling the Myths of Ozone Layer Depletion

    Responses to the most common misunderstandings about ozone depletion.

    Myth #1: CFCs are heavier than air, so they can't reach the ozone layer.

    False! Although CFCs and other ozone depleting substances (ODS) are heavier than air, the winds in the troposphere stir around ODS molecules to mix them with air and lift them up to the ozone layer.

    Myth #2: Volcanoes are causing ozone layer depletion.

    False! Although volcanic eruptions are powerful events that can inject hydrogen chloride into the atmosphere, the vast majority of eruptions are too weak to reach the stratosphere. ODS are the real source of stratospheric chlorine and thus ozone layer depletion. In addition to the link above, a discussion of the role of aerosols from Mt. Pinatubo in ozone depletion is also provided.

    Myth #3: Ozone layer depletion occurs only in Antarctica.

    False! Ozone layer depletion has been measured everywhere outside of the tropics, not just Antarctica.

    Myth #4: No link exists between ozone layer depletion and higher UV levels.

    False! Studies have shown a clear connection between reduced ozone levels and higher levels of UVB, and laboratory experiments have confirmed that ozone absorbs UVB.

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