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Ozone Depletion

  1. What is good ozone
  2. What is Bad ozone (ground-level ozone)
  3. The science of ozone depletion
  4. The effects of ozone depletion
  5. Ozone depletion graphics and images
  6. Ozone Depletion Publications, References and Resources

Questions and Answers on 
Ozone Depletion - FAQs and Publications

bulletWhat is the ozone layer and why is it important? bulletHow does ozone depletion occur? bulletHow do we know that natural sources are not responsible for ozone depletion? bulletWhat is being done about ozone depletion? bulletIs there general agreement among scientists on the science of ozone depletion? bulletWill the ozone layer recover? Can we make more ozone to fill in the hole?

What is the ozone layer and why is it important?
The ozone layer is a concentration of ozone molecules in the stratosphere. About 90% of the planet's ozone is in the ozone layer. The layer of the Earth's atmosphere that surrounds us is called the troposphere. The stratosphere, the next higher layer, extends about 10-50 kilometers above the Earth's surface. Stratospheric ozone is a naturally-occurring gas that filters the sun's ultraviolet (UV) radiation. A diminished ozone layer allows more 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. UV also has other harmful effects.

How does ozone depletion occur?
It is caused by the release of chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODS), which were used widely as refrigerants, insulating foams, and solvents. The discussion below focuses on CFCs, but is relevant to all ODS. Although CFCs are heavier than air, they are eventually carried into the stratosphere in a process that can take as long as 2 to 5 years. Measurements of CFCs in the stratosphere are made from balloons, aircraft, and satellites.

When CFCs reach the stratosphere, the ultraviolet radiation from the sun causes them to break apart and release chlorine atoms which react with ozone, starting chemical cycles of ozone destruction that deplete the ozone layer. One chlorine atom can break apart more than 100,000 ozone molecules.

Other chemicals that damage the ozone layer include methyl bromide (used as a pesticide), halons (used in fire extinguishers), and methyl chloroform (used as a solvent in industrial processes for essential applications). As methyl bromide and halons are broken apart, they release bromine atoms, which are 40 times more destructive to ozone molecules than chlorine atoms.

How do we know that natural sources are not responsible for ozone depletion?
While 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 are not broken down in the lower atmosphere and do not dissolve in water. The chlorine in these human-made molecules does reach the stratosphere. Measurements show that the increase in stratospheric chlorine since 1985 matches the amount released from CFCs and other ozone-depleting substances produced and released by human activities.

What is being done about ozone depletion?
In 1978, the use of CFC propellants in spray cans was banned in the U.S. In the 1980s, the Antarctic "ozone hole" Exit EPA Disclaimer appeared and an international science assessment more strongly linked the release of CFCs and ozone depletion. It became evident that a stronger worldwide response was needed. In 1987, the Montreal Protocol was signed and the signatory nations committed themselves to a reduction in the use of CFCs and other ozone-depleting substances.

Since that time, the treaty has been amended to ban CFC production after 1995 in the developed countries, and later in developing countries. Today, over 180 countries have ratified the treaty. Beginning January 1, 1996, only recycled and stockpiled CFCs will be available for use in developed countries like the US. This production phaseout is possible because of efforts to ensure that there will be substitute chemicals and technologies for all CFC uses.

EPA coordinates numerous regulatory programs designed to help the ozone layer and continues to be active in developing international ozone protection policies. Individuals can also help, primarily by ensuring that technicians working on air conditioning and refrigeration equipment are certified by EPA, that refrigerants are recaptured and not released, and by educating themselves about the issue of ozone depletion. A longer article explains EPA's ozone protection efforts in more detail.

Is there general agreement among scientists on the science of ozone depletion?
Yes. Under the sponsorship of the UN Environment Programme (UNEP) and the World Meteorological Organization (WMO), the scientific community issues periodic reports. Almost 300 scientists worldwide drafted and reviewed the WMO/UNEP Scientific Assessment of Ozone Depletion: 2002. An international consensus about the causes and effects of ozone depletion has emerged. To obtain a copy of the executive summary of the assessment, please visit NOAA's web site

Will the ozone layer recover? Can we make more ozone to fill in the hole?
The answers, in order, are: yes and no. We can't make enough ozone to replace what's been destroyed, but provided that we stop producing ozone-depleting substances, natural ozone production reactions should return the ozone layer to normal levels by about 2050. It is very important that the world comply with the Montreal Protocol; delays in ending production could result in additional damage and prolong the ozone layer's recovery. More detail on these questions is provided elsewhere on this web site.


What is ozone?
Ozone is a gas composed of three atoms of oxygen. Ozone occurs both in the Earth's upper atmosphere and at ground level. Ozone can be good or bad, depending on where it is found;

Good Ozone
Good ozone occurs naturally in the upper atmosphere, 6 to 30 miles above the Earth's surface, where it forms a protective layer that shields us from the sun's harmful ultraviolet rays. This beneficial ozone is gradually being destroyed by manmade chemicals. When the protective ozone "layer" has been significantly depleted; for example, over the North or South pole; it is sometimes called a "hole in the ozone."
Bad Ozone
In the Earth's lower atmosphere, near ground level, ozone is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight. Ozone at ground level is a harmful air pollutant.

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Where does ground-level ozone come from?
Ground-level ozone is formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs), such as xylene, react in the atmosphere in the presence of sunlight. NOx and VOCs are called ozone precursors. Motor vehicle exhaust, industrial emissions, and chemical solvents are the major sources of these chemicals. Ozone pollution is a concern during the summer months when the weather conditions needed to form it - lots of sun, hot temperatures - normally occur. Although these precursors often originate in urban areas, winds can carry NOx hundreds of miles, causing ozone formation to occur in less populated regions as well. For information about how you can reduce your contribution to the ozone problem, please visit

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Are ozone and smog the same thing?
While the two terms are often used interchangeably for general use, smog is more complex. Smog is primarily made up of ground-level ozone combined with other gases and particulate matter.

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What are the health effects of ozone?
Ozone can irritate the respiratory system, causing coughing, throat irritation, and/or an uncomfortable sensation in the chest.

Ozone can reduce lung function and make it more difficult to breathe deeply and vigorously. Breathing may become more rapid and shallow than normal. This may limit a person's ability to engage in vigorous activities.

Ozone can aggravate asthma. When ozone levels are high, more people with asthma have attacks that require a doctor's attention or use of medication. One reason this happens is that ozone makes people more sensitive to allergens, the most common triggers of asthma attacks.

Ozone can increase susceptibility to respiratory infections.

Ozone can inflame and damage the lining of the lungs. Within a few days, the damaged cells are shed and replaced much like the skin peels after a sunburn. Animal studies suggest that if this type of inflammation happens repeatedly over a long time period (months, years, a lifetime), lung tissue may become permanently scarred, resulting in permanent loss of lung function, and a lower quality of life.

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Who is most at risk?
child playing outdoors

Children, people with lung disease, and people who work or play outdoors are at risk from ozone.

Roughly one out of every three people in the United States is at a higher risk of experiencing ozone-related health effects.

One group at high risk is active children because they often spend a large part of the summer playing outdoors. People of all ages who are active outdoors are at increased risk because, during physical activity, ozone penetrates deeper into the parts of the lungs that are more vulnerable to injury.

People with respiratory diseases that make their lungs more vulnerable to ozone may experience health effects earlier and at lower ozone levels than others.

Though scientists don't yet know why, some healthy people are unusually sensitive to ozone. They may experience health effects at more moderate levels of outdoor exertion or at lower ozone levels than the average person.

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How can I find out about ozone levels in my community?
Air quality forecasts are often given with weather forecasts on local television and radio stations, and may be found on the weather page of your newspaper. Another way to learn about unhealthy exposures is to check daily Air Quality Index forecasts.

You can also check ozone levels and other daily air quality information, and find air pollution health effects information at AIRNOW on the EPA web site.

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Designations Process

What does nonattainment mean?
The Clean Air Act identifies six common air pollutants that are found all over the United States. These pollutants can injure health, harm the environment and cause property damage. EPA calls these pollutants criteria air pollutants because the agency has developed health-based criteria (science-based guidelines) as the basis for setting permissible levels. Ozone is a criteria pollutant. There are national ambient air quality standards (NAAQS) for each of the criteria pollutants. These standards apply to the concentration of a pollutant in outdoor air. If the air quality in a geographic area meets or does better than the national standard, it is called an attainment area; areas that don't meet the national standard are called nonattainment areas.

In order to improve air quality, states must draft a plan known as a state implementation plan (SIP) to improve the air quality in nonattainment areas. The plan outlines the measures that the state will take in order to improve air quality. Once a nonattainment area meets the standards and additional redesignation requirements in the CAA [Section 107(d)(3)(E)], EPA will designate the area to attainment as a "maintenance area."

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What does designation mean?
States and tribes submit recommendations to the EPA as to whether or not an area is attaining the national ambient air quality standards for a criteria pollutant. The states and tribes base these recommendations on air quality data collected from monitors at location in urban and rural settings. After working with the states and tribes and considering the information from air quality monitors, EPA "designates" an area as attainment or nonattainment with the ozone standard. If an area is designated as nonattainment it informs the public that the air in the area is unhealthy to breathe, and states, local and tribal governments must develop and implement control plans to reduce ozone-forming pollution.

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What is the designation process for the 8-hour ozone standard??
By July 15, 2003, State governors and tribal leaders examined the latest ozone air quality data, and submitted a recommendation to EPA on what areas are not meeting the 8-hour ozone standards. On April 15, 2004, EPA designated areas as "Unclassifiable/Attainment" or "Nonattainment" for the national ambient air quality standard for ozone. EPA will classify the nonattainment areas according to the severity of their ozone problems. There are five classes of nonattainment areas for ozone, ranging from marginal to severe.

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What is my area going to be required to do to meet the air pollution regulations if it is designated nonattainment for ozone?
In order to meet the requirements of the Clean Air Act, most 8-hour ozone nonattainment areas will be required to submit to EPA a state implementation plan by April 2007. Through this plan, states will design their approach to reducing the ozone level in the air and emissions of ozone precursors. The comprehensive approach to reducing criteria air pollutants taken by the Clean Air Act covers many different sources and a variety of clean-up methods. These air pollution control programs could include nonattainment New Source Review permit program and Federal General Conformity and Transportation Conformity programs. State plans will make sure power plants, factories and other pollution sources meet clean-up goals by working through the air pollution permitting process that applies to industrial facilities. Working with the EPA, states, tribes, areas will also implement programs to further reduce emissions of ozone precursors from sources such as cars, fuels, and consumer/commercial products and activities.

EPA is taking a wide range of national clean air actions that will help all areas across the country significantly improve ozone air quality. Many of these clean air actions will bring local areas into attainment without any additional local controls. These national clean air control programs include:

  bulletEPA's regional ozone transport rule, known as the NOx SIP Call, will significantly reduce NOx emissions in 19 eastern states and the District of Columbia by approximately 600,000 tons starting in the summer of 2004. bulletEPA's proposed Clean Air Interstate Rule would bring many areas into attainment with the fine particle and ozone standards. EPA expects to issue this as a final rule in late 2004. bulletClean Air Diesel Rules targeting diesel emissions from on road and off road diesel engines will help to significantly cut NOx emissions nationwide. bulletEPA is phasing in very stringent tailpipe standards for cars, trucks, and SUVs that also reduce NOx emissions.
After the area is designated as nonattainment, the area must meet the federally mandated deadlines established by the 1990 Amendment to the Clean Air Act for compliance with the national ambient air quality standards. In the interim, the nonattainment area must demonstrate to EPA that they are making reasonable further progress toward improving their air quality.

Dates for 8-hour Ozone Implementation
Classification Attainment Date
(years after designation)
(Subpart 1)
5 to 10 yrs. after designation
Marginal 3 years
Moderate 6 years
Serious 9 years
Severe15 15 years
Severe17 17 years
Extreme 20 years

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What information does EPA use to calculate nonattainment?
The Clean Air Act defines a nonattainment area as an area that is violating the national ambient air quality standard or contributing to a violation of the standard in a nearby area. The 8-hour ozone standard is based on averaging air quality measurements over 8-hour blocks of time. EPA uses the average of the annual 4th highest 8-hour daily maximum concentrations from each of the last three years of air quality monitoring data to determine a violation of the ozone standard. In addition to air quality data, EPA also considers other key factors, including emissions, traffic and commuting patterns, population density, and expected growth as to whether or not additional counties should be included in the consolidated/metropolitan statistical area (C/MSA). The C/MSA is the presumptive nonattainment default boundary for areas classified serious and above as defined in the Clean Air Act.

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How long has this been a problem in my area?
Ozone levels can vary from one area to the next and they can also vary over time or from one season to the next. Ozone is primarily a summertime problem. Some areas have experienced problems with ozone for years, while other areas have not. To learn how long ozone has been a problem where you live, you can visit the Green Book where you will find technical resources and a map of your area to learn more about the problems associated with ozone. And, you can find out how many days your area exceeded the ozone standard.

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How do Early Action Compacts work in relation to nonattainment areas for the 8-hour ozone standard?
Although we are working to deliver air quality improvements to local areas, some areas want to move even faster than required by the Clean Air Act. EPA is working with communities to get clean air as soon as possible by entering into Early Action Compacts to reduce ground-level ozone pollution. Communities with Early Action Compacts will have plans in place to reduce air pollution at least two years earlier than required by the Clean Air Act. In December 2002, a number of states submitted compact agreements pledging to reduce emissions earlier than required for compliance with the 8-hour ozone standard. The states had to meet specific criteria and agreed to meet certain milestones. As long as Early Action Compact Areas meet these milestones, the effective date of ozone nonattainment designations will be deferred. Thirty-three communities in 14 states have volunteered to participate in an Early Action Compact. A rule outlining the details of this deferral was announced April 15, 2004 in the same notice as ozone designations. For more information please visit

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