Ozone is a gas. It can be good or bad, depending on where it is. "Good" ozone occurs naturally about 10 to 30 miles above the Earth's surface. It shields us from the sun's ultraviolet rays. Part of the good ozone layer is gone. Man-made chemicals have destroyed it. Without enough good ozone, people may get too much ultraviolet radiation. This may increase the risk of skin cancer, cataracts, and immune system problems.

"Bad" ozone is at ground level. It forms when pollutants from cars, factories, and other sources react chemically with sunlight. It is the main ingredient in smog. It is usually worst in the summer. Breathing bad ozone can be harmful. It can cause coughing, throat irritation, worsening of asthma, bronchitis, and emphysema. It can lead to permanent lung damage, if you are regularly exposed to it.

What is "good" vs. "bad" ozone?

Ozone can be “good” or “bad” for health and the environment depending on where it’s found in the atmosphere. Stratospheric ozone is “good” because it protects living things from ultraviolet radiation from the sun. Ground-level ozone, the topic of this website, is “bad” because it can trigger a variety of health problems, particularly for children, the elderly, and people of all ages who have lung diseases such as asthma. Learn more about ground-level ozone.

Ozone is a gas composed of three atoms of oxygen (O3). 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.

Called stratospheric ozone, good ozone occurs naturally in the upper atmosphere, where it forms a protective layer that shields us from the sun's harmful ultraviolet rays. This beneficial ozone has been partially destroyed by manmade chemicals, causing what is sometimes called a "hole in the ozone." The good news is, this hole is diminishing. Learn more about stratospheric, or "good" ozone.

Ozone at ground level is a harmful air pollutant, because of its effects on people and the environment, and it is the main ingredient in “smog." Learn more about air emission sources.

How does ground-level ozone form?

Tropospheric, or ground level ozone, is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). This happens when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight.

Ozone is most likely to reach unhealthy levels on hot sunny days in urban environments, but can still reach high levels during colder months. Ozone can also be transported long distances by wind, so even rural areas can experience high ozone levels.

What are the harmful effects of ozone?

What are the health effects of ozone?

Ozone in the air we breathe can harm our health. People most at risk from breathing air containing ozone include people with asthma, children, older adults, and people who are active outdoors, especially outdoor workers. In addition, people with certain genetic characteristics, and people with reduced intake of certain nutrients, such as vitamins C and E, are at greater risk from ozone exposure.

Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and airway inflammation. It also can reduce lung function and harm lung tissue. Ozone can worsen bronchitis, emphysema, and asthma, leading to increased medical care. Learn more about health effects.

What are the environmental effects of ozone?

Ozone affects sensitive vegetation and ecosystems, including forests, parks, wildlife refuges and wilderness areas. In particular, ozone harms sensitive vegetation during the growing season. Learn more about how ozone harms ecosystems.

What are the ozone levels in my community?

Air quality forecasts are often given with weather forecasts on handheld devices, online or in the paper or television. You can check ozone levels and other daily air quality information by visiting www.airnow.gov and in many areas you can receive air quality notifications through www.enviroflash.info.

What can I do to reduce ozone?

There are actions every one of us can take to reduce air pollution and keep the air cleaner and precautionary measures you can take to protect your health. 

What is being done to reduce ozone pollution?

How is ozone pollution controlled?

Ozone is one of the six common air pollutants identified in the Clean Air Act. EPA calls these “criteria air pollutants” because their levels in outdoor air need to be limited based on health criteria.

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.

After working with the states and tribes and considering the information from air quality monitors, EPA "designates" an area as attainment or nonattainment with national ambient air quality standards. 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, EPA will designate the area as a "maintenance area."

What rules or regulations help reduce ozone pollution?

EPA’s national and regional rules to reduce emissions of pollutants that form ground level ozone will help state and local governments meet the Agency’s national air quality standards. Actions include vehicle and transportation standards, regional haze and visibility rules, and regular reviews of the NAAQS.

The Earth's ozone layer protects all life from the sun's harmful radiation, but human activities have damaged this shield. Less ozone-layer protection from ultraviolet (UV) light will, over time, damage crops and lead to higher skin cancer and cataract rates.

I. The Ozone Layer

The Earth's atmosphere is composed of several layers. The lowest layer, the  troposphere , extends from the Earth's surface up to about 6 miles or 10 kilometers (km) in altitude. Virtually all human activities occur in the troposphere. Mt. Everest, the tallest mountain on the planet, is only about 5.6 miles (9 km) high. The next layer, the  stratosphere , continues from 6 miles (10 km) to about 31 miles (50 km). Most commercial airplanes fly in the lower part of the stratosphere.

Most atmospheric ozone is concentrated in a layer in the stratosphere, about 9 to 18 miles (15 to 30 km) above the Earth's surface (see the figure below). Ozone is a molecule that contains three oxygen atoms. At any given time, ozone molecules are constantly formed and destroyed in the stratosphere. The total amount has remained relatively stable during the decades that it has been measured.

The ozone layer in the stratosphere absorbs a portion of the radiation from the sun, preventing it from reaching the planet's surface. Most importantly, it absorbs the portion of UV light called UVB. UVB has been linked to many harmful effects, including skin cancers, cataracts, and harm to some crops and marine life.

Scientists have established records spanning several decades that detail normal ozone levels during natural cycles. Ozone concentrations in the atmosphere vary naturally with sunspots, seasons, and latitude. These processes are well understood and predictable. Each natural reduction in ozone levels has been followed by a recovery. Beginning in the 1970s, however, scientific evidence showed that the ozone shield was being depleted well beyond natural processes.

II. Ozone Depletion

When chlorine and bromine atoms come into contact with ozone in the stratosphere, they destroy ozone molecules. One chlorine atom can destroy over 100,000 ozone molecules before it is removed from the stratosphere. Ozone can be destroyed more quickly than it is naturally created.

Some compounds release chlorine or bromine when they are exposed to intense UV light in the stratosphere. These compounds contribute to ozone depletion, and are called ozone-depleting substances ( ODS ). ODS that release chlorine include  chlorofluorocarbons  (CFCs),  hydrochlorofluorocarbons  (HCFCs),  carbon tetrachloride , and  methyl chloroform . ODS that release bromine include  halons  and  methyl bromide . Although ODS are emitted at the Earth’s surface, they are eventually carried into the stratosphere in a process that can take as long as two to five years.

In the 1970s, concerns about the effects of ozone-depleting substances ( ODS ) on the stratospheric  ozone layer  prompted several countries, including the United States, to ban the use of chlorofluorocarbons ( CFCs ) as  aerosol  propellants. However, global production of CFCs and other ODS continued to grow rapidly as new uses were found for these chemicals in refrigeration, fire suppression, foam insulation, and other applications.

Some natural processes, such as large volcanic eruptions, can have an indirect effect on ozone levels. For example, Mt. Pinatubo's 1991 eruption did not increase stratospheric chlorine concentrations, but it did produce large amounts of tiny particles called  aerosols  (different from consumer products also known as aerosols). These aerosols increase chlorine's effectiveness at destroying ozone. The aerosols in the stratosphere create a surface on which CFC-based chlorine can destroy ozone. However, the effect from volcanoes is short-lived.

Not all chlorine and bromine sources contribute to ozone layer depletion. For example, researchers have found that chlorine from swimming pools, industrial plants, sea salt, and volcanoes does not reach the stratosphere. In contrast, ODS are very stable and do not dissolve in rain. Thus, there are no natural processes that remove the ODS from the lower atmosphere.

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.

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. More information about the global extent of ozone depletion can be found in the Scientific Assessment of Ozone Depletion: 2018 developed by the United Nations Environment Programme.

Ozone in the air we breathe can harm our health, especially on hot sunny days when ozone can reach unhealthy levels. Even relatively low levels of ozone can cause health effects.

Who is at risk?

People most at risk from breathing air containing ozone include people with asthma, children, older adults, and people who are active outdoors, especially outdoor workers. In addition, people with certain genetic characteristics, and people with reduced intake of certain nutrients, such as vitamins C and E, are at greater risk from ozone exposure. 

Children are at greatest risk from exposure to ozone because their lungs are still developing and they are more likely to be active outdoors when ozone levels are high, which increases their exposure.  Children are also more likely than adults to have asthma.

What health problems can ozone cause?

Depending on the level of exposure, ozone can:

• Cause coughing and sore or scratchy throat.
• Make it more difficult to breathe deeply and vigorously and cause pain when taking a deep breath.
• Inflame and damage the airways.
• Make the lungs more susceptible to infection.
• Aggravate lung diseases such as asthma, emphysema, and chronic bronchitis.
• Increase the frequency of asthma attacks.

Some of these effects have been found even in healthy people, but effects can be more serious in people with lung diseases such as asthma. They may lead to increased school absences, medication use, visits to doctors and emergency rooms, and hospital admissions. 

Long-term exposure to ozone is linked to aggravation of asthma, and is likely to be one of many causes of asthma development. Studies in locations with elevated concentrations also report associations of ozone with deaths from respiratory causes.

How can I reduce these health risks?

The AirNow Web site provides daily air quality reports for many areas. These reports use the Air Quality Index (or AQI) to tell you how clean or polluted the air is.

Ozone can affect sensitive vegetation and ecosystems, including forests, parks, wildlife refuges and wilderness areas.  In particular, ozone can harm sensitive vegetation during the growing season. 

What plants are at risk?

Plant species that are sensitive to the effects of ozone on their growth include trees found in many areas of the U.S., such as:

• black cherry
• quaking aspen
• tulip poplar
• white pine
• ponderosa pine
• red alder.

What does ozone exposure do to sensitive plants?

When sufficient ozone enters the leaves of a sensitive plant, it can:

• Reduce photosynthesis, which is the process that plants use to convert sunlight to energy to live and grow.
• Slow the plant's growth.
• Increase sensitive plants' risk of:
  • disease

  • damage from insects
  • effects of other pollutants
  • harm from severe weather.

Also, some plants can show visible marks on their leaves when ozone is present under certain conditions.

What happens to the ecosystem?

The effects of ozone on individual plants can then have negative impacts on ecosystems, including:

• changes to the specific assortment of plants present in a forest
• changes to habitat quality
• changes to water and nutrient cycles.

Breathing in ground-level ozone can be harmful to your health.

How can ozone affect my health?

If you have asthma, bronchitis, or emphysema, ozone can make your symptoms worse. Carefully follow your asthma management plan on days when ozone levels are high.

Ozone has also been linked to:

• Coughing and pain when taking a deep breath
• Lung and throat irritation
• Wheezing and trouble breathing during exercise or outdoor activities

Who is most at risk?

Ozone can affect anyone, but it bothers some people more than others. People most likely to experience health effects caused by ozone include:

• People with asthma or other lung diseases
• Older adults
• People of all ages who exercise or work hard outside
• Babies and children

Protect Yourself and Your Family

The good news is there’s a lot you can do to protect yourself and your family from the health effects caused by ground-level ozone. Start by learning about the Air Quality Index.

Air Quality Index (AQI)

The Air Quality Index (AQI) tells you when air pollution is likely to reach levels that could be harmful. You can use the AQI as a tool to help you avoid particle pollution. Local TV stations, radio programs, and newspapers report the AQI. Try checking it when you’re planning your daily activities.

Take Action

When ground-level ozone levels are high, take steps to limit the amount of air you breathe in while you’re outside. For example:

• Think about spending more time indoors, where ozone levels are usually lower.
• Choose easier outdoor activities (like walking instead of running) so you don’t breathe as hard.
• Plan outdoor activities at times when ozone levels are lower (usually in the morning and evening).