What is 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.

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

Ground-level ozone also damages vegetation and ecosystems. In the United States alone, ozone is responsible for an estimated $500 million in reduced crop production each year.

Under the Clean Air Act, EPA has set protective health-based standards for ozone in the air we breathe. EPA and others 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 and innovative programs also encourage communities to adopt practices, such as carpooling, to reduce harmful emissions.

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

The Clean Air Act requires EPA to set air quality standards to protect both public health and the public welfare (e.g. crops and vegetation). Ground-level ozone affects both.

Health and environment

People with lung disease, children, older adults, and people who are active can be affected when ozone levels are unhealthy. Numerous scientific studies have linked ground-level ozone exposure to a variety of problems, including:

  • airway irritation, coughing, and pain when taking a deep breath;
  • wheezing and breathing difficulties during exercise or outdoor activities;
  • inflammation, which is much like a sunburn on the skin;
  • aggravation of asthma and increased susceptibility to respiratory illnesses like pneumonia and bronchitis; and,
  • permanent lung damage with repeated exposures.
How can ground-level ozone affect your health?
  • Ozone can irritate your respiratory system, causing you to start coughing, feel an irritation in your throat and/ or experience an uncomfortable sensation in your chest.
  • Ozone can reduce lung function and make it more difficult for you to breathe as deeply and vigorously as you normally would. When this happens, you may notice that breathing starts to feel uncomfortable. If you are exercising or working outdoors, you may notice that you are taking more rapid and shallow breaths than normal.
  • Ozone can aggravate asthma. When ozone levels are high, more people with asthma have attacks that require a doctor’s attention or the use of additional medication.
  • One reason this happens is that ozone makes people more sensitive to allergens, which are the most common triggers for asthma attacks. Also, asthmatics are more severely affected by the reduced lung function and irritation that ozone causes in the respiratory system.
  • Ozone can inflame and damage cells that line your lungs. Within a few days, the damaged cells are replaced and the old cells are shed—much in the way your skin peels after a sunburn. Ozone may aggravate chronic lung diseases such as emphysema and bronchitis and reduce the immune system’s ability to fight off bacterial infections in the respiratory system.
  • Ozone may cause permanent lung damage.
Repeated short-term ozone damage to children’s developing lungs may lead to reduced lung function in adulthood.

In adults, ozone exposure may accelerate the natural decline in lung function that occurs as part of the normal aging process.

Are there always symptoms?

Ozone damage also can occur without any noticeable signs. People who live in areas where ozone levels are frequently high may find that their initial symptoms go away over time—particularly when exposure to high ozone levels continues for several days. Ozone continues to cause lung damage even when the symptoms have disappeared.

The best way to protect your health is to find out when ozone levels are elevated in your area and take simple precautions to minimize exposure even when you don’t feel obvious symptoms.

How can you avoid unhealthy exposure to ozone?

Your chances of being affected by ozone increase the longer you are active outdoors and the more strenuous the activity you engage in. If you’re involved in an activity that requires heavy exertion, you can reduce the time you spend on this activity or substitute another activity that requires more moderate exertion (e.g., go for a walk rather than a jog).

In addition, you can plan outdoor activities when ozone levels are lower, usually in the morning or evening.

Examples of activities that involve moderate exertion include climbing stairs, playing tennis or baseball, simplegarden or construction work, and light jogging, cycling,or hiking. Activities that involve heavy exertion includeplaying basketball or soccer, chopping wood, heavy manuallabor, and vigorous running, cycling, or hiking. Becausefitness levels vary widely among individuals, what is moderateexertion for one person may be heavy exertion foranother. No matter how fit you are, cutting back on the level or duration of exertion when ozone levels are high will help protect you from ozone’s harmful effects.

See the Air Quality Guide for Ozone for ways to protect your family's health when ozone levels reach the unhealthy range and ways you can help reduce ozone air pollution.

Environmental effects

Ground-level ozone can have detrimental effects on plants and ecosystems. These effects include:

  • interfering with the ability of sensitive plants to produce and store food, making them more susceptible to certain diseases, insects, other pollutants, competition and harsh weather;
  • damaging the leaves of trees and other plants, negatively impacting the appearance of urban vegetation, as well as vegetation in national parks and recreation areas; and
  • reducing forest growth and crop yields, potentially impacting species diversity in ecosystems.
Are you at risk from ground-level ozone?

Several groups of people are particularly sensitive to ozone— especially when they are active outdoors—because physical activity causes people to breathe faster and more deeply.

Active children are the group at highest risk from ozone exposure because they often spend a large part of the summer playing outdoors. Children are also more likely to have asthma, which may be aggravated by ozone exposure.

Active adults of all ages who exercise or work vigorously outdoors have a higher level of exposure to ozone than people who are less active.

People with asthma or other respiratory diseases that make the lungs more vulnerable to the effects of ozone will generally experience health effects earlier and at lower ozone levels than less sensitive individuals. Scientists don’t yet know why, but some healthy people may experience health effects at more moderate levels of outdoor exertion or at lower ozone levels than the average person.

In general, as concentrations of ground-level ozone increase, more and more people experience health effects, the effects become more serious, and more people are admitted to the hospital for respiratory problems. When ozone levels are very high, everyone is at risk.

MNN Public Information from the U.S. Environmental Protection Agency