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Ozone has the same chemical structure whether it occurs miles above the earth or at ground level. Ozone (O3) is composed of three oxygen atoms. It is not usually emitted directly into the air, but at ground level is created by a chemical reaction between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of heat and sunlight.

VOC + NOx + Heat + Sunlight = Ozone

The summertime pollutant

Peak ozone levels typically occur during hot, dry, stagnant summertime conditions. The length of the ozone season varies from one area of the United States to another. Southern and Southwestern states may have an ozone season that lasts nearly the entire year.

Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOC, that help to form ozone. Sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air. As a result, it is known as a summertime air pollutant.

Many urban areas tend to have high levels of "bad" ozone, but even rural areas are also subject to increased ozone levels because wind carries ozone and pollutants that form it hundreds of miles away from their original sources.

Impacts of ground-level (bad) ozone

Ground-level ozone even at low levels can adversely affect everyone. It can also have detrimental effects on plants and ecosystems.

Health problems

Roughly one out of every three people in the United States is at a higher risk of experiencing ozone-related health effects. Sensitive people include children and adults who are active outdoors, people with respiratory disease, such as asthma, and people with unusual sensitivity to ozone.

  • One group at high risk from ozone exposure is active children because this group often spends a large part of the summer playing outdoors. However, 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 less sensitive individuals.
  • Though scientists don’t yet know why, some healthy people experience health effects at more moderate levels of outdoor exertion or at lower ozone levels than the average person.
  • 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 reduction in lung function may limit a person’s ability to engage in vigorous outdoor activities.
  • 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, 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 less lung elasticity, permanent loss of lung function and a lower quality of life.

Plant and ecosystem damage

  • Ground-level ozone interferes with the ability of plants to produce and store food, which makes them more susceptible to disease, insects, other pollutants and harsh weather.
  • Ground-level ozone damages the leaves of trees and other plants, ruining the appearance of cities, national parks and recreation areas.
  • Ground-level ozone reduces crop and forest yields and increases plant vulnerability to disease, pests and harsh weather.

Note: The source for much of the information on these pages was the U.S. Environmental Protection Agency.

Back to Top Page Last Edited: Fri Dec 18, 2015 11:41:19 AM

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