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Ozone is an invisible gas which is a form of molecular oxygen (three oxygen atoms linked together). It occurs naturally in the upper atmosphere (about nine to 13 miles above the earth’s surface), and protects life on earth by filtering out harmful ultraviolet radiation from the sun. Ozone at ground level, however, is a harmful pollutant. It is a major component of smog and is the result of complex chemical reactions involving vehicle exhaust and sunlight.

Ozone is not emitted directly from mobile or stationary sources but is formed when volatile organic compounds and oxides of nitrogen react in the presence of sunlight. The rate at which the reactions proceed is related to both temperature and intensity of the sunlight. 

Elevated ozone levels typically occur on hot summer afternoons. Oxides of nitrogen and volatile organics (known as precursors to ozone formation) originate from many different sources. Typical urban area sources are emissions from cars, buses and trucks, and off-road mobile sources, such as construction vehicles, planes and trains, power plants and factories. 

Ozone is a severe irritant to the respiratory system and can cause shortness of breath, coughing, wheezing and stinging eyes. It can damage lung tissue and make people more susceptible to respiratory infections. Ozone is especially harmful to children, the elderly, and those with impaired health. This includes people with respiratory problems such as asthma, emphysema, chronic bronchitis and cardiovascular patients. It also inhibits plant growth and can cause damage to crops and forests. 

The EPA strengthened the ozone standards in 2008 and is currently reviewing data to possibly lower this 2008 standard. The EPA is expected to issue a new ozone standard in 2014.

Current ozone standard

  • 0.075 ppm (8-hour average)

Ozone Trends


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