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Skip Navigation LinksDNREC : Division of Air Quality : GoodOzone

Good Ozone, Bad Ozone


Ozone is a naturally occurring gas that is found in two layers of the atmosphere. In the layer surrounding the Earth’s surface—the troposphere—ground level or "bad" ozone is an air pollutant that is a key ingredient of urban smog. The troposphere extends about 6 miles up to the stratosphere, which is where "good" ozone protects life on Earth by absorbing some of the sun’s UV rays. This layer ends at about 30 miles above the surface.

The stratospheric ozone layer acts as a shield, protecting life on Earth from the sun’s ultraviolet (UV) rays. In the 1980s, scientists began accumulating evidence that the ozone layer was being depleted by man-made chemicals. Depletion of the ozone layer results in increased UV radiation reaching the Earth’s surface, which in turn can lead to a greater chance of overexposure to UV radiation and the related health effects of skin cancer, cataracts, and immune suppression. To prevent further ozone depletion, the government has phased out the manufacture and use of ozone-depleting chemicals.

Ground-level (bad!) ozone (the primary constituent of smog) continues to be a pervasive pollution problem throughout many areas of the United States. Ozone is not emitted directly into the air but is formed by the reaction of VOCs and NOx in the presence of heat and sunlight. Ground-level ozone forms in the atmosphere all year round but most readily during hot summer weather. VOCs are emitted from a variety of sources, including motor vehicles, chemical plants, refineries, factories, consumer and commercial products (such as hair spray, cleaners and pesticides), and other industrial sources. Nitrogen oxides are emitted from motor vehicles, power plants, and other kinds of engines. Ozone and the precursor pollutants that cause ozone can be transported into an area from pollution sources found hundreds of miles upwind. Of the 492 tons per day of emissions, which form ozone, 27% come from stationary sources (e.g., stacks), 37% from mobile sources (i.e. vehicles), and the balance from natural emissions.

Short-term (1-3 hours) and prolonged (6-8 hours) exposures to ambient ozone have been linked to a number of adverse health effects. Repeated exposures to ozone can make people more susceptible to respiratory infection, result in lung inflammation, and aggravate pre-existing respiratory diseases such as asthma.

Ozone also affects vegetation and ecosystems, leading to reductions in agricultural and commercial forest yields, reduced growth and survivability of tree seedlings, and increased plant susceptibility to disease, pests, and other environmental stresses (e.g., harsh weather). Ground-level ozone damage to the foliage of trees and other plants also can decrease the aesthetic value of ornamental species as well as the natural beauty of our national parks and recreation areas. Ozone pollution is responsible for over $500 million a year in crop losses in the U.S.

Ambient ozone trends are influenced by year-to-year changes in meteorological conditions, population growth, loadings of VOC and NOx in the atmosphere, and by changes in emissions from ongoing control measures.

When ozone approaches unhealthy levels, an Air Quality Action Day is called. Air Quality Action Days for Ozone occur when the predicted weather the next day is ripe for ozone formation. On those days, the public is asked to curtail activities that could form ozone and avoid outdoor exposure. Air Quality Action Days are announced at 2:30 PM each day for the following day.

To find out what the ozone forecast is for the region, check the weather segment of your local television news program or log on to the Air Quality Partnership of the Delaware Valley Air Quality Forecast at, or call the Hotline at 1-800-872-7261.  Current hourly air monitoring information for Delaware’s air monitoring stations also include near real time ozone levels. This information is posted on the Department of Natural Resources and Environmental Control’s web page at

For more information contact:

Joseph Martini
(302) 323-4542

Revised: 01/11/13 

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