The Photochemical smog reference article from the English Wikipedia on 24-Apr-2004
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Photochemical smog

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Photochemical smog is the term to represent a multitude of chemical agents which are considered to be detrimental to the environment and health. They are called photochemical smog because they are produced by the photolysis of primary pollutants. Photochemical smog includes the following:

All of these are usually highly reactive and/or oxidising, and so photochemical smog is considered to be a problem of modern industrialization.