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Photochemical Smog forms when primary pollutants react with ultraviolet light to create a variety of toxic and reactive compounds. It is yellowish brown smog which is formed under oxidising conditions and high temperature over cities and towns that are releasing a lot of nitrogen oxides and unburnt hydrocarbons. In still air the two interact photochemically to produce photochemical oxidants-ozone, PAN, aldehydes and phenols. .
 Effect on biological world -  It causes bleaching of foliage of certain plants, silvering and necrosis(death) of the leaves. It causes respiratory problems in humans.