A combination of oxides of nitrogen and sulphur, partially oxidised hydrocarbons and their derivatives produced by industries and automobiles form dark, thick dust and soot-laden fog known as smog.
Damaging effect: Smog is noxious and irritating. It reduces visibility, induces respiratory troubles and can cause death by suffocation.
Photochemical smog: Damages the tissues of certain plants and even decreases the yield of citrus fruits and grapes.