Smog, When smoke mixes with fog, it is called smog. It is of two types:
1. Classical smog: It occurs in cool humid climate and is the result of build-up of sulphur dioxides and particulate matter from fuel combustion. It is reducing in nature.
2. Photochemical smog : It occurs in warm, dry and sunny climate and results from the action of sunlight on the nitrogen oxides and hydrocarbons produced by automobiles and factories. It is oxidizing in nature.