Smog is a form of air pollution.
The term was first used in 1905 to describe the combination of smoke and thick fog that hung over London and other cities in Great Britain. Today, smog also refers to a condition caused by the action of sunlight on the exhaust gases from automobiles and factories.
Weather conditions such as a lack of wind or a thermal inv.ersion may cause smog to build up in an area. A thermal inversion occurs when a layer of warm air settles over a layer of cool air that lies near the ground. This condition prevents the smog from rising and scattering.
Smog also destroys plant life and causes building materials to deteriorate faster than usual.
There are two kinds of smog:
The occurs when moisture in air condenses on smoke particles produced by the burning of coal, forming tiny smog droplets. A dangerous part of London-type smog is sulphur dioxide, a gas that attacks the lungs and makes breathing difficult. For this reason, it is sometimes called sulphur smog.
About 4000 people in London died within 5 days as a result of a thick smog in 1952.
The latter forms when various types of air pollution from industry and automobiles mix. When this occurs in the presence of sunlight and heat it causes chemical reactions that create toxic chemicals such as ozone. Ozone forms the most abundant oxidant in photochemical smog. It can irritate the eyes, nose and throat and damage the lungs.