Smog is a form of air pollution. The term was first used in 1905 to describe the combination of smoke and thick fog that at times hung over London and other cities in the United Kingdom. Today, smog also refers to a condition caused by the action of sunlight on the exhaust gases from automobiles and factories. This type of smog is sometimes called photochemical smog.
Weather conditions such as a lack of wind or a thermal inversion 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. Mountain ranges near cities may also trap smog in an area.
Heavy concentrations of smog are poisonous. In 1948, 20 people died and nearly 6,000 became ill from a photochemical smog over Donora, Pennsylvania. About 4,000 Londoners died within five days as a result of a thick smog in 1952. Smog also destroys plant life. In addition, smog causes building materials to deteriorate faster than usual.
London-type smog
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 sulfur dioxide, a gas that attacks the lungs and makes breathing difficult. For this reason, London-type smog is sometimes called sulfur smog.
Photochemical smog
involves the action of sunlight on hydrocarbons and nitrogen oxides in the air. These compounds are chemicals released as emissions from automobiles and certain industrial processes. When activated by sufficient sunlight, the compounds undergo a chemical reaction that produces gases called oxidants. Ozone forms the most abundant oxidant in photochemical smog. It can irritate the eyes, nose, and throat and damage the lungs. Other oxidants in smog include nitrogen dioxide and peroxyacetyl nitrate (PAN).
