Smog

Smog, mixture of solid and liquid fog and smoke particles formed when humidity is high and the air so calm that smoke and fumes accumulate near their source. Smog reduces natural visibility and often irritates the eyes and respiratory tract. In dense urban areas, the death rate may rise considerably during prolonged periods of smog, particularly when a process of heat inversion creates a smog-trapping ceiling over a city. Smog occurs most often in and near coastal cities and is an especially severe air pollution problem in Athens, Los Angeles, and Tokyo.

Smog prevention requires control of smoke from furnaces; reduction of fumes from metal-working and other industrial plants; and, increasingly, control of noxious emissions from motor vehicles and incinerators. Internal-combustion engines are regarded as one of the main contributors to the smog problem, emitting large amounts of contaminants, including unburned hydrocarbons and oxides of nitrogen. The number of undesirable components in smog, however, is considerable, and the proportions highly variable. They include ozone, sulphur dioxide, hydrogen cyanide, and hydrocarbons and their products formed by partial oxidation. Fuel obtained from fractionation of coal and petroleum produces sulphur dioxide, which is oxidized by atmospheric oxygen, forming sulphur trioxide (SO₃). Sulphur trioxide is in turn hydrated by the water vapour in the atmosphere to form sulphuric acid (H₂SO₄).

So-called photochemical smog, which irritates sensitive membranes and damages plants, is formed when nitrogen oxides in the atmosphere undergo reactions with the hydrocarbons energized by ultraviolet and other types of radiation from the Sun.