Smog refers to an atmospheric condition of atmospheric instability, poor visibility, and large concentrations of gaseous and particulate air pollutants. The word "smog" is an amalgam of the words "smoke" and "fog." There are two types of smog: reducing smog characterized by sulfur dioxide and particulates, and photochemical smog characterized by ozone and other oxidants.

Reducing smog refers to air pollution episodes characterized by high concentrations of sulfur dioxide and smoke (or particulate aerosols). Reducing smog is also sometimes called London-type smog, because of famous incidents that occurred in that city during the 1950s.

Reducing smogs first became common when industrialization and the associated burning of coal caused severe air pollution by sulfur dioxide and soot in European cities. This air pollution problem first became intense in the nineteenth century, when it was first observed to damage human health, buildings, and vegetation.

There have been a number of...