Originally, the term smog was coined to describe the mixture of smoke and fog that lowered visibility and led to respiratory problems in industrial cities. More recently, the term has come to mean any decrease in air quality whether associated with reduced visibility or a noticeable impact on human health. Smog occurs when emissions of gases and particles from industrial or transportation sources are trapped by the local meteorology so the concentrations rise and chemical reactions occur. It is common to distinguish between two types of smog: London smog and Los Angeles smog.

London, or sulphurous, smog was noted following the introduction of coal into cities. It is most prevalent in the fall or winter when cool conditions naturally produce a thick surface fog. This fog mixes with the smoke and gases from burning coal to produce a dark, thick, acrid sulphurous atmosphere. Normally, the unpolluted fog would...