A combination of several gases and particulate matter that forms during a cover of fog or in the event of a low lying temperature inversion. The gases present include sulfur dioxide as a result of the burning of coal for use as energy, and the particulate matter included calcium sulfide as a result of acid deposition. In 1952, over a four day period, a temperature inversion trapped smoke and sulfur dioxide at dangerous levels, >4 mg/m³ and >1. 3 ppmv respectively, both ten times the normal levels.