Smog

Smog is a severe form of air pollution whose name is a portmanteau of smoke and fog, coined in the early twentieth century for a smoky haze notable for both its opacity and its smell; it typically combines nitrogen oxides, sulfur oxides, ozone, smoke, and particulates. It is produced by coal burning, vehicle exhaust, agricultural burning, industrial emissions, and forest fires, and further pollutants form when these emissions react photochemically in sunlight. Winter smog builds up in cold weather when a temperature inversion traps emissions from heating fuel near the ground, while summer or photochemical smog forms when vehicle and industrial emissions react with sunlight to create secondary pollutants, the hazy conditions long associated with cities such as Los Angeles.