What is smog?

Smog is a serious form of air pollution that can cause breathing problems. The tem smog was frist used to describe a type of air pollution in London, when smoke from coal fires combined with fog to produce dense yellow clouds that sometimes persisted for days. The term now describes the hazy fog that is caused mainly by vehicle exhausts and smoke from factories. It builds up when there is little air movement, and can happen when a layer of warm air traps cooler air from the ground, preventing the smog from being blown away. Cities in bowls or sheltered valleys are particularly liable to this condition.