Smog is caused by the exhaust of fossil fuel engines, the emmitance of volatile organic compounds from paints and solvents, chemicals from pesticides and other chemical reactions and industrial processes. Smog can form in almost every climate, but is far worse in the warmer months.
Smogs were common in London, England in the beginning the 20th century and were nicknamed as "London Peculiars". From December 8, 1952 to March 1953 poisonous smog darkened the skies over London, and killed maybe 12,000 residents. Reluctant to admit coal smoke was to blame, British government blamed a flu epidemic.
Smog is still a problem in a number of cities and continues to end lives prematurely. Many local governments are still unwilling to accept the realities of smog.
For the rock band named Smog, see Smog (band).