If you've ever been to Los Angeles, you know that a chemical haze called smog often pollutes the air. The word "smog" is a mixture of "smoke" and "fog."Coined in England during the industrial revolution, it was first used to describe the densely polluted air that killed over 1000 people in industrial Scottish towns.

There are actually two types of smog: sulfurous smog and photochemical smog. Sulfurous smog results from burning sulfur-bearing fossil fuels, which release a high concentration of sulfur oxides in the air. When the sulfur oxides mix with dampness, smog results. This is the type of smog that polluted towns in Britain in the early part of the last century.

Photochemical smog results mainly from exhaust emitted by cars in dense urban areas such as Los Angeles. When nitrogen oxides and hydrocarbon vapors from cars react with sunlight in the lower atmosphere, they produce ozone and nitrogen dioxide, both highly toxic gases. Together, these gases result in smog that pollutes the atmosphere, reduces visibility, and even causes breathing problems.

Perhaps the worst case of smog on record was the so-called "Killer Fog" in London in 1952. Actually, it was killer smog. Coal burning factories, combined with the introduction of diesel buses, caused a thick combination of sulfurous and photochemical smog to settle over London for over a week. Thousands of people died as a result. The main problem was breathing: reduced oxygen and more particles in the air left Londoners literally gasping for breath.