Pollution from cars is one of the main causes of smog. Click on image for full size (52 Kb JPEG) Image courtesy UCAR.

Smog is a type of air pollution. Smog is a mixture of smoke and fog. Smog usually forms when smoke from pollution mixes with fog. For example, London, England, is often very foggy. Most people in London used to heat their homes by burning coal. The coal made lots of smoke, which mixed with fog to form smog. London used to have a lot of smog.

There is a special kind of smog called photochemical smog. It forms when photons of sunlight hit molecules of different kinds of pollutants in the atmosphere. The photons make chemical reactions happen. The pollution molecules turn into other kinds of nasty chemicals. That mixture of bad chemicals is called photochemical smog.

The chemicals in photochemical smog include nitrogen oxides, Volatile Organic Compounds (VOCs), ozone, and PAN (peroxyacytyl nitrate). Nitrogen oxides mostly come from the engines of cars and trucks. VOCs are given off by paint, gasoline, and pesticides. Ozone is a form of oxygen that is harmful. PAN is a type of pollution that is made by chemical reactions between other kinds of pollution.

Smog smells bad and makes it hard for people to breath. It can also damage materials. Smog is a very harmful kind of air pollution.