Air pollution may be defined as the introduction into the atmosphere of any material that brings about a negative effect on plants, animals, or people. The effect can vary from lowered visibility at the Grand Canyon to severe pollution outbreaks which may force city residents to stay indoors. Some pollutants--like soil dust, forest fire smoke, and the emissions of volcanoes--occur naturally, but most air pollution is caused by humans. Air pollution has become a serious problem all over the world and is especially problematic in large urban areas such as Mexico City, Mexico, and Athens, Greece. Many people suffer from illnesses caused by smog and air pollution in these areas.

The normal composition of unpolluted air close to the surface of Earth is about 78% nitrogen gas (N2) and 21% oxygen gas (O2). The remaining one percent is composed of a large number of minor gases, including argon...