In dry air that does not contain pollution, the predominant gas is nitrogen, which makes up 78 percent of the atmosphere. The next two most voluminous gases are oxygen (21 percent) and argon (1 percent). Other gases appearing in trace amounts include carbon dioxide.
The air naturally contains water vapor as well. The proportion of water vapor in the area varies widely between different areas, depending on the climate.
Human activity has had a significant impact on the gases in the atmosphere, particularly through the burning of fuels and the production of pollutant gases. Burning fossil fuels sends a great deal of carbon dioxide into the atmosphere.
Other gases that enter the atmosphere as pollution include sulfur dioxide, nitrogen oxides and carbon monoxides. When fuels do not burn completely, they send solid particles, as with carbon, directly into the air, causing a dirty film to form on buildings and on the inside of lungs.
Carbon monoxide is one of the most toxic by-products among these pollutants. Inhaling carbon monoxide can lead to death. However, the other pollutants also are caustic to the environment; sulfur dioxide leads to acid rain, which can maim or even kill crops, trees and other plants.