oxygen
"Air pollution" is not just one gas. It can be any number of different gasses or particulates. For example, smoke from coal power plants contains large amounts of Sulphur Dioxide (SO2.) You could...
What gases cause air pollution
Oxygen, carbon dioxide,carbon monoxide, sulfuric acid, nitrogen, etc.
Worldwide Air Pollution
Air Pollution's Effect on Health
How Does Air Pollution Affect Skin
Dangers of Indoor Air Pollution to Children