Substances released by humans into the Earth’s atmosphere that can be harmful to people, as well as animals, plant life, and the environment in general. There many types of sources of air pollution, but topping the list of contributors are fossil fuels. Among the long list of pollutants that the burning of fossil fuels emits into the air are sulfur oxides, nitrogen oxides, carbon monoxide, and carbon dioxide. When sulfur oxides and nitrogen oxides are present together in the atmosphere, they combine to form the compounds that are the source of acid rain. Carbon emissions have also been blamed for global warming.