Burning coal introduces pollutants such as nitrogen oxides, sulfur dioxide, and mercury into the atmosphere. A large amount of carbon dioxide, the principal pollutant implicated in global warming, is also produced by coal power plants. Aside from global warming, many of the pollutants from coal power plants are bad for human health and also cause smog, soot, and acid rain. The solid waste produced by burning coal can also be harmful to the environment. The ash from coal power plants is typically composed of about 5% pollutants or dangerous substances such as arsenic, cadmium, chromium, lead, and mercury. This can also cause pollution in water if it is not disposed of properly, and there have been many cases of water contamination caused by improper disposal of this ash.