Human effects on the air are varied. Human activities, particularly the burning of fossil fuels by power plants and vehicles, emit many different gases such as carbon dioxide (CO₂), nitrogen oxides (NO_x), sulfur oxides (SO₂), and hydrocarbons (HC) such as methane. These are responsible for smog, acid rain, global warming, and ozone depletion. Human activities also produce particulates, such as dust, which contribute to respiratory disease.