Air pollution usually refers to the presence of any chemical or particulate that alters the normal make-up of our atmosphere, causing direct threats to human health (such as breathing difficulties) or longer-term damage through its effects on our planet's ecosystem. Pollutants include smoke and dust, nitrogen oxides, methane, and the fumes from aerosol sprays and other solvents. Industrial processes and transport are major contributors to air pollution, but it can also be caused by natural processes such as forest fires and volcanoes. See also acid rain.