Air pollution is any type of pollution that hangs in the atmosphere. Some air pollution results in the creation of smog; visible haze made up of smoke and other pollutants that looms over cities, and causes some people to need to wear breathing masks. Not all air pollutants are visible though; some, such as particulate matter, are not visible to the human eye, but are no less harmful. All types of pollution are harmful in one way or another, but air pollution is the most directly so. Pollutants get into the lungs and cause sickness, disease, and occasionally even death.