Air pollution is the contamination of the atmosphere, or the air in a building, by harmful gases, vapors, aerosols, and dust particles, resulting principally from human activities but to a lesser extent from natural processes. Natural pollutants include pollen particles, salt-water spray, wind-blown dust, and fine debris from volcanic eruptions. Most human-made air pollutants are the products of combustion – smoke (from burning wood, coal, and oil in municipal, industrial, and domestic furnaces); carbon monoxide from automobiles; and oxides of nitrogen and sulfur dioxide (mainly from burning coal) – though other industrial processes, crop-spraying, and (in the past) atmospheric nuclear explosions also contribute. Most air pollution arises in the urban environment, with a large portion of that coming from road vehicles. See also air pollution control.