Sulfur is an abundant element in Earth's crust, occurring mostly in minerals, but also as chemicals in water, the atmosphere, and organisms. The storage of sulfur in the various compartments of Earth and its biosphere, and the many transfers occurring among them, is referred to as the sulfur cycle.

Sulfur (S) occurs in the environment in many chemical forms, including organic and mineral compounds, either of which can be chemically transformed by both biological and inorganic processes. Sulfur dioxide (SO ₂) is a gas that is toxic to plants at concentrations smaller than one part per million (ppm) in the atmosphere, and to animals at larger concentrations. Natural sources of emissions of sulfur dioxide include volcanic eruptions and forest fires. Large emissions are also associated with human activities, especially the burning of coal and oil to generate electricity and the processing of sulfide metal ores. Gaseous hydrogen...