Small amounts of hydrogen sulphide occur in crude petroleum, but natural gas can contain up to 90%. Volcanoes and some hot springs (as well as cold springs) emit some H2S, where it probably arises via the hydrolysis of sulphide minerals, i.e. MS + H2O → MO + H2S. Hydrogen sulphide can be present naturally in water, often as a result of the action of sulphate-reducing bacteria. About 10% of total global emissions of H2S is due to human activity. The largest industrial emission of H2S is from petroleum refineries. The hydrodesulphurization process liberates sulphur from petroleum by the action of hydrogen. The resulting H2S is converted to elemental sulphur by partial combustion via the Claus process, which is a major source of elemental sulphur. Hydrogen sulphide arises from anywhere where elemental sulphur comes in contact with organic material, especially at high temperatures.