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Sour crude oil is crude oil containing a high amount of the impurity sulfur. It is common to find crude oil containing some impurities. When the total sulfur level in the oil is more than 0.5% , the oil is called "sour".
The impurities need to be removed before this lower-quality crude can be refined into petrol, thereby increasing the cost of processing. This results in a higher-priced gasoline than that made from sweet crude oil.
Current environmental regulations in the United States strictly limit the sulfur content in refined fuels such as diesel and gasoline.
The majority of the sulfur in crude oil occurs bonded to carbon atoms, with a small amount occurring as elemental sulfur in solution and as hydrogen sulfide gas. Sour oil can be toxic and corrosive, especially when the oil contains higher levels of hydrogen sulfide, which is a breathing hazard. At low concentrations the gas gives the oil the smell of rotting eggs. For safety reasons, sour crude oil needs to be stabilized by having hydrogen sulfide gas removed from it before being transported by oil tankers.