Control of sulfur emissions is a key environmental concern in the retorting of oil shale. Oil shale may contain 2% or more sulfur, while a typical shale in the Green River Formation of Colorado contains about 0.7% sulfur. Although this sulfur concentration is not large compared to some high sulfur coals, large-scale production of shale oil will require processing millions of tonnes of oil shale; thus, sulfur emissions would be very great if controls were not applied. About 16 to 30% of the sulfur in oil shale is liberated to the vapors produced during retorting, and the remainder stays with the spent shale. Retort gases generally contain hydrogen sulfide as the major sulfur compound and lesser amounts of other organic sulfur compounds such as sulfur dioxide, carbonyl sulfide, carbon disulfide, and mercaptans.