statues and structures. Fuel standards increasingly require that fuel producers extract sulfur from fossil fuels to prevent acid rain formation. This extracted and refined sulfur represents a large portion of sulfur production. In coal-fired power plants, flue gases are sometimes purified. More modern power plants that use synthesis gas extract the sulfur before they burn the gas. Hydrogen sulfide is about one-half as toxic as hydrogen cyanide, and intoxicates by the same mechanism (inhibition of the respiratory enzyme cytochrome oxidase), though hydrogen sulfide is less likely to cause sudden poisonings from small inhaled amounts (near its permissible exposure limit — PEL — of 20 ppm) because of its disagreeable odor. However, its presence in ambient air at concentration over 100-150 ppm quickly deadens the sense of smell, and a victim may breathe increasing quantities without noticing until severe symptoms cause death. Dissolved sulfide and hydrosulfide salts are toxic by the same mechanism. See also Blue lava Stratospheric sulfur aerosols Sulfur assimilation Ultra-low sulfur diesel References Further reading External links Sulfur at The Periodic Table of Videos (University of Nottingham) Atomic Data for Sulfur, NIST Physical Measurement Laboratory Sulfur phase diagram , Introduction to Chemistry for Ages 13–17 Crystalline, liquid and polymerization of sulfur on Vulcano Island, Italy Sulfur and its use as a pesticide The Sulphur Institute Nutrient Stewardship and The Sulphur Institute Chemical elements Chalcogens Reactive nonmetals Polyatomic nonmetals Agricultural chemicals Anti-acne preparations Dietary minerals Industrial minerals Inorganic polymers Native element minerals Orthorhombic minerals Minerals in space group 70 Pyrotechnic fuels Chemical elements with primitive orthorhombic structure