This fuel consists mostly of hydrocarbons and is sometimes enhanced with benzene to increase octane ratings and is used as fuel in internal combustion engines. Unleaded gasoline is manufactured to meet the requirement of new model automobiles in the US since 1975, because in that year catalytic converters were made mandatory for all vehicles sold in the US. Lead was removed from gasoline because lead emissions poison the metals used in catalytic converters. Lead had been added to gasoline since the 1920's as tetraethyllead (TEL), historically referred to as "ethyl. " Exposure to high levels of lead can cause vomiting, diarrhea, convulsions, coma or even death, but lead emissions from automobiles consisted of lead oxide particles which settle out near highways. Lead removal from gasoline in the US has resulted in a clear, measurable decrease in lead in the urban environments in the US.