Gasoline (American English), or petrol (British English), is a colorless petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. On average, a 42-U.S.-gallon (160-liter) barrel of crude oil yields about 19 U.S. gallons (72 liters) of gasoline (among other refined products) after processing in an oil refinery, though this varies based on the crude oil assay. The characteristic of a particular gasoline blend to resist igniting too early (which causes knocking and reduces efficiency in reciprocating engines) is measured by its octane rating which is produced in several grades. Tetraethyl lead and other lead compounds are no longer used in most areas to increase octane rating (still used in aviation and auto-racing). Other chemicals are frequently added to gasoline to improve chemical stability and performance characteristics, control corrosiveness and provide fuel system cleaning. Gasoline may contain oxygen-containing chemicals such as ethanol, MTBE or ETBE to improve combustion. Gasoline used in internal combustion engines can have significant effects on the local environment, and is also a contributor to global human carbon dioxide emissions. Gasoline can also enter the environment uncombusted, both as liquid and as vapor, from leakage and handling during production, transport and delivery (e.g., from storage tanks, from spills, etc.). As an example of efforts to control such leakage, many underground storage tanks are required to have extensive measures in place to detect and prevent such leaks. Gasoline contains benzene and other known carcinogens.