Gasoline /ˈɡæsəliːn/, or petrol /ˈpɛtrəl/, is a transparent, petroleum-derived liquid that is used primarily as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives; a 42 gallon barrel of crude oil yields about 19 gallons of gasoline, when processed in an oil refinery.The 'anti-knock' characteristic of a particular gasoline blend, when used as a fuel in internal combustion engines, is measured by its octane rating. Gasoline is produced in several grades of octane rating. Tetraethyl lead and other lead compounds are no longer used in most areas to regulate and increase octane-rating, but many other additives are put into gasoline to improve its chemical stability, control corrosiveness and provide fuel system 'cleaning,' and determine performance characteristics under intended use. Sometimes, gasoline also contains ethanol as an alternative fuel, for economic or environmental reasons.Gasoline, as used worldwide in the vast number of internal combustion engines used in transport and industry, has a significant impact on the environment, both in local effects (e.g., smog) and in global effects (e.g., effect on the climate). Gasoline may also enter the environment uncombusted, as liquid and as vapors, from leakage and handling during production, transport and delivery, 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. The material safety data sheet for unleaded gasoline shows at least 15 hazardous chemicals occurring in various amounts. Benzene and many anti-knocking additives are carcinogenic.Inhaled (huffed) gasoline vapor is a common intoxicant that has become epidemic in some poorer communities and indigenous groups in Australia, Canada, New Zealand, and some Pacific Islands. In response, Opal fuel has been developed by the BP Kwinana Refinery in Australia, and contains only 5% aromatics (unlike the usual 25%), which weakens the effects of inhalation.