'''Natural gas''' is a [[fossil fuel]] that can be a byproduct of [[petroleum]] extraction and production (known as associated natural gas), extracted from separate natural gas fields (non-associated natural gas), or extracted from other areas such as coal-beds or shale (continuous or unconventional gas).<ref>U.S. Geological Survey. ''Natural Gas Production in the United States''. 2002. http://pubs.usgs.gov/fs/fs-0113-01/fs-0113-01.pdf</ref> It consists of about 75% [[methane]], and the rest is made up of [[ethane]] (15%) and other [[hydrocarbon]]s,<ref>http://scifun.chem.wisc.edu/chemweek/methane/methane.html</ref> but this varies by region and degree of refinement. It is naturally oderless, so odor is added to make a leak detectable.
When burned for electricity production (its most common use), natural gas produces significantly less [[pollution]] than [[coal]], the most popular source of energy in the United States for electricity production. Natural gas produces no sulfur dioxide (the main contributor to acid rain, and which coal produces, though in much smaller quantities after the 1990 amendment to the [[Clean Air Act]]). Natural gas also produces over 77% less nitrogen oxide air pollutants (important contributors to smog and respiratory health problems) compared to coal, on average.<ref>Proops, J.L.R., et. al. "The lifetime pollution implications of various types of electricity generation: an input-output analysis." 1996. ''Energy Policy'' Vol. 24, No. 3, pp. 229-237.</ref> As such, it is especially useful in urban environments (e.g., in fueling buses) where local concentrations of airborne pollutants can cause certain health issues. If considering [[carbon dioxide]] as a pollutant, natural gas produces over 36% less CO<sub>2</sub> emissions from electricity generation compared to coal, on average. In the U.S. in 2008, natural gas made up about 20% of electricity production<ref>U.S. Energy Information Administration. ''Annual Energy Outlook 2010'' - Electricity Demand. http://www.eia.doe.gov/oiaf/archive/aeo10/electricity.html</ref> and 16% of CO<sub>2</sub> emissions from electricity production, compared to 48% and 82%, respectively, for coal. CO<sub>2</sub> emissions from electricity generation make up 41% of total U.S. CO<sub>2</sub> emissions.<ref>U.S. Energy Information Administration. ''Annual Energy Outlook 2010'' - Emissions Projections. http://www.eia.doe.gov/oiaf/archive/aeo10/emission.html</ref>