'''Natural gas''' is a [[fossil fuel]]
found naturally in the Earth, consisting of about 70% [[methane]] . The rest is made up of [[ethane]] (15%) and other [[hydrocarbon |hydrocarbons]] .<ref>http://scifun.chem.wisc.edu/chemweek/methane/methane.html</ref> It is naturally oderless, so odor is added to make a leak detectible.
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.
<i>Energy Policy </i> 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. <i>Annual Energy Outlook 2010 </i> - 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. <i>Annual Energy Outlook 2010 </i> - Emissions Projections. http://www.eia.doe.gov/oiaf/archive/aeo10/emission.html</ref> .