In a nation-wide study done by the Environmental Research and Education Foundation in 2013, only 1,540 operational municipal solid waste landfills were counted throughout the United States. Decomposing waste in these landfills produce landfill gas, which is a mixture of about half methane and half carbon dioxide. Landfills are the third largest source of methane emissions in the United States, with municipal solid waste landfills representing 95 percent of this fraction.[5][6][citation needed]