 The United States holds less than 5% of the world's population, but due to large houses and private cars, uses more than 25% of the world's supply of fossil fuels.As the largest source of U.S. greenhouse gas emissions, CO2 from fossil fuel combustion accounted for 80 percent of weighted emissions in 1998.Combustion of fossil fuels also produces other air pollutants, such as nitrogen oxides, sulfur dioxide, volatile organic compounds and heavy metals.