During 2021, natural gas supplied 32% of all energy used in the U.S., as compared to 36% for petroleum, 11% for coal, 8% for nuclear, 3% for wind, 2% for biofuels, 2% for hydroelectric, 2% for wood, 2% for solar, and less than 1% for biowaste and geothermal. Natural gas combustion generates the lowest air pollutant emissions of all fossil fuels. Prior to the early-to-mid 1900s, natural gas was not widely used because it was difficult to transport large amounts of it over long distances. Since then, advances in pipeline technology and infrastructure have made it economical to transport large quantities of natural gas.