Natural gas tens of millions of structures that use the fuel, the individual risk of using natural gas is very low. Natural gas heating systems may cause carbon monoxide poisoning if unvented or poorly vented. In 2011, natural gas furnaces, space heaters, water heaters and stoves were blamed for 11 carbon monoxide deaths in the US. Another 22 deaths were attributed to appliances running on liquified petroleum gas, and 17 deaths on gas of unspecified type. Improvements in natural gas furnace designs have greatly reduced CO poisoning concerns. Detectors are also available that warn of carbon monoxide and/or explosive gas (methane, propane,
Natural gas and shallow sediments. Deeper in the earth, at greater temperature and pressure, thermogenic gas is created from buried organic material. In petroleum production gas is often burnt as flare gas. The World Bank estimates that over 150 cubic kilometers of natural gas are flared or vented annually. Before natural gas can be used as a fuel, most, but not all, must be processed to remove impurities, including water, to meet the specifications of marketable natural gas. The by-products of this processing include: ethane, propane, butanes, pentanes, and higher molecular weight hydrocarbons, hydrogen sulfide (which may be converted into pure sulfur),