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 etc.). Quantities of natural gas are measured in normal cubic meters (cubic meter of gas at "normal" temperature and pressure ) or standard cubic feet (cubic foot of gas at "standard" temperature and pressure ), . The gross heat of combustion of commercial quality natural gas is around , but this can vary by several percent. This is about (assuming a density of , an approximate value). Gas prices for end users vary greatly across the EU. A single European energy market, one of the key objectives of the EU, should level the prices of gas in all EU member