"technical-grade" (five nines pure, 99.999%) produced by methane pyrolysis or electrolysis, which is suitable for applications such as fuel cells, and "commercial-grade", which has carbon- and sulfur-containing impurities, but which can be produced by the slightly cheaper steam-reformation process that releases carbon dioxide greenhouse gas. Fuel cells require high-purity hydrogen because the impurities would quickly degrade the life of the fuel cell stack. Much of the interest in the hydrogen economy concept is focused on the use of fuel cells to power hydrogen vehicles, particularly large trucks. Hydrogen fuel cells suffer from a low power-to-weight ratio. Fuel cells are more efficient than internal combustion engines. If a practical method of hydrogen storage is introduced, and fuel cells become cheaper, they can be economically viable to power hybrid fuel cell/battery vehicles, or purely fuel cell-driven ones. The combination of the fuel cell and electric motor is 2-3 times more efficient than an internal-combustion engine. Capital costs of fuel cells have reduced significantly over recent years, with a modeled cost of $50/kW cited by the Department of Energy. Other fuel cell technologies based on the exchange of metal ions (e.g. zinc–air fuel cells) are typically more efficient at energy conversion than hydrogen fuel cells, but the widespread use of any electrical energy → chemical energy → electrical energy systems would necessitate the production of electricity. In the short term hydrogen has been proposed as a method of reducing harmful diesel exhaust. For example, by 2050, the energy requirement for transportation might be between 20% and 30% fulfilled by hydrogen and synthetic fuels. Safety Hydrogen has one of the widest explosive/ignition mix range with air of all the gases with few exceptions such as acetylene, silane, and ethylene oxide. This means that whatever the mix proportion between air and hydrogen, when ignited in an enclosed space a hydrogen leak will most likely lead to an explosion, not a mere flame. This makes the use of hydrogen particularly dangerous in enclosed areas such as tunnels or underground parking. Pure hydrogen-oxygen flames burn in the ultraviolet color range and are nearly invisible to the naked eye, so a flame detector is needed to detect if a hydrogen leak is burning. Like natural gas, hydrogen is odorless and leaks cannot be detected by smell. This is the reason odorant chemical is injected into the natural gas to deliver the rotten-egg odor. Hydrogen codes and standards are