Some types of fuel cells produce only water, which means no pollution. Most types of fuel cell cause much less emissions than classic ("caloric") power generation. They can consume the same fuel types as classic power generators, for example Diesel engines, but they are about two times as efficient, meaning they can produce the same amount of energy with half as much fuel, and thus at least half as much pollution. Additionally, the direct conversion fuel cells use has less risk of producing secondary emissions like NOx, SOx, and particulate matter, which are side effects of combustion, contribute to global warming, and are known as . 