The self-ignition temperature of hydrogen is 550 degrees Celsius. Gasoline self-ignition temperature varies from 228-501 degrees Celsius, depending on the grade. Hydrogen burns readily with oxygen, releasing considerable energy and producing only water as exhaust. When hydrogen burns in air (which is mostly nitrogen), some oxides of nitrogen (NOx, contributors to smog and acid rain) can be formed, but much fewer pollutants are formed than when normal hydrocarbon fuels such as gasoline and diesel are burned. Because no carbon is involved, using hydrogen fuel eliminates carbon monoxide and carbon dioxide, and therefore does not contribute to global warming. In fact, when a hydrogen powered engine operates, it actually cleans the ambient air, by completing combustion of the unburned hydrocarbons that surround us.