The following are often quoted facts about hydrogen:

A flame resulting from the combustion of a gas mixture containing 77% hydrogen and 23% methane. Image courtesy of  Sandia National Laboratories, NREL/DOE

• Hydrogen is the most abundant element in the universe, making up 75% of matter by mass and over 90% by number of atoms.
• In stars, hydrogen nuclei combine with each other in nuclear fusion reactions to build helium atoms. These high-energy reactions create the light and heat of the Sun and most other stars.
• Hydrogen is an energy carrier not a source because it takes energy to isolate hydrogen from other compounds such as water or fossil fuels.
• World production of hydrogen is right around 50 million tons per year.
• Hydrogen may be produced through electrolysis of water which requires abundant and inexpensive electricity which may promote further development of renewable energy resources.
• Hydrogen is the lightest element and molecule.
• When released into the atmosphere, hydrogen dissipates quickly and therefore causes little danger of reaching combustible levels.
• Molecular hydrogen (H₂) is eight times lighter than natural gas.
• With light weight comes tremendous bulk, per unit volume at atmospheric pressure, hydrogen gas contains only 30% as much energy as natural gas.
• Storage of hydrogen onboard vehicles is considered a major hurdle in launching the hydrogen economy.
• Per unit weight, hydrogen has the highest energy content of any known fuel-120.7 kJ/g, while natural gas is 51.6 kJ/g and petroleum is 43.6 kJ/g.