Lifting gas
Gases theoretically suitable for lifting
The diffusion issue shared with Hydrogen (though, as Helium's molecular radius is smaller, it diffuses through more materials than Hydrogen). Helium is expensive. Although abundant in the universe, helium is very scarce on Earth. The only commercially viable reserves are a few natural gas wells, mostly in the US, that trapped it from the slow alpha decay of radioactive materials within Earth. By human standards helium is a non-renewable resource that cannot be practically manufactured from other materials. When released into the atmosphere, e.g., when a helium-filled balloon leaks or bursts, helium eventually escapes into space and is lost. Hydrogen (density 0.090 g/L at STP, average molecular mass 2.016 g/mol) and helium (density 0.179 g/L at STP, average molecular mass 4.003 g/mol) are the most commonly used lift gases. Although helium is twice as heavy as (diatomic) hydrogen, they are both so much lighter than air that this difference only results in hydrogen having 8% more buoyancy than helium.
Lifting gas-6