High Purity Helium

After hydrogen, helium is the second lightest and second most abundant element in the observable universe.
One industrial application for helium is leak detection. Because helium diffuses through solids three times faster than air, it is used as a tracer gas to detect leaks in high-vacuum equipment (such as cryogenic tanks) and high-pressure containers.
Because it is lighter than air, airships and balloons are inflated with helium for lift. While hydrogen gas is more buoyant, and escapes permeating through a membrane at a lower rate, helium has the advantage of being non-flammable, and indeed fire-retardant.

Helium as a breathing gas has no narcotic properties, so helium mixtures such as trimix, heliox and heliair are used for deep diving to reduce the effects of narcosis, which worsen with increasing depth.[143][144] As pressure increases with depth, the density of the breathing gas also increases, and the low molecular weight of helium is found to considerably reduce the effort of breathing by lowering the density of the mixture