Although trace amounts of helium can be produced through nuclear reactions, helium cannot be chemically synthesized or produced industrially. Instead, it can only be obtained at varying purity through physical purification. The concentration of helium in the air is merely 5.2 mL/m3(ppmV). Although helium can be industrially separated using non-condensable gases from large-scale air liquefaction and separation units, extracting helium from helium-containing natural gas is economically more feasible and remains the primary source of helium today. Helium content in helium-bearing natural gas can reach 0.3–8%. In some cases, even if the helium content in natural gas is less than 0.1%, helium can accumulate in the BOG from large-scale natural gas liquefaction plants, making helium recovery from BOG economically attractive.