大气里的氦只有几个PPM,从大气里提取氦气，代价太大了。工业用氦气基本上都是从天然气里提取的。
In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million.[65][66] The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth's atmosphere escapes into space by several processes.[67][68] In the Earth's heterosphere, a part of the upper atmosphere, helium and other lighter gases are the most abundant elements.
Nearly all helium on Earth is a result of radioactive decay, and thus an Earthly helium balloon is essentially a bag of retired alpha particles. Helium is found in large amounts in minerals of uranium and thorium, including cleveite, pitchblende, carnotite and monazite, because they emit alpha particles (helium nuclei, He2+) to which electrons immediately combine as soon as the particle is stopped by the rock. In this way an estimated 3000 metric tons of helium are generated per year throughout the lithosphere.[69][70][71] In the Earth's crust, the concentration of helium is 8 parts per billion. In seawater, the concentration is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. Because helium is trapped in a similar way by non-permeable layer of rock like natural gas the greatest concentrations on the planet are found in natural gas, from which most commercial helium is derived. The concentration varies in a broad range from a few ppm up to over 7% in a small gas field in San Juan County, New Mexico.[72][73]
link: http://en.wikipedia.org/wiki/Helium
