Taken from "Ban helium balloons this Christmas, academic warns" by the Guardian:
Helium is a non-renewable gas that is used to cool magnets in MRI scanners in hospitals. It is also mixed with oxygen to make breathing easier for ill patients and can help save new-born babies’ lives.
However, there is currently a global shortage of the gas, which cannot be synthesized. The gas has to be extracted from beneath the earth’s crust and 75 per cent of the world’s helium comes from the US.