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Helium (Chemical symbol He) is the lightest of all noble gases.

As well as a very low density, helium has a viscosity markedly less than that of air(or Nitrogen).

  • Used by cardiologists to fill the balloon of the Intra-aortic balloon pump.
  • Used in a mixture with oxygen to help people with laryngeal obstruction to breath more easily as the resistance to flow in either laminar or turbulent flow of the resulting 'heliox' mixture is markedly less than room or medical air or pure oxygen. The same physics provides a benefit for divers at great depths, where the effort of drawing conventional breathing mixtures through normal airways would be considerable. (The anaesthetic effect of Nitrogen is more of a problem there, Helium unlike Xenon is not an anaesthetic, perhaps because the molecule is so small it does not expand cell membranes - see theories of anaesthetic action.)

See also Poiseuille's law for laminar flow (viscosity).

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