He hazard to airplanes

Date:         18 Mar 97 03:14:53 
From:         harrisp@nucleus.com
Organization: bovar
Followups:    1
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A co-worker of mine tried to bring a HeNe [Helium-Neon] laser on a
NorthWest Flight recently and was told that He was a hazard to air flight.
He couldn't even leave it in with his luggage.  My question is why is He,
a hazard to airplanes?  Is there some sort of sensing system that He will
interfere with?