Well, some years ago helium was a preferred way for suicide. This reflected very bad on the producers of party balloon helium tanks, so they added an amount of oxygen and it was no longer an effective way.
So the question becomes: How much of that atmosphere is helium?
> helium was a preferred way for suicide
The era of ridiculous sounding last words came to an end
Hmm, really? That's interesting.... [time passes] ... I found more information than I really needed on how to kill one's self with helium, and I saw some places making suggestions that helium be cut with oxygen, seemingly starting with a New Zealand coroner in 2011, but nothing suggesting this had been implemented at any sort of scale. The links I found on Amazon for party balloon helium tanks all mostly proudly state they are 99%+ helium.