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.