Comment by teraflop

I'm not sure that's true.

If you displace 30% of the atmosphere with helium, the O2 concentration drops from 21% to 15%. That's low enough that you'll probably start to see mild cognitive impairment, but it's still plenty capable of supporting life. However, a 30/70 mixture of helium and air is about 67% as dense as normal air, which means voices would be pitched more than half an octave higher.