Eventually. But I would expect that by the time the effect is noticeable, the concentrations will be too high to support consciousness and will have been for some time.
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.
Eventually. But I would expect that by the time the effect is noticeable, the concentrations will be too high to support consciousness and will have been for some time.
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.