Is breathing hydrogen and oxygen healthier than air?
What you breathe has to have, nominally, roughly a 20 kilopascal partial pressure of oxygen. You could breathe pure oxygen at 1/5th normal air pressure, and you’d be OK. How much the overall gas mixture weights per unit volume is not relevant, so replacing nitrogen with hydrogen would not cause problem as such. A lot more oxygen than what we have in the atmosphere would be toxic, not much better than not enough actually. That said, if there is any source of ignition around the hydrogen/oxygen mixture, a mere spark, a bit of heat, then the whole thing, including the content of your lungs would go kaboom. So that may not be the best of choice.
