Originally Posted by Nick Maclaren

A while ago, there was a debate of whether an air mixture of 50:50
oxygen and nitrogen (with appropriate water vapour and carbon
dioxide) at a pressure of 0.3-0.4 bar would have harmful effects
on humans if breathed for long periods (years or decades).
I never saw a resolution of that.

That is a very low pressure, but the partial pressure of oxygen
is comparable to a relatively modest altitude, so there isn't a
problem in that respect. The speculation (and it was no more)
was that that high a proportion of oxygen could burn the lungs.

Are there any known facts, or is it still guesswork?


Regards,
Nick Maclaren.