They had reasons. See: https://en.wikipedia.org/wiki/Apollo_1#Choice_of_pure_oxygen_atmosphere https://en.wikipedia.org/wiki/Apollo_1#Choice_of_pure_oxygen...

They had reasons. See: https://en.wikipedia.org/wiki/Apollo_1#Choice_of_pure_oxygen_atmosphere https://en.wikipedia.org/wiki/Apollo_1#Choice_of_pure_oxygen... Even after the fire, the Apollo spacecraft still used 100% oxygen when in space. The cabin was 60% oxygen / 40% nitrogen at 14.7 psi at launch, reducing to 5 psi on ascent by venting, with the nitrogen then being purged and replaced with 100% oxygen. > See Murpheys Law... Indeed. I hope that was a joke.