Pretty much all modern EVA suits (like the ones used by US, China, Russia) run a pure oxygen at about .20 ATM so that the internal pressure is lower but the amount of oxygen the astronaut is breathing is the same density of oxygen as at 1 ATM. This allows much better maneuverability in the space suit, because any air mixture at 1 ATM makes it nearly immediately to move in a space suit (they become too stiff).

Point is, we have been doing spacewalks like this since spacewalks started and no one has spontaneously combusted on one.