Oxygen
Pure oxygen at about 5 psi was the standard Apollo atmosphere in space, although I understand there was some mixed gas combination used on the pad while the astronauts breathed pure oxygen in their suits. As for how the flammability problem compares to a normal atmosphere at normal pressure, I found this article on the subject. The bit about the amount of oxygen in Earth's atmosphere sounds a bit spooky at first, but a little reflection would show that this would be self-regulating.