“Highly concentrated sources of oxygen promote rapid combustion and therefore are fire and explosion hazards in the presence of fuels. The fire that killed the Apollo 1 crew on a test launchpad spread so rapidly because the pure oxygen atmosphere was at normal atmospheric pressure instead of the one third pressure that would be used during an actual launch.”

LENNTECH.com http://www.lenntech.com/periodic/elements/o.htm#ixzz48HfY7per Water Treatment and Purification Company