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The Orion Crew Exploration Vehicle Module (CM) is being designed to operate in an atmosphere of up to 30% oxygen at a pressure of 10.2 psia for lunar missions. Spacecraft materials selection is based on a normal gravity upward flammability test conducted in a closed chamber under the worst expected conditions of pressure and oxygen concentration. Material flammability depends on both oxygen concentration and pressure, but since oxygen concentration is the primary driver, all materials are certified in the 30% oxygen, 10.2 psia environment. Extensive data exist from the Shuttle Program at this condition, which used essentially the same test methodology as the Constellation Program is currently using. Raising the partial pressure of oxygen in the Orion CM immediately before reentry, while maintaining the total cabin pressure at 14.7 psia, has been proposed to maximize the time the crew is able to breathe cabin air after splashdown.