Flames are a serious hazard to crewmembers confined in sealed cabins. The flame hazard can be diminished by lowering the oxygen concentration, this being accomplished by reducing the partial pressure of oxygen or raising the partial pressure of nitrogen in the chamber. Excessive modification of the atmosphere can cause one of the following medical problems; hypoxia, barotrauma, nitrogen narcosis, or decompression sickness. These conditions establish the basic medical criteria for designing habitable atmospheres to reduce the flame hazard of fires. Experimental evidence supports the use of 130 torr oxygen to design habitable, flame-suppresant atmospheres.