To supplement the onboard environmental control system, he wore a pressure suit with its own oxygen supply, which would also cool him. A cabin atmosphere of pure oxygen at a low pressure of (equivalent to an altitude of ) was chosen, rather than one with the same composition as air (nitrogen/oxygen) at sea level. This was easier to control, avoided the risk of decompression sickness ("the bends"), and also saved on spacecraft weight. Fires (which never occurred) would have to be extinguished by emptying the cabin of oxygen. In such case, or failure of the cabin pressure for any reason, the astronaut could make an emergency return to Earth, relying on his suit for survival. The astronauts normally flew with their visor up, which meant that the suit was not inflated. With the visor down and the suit inflated, the astronaut could only reach the side and bottom panels, where vital buttons and handles were placed.