Flak and enemy fighter attacks were certainly concerning but to me the cold temperatures and use of oxygen were at the top of the list. We could dress for the cold and the 24 volt electric heated suits off set the discomfort of the cold. The use of oxygen was another story. A crewman could die in 7 minutes with out oxygen at mission altitudes. we went on oxygen at 10,000 feet and remained on it until letting down to about 15,000 feet on the return. If by chance your oxygen mask hose became disconnected you simply went to sleep without any warning. To off set this possibility, we did oxygen checks. I was assigned that responsibility. Beginning at 18,000 feet I would call for a tail to nose check from the tail, belly turret, waist gunners, radio operator and flight engineer. Each would reply "okay". I called for a check every 4 minutes during the flight. The only exception was when on the bomb run and I was preoccupied. The pilots and navigator did not respond because I could see them from the bombardiers position. We had no emergencies on the missions I flew. However, had there been one, I would have asked a near by crewman to check it out.