§135.89 Pilot requirements: Use of oxygen. (4) If one pilot leaves a pilot duty station of an aircraft when operating at altitudes above 25,000 feet MSL, the

§135.89 Pilot requirements: Use of oxygen. (4) If one pilot leaves a pilot duty station of an aircraft when operating at altitudes above 25,000 feet MSL, the remaining pilot at the controls shall put on and use an approved oxygen mask until the other pilot returns to the pilot duty station of the aircraft. Additionally, it's either US airline operating procedure (or perhaps FAA regulations, I couldn't find it though) for a second crew member to stay in the cockpit when one of the pilots has to leave.