At the high cruising altitudes of jet airliners and other airplanes the air pressure is so low that lungs don't absorb enough oxygen. Crewmembers and passengers can die without an oxygen mask or cabin pressurization. Aircraft crews are trained to recognize and avoid the dangers of hypoxia in altitude chambers. The crew sits inside while much of the air is pumped out, forming a vacuum that simulates high altitudes. Some humane societies and animal shelters use similar chambers for euthanasia of unwanted pets.
Few people realize what it is like to be in such a chamber.
When I was a flight crewmember in the Air Force, annual altitude chamber training was required. About a half dozen of us would duck our heads and file through the small hatch into the tubular tank. Technicians directed us to seats and briefed us on the oxygen equipment and schedule of events. We put on our oxygen masks and adjusted the settings. Communication, by intercom mikes and headsets, was checked.