 Clarification of OSHA's requirement for breathing air to have at least 19.5 percent oxygen content
  Clarification of OSHA's requirement for breathing air to have at least 19.5 percent oxygen content    "Human beings must breathe oxygen . . . to survive, and begin to suffer adverse health effects when the
oxygen level of their breathing air drops below [19.5 percent oxygen]."
https://www.osha.gov/laws-regs/standardinterpretations/2007-04-02-0