Is breathing air with less than 19.5% oxygen content dangerous?

The rulemaking record for the Respiratory Protection Standard clearly justifies adopting the requirement that air breathed by employees must have an oxygen content of at least 19.5 percent. A lesser concentration of oxygen in employees' breathing air could endanger them physiologically and diminish their ability to cope with other hazards that may be present in the workplace. The rulemaking record also demonstrates that any workplace atmosphere controlled at or near your recommended minimal oxygen level of 100 mm of mercury at sea level (equivalent to about 13 percent oxygen at sea level) is not safe and healthful for all employees. Exposing employees to partial pressures of oxygen that approach 100 mm of mercury at sea level leaves them with no margin of safety from potentially debilitating effects, which could appear suddenly and without warning.

https://www.osha.gov/laws-regs/standardinterpretations/2007-04-02-0#:~:text=The%20rulemaking%20record%20for%20the,of%20at%20least%2019.5%20percent.