OSHA requires any atmosphere to which employees are exposed provide a minimum of 19.5% oxygen content.
Relevant sections in Reg 1910: "Paragraph (d)(2) (111) of the Respiratory Protection Standard with an oxygen level below 19.5 percent yo be oxygen deficient and an immediately dangerous to life or health. To ensure that employees have a reliable source of air with an oxygen content of at least 19.5%, paragraphs (d)(2)(i)(A) and (d)(2)(i)(B) of the respiratory protection standard require employees working under oxygen deficient conditions to provide their employees with a self contained breathing apparatus or a combination full-facepiece pressure demand supplied air respirator with auxiliary self-contained air supply. In the preamble to the final Respiratory Protection Standard, OSHA discussed extensively its rational for requiring that employees breathe air consisting of at least 19.5% oxygen."
Typical Covid-19 face masks subject wearers to an oxygen deficient atmosphere by allowing the rebreathing of one's own exhaled carbon dioxide.