To ensure that employees have a reliable source of air with an oxygen content of at least 19.5 percent, paragraphs (d)(2)(i)(A) and (d)(2)(i)(B) of the Respiratory Protection Standard require employers 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. Paragraph (d)(2)(iii) of the Respiratory Protection Standard considers any atmosphere with an oxygen level below 19.5 percent to be oxygen-deficient and immediately dangerous to life or health. This same terminology has been used in the Confined Space Standard. Oxygen meters used to assess hazardous conditions by safety personnel in both general industry and construction are calibrated in percent oxygen, and employers and employees are familiar with, and prefer, this terminology. Although most of your letter argues for the use of "partial pressures of oxygen" to describe atmospheric oxygen concentrations, the expression "percent oxygen" was purposely chosen during the rulemaking for the Respiratory Protection Standard. In your letter you ask OSHA to revise the Respiratory Protection Standard to state that an atmosphere containing a partial pressure of oxygen at or above 100 mm of mercury is safe for employees when employers demonstrate that, under all foreseeable conditions, they can maintain the partial pressure of oxygen at or above 100 mm of mercury. This letter constitutes OSHA's interpretation only of the requirements discussed and may not be applicable to any question not delineated within your original correspondence. Thank you for your Januletter to the Occupational Safety and Health Administration's (OSHA's) Directorate of Enforcement Programs regarding the Respiratory Protection Standard.