Compressed Air Best Practices, May 2017, by Nancy Aulisa.
http://www.airbestpractices.com/standards/nfpa-99-medical-air/osha-standard-29-cfr-1019134-grade-d-compressed-breathing-air-requirem

"Compressed air, commonly called Industry’s fourth utility, is the most common utility used in a typical industrial facility. Compressed air is used in more than 70 percent of all manufacturing activities including supplying breathing air to personnel using supplied air respirators. Hazardous breathing conditions exist in many routine industrial operations, such as chemical manufacturing, hospitals, abrasive blasting, paint spraying, industrial cleaning, and arc welding. In these and other operations that introduce contaminants into the workplace, supplied-air respirators, air filtration systems and carbon monoxide monitors are frequently used for worker protection."