Clemco 6 cu ft Contractor Blast Machine, 150-psi maximum working pressure, 1-1/2-inch Millennium valve provides unrestricted air flow, CO Monitor Required: OSHA requires Grade D quality breathing air and NIOSH-approved Type CE continuous-flow supplied-air respirators for abrasive blast operators. OSHA regulations call for a maximum exposure limit to carbon monoxide of 10 parts per million (ppm).