Criteria and recommended standards based thereon developed by the National Institute for Occupational Safety and Health to meet the need for providing adequate protection for the safety and health of employees exposed to carbon monoxide. Attention is drawn to the two main sources of carbon monoxide, cigarette smoke and the internal combustion engine. Employee exposure to carbon monoxide at the workplace has to be controlled by complying with the standard set forth in eight sections of this report. Recommended standard states that no worker shall be exposed to a concentration greater than 35 ppm determined as a time-weighted average exposure for an 8-hour workday, as measured with a portable, direct reading, hopcalite-type carbon monoxide meter calibrated against known concentrations of carbon monoxide, or with gas detector tube units certified under title of 42 of the Code of Federal Regulations, Part 84. No level of carbon monoxide to which workers are exposed shall exceed a ceilig concentration of 200 ppm. Procedures for calibration of equipment, air sampling and analysis are given in an appendix.