The Occupational Safety and Health Administration standard for carbon monoxide in 1987 allowed exposure of 50 parts per million (ppm) time-weighted average over an 8-hour day with no ceiling level limit. The margin of safety of this standard for workers with cardiovascular disease was questioned by a series of controversial studies of exposures averaging 50 ppm which demonstrated a decrease in exercise necessary to induce angina in experimental subjects with underlying cardiovascular disease. NIOSH recommended a standard for carbon monoxide exposure of 35 ppm as an 8-hour time-weighted average with a ceiling limit of 200 ppm.