Allowable (Legal) Limits for Carbon Monoxide

Occupational Safety & Health Administration, USA (OSHA) (for industrial situations) -

Permissible Exposure Limit (PEL, by TWA) - 50 ppm, 8 hrs.
Old PEL Standard (by TWA) - 35 ppm, 8 hrs.
Threshold Limit Value (TLV, by TWA) - 25 ppm (29 mg/cu. m)
(recommended by American Conference of Governmental & Industrial Hygienists ACGIH)
Ceiling (max. value, 15 min.) - 200 ppm (229 mg/cu. m)

MI Occupational Safety & Health Administration (MIOSHA) -

PEL (Industry, by TWA) - 35 ppm, 8 hrs. (38.5 mg/cu. m)
PEL (Construction, by TWA) - 50 ppm, 8 hrs. (55 mg/cu. m)
Ceiling (max. value) - 200 ppm (229 mg/cu. m)

Environmental Protection Agency, USA (EPA) -

Domestic, outdoor air, all ages (TWA) - 9 ppm*, 8 hrs.
Domestic, outdoor air, all ages (TWA) - 35 ppm, 1 hr.

World Health Organization (WHO) -

Domestic, outdoor air, all ages (TWA) - 9 ppm*, 8 hrs.

American Society of Heating, Refrigeration & Air Conditioning Engineers (ASHRAE) -

Indoor air (leakage at a heat register) - 9 ppm

American Gas Association -

Indoor air (leakage at a heat register) - 15 pm