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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 Amer. Conf. of Governmental & Industrial Hygienists ACGIH)
Ceiling (max. value, 15 min.) - 200 ppm (229 mg/cu. m)

MI Occup. Safety & Health Admin. (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.

Am. Soc. of Heating, Refrigeration & Air Cond. Engineers (ASHRAE) -

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

American Gas Association -

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

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TWA - Computed by making measurements at intervals over 8 hours, then adding the sums of the concentrations and the intervals, and dividing by 8 hours (480 min.).

* Based on several published studies of people with coronary ischemic disease showing ECG changes during moderate exercise when breathing concentrations of CO giving 3% COHb.