Carbon monoxide exposure is the most common method of death by poisoning in the world.
The effects of carbon monoxide poisoning are well understood. CO gas competes with oxygen to bind with hemoglobin in the blood leading to a reduction of oxygen in the brain. Even low carbon monoxide levels over long periods can have an impact on brain chemistry.
While exact short term and long term CO levels recommended by ASHRAE, OSHA, NIOSH and other organizations differ, the consensus is that 9 ppm (parts-per-million) is the maximum indoor safe carbon monoxide level over 8 hours.
Click the links below to see each organization's safe carbon monoxide levels and exposure limits:
- OSHA: Occupational Safety and Health Association
- ASHRAE: American Society of Heating, Refrigerating and Air-Conditioning Engineers
- NIOSH: National Institute for Occupational Safety and Health
- ACGIH: American Conference of Governmental Industrial Hygienists
- EPA: Environmental Protection Agency
- WHO: World Health Organization
- UL, CSA, ANSI: Underwriters Laboratories
The Occupational Safety and Health Association OSHA sets standards for working conditions in the US including safe carbon monoxide levels.
The OSHA personal exposure limit (PEL) for CO is 50 parts per million (ppm). OSHA standards prohibit worker exposure to more than 50 parts of CO gas per million parts of air averaged during an 8-hour time period. The 8-hour PEL for CO in maritime operations is also 50 ppm. Maritime workers, however, must be removed from exposure if the CO concentration in the atmosphere exceeds 100 ppm. The peak CO level for employees engaged in roll-on roll-off operations during cargo loading and unloading is 200 ppm.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers ASHRAE is a voluntary world-wide organization that sets standards for members focused on building systems, energy efficiency, indoor air quality, refrigeration and sustainability. The ASHRAE Standard 62.1-2016, "Ventilation for Acceptable Indoor Air Quality" agrees with the US Environmental Protection Agency and the World Health Organization limit of 9 ppm over an 8 hour exposure.
The National Institute for Occupational Safety and Health NIOSH is the US federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness. The Occupational Safety and Health Act of 1970 established NIOSH as a research agency focused on the study of worker safety and health, and empowering employers and workers to create safe and healthy workplaces.
NIOSH recommends no more than 35 ppm time weighted average TWA exposure limit. In addition, NIOSH recommends no more than 200 ppm short term exposure limit STEL for 15 minutes.
The American Conference of Governmental Industrial Hygienists ACGIH is a non-profit organization that advances occupational and environmental health.
The ACGIH recommends a Threshold Limit Value – Time-Weighted Average (TLV-TWA) 50 ppm with a TLV- short term exposure limit of 400 ppm. A TLV-TWA is defined as the concentration of a hazardous substance in the air averaged over an 8-hour workday and a 40-hour workweek to which it is believed that workers may be repeatedly exposed, day after day, for a working lifetime without adverse effects.
The United States Environmental Protection Agency EPA sets national ambient air quality standards (NAAQS) under authority of the Clean Air Act (42 U.S.C. 7401 et seq.). NAAQS is applied for outdoor air throughout the country.
The NAAQS standard for carbon monoxide in outdoor air is 9 ppm over 8 hours and 35 ppm over 1 hour not to be exceeded more than once a year.
The World Health Organization WHO suggests maximum carbon monoxide levels both in response to their mandate for outdoor clean air and to help countries develop indoor air quality building standards. In indoor environments, space heaters fueled with oil, gas or kerosene, gas stoves, combustion appliances and tobacco smoking are known to cause significant emissions of carbon monoxide. The WHO recommended limits are:
- 9-10 ppm for no more than 8 hours
- 25-35 ppm for no more than 1 hour
- 90-100 ppm for no more than 15 minutes
Underwriters Laboratories UL is the largest and best known independent, not-for-profit testing laboratory in the world. It conducts product evaluations to confirm that the products meet the “UL” standard in electrical safety, fire testing, medical device testing, EMC testing and more.
American National Standards Institute ANSI works in close conjunction with Underwriters Laboratories, and many of the standards are listed as ANSI/UL. UL also has a close association and shares product ratings with the CSA Group which is accredited by the Standards Council of Canada.
The ANSI/UL 2034, ANSI/UL 2075 and CSA 6.19-01 “Standard for Safety for Single and Multiple Station Carbon Monoxide Alarms” requirements are purchased by manufacturers before submitting their devices for testing. They are not normally available to the public. However, UL-certified CO alarms meet a different range of standards that balance the responsiveness of the alarm with the requirement that they not generate nuisance alarms due to background CO caused by outdoor pollution, the normal use of fuel burning appliances or other vapors like methane or ammonia.
The alarm thresholds, set by CO concentration measured in parts per million (ppm), are: no alarm below 30 ppm until after 30 days; 70 ppm for one to four hours (but not less than one hour); 150 ppm for 10 to 50 minutes; 400 ppm for four to 15 minutes.