CO Knowledge Center

There are several goverment organizations and professional organizations that have posted recommended exposure limits to Carbon Monoxide (mostly for the work place). There is no single listing of all of these limits, and so we’ve tried to summarize many of them on this page.

First, here are descriptions of some of the organizations that publish these limits. Click on the logos to visit their official websites.

Occupational Health and Safety Administration (OSHA) – A Federal agency under the Department of Labor that develops and enforces federal standards for health and safety in the work place. Diesel Exhaust Diesel Exhaust Hazard Alert
National Institute for Occupational Safety and Health (NIOSH) – A Federal agency under the Department of Health and Human Services that trains occupational health and safety professionals, conducts research on health and safety concerns, and tests and certifies respirators for workplace use.
American Conference of Governmental Industrial Hygenists (ACGIH) – An organization of professional industrial hygiene personnel in government or educational institutions. The ACGIH develops and publishes recommended occupational exposure limits each year called Threshold Limit Values (TLV’s) for hundreds of chemicals and physical agents.
American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) – An international trade association that provides information and sets standards for the uniform testing and rating of heating, ventilation, air conditioning, and refrigeration equipment.
Environmental Protection Agency (EPA) – A part of the US federal government that enforces environmental laws and provides information and guidance to policy makers.

Each organization has its own vocabulary to describe the type of limit or level. Here is a summary of some commonly seen terms:

• TWA – Time Weighted Average. This is the level of CO (in PPM) that a person has been exposed to on average over some period of time (usually 8 hours).
• PEL – Permissible Exposure Limit. This is the term OSHA uses for the limits it publishes. For CO, it happens that all of OSHA’s PELs are actually TWA limits.
• Ceiling – An upper limit on the concentration of CO at any moment. For example, a ceiling value of 200ppm means that the level of CO should never rise above 200ppm.
• STEL – Short Term Exposure Limit. This is a TWA defined over a short time period of 15 minutes. According to ACGIH, it should not be exceeded during any part of a workday, even if the the 8-Hour TWA is within its limit.
• Excursion Limit – An ACGIH term that refers to the Ceiling limit for a short period of time (typically 15 – 30 minutes). This limit is used when no STEL is published, and is defined as 5 times the 8-Hour TWA limit (Source: ACGIH Handbook of TLVs and BEIs, 2004).

The table below summarizes some of the most up-to-date exposure limits, including their date of publication.

For more information on Carbon Monoxide, and the health effects associated with different concentrations, click here.