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What are the most common toxic gases in confined spaces?

By Bob Henderson | Last updated: July 14, 2021

Toxic gases can be produced by materials deliberately used or stored in confined spaces, can be produced by natural processes, can be accidentally introduced into the space, or in the case of sewers and large interconnected systems, can migrate into the area where work is being performed.

The most common toxic gases found in confined spaces are carbon monoxide (CO) and hydrogen sulfide (H2S). These gases are usually measured by means of substance-specific electrochemical toxic gas sensors. Gas that enters the sensor undergoes a reaction that produces an electric current (output) from the sensor that is proportional to the concentration of gas.


Volatile organic chemical (VOC) vapors are potentially present in many confined spaces as well, especially spaces associated with the oil and petrochemical industry. VOC vapors are often toxic at very low concentrations. VOCs are normally measured by means of photoionization detector (PID) sensors that measure in parts per million (ppm) or even smaller increments.

Carbon Monoxide (CO)

Carbon monoxide is a colorless, odorless and highly toxic gas that is produced as a by-product of incomplete combustion. Carbon monoxide bonds to the hemoglobin molecules in red blood cells, preventing them from properly transporting oxygen. CO is potentially present whenever combustion occurs. It is particularly associated with internal combustion engine exhaust.

Carbon monoxide can be generated by hot work that involves combustion, operating internal combustion engines within the confined space, or introduced into the space by improper use of ventilation equipment. Vehicle exhaust has been implicated in many accidents. Verify that blowers and ventilation equipment introduce only fresh air into the space, and that atmosphere evacuated from the space is vented safely.

Carbon monoxide is a chronically toxic gas. Prolonged or repeated exposure to relatively low concentrations of CO can eventually lead to injury, illness, or death. Although high concentrations of carbon monoxide may be acutely toxic and lead to immediate respiratory arrest or death, it is the long-term physiological effects due to chronic exposure at lower concentrations that take the greatest toll on affected workers. Even when exposure levels are too low to produce immediate symptoms, small repeated doses can reduce the oxygen carrying capacity of the blood over time to dangerously low levels. This partial impairment of the blood supply may lead to serious physiological consequences over time.

OSHA permissible exposure limits (PELs) are published in 29 CFR 1910 Subpart G, “Occupational Health and Environmental Control,” or in Subpart Z, “Toxic and Hazardous Substances.” If the toxic gas concentration exceeds the PEL the atmosphere is hazardous.

The OSHA PEL for carbon monoxide is 50 ppm, calculated as an 8-hour TWA limit. The NIOSH Recommended Exposure Limit (REL) consists of a two part definition, an 8 hour TWA limit of 35 ppm, and a ceiling limit of 200 ppm. The American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value (TLV) for CO is 25 ppm, calculated as an 8-hour TWA.

Hydrogen Sulfide (H2S)

Hydrogen sulfide (H2S) is produced by the action of anaerobic, sulfur fixing bacteria on materials that contain sulfur. It is commonly associated with raw sewage, animal products, and the pulp and paper industry, but can occasionally be encountered in almost any confined space. It is a constituent of natural gas, petroleum, sulfur deposits, volcanic gases, and sulfur springs. It is especially associated with oil production and refining activities.

Exposure limits for H2S vary widely as a function of jurisdiction and workplace activity. The most widely recognized standards for H2S reference an 8-hour TWA of either 10 ppm, and a 15-minute short term exposure limit (STEL) of no more than 15 ppm. The ACGIH TLV for H2S is much more conservative. It consists of an 8-hour TWA limit of 1.0 ppm, and a 15-minute STEL of 5.0 ppm.

When in doubt, be conservative! Concentrations above 100 ppm should be regarded as immediately dangerous to life and health, with the potential for causing irreversible physiological harm to the exposed individual. Many monitoring programs use instruments with the alarms set to sound immediately if the concentration reaches 10 ppm, in which case the workers immediately leave the affected area. This approach essentially eliminates the potential of ever reaching STEL or TWA exposure limits.

Toxic VOC Vapors

VOCs are organic compounds characterized by their tendency to evaporate easily at room temperature. Familiar VOCs include solvents, paint thinner, and nail polish remover, as well as the vapors associated with fuels such as gasoline, diesel, heating oil, kerosene, and jet fuel. The category also includes many specific toxic chemicals such as benzene, butadiene, hexane, toluene, xylene, and many others. Increased awareness of the toxicity of these common contaminants has led to lowered exposure limits, and increased requirements for direct measurement of these substances at their exposure limit concentrations. Instruments equipped with photoionization detectors are increasingly being used as the detection technique of choice in these applications.

VOCs present multiple potential threats in the workplace environment. Many VOC vapors are heavier than air, and can act to displace the atmosphere in an enclosed environment or confined space. Oxygen deficiency is a leading cause of injury and death in confined space accidents. The literature contains many examples of fatal accidents caused by oxygen deficiencies due to displacement by VOC vapors.

Most VOC vapors are flammable at surprisingly low concentrations. For instance, the lower explosion limit (LEL) concentrations for toluene and hexane are only 1.1% (11,000 PPM). By comparison, it takes 5% volume methane (50,000 PPM) to achieve an ignitable concentration in the air. Because most VOCs produce flammable vapors, in the past, the tendency has been to measure them by means of combustible gas measuring instruments. Combustible gas reading instruments usually provide readings in percent LEL increments, where 100% LEL indicates a fully ignitable concentration of gas. Combustible gas instrument alarms are usually set to go off if the concentration exceeds 5% or 10% LEL. Unfortunately, most VOC vapors are also toxic, with PEL values that are much lower than 10% LEL.

VOC vapors are commonly measured by means of photoionization detector (PID) sensors. Photoionization detectors use high-energy ultraviolet light from a lamp housed within the detector as a source of energy used to remove an electron from neutrally charged VOC molecules, producing a flow of electrical current proportional to the concentration of the contaminant. The amount of energy needed to remove an electron from the target molecule is called the ionization energy (IE) for that substance. The larger the molecule, or the more double or triple bonds the molecule contains, the lower the IE. Thus, in general, the larger the molecule, the easier it is to detect. This is very different than the performance characteristics of the catalytic type combustible sensor.

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Written by Bob Henderson | President

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Bob Henderson is President of GfG Instrumentation, Inc. in Ann Arbor, Michigan.Robert has been a member of the American Industrial Hygiene Association since 1992. He is an active member of the AIHA Real Time Detection Systems Technical Committee, and the AIHA Confined Spaces Committee. He is also a past chair of the Instrument Products Group of the International Safety Equipment Association. Robert has over 37 years of experience in the design, sale and marketing of atmospheric monitoring instruments used in confined space, industrial safety, and industrial hygiene monitoring applications.

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