To read the full article please visit
www.safeopedia.com/what-are-t...
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.