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URL: https://www.sciencedirect.com/science/article/pii/B9780124158412000374
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URL: https://www.sciencedirect.com/science/article/pii/B9781933762166500062
A Short Term Exposure Limit (STEL) is defined by ACGIH as the concentration to which workers can be exposed continuously for a short period of time without suffering from irritation, chronic or irreversible tissue damage, or narcosis of sufficient degree to increase the likelihood of accidental injury, impair self-rescue or materially reduce work efficiency.
Whether to establish medical surveillance programs for employees depends on the circumstances. Employers must provide a medical surveillance program for all employees who perform specific duties.
Engaging in Class I, II, or III work or being exposed at or above the PEL or STEL for a combined total of 30 or more days per year is one factor. Wearing negative-pressure respirators is another factor. In addition, a licensed physician must perform or supervise all medical exams and procedures that employers provide at no cost to employees and at a reasonable time. Employers must make medical exams and consultations available to employees as follows:
Prior to employee assignment to an area where negative-pressure respirators are worn.
Within 10 working days after the thirtieth day of combined engagement in Classes I, II, and III work and exposure at or above a PEL, and at least annually thereafter.
When an examining physician suggests them more frequently.
If an employee was examined within the past 12 months and that exam meets the criteria of the standard, however, another medical exam is not required.
The long-term exposure limit (LTEL) is the time-weighted average concentration of a substance over an 8-h period thought not to be injurious to health.
The short-term exposure limit (STEL) is the time-weighted average concentration of a substance over a 15 min period thought not to be injurious to health.
The HSE Publication EH40 gives exposure limits for a wide range of chemicals (see “Further Reading”).
If we identify excessive exposure to toxic chemicals in our design, we should first consider substituting the materials which produce toxic hazards. Failing that, we can use engineering controls such as ventilation, avoidance of enclosure, controlling access to contaminated areas, and so on.
Note that there are many substances which are both toxic and flammable, and both hazards should be considered simultaneously.
There may be some substantially enclosed areas which may have flammable, toxic or asphyxiating atmospheres which we cannot design out. These are classified as confined spaces, and access to them has to be tightly controlled.
Tilman Hahn , ... Kaye H. Kilburn , in Handbook of Solvents (Third Edition) , 2019
Several organizations and governmental agencies prepare regulations or recommendations limiting exposure to toxic chemicals. The regulations and recommendations for limiting benzene exposure are summarized below.
Occupational Safety and Health Administration (OSHA) defines the Permissible Exposure Limit to be 1 ppm (8 hour time weighted average) and Short Term Exposure Limit to be 5 ppm (15 minute average). These limits have the effect of law and represent maximum allowable exposures in the workplace.
The National Institute for Occupational Safety and Health (NIOSH) has adopted a Recommended Exposure Limit for benzene of 0.1 ppm (8 hour time-weighted average) and a Short Term Exposure Limit of 1 ppm (15 minutes). 70
The Agency for Toxic Substances Disease Registry (ATSDR) has derived an Acute Inhalation Minimal Risk Level (MRL) of 0.05 ppm for benzene based on a Lowest Observed Adverse Effect Level (LOAEL) for immunological effects in mice. 71 The ATSDR has derived an Intermediate Inhalation MRL of 0.004 ppm for benzene based on a LOAEL for neurological effects in mice. 72
The American Conference of Governmental Industrial Hygienists (ACGIH) has adopted a Threshold Limit Value (TLV) of 0.5 ppm (8 hour time-weighted average) and a Short Term Exposure Limit (STEL) of 2.5 ppm (15 minute time-weighted average). These are recommendations for breathing zone concentrations, which should not be exceeded in the workplace. As recommendations for industry, they do not have the force or effect of law. 73
The California Environmental Protection Agency (Cal/EPA) has adopted a No Significant Risk Level (NSRL) of benzene of 13 μg/day by inhalation. This represents the maximum amount of benzene that the State of California has determined a person may be exposed daily that poses no significant risk of cancer. 74
The Collegium Ramazzini has recommended a Threshold Limit Value (TLV) for benzene of 0.1 ppm. 75 However, recently, the Collegium Ramazzini issued a Statement calling for reduction of benzene exposure to the lowest level possible. 76
The World Health Organization (WHO) has concluded that “no safe level of exposure can be recommended.” 77
Even the American Petroleum Institute (API) recognizes that there is no safe level of exposure to benzene. 19 As early as 1948, the API published a Toxicological Review on Benzene, which acknowledged that “as much as the body develops no tolerance to benzene, and as there is a wide variation in individual susceptibility, it is generally considered that the only absolutely safe concentration for benzene is zero.”
Many of the process and utility streams used in process facilities contain toxic gases of sufficiently high concentration that they could create a hazard if released. Therefore, gas detectors are required to alert workers that a release has occurred. Small releases of toxic gas may cause injury to personnel working directly where the leak occurs. Medium releases may affect other personnel working on the unit but not directly at the release source. Large releases may produce toxic concentrations outside the unit or outside the plant fence line.
Detailed information to do with toxic gas terminology and the effects of these gases on the human body are provided in Process Risk and Reliability Management.
Much of the guidance to do with flammable detectors can be applied to toxic gas detection also. However, the following additional issues should be considered:
There can be two levels of alarm—similar to that discussed for flammable releases. The first level is a “toxic gas release” alarm given when one detector is at the STEL concentration. The second is a “confirmed gas release” alarm. This signal can be given for any of the following:
One detector at the immediately dangerous to life and health (IDLH) concentration
Two (or more) detectors at the STEL concentration
Detectors should be located within 1.5 meters of the leak point.
When multiple sources are located together, e.g., pumps, a single monitor can serve two sources, provided the sources are located within 3 meters of one another.
Potential releases from passive equipment such as pipe flanges and pipe/vessel wall ruptures do not usually require monitoring. However, manifolds with large number of valves and flanges, representing a concentration of leak sources, may be locally monitored if the toxic gas concentrations are high.
Fixed H 2 S detectors should be provided in high potential hazard H 2 S areas defined as:
H 2 S concentrations in the vapor phase of the contained stream are above 2% by volume.
Flash of a released liquid at atmospheric pressure produces a vapor with more than 2% by volume H 2 S.
Low-lying, poorly ventilated areas where H 2 S could accumulate, that are not under Permit to Work access control.
Perimeter monitoring using open path detectors should be considered in circumstances such as close proximity to a fence line or occupied buildings. Open path detectors should be considered.
Benzene is used in the manufacture of many different industrial chemicals, including plastics, detergents, and insecticides. It is a carcinogen that can cause leukemia and other illnesses.
OSHA regulations have established a PEL of 1.0 ppm, as an 8-hour TWA and a 15-minute STEL of 5.0 ppm. The OSHA regulation contains an action level of 0.5 ppm 8-hour TWA, which triggers requirements for additional monitoring, medical surveillance, and annual employee training.
The standard exempts containers and pipelines carrying mixtures with less than 0.1% benzene and natural gas processing plants processing gas with less than 0.1% benzene. Also exempted are work operations where the only exposure to benzene is from liquid mixtures containing less than 0.1% benzene and oil and gas drilling, production, and servicing operations. These exempted operations still fall under the OSHA 1910.1000 PELs of 10 ppm PEL or 25 ppm ceiling.
One company has elected to voluntarily impose 1.0 ppm PEL and 5.0 ppm STEL across all of its operations.
The Council Directive 2000/39/EC, established on 8th June 2000 and based on Directive 98/24/EC, aims to protect the workers from exposure to chemical agents which pose risks to human health and the environmental. According to this Directive, all EU members must implement legislations that establish maximum values of exposure. The Directive provides average values for an eight-hour shift. The available values are for numerous chemical compounds including anhydrous ammonia. The directive sets an 8-hour time weighted average (TWA) of 20 ppm and a short-term exposure limit of 50 ppm. Under the directive, member states are obliged to establish their own National TWA and short-term exposure limit (STEL) which comply with the community values. All environments where ammonia is present must have TWA values less than 20 ppm to be deemed safe and to adherent to working standards.
Particular to the United Kingdom, the Control of Substances Hazardous to Health (COSHH) [ 28 ] regulation sets a TWA of 25 ppm and STEL of 35 ppm for anhydrous ammonia. The substances considered to be hazardous to health take numerous forms, some of which are fumes, dusts, vapours, mists and gases (including asphyxiating gases). One of the reasons why ammonia is included in the COSHH list is the injuries it can potentially cause, as previously described.
In terms of working in a facility that employs ammonia refrigeration systems, people shall be provided with the appropriate H&S information about the risks, precautions and safety systems of the plant, with enough information in case of abnormal events. Further information can be found in standards such as the BS EN 13313:2001. Moreover, particular attention should be taken in terms of classification and approach to Explosive Atmospheres (BS EN 60079) [ 29 ]. Electrical Installations close to ammonia systems are also considered in this code. Installation, maintenance and inspection (with scheduled check-lists) need to be performed by experienced and trained personnel, with equipment properly selected to work in explosive atmospheres and hazardous areas (previously identified), thus avoiding explosions caused by ammonia leaks.
Similar to the United States, directives and regulations are intended to minimize risks associated with the use of ammonia. Formal risks assessments combined with adequate training and well-established procedures for incident investigations are documented in European legislations. However, as previously presented and different from the United States, Seveso III does not require a site-specific hazard and safety plan unless a large quantity of ammonia is stored. Moreover, the hazard classification (Acute Toxic [ 30 ]) differs from the United States standard that treats the chemical more like a ‘flammable gas’ [ 6 ].
Many of the process and utility streams used in process facilities contain high concentrations of toxic gases. Therefore gas detectors are required to alert workers that a release has occurred. Small releases of toxic gas may cause injury to personnel working directly where the leak occurs. Medium releases may affect other personnel working at the site but not directly at the release source. Large releases may produce toxic concentrations that could enter public areas.
Information to do with the detection and response to hydrogen sulfide (H 2 S) leaks is provided in Chapter 17 : Chemicals. A detailed discussion to do with toxic gas terminology and the effects of these gases on the human body are provided in the companion book to this one: Process Risk and Reliability Management .
Much of the guidance that was provided above with regard to flammable detectors can be applied to toxic gas detection also. However, the following additional issues should also be considered:
There can be two levels of alarm—similar to that discussed for flammable releases. The first level is a “gas release” alarm given when one detector is at the short-term exposure limit (STEL) concentration. The second is a “confirmed gas release” alarm. This signal can be given either if one detector records a concentration of gas that is at the immediately dangerous to life and health concentration, or if two or more detectors record STEL concentrations.
Detectors should be located within 1.5 m of the leak point.
When multiple sources such as pumps are in the same location, a single monitor can be used provided that those sources are located close to one another — say less than 3 meters apart.
Unless there is a probability of escalation, there is not generally a need to install dedicated detectors around low-probability leak sources. Potential releases from passive equipment such as pipe flanges and pipe/vessel wall ruptures do not usually require monitoring. However, manifolds with large number of valves and flanges, representing a concentration of leak sources, may be locally monitored if the toxic gas concentrations are potentially high. The following may also be considered to be an exception:
Small pipes and connections subject to failure due to vibration.
Manifolds with a large number of flanges.
Pipes subject to external corrosion.
Control valves operating above 15 bar(g).
Areas containing a large number of small bore connections in a concentrated area.
Perimeter monitoring using open path detectors should be considered in circumstances such as close proximity to a fence line or occupied buildings.
PER OLAF TJELFLAAT , ... MAMDOUH EL HAJ ASSAD , in Industrial Ventilation Design Guidebook , 2001
Requirements for indoor environment quality must be discussed and decided before the air conditioning design is performed. Criteria for acceptable indoor air quality and thermal comfort must be set.
Health hazard due to exposure to contaminants by inhalation is the most important issue. International or national health and building regulations or codes and recommendations are used as basis for the discussion of what requirements should be used for design. In regulations, worker exposure limits for airborne gases are normally expressed as
Time-weighted average concentration, TWA, based on a 40-hour work week,
Short-term exposure limit , STEL, which can be exceeded for periods of up to 15 minutes, and
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