Emergency response guidelines are often needed during a spill incident, in order to help determine the level of concern (LOC) in case of a possible population exposure. The LOC selected may greatly affect the response activity. Choosing a LOC that is unnecessarily low may result in unneeded evacuation. An evacuation of a population center is a risky operation that may have serious consequences (consider evacuating hospitals, with patients on life support systems). On the other hand, selecting a LOC that is too high may lead to harmful exposure to the population.
Over the years, several sets of exposure guidelines have been developed for both the work force and the general public. Since these two groups differ in many aspects, exposure guidelines that serve one group may not be applicable to the other.
| Occupational Exposure Guidelines
Legal standards for workplace exposures are established by the Occupational Safety and Health Administration (OSHA). These standards are known as Permissible Exposure Limits (PELs). These limits are a legal requirement for occupational exposures, and exceeding them is a violation of the law, for which fines may be imposed. Most OSHA PELs are for airborne substances with allowable exposure limits averaged over an 8-hr day, 40-hr week. This is known as the Time-Weighted-Average (TWA) PEL. Adverse effects should not be encountered with repeated exposures at the TWA PEL. OSHA also issues Short Term Exposure Limit (STELs) PELs, Ceiling Limit PELs, and PELs that carry a skin designation. PEL STELs are concentration limits of substances in the air that a worker may be exposed to for 15 minutes without suffering adverse effects. The 15 minute STEL is usually considerably higher than the 8-hour TWA exposure level. For example, for trichloroethylene the PEL-STEL is 200 ppm whereas the PEL-TWA is 50 ppm. Ceiling Limit PELs are concentration limits for airborne substances that should never be exceeded. A skin designation indicates that the substance can be readily absorbed through the skin, eye or mucous membranes, and substantially contribute to the dose that a worker receives from inhalation of the substance.
An occupational exposure guideline developed by the OSHA Standards Completion Program is the Immediately Dangerous to Life and Health (IDLH). The IDLH is a condition "that poses a threat of exposure to airborne contaminants when that exposure is likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from such an environment." For concentrations above the IDLH, a self-contained breathing apparatus (SCBA) is required. Below that level, air-purifying respirators may be used, if appropriate. Unlike previous definition of the IDLH, which incorporated a 30-minute time period, the new definition (1994) does not have an exposure duration associated with it. If you are in an IDLH condition, you need to get out of there immediately! Using IDLH as a LOC may not be appropriate when the general public is the population at risk because IDLHs were developed for the typical worker (a healthy adult) and allow for some discomfort or even temporary health effect at the level suggested. When OSHA was formed in 1971, it immediately adopted existing occupational heath guidelines for its PELs. These guidelines were those of the American National Standards Institute (ANSI), American Conference of Governmental Industrial Hygienists (ACGIH), and National Institute for Occupational Safety and Health (NIOSH). OSHA also developed health standards for over 30 other workplace hazards based on risk assessments that they conducted. The guidelines issued by the ACGIH are known as Threshold Limit Values (TLVs). NIOSH guidelines are designated as NIOSH Recommended Exposure Limits (RELs). Three types of TLVs exist as previously described for OSHA PELs. They are: Threshold Limit Value Time-Weighted Average (TLV-TWA), TLV as a Short-Term Exposure Limit (TLV-STELs), and Threshold Limit Value as a Ceiling Limit (TLV-C). The NIOSH RELs are also designated as time-weighted average, short-term exposure limits and ceiling limits. TLV-TWA Time-Weighted Average (TWA) exposure to a substance is the level to which workers may be exposed for an 8-hour work shift without suffering an adverse effect. TLV-TWA is meant to regulate exposure over an 8-hour period. Don't extrapolate to shorter periods of time. Don't assume that if a certain limit applies for 8 hours, then eight times that limit may be applied if the exposure lasts for only 1 hour. It simply doesn't work that way. Therefore, the 8-hour limits may not be very useful for spill response, where exposure durations are usually much shorter than 8 hours. TLV-STEL The STEL is a 15-minute exposure limit that should not be exceeded even if the 8-hour TLV remains within the limit. Such limits were assigned to substances exerting toxic effects even over a short period of time. Where a STEL limit is not available (but is believed to be justified), it is recommended that a limit three times as high as the TLV for a 15-minute exposure be used. Ceiling A TLV with a ceiling notation ("C") represents an "excursion limit" that may not be exceeded at any time. Ceiling values are used for substances known to be fast-acting such as irritating gases. While the TLV-TWA allows excursions above the limit as long as they are compensated for by periods of low exposure, ceiling limits do not allow such excursions. REL The Recommended Exposure Limits (RELs) were developed by the National Institute of Occupational Safety and Health (NIOSH). Those standards are similar to TLVs in the way that they were derived, but are often stricter. The RELs are published in the NIOSH Pocket Guide To Chemical Hazards, which is updated every few years, and in other NIOSH publications.
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NIOSH - National Institute for Occupational Safety and Health The "immediately dangerous to life or health air concentration values (IDLHs)" used by the National Institute for Occupational Safety and Health (NIOSH) as respirator selection criteria were first developed in the mid-1970's. The Documentation for Immediately Dangerous to Life or Health Concentrations (IDLHs) is a compilation of the rationale and sources of information used by NIOSH during the original determination of 387 IDLHs and their subsequent review and revision in 1994.
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NPG - NIOSH Pocket Guide to Chemical Hazards The NPG is intended as a source of general industrial hygiene information on several hundred chemicals/classes for workers, employers, and occupational health professionals. The NPG does not contain an analysis of all pertinent data, rather it presents key information and data in abbreviated or tabular form for chemicals or substance groupings (e.g. cyanides, fluorides, manganese compounds) that are found in the work environment. The information found in the NPG should help users recognize and control occupational chemical hazards.
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Public Exposure Guidelines
Public exposure guidelines are meant to protect all segments of the population, including the very young and very old, pregnant women, and hypersensitive individuals. However, relatively few public exposure guidelines have been developed, so spill responders commonly use occupational standards and their own best judgment to select a LOC. | |
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ERPG
The Emergency Response Planning Guidelines (ERPG) were developed under the guidance of a committee within the American Industrial Hygiene Association (AIHA). It is a three-tier standard with one common denominator, a 1-hour contact duration. Each standard identifies the substance, its chemical and structural properties, animal toxicology data, human experience, existing exposure guidelines, the rationale behind the selected value, and a list of references. The ERPG does not contain safety factors usually incorporated into exposure standards such as the TLV. At the ERPG-1, for example, most people would detect the chemical and may have temporary mild effects. At ERPG-3, on the other hand, it is estimated that the effects would be severe, although not life threatening. The TLV, on the other hand, incorporate a safety factor into its standards, to prevent any ill effects. The ERPG should serve as a planning tool, not a standard to protect the public. ERPG standards have been defined for only 35 substances. EEGL Emergency Exposure Guidance Levels (EEGLs) were developed by the National Research Council Committee on Toxicology for the Department of Defense for planning operations under emergency conditions such as spills, fires, and other contamination. Exposure duration was set at 1 to 24 hours. The exposures allowed are not safe but tolerable, and temporary effects are tolerated. The EEGLs were developed for young, healthy military personnel, so the same logic that applies to the IDLH applies to EEGLs: exposure that may be a nuisance to a young and healthy adult may be a real problem for a compromised individual. EEGL standards were developed for 41 substances, some of them used almost exclusively by the military. SPGEL The Short-term Public Exposure Guidance Levels (SPEGL) were developed as public exposure guidelines for civilian populations. Effects were considered for all groups of the public. Only five SPEGLs have been developed: hydrazine, dimethylhydrazine, monomethyl hydrazine, nitrogen dioxide, and hydrogen chloride.
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