Safety and risk analysis studies involving toxic chemicals often require the use of limiting toxicity criteria to develop safe exclusion zones, individual risk profiles, and/or societal risk profiles. For a short duration release, a fixed concentration criteria may overpredict the extent of hazard or risk. For a long duration release the extent of hazard may be underpredicted if the dose response is nonlinear. The most appropriate and direct approach is to use a dose response curve to determine the extent of risk. This is typically referred to as a vulnerability model or a probit (see Section 3) where the extent of risk or hazard is expressed as a probability of receiving a specific dose which depends on both concentration and exposure time.
Adequate probit models are only available for a limited number chemicals. They are mostly used in quantitative risk assessment studies. Probit models are not typically used to develop safe separation zones for safety studies in the United States because they relate the probability of fatality or the probability of receiving a dangerous dose to an exposure level and duration. Concentration based criteria such as emergency response planning guidelines (ERPG) are typically used because of the quality and the level of peer review provided by the American Industrial Hygiene Association (AIHA). ERPG guidelines are based on 60 minutes of exposure time and do not explicitly provide the nonlinearity of dose-response to use directly in transient dispersion models.
AEGLs provide concentration limits for different exposure durations. They are designed to protect the elderly, children, and other individuals who may be susceptible. Similar to ERPG levels, AEGLs are assigned levels of 1, 2 or 3 according to severity of effects. AEGLs are calculated for five relatively short exposure periods 10 minutes, 30 minutes, 1 hour, 4 hours, and 8 hours. AEGL levels are dictated by the severity of the toxic effects caused by the exposure, with Level 1 being the least and Level 3 being the most severe. AEGLs are developed based on formal guidance provided by the national research council (NRC). Published AEGL values can be found on the EPA’s website. In 2001, the National Academies published procedural guidance to make development of AEGLs systematic, consistent, documented and transparent to the public.
AEGL levels can be expressed in parts per million (ppm) or milligrams per cubic meter (mg/m3) of a substance above which it is predicted that the general population (including susceptible individuals) could experience one of the following responses to exposure:
Selective regression of AEGL data can be calculated with our software Process Safety Office® SuperChems™. A table with the AEGL compiled values is included in this paper along with a case study.
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