Maximally Exposed Individual

Maximally Exposed Individual (MEI) The single individual with the highest e.xposure in a given population (also, maximum exposed individual). This term has historically been defined in various ways, including as defined here and also synonymously with worst case or bounding estimate. 

EPA proposed standard in 40CFR197 (EPA, 1999). The standard here is based on the dose to the reasonably maximally exposed individual. The proposed limit is set at 0.15 mSv per year for the next 10,000 years. The U.S. Nuclear Regulatory Commission (NRC) has made numerous specific criticisms of the EPA proposal, including suggestions that the dose limit be raised to 0.25 mSv per year (Travers, 1999). Such criticisms, and inputs from other sources, may significantly delay the promulgation of the final standard. 

For nuclear waste disposal, in a site such as Yucca Mountain, if the maximally exposed individual receives the proposed annual limit of 0.15 mSv, present estimates (based on the linearity hypothesis) suggest a 0.00 1 % risk of an eventual fatal cancer. The maximum dose is reached only if the wastes are dissolved in a small volume of water, and therefore only a limited number of people would receive this dose. If this number were as high as 1000, the implied toll for Yucca Mountain neighbors would be one cancer fatality per century per repository site.19 This toll would not start for many centuries, when the waste canisters begin to fail, and it not unreasonable to expect that cancer prevention and treatment will be much improved by then. Ignoring this prospect, and assuming many repositories and some doses above the prescribed limit, it still appears that the expected toll would be well under a thousand deaths per century. 

NCRP (1993a) also has emphasized the importance of source constraints in radiation protection of the public. NCRP has reaffirmed a previous recommendation (NCRP, 1984b 1987a) that whenever the potential exists for routine exposure of an individual member of the public to exceed 25 percent of the limit on annual effective dose as a result of irradiation attributable to a single site, the site operator should ensure that the annual effective dose to the maximally exposed individual from all man-made sources combined does not exceed 1 mSv on a continuous basis. Alternatively, if such an assessment is not conducted, no single source or set of sources under one control should result in an individual receiving an annual effective dose of more than 0.25 mSv. 

In early exposure assessments used in risk characterization, practitioners often used single point estimates of the maximum exposure of individuals to compare with measures of dose-response. Such estimates often lacked transparency in the context of the assumptions on which they were based and led to confusion in terminology (employing concepts such as upper-bound exposure and maximally exposed individual ). 

An estimate (e.g. of exposure) that is incurred by an entity or person who is at an upper percentile with respect to the distribution of interindividual variability. A high-end bounding estimate would typically refer to an estimate at or above the 90th percentile of variability. A bounding estimate would typically refer to an estimate that is at least as high as the maximum real-world value and in some cases could be much higher (e.g. a maximally exposed individual). 

Hawkins NC. 1991. Conservatism in maximally exposed individual (MEI) predictive exposure assessments a first-cut analysis. Regul Toxicol Pharmacol 14(2) 107-117. 

Screening analysis was conducted for different exposure pathways and chemicals of concern. Using a screening level cutoff of a one-in-a-million excess risk for a 70-year lifetime exposure for the maximally exposed individual (MEI), a set of carcinogenic chemicals was identified for further analysis. For noncarcinogens, MEI exposure levels were compared to health thresholds. If the MEI exposure exceeded the established health threshold, further analysis was done. 

As shown previously, radioactive hazards associated with SF and HLW decrease exponentially over time (see Figure 2). After 10" -10 yr, the risk to the public of a nuclear waste disposal vault approaches that of a high-grade uranium ore deposit and is less than the time invariant toxicity risk of ore deposits of mercury and lead (Langmuir, 1997a). The new ERA standard for nuclear waste repositories seeks to limit exposures from all exposure pathways for the reasonably maximally exposed individual living 18 km from a nuclear waste repository to 0.15mSvyr (15 mrem yr ) (US EPA, 2001). For comparison. 

In its simplest form, risk assessment asks, How much exposure can we allow without causing irreparable harm Harm to whom Initially, to a maximally exposed individual and more recently to a sensitive and maximally exposed individual. For example, to keep chemical contamination to acceptable levels in a river, EPA would define acceptable risk to a maximally exposed individual. Acceptable risk would be defined as an exposure to a contaminant below its threshold for causing damage or, more often, a one-in-a-million risk of getting cancer from a... 

At present, the Dq>artment of Energy internal standards require that sites be remediated to an annual committed effective dose equivalent not to exceed 1 mSv per year to a maximally exposed individual, plus a requirement that the potential exposure be As Low As Reasonably Achievable (ALARA). An analysis of previous remedial actions reveals that, in many cases, the actual committed effective dose equivalent is less than 0.01 mSv per year following remediation. 

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