Calculating the Concentration Term in Risk Assessment: The Importance of Incorporation of the Total Number of Exposure Events. D. Burmistrov, Menzie-Cura & Associates, Inc.; I. Linkov, ICF Consulting; and M. Small, Carnegie-Mellon University
The current EPA guidance for calculating the concentration term for risk assessment recommends the use of the 95% Upper Confidence Limit (UCL) on the mean to account for an uncertainty about the true mean concentration of a chemical in an environmental medium. This is done by incorporating into the assessment an estimate of variability of concentration in the environmental medium as represented by several samples collected in the field. However, this technique solely employs the results of the sampling program, and does not take into account the actual number of exposure events experienced by the individuals at risk. We will discuss the influence of the explicit account of the number of exposure events in calculating exposure point concentration for human health risk assessment. The current EPA approach is adequate for estimating the mean exposure concentration. Its validity for estimation of the reasonable maximum exposure (RME, or upper confidence limit on mean) is limited to the cases when the number of exposure events that individuals experience during the specified averaging time is very high. Underestimation or overestimation of the RME can occur when the number of samples collected in the field is higher or lower than the number of exposure events. We develop a methodology that explicitly incorporates number of exposure events into the calculation of the concentration term distribution and point estimates in risk assessment. We then use this methodology to derive distributions and calculate mean and RME estimates for a case study.
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