Human and Ecological Risk Assessment - Earthjustice

Appendix FHuman Exposure FactorsF.1.4.4 Exposure DurationTable F-6 presents exposure duration data and distributions. Exposure duration wasassumed to be equivalent to the average residence time for each receptor. Exposure durations foradult and child residents were determined using data on residential occupancy from the EFHTable 15-168 (U.S. EPA, 1997c). The data represent the total time a person is expected to live ata single location, based on age. The table presents male and female data combined. Adultresidents aged 21 to 90 were pooled. For child residents, the 3-year-old EFH age group was usedfor the 1- to 5-year-old CCW cohort. The 6- and 9-year-old EFH age groups were pooled for the6- to 11-year-old CCW cohort.Table F-6. Exposure Duration Data and DistributionsEFH DataData MeanAge Cohort (yr) DistributionDistributionsPop-Estd Shape(yr) aPop-Estd Scale(yr)1–5 6.5 Weibull 1.32 7.0596–11 8.5 Weibull 1.69 9.467Adult 16.0 Weibull 1.34 17.38Pop-Estd = Population-estimated.a Distributions used in risk assessment.In an analysis of residential occupancy data, Myers et al. (U.S. EPA, 2000) found that thedata, for most ages, were best fit by a Weibull distribution. The Weibull distribution asimplemented in Crystal Ball is characterized by three parameters: location, shape, and scale.Location is the minimum value and, in this case, was presumed to be 0. Shape and scale weredetermined by fitting a Weibull distribution to the pooled data, as follows. To pool residentialoccupancy data for the age cohorts, an arithmetic mean of data means was calculated for eachage group. Then, assuming a Weibull distribution, the variance within each age group (e.g., 6-year-olds) was calculated in the age cohort. These variances in turn were pooled over the agecohort using equal weights. This is not the usual type of pooled variance, which would excludethe variation in the group means. However, this way, the overall variance reflected the varianceof means within the age groups (e.g., within the 6-year-old age group). The standard deviationwas estimated as the square root of the variance. The coefficient of variation was calculated asthe ratio of the standard deviation divided by the Weibull mean. For each cohort, the populationestimatedparameter uncertainty information (e.g., shape and scale) was calculated based on aWeibull distribution, the calculated data mean for the age cohort, and the CV.F.2 Exposure Parameters Used in Screening AnalysisThe 50th percentile values used for the human exposure factors in the screening analysisare presented in Table F-7.April 2010–Draft EPA document. F-10