Human and Ecological Risk Assessment - Earthjustice

Appendix FHuman Exposure Factorswere imposed on the distributions. The minimum and maximum values are included inTable F-1.F.1.2 Exposure Parameter Distribution MethodologyThis section describes how stochastic or distributed input data for each exposure factorwere collected and processed. Exposure parameter distributions were developed for use in theMonte Carlo analysis. For most variables for which distributions were developed, exposurefactor data from the EFH were analyzed to fit selected parametric models (i.e., gamma,lognormal, Weibull). Steps in the development of distributions included preparing data, fittingmodels, assessing fit, and preparing parameters to characterize distributional uncertainty in themodel inputs.For many exposure factors, EFH data include sample sizes and estimates of the followingparameters for specific receptor types and age groups: mean, standard deviation, standard error,and percentiles corresponding to a subset of the following probabilities: 0.01, 0.02, 0.05, 0.10,0.15, 0.25, 0.50, 0.75, 0.85, 0.90, 0.95, 0.98, and 0.99. These percentile data, where available,were used as a basis for fitting distributions. Although in no case were all of these percentilesactually provided for a single factor, seven or more are typically present in the EFH data.Therefore, using the percentiles was a fuller use of the available information than fittingdistributions simply based on the method of moments (e.g., selecting models that agree with thedata mean and standard deviation). For some factors, certain percentiles were not used in thefitting process because sample sizes were too small to justify their use. Percentiles were usedonly if at least one data point was in the tail of the distribution. If the EFH data repeated a valueacross several adjacent percentiles, only one value (the most central or closest to the median) wasused in most cases (e.g., if both the 98th and 99th percentiles had the same value, only the 98thpercentile value was used).The EFH does not use standardized age cohorts across exposure factors. Data fordifferent exposure factors are reported for different age categories. Therefore, to obtain thepercentiles for fitting the four standardized age cohorts (i.e., ages 1 to 5, 6 to 11, 12 to 19, andmore than 20), each EFH cohort-specific value for a given exposure factor was assigned to oneof these four cohorts. When multiple EFH cohorts fitted into a single CCW cohort, the EFHpercentiles were averaged within each CCW cohort (e.g., data on 1- to 2-year-olds and 3- to 5-year-olds from EFH were averaged for the CCW 1- to 5-year-old cohort). If sample sizes wereavailable, weighted averages were used, with weights proportional to sample sizes. If samplesizes were not available, equal weights were assumed (i.e., the percentiles were simplyaveraged).April 2010–Draft EPA document. F-2