Frequent Questions

How does EPA estimate the relative source contribution (RSC)?

e total daily exposure (54 FR 22062, 22069; May 22, 1989). For drinking water contributions between eighty and one hundred percent, EPA uses an eighty percent ?ceiling? (i.e., maximum drinking water contribution). The ?ceiling? accounts for the possibility of unusual exposures (e.g., individuals exposed to higher than currently indicated levels of a contaminant in food) or for changes in the distribution of a contaminant in the environment. The ?ceiling? provides a margin of safety for those individuals. For drinking water contributions between twenty and eighty percent, EPA uses the actual data as the estimate for the RSC. For drinking water contributions less than twenty percent, EPA uses a twenty percent ?floor? (i.e., minimum drinking water contribution). The ?floor? represents a level below which additional incremental protection is negligible. It also indicates that control of other more contaminated media (e.g., air) will have greater reduction in daily exposure (56 FR 3526, 3535; January 30, 1991).
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