Human and Ecological Risk Assessment - Earthjustice

Section 3.0Analysis• The RfD is an estimate (with uncertainty spanning perhaps an order of magnitude) of adaily exposure to the human population (including sensitive subgroups) that is likely tobe without appreciable risk of deleterious noncancer effects during a lifetime. The RfDprovides a reference point to gauge the potential effects (U.S. EPA, 2002c). At exposuresincreasingly greater than the RfD, the potential for adverse health effects may increase,although this potential cannot be quantified. Lifetime exposure above the RfD does notimply that an adverse health effect would necessarily occur.• The CSF is an upper-bound estimate (approximating a 95 percent confidence limit) of theincreased human cancer risk from a lifetime exposure to an agent. Because this is anupper-bound estimate, true risk is likely lower. This estimate is usually expressed in unitsof proportion (of a population) affected per milligram of agent per kilogram body weightper day (mg/kg-d). Unlike RfDs, CSFs do not represent “safe” exposure levels; rather,they relate levels of exposure with a probability of effect or risk.Human health benchmarks are available from several sources. Health benchmarksdeveloped by EPA were used whenever they were available. Sources of human healthbenchmarks were used in the following order of preference:• Integrated Risk Information System (IRIS) (U.S. EPA, 2002c)• Superfund Technical Support Center Provisional Benchmarks• Health Effects Assessment Summary Tables (HEAST) (U.S. EPA, 1997f)• Various other EPA health benchmark sources• ATSDR minimal risk levels (MRLs) (ATSDR, 2002).These sources and the selection hierarchy are described in more detail in Appendix G.The chronic human health benchmarks used in the screening and full-scale analysis aresummarized in Table 3-1. For most constituents, human health benchmarks were available fromIRIS. Benchmarks for a few constituents were obtained from ATSDR. For chemicals for whichpurely health-based benchmarks were not available (lead), a drinking water action level was used(U.S. EPA, 2002d).ConstituentTable 3-1. Human Health Benchmarks Used in the CCW Risk AssessmentCancer BenchmarkType ofBenchmark a Value Units Source bArsenic CSF 1.5E+00 (mg/kg-d) -1 IRISNoncancer BenchmarksAluminum RfD 1.0E+00 mg/kg-d PPRTVAntimony RfD 4.0E-04 mg/kg-d IRISArsenic RfD 3.0E-04 mg/kg-d IRISBarium RfD 2.0E-01 mg/kg-d IRISBeryllium RfD 2.0E-03 mg/kg-d IRIS(continued)April 2010–Draft EPA document. 3-4