EVALUATION OF THE USE OF UNCERTAINTY FACTORS IN DERIVING RFDS FOR SOME CHLORINATED COMPOUNDS

Risk assessment of exposure to chemicals having a toxic endpoint routi nely uses the reference dose (RfD) approach based on uncertainty facto rs of 10. The purpose of this investigation was to evaluate whether th e magnitude of the U.S. Environmental Protection Agency (EPA) 10x unce rtainty factors has scientific merit when compared with data from rece nt human and animal experimental studies. A compilation and comparison of ratios between LOAEL/NOAEL (lowest observed adverse effect level/n o observed adverse effect level) and subchronic/chronic values was mad e for six chlorinated compounds, namely, carbon tetrachloride, methyle ne chloride, pentachlorophenol, monochlorobenzene, chlorpyrifos, and 1 ,1-dichloroethane. Data sets demonstrated that 91.3% of the LOAEL/NOAE L ratios were less than or equal to 6 while 87% of the ratios for the same parameter were less than or equal to 5. Furthermore, subchronic/c hronic ratios were less than or equal to 3.5. From our investigation w e concluded that automatic safety factors of 10x are not scientificall y supportable and are overly conservative for the chlorinated compound s studied here.