Consideration of Sensitive Subgroups in Noncancer RfD And RfC Assessments. J. S. Gift and G. L. Foureman, U.S. EPA, NCEA, RTP, NC 27711
The U.S. EPA’s RfD/C assessments become more quantitatively meaningful as areas of uncertainty are resolved. For data rich chemicals, uncertainty factors associated with the use of nonhuman data, subchronic studies, a limited database and adverse effect levels are often unnecessary. However, the uncertainty factor for sensitivity among humans typically remains and is accommodated in RfD/C assessments with a default factor of 10 unless usable relevant information exists. For example, information available in animals showing more severe effects in young rather than adult animals allowed for partial obviation of this factor in the strontium RfD. Clear biological evidence that human infants are the susceptible population to the effects from nitrite exposure and that children are the most susceptible population for fluorosis allowed total obviation of this factor in RfDs associated with these effects . Information from human populations, which generally comprise sensitive subgroups, also has been used to address this factor. In the selenium RfD, this factor was partially obviated based on the observation that the same NOAEL was observed in two moderately-sized human populations. Such information on general populations is also available for renal tubular dysfunction from cadmium exposure (Jarrup et al., 1998, Scand J Work Environ Health 24, suppl 1). Addressing uncertainty about sensitivity among humans with distributions of responses in human populations of adequate size and that were inclusive of sensitive subgroups is an important advance towards improving the quantitation of RfD/Cs.
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