Studies
that directly examined the joint toxic action of BTEX chemicals on the nervous system are essentially
limited to a few human and animal inhalation studies of some binary mixtures of components, particularly
benzene/toluene, toluene/xylene, and ethylbenzene/xylene (Dudek et al. 1990; Frantik and Vodickova
1995; Frantik et al. 1988; Korsak et al. 1988, 1992; Toftgard and Nilsen 1981, 1982). As discussed in
Section 2.3, the neurotoxicity studies of the binary mixtures provide no data that conflict with the
predictions of the PBPK studies (i.e., that joint action is expected to be additive at BTEX concentrations
below approximately 20 ppm of each component). In summary, based on evidence from PBPK and
neurotoxicity studies supporting the plausibility of additive joint action at the shared target of toxicity at
relatively low levels of exposure, the hazard index approach is recommended for assessing possible
neurotoxic health hazards from environmental exposures to BTEX. This is a conservative approach for
assessing BTEX due to the protective nature of the MRLs and guidance values on which it is based, the