The hazard index method is recommended for assessing the joint neurotoxic hazard of BTEX because this
approach is most appropriately applied to mixture components that cause the same effect by the same
mechanism of action (ATSDR 2001a). A hazard index is calculated for each exposure scenario of
concern by first determining a hazard quotient for each of the mixture components. A hazard quotient is
the ratio of an exposure estimate to the appropriate MRL, reference dose (RfD)/reference concentration
(RfC), or guidance value. Available inhalation MRLs and guidance values for BTEX components are
summarized in Table 3-1. The assessment should proceed if two or more of the individual components