Appropriateness of Critical End Point
The LOAELadj (0.022 mg/kg per day) used by ORNL for derivation of the RfD for sulfur mustard was based on the dose that caused mild epithelial acanthosis in the forestomach of rats (Sasser et al. 1989a). The subcommittee believes that those lesions were probably a result of administering doses of sulfur mustard directly to the forestomach, which is typically more toxic than delivering doses at a slower rate throughout the day. If rats were administered sulfur mustard at a slower rate, in feed, for example, the daily dose required to induce the same mild lesions would likely be higher. Although humans do not have forestomachs, the subcommittee believes that the primary mechanism of toxicity of sulfur mustard is epithelial tissue damage from direct contact and agrees with ORNL that epithelial acanthosis of the forestomach in rats can be used as the critical noncancer toxicity end point for deriving the RfD. However, that end point resulting from direct administration to the forestomach is likely to overestimate the toxicity of sulfur mustard, resulting in an RfD that might be overprotective for noncancer health effects.