Sulfur mustard is a chemical-warfare agent. It is classified as a vesicating agent because of its ability to cause blisters on exposed skin. Sulfur mustard is the chemical-warfare agent present at most stockpile and nonstockpile munition sites in the United States and it territories. At the request of the U.S. Army, Oak Ridge National Laboratory (ORNL) conducted a health risk assessment of sulfur mustard. The assessment included a detailed analysis sulfur mustard's physical and chemical properties, environmental fate, toxicokinetics, mechanism of action, animal and human toxicity, and carcinogenicity (see Appendix E, Health Risk Assessment of Sulfur Mustard, ORNL 1996). On the basis of that assessment, ORNL proposed a reference dose (RfD) of 7 × 10-6mg/kg of body weight per day for noncancer health effects and a slope factor (SF) of 9.5 per mg/kg per day for the carcinogenic potency of sulfur mustard. The Army's Surgeon General accepted ORNL's proposed RfD and SF as interim exposure values until an independent evaluation of the proposed RfD and SF was conducted by the National Research Council (NRC). This chapter contains the NRC's independent assessment of the scientific validity of the Army's interim RfD and SF for sulfur mustard.