The evidence indicates a causal relation between exposure to sufficient concentrations of sulfur mustard (and presumably nitrogen mustard and Lewisite) and chronic nonreversible respiratory effects in humans. It is well known that pulmonary injury is the principal cause of mortality in the first few days to weeks after intense exposure to sulfur mustard. In addition, the evidence is consistent with a causal relationship between occupational exposure to these agents and chronic obstructive lung diseases, including chronic bronchitis, asthma, and pneumonia. Finally, evidence from studies of combat survivors of gas attacks is consistent with a causal relationship between acute overexposure and bronchitis and emphysema. Further, given the likely exposure levels outlined in Chapter 3, the evidence is consistent with a causal relation between the concentrations of sulfur mustard (and Lewisite) used in the WWII experiments and chronic nonreversible lung diseases.

Indirect evidence, based on a review of the relationships between acute and chronic effects caused by other substances, suggests that the likelihood of long-term respiratory effects may not necessarily be linked to the presence of an acute respiratory response. Review of the evidence does not support a minimum  magnitude of acute response necessary in order for there to be long-term sequelae. Further, if the disease model requires the acute exposure to cause acute irreversible damage, then the magnitude of acute response might well predict the magnitude of the chronic response. However, if acute exposure led to an alteration in individual risk factors, then it is possible that the magnitude of the acute and chronic responses would be unrelated. Finally, indirect evidence suggests that only an unusual disease model would exclude the possible mechanism of change in individual risk factors so that the absence of an acute reaction would eliminate the possibility of a chronic effect related to an acute exposure. Thus, there is insufficient evidence to conclude that long-term respiratory responses occur only in cases where an earlier acute response has been documented.