respiratory infections. Those disorders could be increased in frequency and severity when the normal defense mechanisms of the lower respiratory tract are compromised. Chronic non-malignant respiratory disorders generally take one of two forms: Airways disease is a general term for disorders characterized by obstruction of the flow of air out of the lungs, among them asthma and chronic obstructive pulmonary disease (COPD), which includes emphysema and chronic bronchitis. Parenchymal disease, or interstitial disease, generally includes numerous disorders that cause inflammation and scarring of the deep lung tissue, including the air sacs and supporting structures. Parenchymal disease is less common than is airways disease, and its disorders are characterized by reductions in lung capacity, although they can include a component of airway obstruction. Some severe chronic lung disorders, such as cystic fibrosis, are hereditary. Because Vietnam veterans received health screenings before entering military service, few severe hereditary chronic lung disorders are expected in that population.
The major risk factor for many non-malignant respiratory disorders is cigarette-smoking. Although cigarette-smoking is not associated with every disease of the lungs, it is the major cause of many airways disorders, it contributes to some interstitial disease, and it compromises host defenses in such a way that people who smoke are generally more susceptible to some types of pneumonia. Cigarette-smoking also makes almost every respiratory disorder more severe and symptomatic than would otherwise be the case. The frequency of habitual cigarette-smoking varies with occupation, socioeconomic status, and generation. For those reasons, cigarette-smoking is a major confounding factor in interpreting the literature on risk factors for respiratory disease. Vietnam veterans are reported to smoke more heavily than are non-Vietnam veterans (McKinney et al., 1997).
The committee responsible for VAO concluded that there was inadequate or insufficient information to determine an association between exposure to the compounds of interest (2,4-D, 2,4,5-T, TCDD, picloram, or cacodylic acid) and the respiratory disorders specified above. Additional information available to the committees responsible for Update 1996 and Update 1998 did not change that finding. Update 2000 drew attention to findings from the Seveso cohort that suggested a higher mortality from non-malignant respiratory disorders among study subjects, particularly males, who were more heavily exposed to TCDD. Those findings were not replicated in several other relevant studies, although one showed an increase that did not attain statistical significance. The committee for Update 2000 concluded that although new evidence suggested an increased risk of non-malignant respiratory disorders, particularly COPD, among people exposed to TCDD, the observation is tentative and the information insufficient to determine an association between the exposures of interest and respiratory dis-