with the results; design information on other studies can be found in Chapter 5. A synopsis of toxicologic and clinical information related to the biologic plausibility that the chemicals of interest can influence the occurrence of a health outcome is presented next and followed by a synthesis of all the material reviewed. Each health-outcome section ends with the present committee’s conclusions regarding the strength of the evidence that supports an association with the chemicals of interest. The categories of association and the committee’s approach to categorizing the health outcomes are discussed in Chapters 1 and 2.
For the purposes of this report, noncancerous respiratory disorders comprise acute and chronic lung diseases other than cancer. Acute noncancerous respiratory disorders include pneumonia and other respiratory infections; they can increase in frequency and severity when the normal defense mechanisms of the lower respiratory tract are compromised. Chronic noncancerous respiratory disorders generally take two forms: airways disease and parenchymal disease. Airways disease encompasses disorders, among them asthma and chronic obstructive pulmonary disease (COPD), characterized by obstruction of the flow of air out of the lungs. COPD is also known as chronic obstructive airways disease and includes emphysema and chronic bronchitis. Parenchymal disease, or interstitial disease, generally includes disorders that cause inflammation and scarring of the deep lung tissue, including the air sacs and supporting structures; parenchymal disease is less common than airways disease and is characterized by reductions in lung capacity, although it 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 most important risk factor for many noncancerous respiratory disorders is inhalation of cigarette smoke. Although exposure to cigarette smoke is not associated with all diseases of the lungs, it is the major cause of many airways disorders, especially COPD; 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 it would otherwise be. The frequency of habitual cigarette-smoking varies with occupation, socioeconomic status, and generation. For those reasons, cigarette-smoking can be 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 (Kang et al., 2006; McKinney et al., 1997).
It is well known that causes of death from respiratory diseases, especially chronic diseases, are frequently misclassified on death certificates. Grouping various respiratory diseases for analysis, unless they all are associated with a given