The respiratory system under normal conditions has the ability to maintain sterility of the deep lung even when large numbers of microbes are deposited under laboratory conditions. A number of mechanisms have been shown to be responsible for this defense of the lung, namely, mechanical clearance via the mucociliary escalator, biological clearance mediated through the alveolar macrophage (AM), and associated cellular and humoral immunological events. The adverse effects of numerous gaseous pollutants on these pulmonary defense mechanisms against infectious disease have long been recognized. Recently, researchers have begun to investigate the propensity of trace metals for causing similar effects.