PULMONARY EFFECTS OF INHALED ZINC-OXIDE IN HUMAN-SUBJECTS, GUINEA-PIGS, RATS, AND RABBITS

Occupational exposure to freshly formed zinc oxide (ZnO) particles (< 1.0-mu-m aerodynamic diameter) produces a well-characterized response known as metal fume fever. An 8-hr threshold limit value (TLV) of 5 mg/m3 has been established to prevent adverse health effects because of exposure to ZnO fumes. Because animal toxicity studies have demonstrated pulmonary effects near the current TLV, the present study examined the time course and dose-response of the pulmonary injury produced by inhaled ZnO in guinea pigs, rats, rabbits, and human volunteers. The test animals were exposed to 0, 2.5, or 5.0 mg/m3 ZnO for up to 3 hr and their lungs lavaged. Both the lavage fluid and recovered cells were examined for evidence of inflammation or altered cell function. The lavage fluid from guinea pigs and rats exposed to 5 mg/m3 had significant increases in total cells, lactate dehydrogenase, beta-glucuronidase, and protein content. These changes were greatest 24 hr after exposure. Guinea pig alveolar macrophage function was depressed as evidenced by in vitro phagocytosis of opsonized latex beads. Significant changes in lavage fluid parameters were also observed in guinea pigs and rats exposed to 2-5 mg/m3 ZnO. In contrast, rabbits showed no increase in biochemical or cellular parameters following a 2-hr exposure to 5 mg/m3 ZnO. Differences in total lung burden of ZnO, as determined in additional animals by atomic absorption spectroscopy, appeared to account for the observed differences in species responses. Although the lungs of guinea pigs and rats retained approximately 20% and 12% of the inhaled dose, respectively, rabbits retained only 5%. Adverse responses to ZnO inhalation were also observed in human subjects (n = 4). Each of the subjects responded with one or more of the classic symptoms of metal fume fever approximately 6 to 10 hr after a 2-hr exposure to 5 mg/m3 ZnO, although no changes in pulmonary function were observed. Together, these studies demonstrate that a single 2- to 3-hr exposure to ZnO at the current TLV does elicit adverse health effects in several mammalian species.