Recurrent exposure to welding fumes induces insufficient recovery from inflammation.

Previous studies on welding-fume-induced lung fibrosis have indicated that recovery is possible when the degree of exposure is short-term and moderate. However, this study investigated the recovery after recurrent exposure to welding fumes, as welders are invariably re-exposed to welding fumes after recovering from radiographic pneumoconiosis. Thus, to investigate the disease and recovery processes of welding-fume-induced pneumoconiosis in the case of recurrent welding-fume exposure, rats were exposed to manual metal arc-stainless steel (MMA-SS) welding fumes with a total suspended particulate (TSP) concentration of 51.4 +/- 2.8 mg/m(3) (low dose) or 84.6 +/- 2.9 mg/m(3) (high dose) for 2 h/day in an inhalation chamber for 1 mo and then allowed to recover from the inflammation for 1 mo. Thereafter, the rats were exposed again to MMA-SS with a TSP concentration of 44.1 +/- 8.8 mg/m(3) (low dose) or 80.1 +/- 9.8 mg/m(3) (high dose) for another 30 d and then allowed to recover from the inflammation for 1 mo. The recovery from the first exposure was then compared with that from the second exposure. The first and second exposures to MMA-SS welding fumes were found to produce significant increases in the lung weights and inflammatory parameters, including total cell numbers, alveolar macrophages (AMs), polymorphonuclear cells (PMNs), lymphocytes, and lactate dehydrogenase (LDH) in the bronchoalveolar lavage fluid (BALF) when compared with the unexposed controls. Following the first and second recovery, a significant reduction in inflammatory parameters of BALF was observed between the exposure and recovery groups. Histopathological observations showed foamy or pigmented macrophage accumulation, cellular debris, or pigment from burst macrophages after the first or second exposure. Following the first or second recovery, cellular debris or pigment from burst macrophages was cleared away from the lungs and accumulation of foamy or pigmented macrophages was decreased when compared to previous exposure. Reactive hyperplasia was noticed after second exposure or either recovery. However, significant differences were observed between the first and second exposure or the first and second recovery. In particular, the number of PMNs was significantly higher after the second exposure than after the first exposure. Also, all cell types in the BALF were significantly elevated in the high-dose second recovery group than in the first recovery group, indicating an incomplete recovery from second exposure. In conclusion, these results indicated that the lung damage caused by the second welding-fume exposure was more difficult to recover from than the first exposure.