OBJECTIVES--To examine whether or not workers with pre-existing mild pulmonary fibrosis have accelerated decline in forced expiratory volume in one second (FEV1) or forced vital capacity (FVC), under low level exposure to chrysotile asbestos. METHODS--All male workers in two asbestos manufacturing factories were followed up annually for six years to compare their declines in FEV1 and FVC. The values of FEV1 and FVC were divided by the square of the person's height to adjust for body size differences (FEV1/Ht2 and FVC/Ht2, respectively). Annual change was calculated for each subject as a slope of the simple linear regression with FEV1/Ht2 or FVC/Ht2 regressed according to age. Analysis was conducted on 242 middle aged workers who had normal routine spirometry values, normal chest radiographs or mild pneumoconiosis up to 1/2 grade, without changes either in smoking habit or severity of pneumoconiosis during the study period, and with acceptable spirograms in three or more surveys. The occupational environment, in terms of chrysotile asbestos, had been well controlled below the threshold limit value of Japan at that time--namely, 2 fibres/micromilligrams. RESULTS-There was no significant effect from the interaction between pre-existing mild pulmonary fibrosis and low level of exposure to chrysotile asbestos on the accelerated annual decline of FEV1/Ht2, or FVC/Ht2. Fibrosis significantly contributed to annual changes in FEV1/Ht2, even after adjustment for mean FEV1 and smoking. The point estimate of the contribution was - 4.9 ml/m2/y. No significant independent contribution of exposure was found in decline of either FEV1/Ht2 or FVC/Ht2. CONCLUSIONS--Pre-existing pulmonary fibrosis is an independent risk factor for accelerated annual decline of FEV1, even when mild and stable. Additional decline due to exposure to chrysotile asbestos is less probable if it is well controlled under the current threshold limit value.
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