The potential significance of airborne fiber size parameters to the development of pleural plaques in workers who manufacture refractory ceramic fiber

Increasing production of refractory ceramic fiber (RCF), a synthetic material with industrial applications (e.g., kiln insulation), has created interest in investigating any potential respiratory effects of exposure to airborne fibers during manufacturing. While RCF differs from naturally occurring fibers by physicochemical characteristics, one hypothesis is that toxicity is determined by the dose, durability and dimensions of fibers reaching target tissues of the lungs.;Study of RCF manufacturing workers has indicated an association between cumulative fiber exposure and pleural plaques. Air sampling information for the study includes fiber sizing data obtained from electron microscopy analyses of 118 samples collected from three independent studies over a 20-year period (1976-1995). Mean fiber length and diameter were found to differ among the three studies, although analyses of variance showed these differences might be attributed to specific production processes and tasks associated with workers who were sampled. Sizing data were used with the Harris-Fraser deposition model to estimate pulmonary dose of fibers of specified dimensions for 709 workers. From review of the literature, critical dimensions (length $<$ 10$mu$m, diameter $<$ 0.4$mu$m) were defined for fibers which may translocate to the parietal pleura. Separate dose correction factors reflecting differences in fiber dimensions in six Uniform Job Title Groups were used with data on airborne fiber concentration and employment duration to calculate cumulative dose for each worker. Each of three exposure/dose metrics treated as continuous variables in separate logistic regression models was significantly related to plaques, even after adjusting for possible past asbestos exposure: cumulative fiber exposure, $rm Xsp2=15.0 (p<0.001)$; cumulative pulmonary dose (all fibers), $rm Xsp2=16.2 (p<0.001)$; cumulative pulmonary dose (critical dimension fibers), $rm Xsp2=15.3 (p<0.001).$ Odds ratios (ORs) were calculated for levels of each metric. Increasing ORs were statistically significant for the two highest dose levels of critical dimension fibers (level three, OR = 9.5, 95%CI = (1.2, 76); level four, OR = 20, 95%CI = (2.8, 152)). Similar associations existed for all metrics after adjustment for possible asbestos exposure. It was concluded that development of pleural plaques follows exposure- and dose-response patterns, and that airborne fibers in RCF manufacturing facilities include those with critical dimensions. Analysis of additional air samples could improve estimates of the dose-response relationship.