Estimates of historical exposures by phase contrast and transmission electron microscopy for pooled exposure-response analyses of North Carolina and South Carolina, USA asbestos textile cohorts

Objectives To develop pooled size-specific asbestos fiber exposure estimates for North Carolina and South Carolina asbestos textile plants. Methods Airborne sample data and prior exposure estimates by phase-contrast microscopy (PCM) for the two cohorts were reviewed and compared. Estimates by transmission electron microscopy (TEM) for 160 membrane filter samples from all plant were pooled. Poisson regression models were developed to predict bivariate diameter/length airborne fiber size distributions based on independent categorical variables for fiber diameter, fiber length, plant, and exposure zone. The model predicted bivariate diameter/length distributions were expressed as the proportion of fibers in 28 size-specific cells and these data were used to calculate PCM to TEM adjustment factors in order to estimate fiber size-specific exposures for the pooled cohort. Results Exposure levels in the North Carolina plants were in excess of 50 f/cc for many operations through about 1955 owing to lack of dust control measures in early years whereas levels in the South Carolina plant were generally less than 10 f/cc by about 1950. The Poisson regression models found covariates for plant department to be a stronger predictor of bivariate size proportions than plant; however, a plant effect was observed. The final Poisson models demonstrated good fit to the observed data. Conclusions Consistent with early studies, fiber exposures in the North Carolina plants were much higher than in South Carolina plant. Use of the predicted size-specific TEM exposures by plant and department based on the Poisson model predictions should reduce exposure.