Evaluation of Airborne Crocidolite Fibers at an Asbestos-Cement Plant

Introduction

Mineral fibers exposures are regulated in the United States to reduce the possible risk to workers of contracting disease. The bases for these regulations are retrospective epidemiological studies of workers exposed to airborne asbestos fibers (primarily chrysotile, but also reflect exposures to crocidolite and amosite used in mining, textiles, insulation, cement, and friction products) located in North America and Europe. These studies were examined and used to establish a unit risk factor (slope factor) for asbestos exposures,^1–5 leading to contemporary regulatory exposure levels. These models predict a risk of one in 10 000 for a dose of 0.1 f/ml over a working lifetime. The exposures used to establish the dose response were based on data derived using optical microscopy to count particles 1 µm and larger (midget impinger samples); very few membrane filter analyses were incorporated into these studies (the method was not developed until the late 1960s, after the exposures of interest occurred). The unit risk factor was developed after converting the available impinger data to fiber data by using an assumed conversion factor or by establishing exposure levels on the basis or employee interviews.¹ This lack of data created an inherent uncertainty in the estimated conversion factors. More recent reviews of a wider range of epidemiology studies are also affected by assumptions necessary to evaluate the reported exposures.⁶ No worker exposures were determined using electron microscopy analytical procedures, though these procedures have recently been applied to retained filters from a textile mill.⁷

Abstract

In many developing countries, asbestos is still used for friction products, insulation, and asbestoscement products. However, there is very limited modern data evaluating the airborne concentrations of asbestos fibers in operating asbestos plants. We have conducted an Industrial Hygiene survey, using state of the art PCM and TEM analysis, of a Bolivian asbestos cement plant that uses crocidolite during the production of roofing shingles and water tanks. The study is the first to use modern analytical measurements in an uncontrolled asbestos environment. Thus, this study offers insight into the historical airborne exposures at asbestos plants operating before modern dust control measures were introduced or airborne fiber levels regulated. The results suggest that fiber concentrations are far in excess of those for which current models would predict the onset of asbestos disease and mesothelioma. Significantly, the plant and community do not have an elevated incidence of asbestos disease or mesothelioma. Airborne fiber levels during comminution and fiber sizing ranged from 100 to as much as 1,000 f/cc. Fiber levels in the mixing, forming and finishing areas ranged from 0.5 to 9 f/cc. Based on the number of overloaded samples and estimates of their concentration, historical estimates of exposure may be underestimated by an order of magnitude. Also, an excellent correlation (1:1) between the PCM and TEM fiber concentrations (for fibers >0.25 um diameter) were obtained, raising questions about the conventional wisdom that the PCM systematically undercounts fibers. The results of this study further indicate that current models may overestimate the risk of asbestos disease and mesothelioma.

Keywords

Asbestos, crocidolite, Bolivia, cement, shingles, amphibole, sanding, sieving