The Random-Effects Model Applied to Refractory Ceramic Fiber Data

Abstract

Refractory ceramic fiber (RCF) is a valuable, high-temperature, insulating material with a variety of industrial uses. Because some fibers are respirable by humans and RCF is relatively durable in simulated lung fluids, RCF may pose a health hazard in the workplace. The RCF industry has established a comprehensive product stewardship program (PSP) to identify, quantify, and manage risks. One key element of this PSP is a workplace monitoring program. This paper analyzes monitoring data collected as part of a Consent Agreement with the U.S. Environmental Protection Agency over the period from 1993 to 1998. More specifically, this paper applies the random-effects model (REM) to data collected at several Unifrax plants and applicable to several groups of workers. The REM fits the RCF data well. Depending upon the plant and the functional job category values of the variance of the log-transformed time-weighted average workplace concentrations range from slightly less than 0.5 to 1.0. The estimated intraclass correlations (ratio of the between-worker variance to the total variance) were less than 0.4, and most were less than 0.2. Implications of these findings are examined. Use of the REM in the development of a workplace respiratory policy is described. Finally, two possible criteria for measuring compliance with an occupational exposure limit are reviewed: an “overexposure” criterion developed by Rappaport and co-workers and a conventional “no exceedance” criterion reportedly used by regulatory agencies. The overexposure criterion is logically correct for potential toxicants with chronic effects. For representative values of statistical parameters for RCF from the plants considered, the overexposure criterion is less stringent.