Interpreting Levels of Exposures To Chemical Agents

The past half-century has witnessed great advances in the characterization of exposures to toxic chemicals in the workplace. With the development of robust personal monitors, it is now a relatively simple matter to measure the daily air levels among various workers in an observational group (a group of workers sharing observable factors such as job, location, department, etc.). Unfortunately, the interpretation of levels of exposure has not achieved the same degree of sophistication. In fact, decisions still tend to rely upon one-to-one comparisons of the highest measured air concentration with occupational exposure limits (OELs), much as they did 50 years ago (see, for example, Refs. (10–14)). If the highest air concentration is less than the OEL, then exposure is acceptable (in compliance) and vice versa. While such interpretations would be reasonable if occupational exposures were relatively constant, they seem remarkably naive when considered in light of daily exposure levels which vary between 10 and 4000 fold. Because exposures must be evaluated in the face of great variability within and between workers in observational groups, statistical methods are central to proper assessment procedures. Likewise, since monitoring is motivated by, and must ultimately relate to, exposure limits, the impact of exposure variability on the interpretation of OELs is important. The purpose of this chapter is to integrate statistical methods and the philosophical bases for exposure limits within a conceptual framework for interpreting exposure data. The methods will be illustrated with the four sets of exposure data depicted in graphical form. The sources and characteristics of these measurements are summarized and all data are listed in Appendix A for those who wish to work through the various examples in the text. Keywords: Random process; Compliance testing; Variations; Sampling; Statistical models; Exposure limits; Testing relative to limits; Nomenclature