Exposure to chemical, physical, and biological agents in the workplace is difficult to characterise. A worker’s exposure is never constant over time. Workers within groups with similar tasks and working environments are rarely uniformly exposed. Hence, assigning workers to “exposed” and “unexposed” groups or to exposure categories becomes a problem. Insight is required into the reasons why exposure variability exists, how large this variability is, and which factors determine differences in exposure levels among workers. This knowledge is essential in design, conduct, and interpretation of epidemiological studies and workplace intervention programmes. In recent years statistical techniques have become available that allow simultaneous evaluation of the magnitude of variance components as well as determinants of this variability. These techniques are powerful instruments in the design of measurement strategies in epidemiological studies and in implementation of control and prevention strategies to reduce hazardous exposure.¹

EXPOSURE VARIABILITY AND EXPOSURE MODELS

Assessment of exposure to hazardous substances at the workplace has shown that exposure is rarely constant. In workplaces tasks, activities, work processes, and locations change over time, resulting in occupational exposures that vary both within workers over time and between workers in the same job. Figure 1 depicts the variability in exposure to inhalable flour dust among and between workers in Swedish bakeries,² illustrating that the (geometric) mean and (geometric) standard deviation of an exposure parameter in an occupational group present only limited information on the underlying exposure pattern among workers in this occupational group. The phenomenon of exposure variability needs to be understood for a number of purposes, including planning exposure measurements, assigning estimates of exposure to subjects in a study, identifying determinants of exposure, evaluating compliance with exposure limits, and establishing efficiency of control measures.