Researchers are fortunate when they have at their disposal a detailed chronology of the worklife experience of workers that provides an historic review of jobs they have held over time. For these workers a job exposure matrix can then be set up that allows each and every job change that a worker has gone through to be associated with specific exposure information.
Detailed exposure histories must be summarized for analysis purposes in order to determine whether patterns are evident that could be related to health and safety issues in the workplace. We can visualize a list of, say, 20 job changes that a worker had experienced in his or her working lifetime. There are then several alternative ways in which the exposure details (for each of the 20 job changes in this example) can be summarized, taking duration and/or concentration/dose/grade of exposure into account.
It is important to note, however, that different conclusions from a study could be reached depending on the method selected (Suarez-Almazor et al. 1992). An example of five summary worklife exposure measures is shown in table 1.
Table 1. Formulae and dimensions or units of the five selected summary measures of worklife exposure
|
Exposure measure |
Formula |
Dimensions/Units |
|
Cumulative exposure index (CEI) |
Σ (grade x time exposed) |
grade and time |
|
Mean grade (MG) |
Σ (grade x time exposed)/total time exposed |
grade |
|
Highest grade ever (HG) |
highest grade to which exposed for ≥ 7 days |
grade |
|
Time-weighted average (TWA) grade |
Σ (grade x time exposed)/total time employed |
grade |
|
Total time exposed (TTE) |
Σ time exposed |
time |
Adapted from Suarez-Almazor et al. 1992.
Cumulative exposure index. The cumulative exposure index (CEI) is equivalent to “dose” in toxicological studies and represents the sum, over a working lifetime, of the products of exposure grade and exposure duration for each successive job title. It includes time in its units.
Mean grade. The mean grade (MG) cumulates the products of exposure grade and exposure duration for each successive job title (i.e., the CEI) and divides by the total time exposed at any grade greater than zero. MG is independent of time in its units; the summary measure for a person exposed for a long period at a high concentration will be similar to that for a person exposed for a short period at a high concentration. Within any matched set in a case-control design, MG is an average grade of exposure per unit of time exposed. It is an average grade for the time actually exposed to the agent under consideration.
Highest grade ever. The highest grade ever (HG) is determined from scanning the work history for the highest grade assignment in the period of observation to which the worker was exposed for at least seven days. The HG could misrepresent a person’s worklife exposure because, by its very formulation, it is based on a maximizing rather than on an averaging procedure and is therefore independent of duration of exposure in its units.
Time-weighted average grade. The time-weighted average (TWA) grade is the cumulative exposure index (CEI) divided by the total time employed. Within any matched set in a case-control design, the TWA grade averages over total time employed. It differs from MG, which averages only over the total time actually exposed. Thus, TWA grade can be viewed as an average exposure per unit of time in the full term of employment regardless of exposure per se.
Total time exposed. The total time exposed (TTE) accumulates all time periods associated with exposure in units of time. TTE has appeal for its simplicity. However, it is well accepted that health effects must be related not only to duration of chemical exposure, but also to the intensity of that exposure (i.e., the concentration or grade).
Clearly, the utility of a summary exposure measure is determined by the respective weight it attributes to either duration or concentration of exposure or both. Thus different measures may produce different results (Walker and Blettner 1985). Ideally, the summary measure selected should be based on a set of defensible assumptions regarding the postulated biological mechanism for the agent or disease association under study (Smith 1987). This procedure is not, however, always possible. Very often, the biological effect of the duration of exposure or the concentration of the agent under study is unknown. In this context, the use of different exposure measures may be useful to suggest a mechanism by which exposure exerts its effect.
It is recommended that, in the absence of proved models for assessing exposure, a variety of summary worklife exposure measures be used to estimate risk. This approach would facilitate the comparison of findings across studies.