Current recommendations for limits on metalworking fluids may provide insufficient protection from workplace-related illness. Chronic obstructive pulmonary disease (COPD) is a challenging outcome in occupational cohorts because its long period of worsening pulmonary function allows sicker workers to reduce exposure, causing a healthy worker survivor bias. G-estimation is a statistical method that reduces this bias. We introduce a public health approach using g-estimation to compare a series of potential exposure-reducing interventions.
Autoworkers at three General Motors plants in Michigan were followed for COPD mortality from 1 January 1941 to 31 December 1994. For each of the three fluid types (straight, soluble, synthetic), a series of binary variables indicated whether exposure exceeded a series of potential limits. Separate g-estimation analyses for each limit yielded results expressed as the total number of years of life that could have been saved among those who died from COPD had that exposure limit been enforced.
Lower limits would have had greater effect than higher limits. A ban on soluble fluids (the most common type) would have had the greatest effect, saving an estimated 1550 years of life. Corresponding estimates were 737 and 260 years for straight and synthetic fluids, respectively. Few workers were exposed to synthetic fluids, limiting analytic power.
This application of g-estimation suggests that limiting exposure to metalworking fluids could have saved many years of life lost to COPD in this cohort. The approach permits comparison of different interventions. Separate limits should be considered for different types of fluids.