O10.1 YEARS OF LIFE LOST DUE TO EXPOSURE: CAUSAL CONCEPTS AND EMPIRICAL SHORTCOMINGS

P. Morfeld.Institut für Arbeitswissenschaften der RAG Aktiengesellschaft, Institut und Poliklinik für Arbeits- und Sozialmedizin der Universität zu Köln, Germany

Introduction: Excess years of potential life lost due to exposure (EYPLL) are an important measure of health impact in occupational epidemiology complementing rate or risk statistics. They are calculated in two steps: firstly, for each age group the product of the number of excess deaths in the exposed is multiplied by the expected remaining years of life at age of death; secondly, these products are summed up over all age categories. Recently, following a presentation at EPICOH 2001, this approach was extended by Park et al¹ from SMR based calculations to EYPLL estimates based on Poisson regression models to estimate age specific and endpoint specific exposure effects. We investigated the limitations of this concept.

Methods: Counterfactual logic is used to explore whether EYPLL does measure the true excess years of life lost due to exposure (EYLL) without bias. This approach follows an abstract reasoning presented by Robins and Greenland.²

Results: I show that the total EYLL can be estimated unbiasedly by calculating the corresponding EYPLL. I further demonstrate by life table examples that the EYLL conditional on age at death and the EYLL for a specific cause of death, such as lung cancer, cannot be estimated unbiasedly without adopting speculative causal models. This potential bias can be fairly extreme.

Conclusions: EYLL estimates calculated from life tables or regression models, as presented by some authors for lung cancer or after stratification for age, are potentially biased. Although statistics conveying information about the advancement of disease onset are helpful in exposure impact analysis and especially worthwhile in exposure impact communication, we believe that attention should be drawn to the difficulties involved and that epidemiologists should always be aware of these …