RESPONSE: Re: Extended Follow-up of a Cohort of British Chemical Workers Exposed to Formaldehyde

Extract

As Dr. Greenberg points out, the optimal choice of reference rates in occupational cohort studies is not always straightforward. The advantage of comparison with data for the general population is that these data are often readily available and statistically stable. Bias may occur, however, if a substantial proportion of the reference population is materially exposed to the agent or agents under study or if they differ importantly in their exposure to other risk factors for the disease.

In our study, we calculated expected numbers of deaths for lung cancer from national rates, both with and without adjustment for local differences in mortality. Neither method is ideal. On the one hand, comparisons with the national population are potentially more prone to confounding by differences in smoking habits and in exposure to outdoor air pollutants. On the other hand, the local populations whose mortality was used to adjust expected numbers will have included proportionately more cohort members and therefore a higher percentage of individuals with exposure to formaldehyde. This proportion was still relatively small, however, and we therefore give greater weight to the locally adjusted analysis, although as we indicated, there remains a possibility of residual confounding.