A Historical cohort study of cancer mortality and offspring sex ratio among municipal solid waste incinerator workers

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1. Construction of the cohortWith the collaboration of the trade union, we asked municipal solid waste workers to join the cohort study. Finally, 2,866 workers having a work history at municipal solid waste incinerators (exposed group) and 6,239 waste collection workers (reference group) answered to the baseline survey questionnaire. To make rapid evaluation of cancer mortality, we tried to construct a retired workers cohort but it was not possible mainly due to strong social concern on privacy protection.2. Assessment of exposure to dioxins among incinerator workersIt is not feasible to measure blood dioxin levels for all cohort members. Therefore, we generated several surrogate exposure indices from baseline data such as duration / frequency of exposure to fly ash obtained by self-administrated questionnaire. Those indices were validated by comparing with blood dioxin levels in the subgroup of 58 waste incinerator workers. A correlation was observed between duration of exposure (sum of the period of work exposed to fly ash) and HpDF (one of the PCDD/F congeners) concentration.3. Evaluation of cancer mortalitySince the cohort consists of active workers only, the population is too yang to evaluate risks of cancer mortality. We need further follow-up for more than 10 years to reach conclusions.4. Effects on offspring sex ratioWe examined offspring sex ratio as an index of effect on reproductive system. Analysis using surrogate exposure indices showed association between longer period of exposure and lower sex ratio (fewer male offspring), though it was not statistically significant. The association did not change after adjusting maternal parity, paternal age at birth and the year of birth. However, the sex ratio was higher in the shortest exposure group than in the control group. The evidence is not strong enough to discuss relationship between fly ash exposure and offspring sex ratio. Further research is necessary.