Cohort study of workers at a New Zealand agrochemical plant to assess the effect of dioxin exposure on mortality

Objectives

To describe how the exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) influenced mortality in a cohort of workers who were exposed more recently, and at lower levels, than other cohorts of trichlorophenol process workers.

Design

A cohort study.

Setting

An agrochemical plant in New Zealand

Participants

1,599 men and women working between 1 January 1969 and 1 November 1988 at a plant producing the herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) with TCDD as a contaminant. Cumulative TCDD exposure was estimated for each individual in the study by a toxicokinetic model.

Primary outcome measures

Calculation of cause-specific standardised mortality ratios (SMRs) and 95% confidence intervals (95% CI's) compared those never and ever exposed to TCDD. Dose–response trends were assessed firstly through SMRs stratified in quartiles of cumulative TCCD exposure, and secondly with a proportional hazards model.

Results

The model intercept of 5.1 ppt of TCDD was consistent with background TCDD concentrations in New Zealand among older members of the population. Exposed workers had non-significant increases in all-cancer deaths (SMR=1.08, 95% CI 0.86 to 1.34), non-Hodgkin lymphoma (SMR=1.57, 95% CI: 0.32 to 4.59), soft tissue sarcoma (one death) (SMR=2.38, 95% CI: 0.06 to 13.26), diabetes (SMR=1.27, 95% CI: 0.55 to 2.50) and ischaemic heart disease (SMR=1.21, 95% CI: 0.96 to 1.50). Lung cancer deaths (SMR=0.95, 95% CI: 0.56 to 1.53) were fewer than expected. Neither the stratified SMR nor the proportional hazard analysis showed a dose–response relationship.

Conclusion

There was no evidence of an increase in risk for 'all cancers', any specific cancer and no systematic trend in cancer risk with TCDD exposure. This argues against the carcinogenicity of TCDD at lower levels of exposure.

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