To assess the risk of Parkinson's disease (PD) and update information on mortality from major causes of death among a UK workforce who manufactured paraquat (PQ) between 1961 and 1995. There have been no previous studies of the incidence of PD among PQ production workers, although much epidemiological literature exists concerning the relationship between pesticides and PD, and interest has focused on PQ and its users.
The cohort included all employees who had ever worked on any of the four plants at Widnes where PQ was manufactured between 1961 and 1995, and 926 male and 42 female workers were followed through 30 June 2009. Mortalities for males were compared with national and local rates, including rates for PD as a mentioned cause of death.
Overall, 307 workers had died by 30 June 2009. One male death was due to PD, and no other death certificate mentioned PD. At least 3.3 death certificates of male employees would have been expected to have mentioned PD (standardised mortality ratio=31; 95% CI 1 to 171). Personal monitoring results were indicative that the exposure of a PQ production worker on a daily basis was at least comparable with that of a PQ sprayer or mixer/loader. Reduced mortalities compared with local rates were found for major causes of death.
The study provided no evidence of an increased risk of PD, or increased mortalities from other causes.
Many epidemiological studies have been conducted to investigate the relationship between exposure to pesticides and Parkinson's disease (PD) since a report that the toxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) caused acute parkinsonism in a small group of drug addicts, and several have focused on paraquat (PQ) in part because of its structural similarity to a toxic metabolite of MPTP.
Case–control studies provide much of the information about a possible association between PD and exposure to PQ, but most have small numbers of subjects exposed to PQ and/or limited exposure information.
This study is the first investigation of mortality from PD among a cohort of PQ manufacturing workers.
The study provided no evidence of increased mortalities from major causes of death. There was no evidence of increased mortality (underlying and mentioned cause) from PD.
Personal monitoring results indicated that workers engaged in PQ production were likely to have had a higher exposure to PQ than many of the subjects in case–control studies classified as exposed to PQ.
Strengths and limitations of this study
A major strength of the study is that it is a cohort study. Exposure of workers to PQ is confirmed by comprehensive job histories and the availability of personal monitoring information.
Limitations of the study include its size and power, although the upper confidence limit of the standardised mortality ratio for mentions of PD is relatively low (171). In addition, only information from death certificates of deceased workers was available, and it was not possible to study the morbidity of the entire group.