Occupational exposure to the herbicide paraquat was not found to be associated with a greater risk of Parkinson disease or increased risk of mortality related to other causes.
Amid ongoing legal proceedings examining paraquat (PQ) exposure with risk of Parkinson disease (PD), a recent study published in the Journal of Occupational Medicine and Toxicology indicates that the herbicide has no association with increased risk of PD or death from any cause.
With research over the past 35 years having evaluated environmental factors, including rural living, well-water consumption, farming, and pesticide exposure with risk of PD, researchers highlight that notable interest has focused on the quaternary ammonium herbicide PQ due to its structural similarity with a toxin, called 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (or MPTP), that has been shown to cause permanent parkinsonism in humans.
“Many experimental animal and in vitro studies of PQ have been performed looking for effects, such as deficits in dopaminergic neurons, but there remain many challenges in interpreting the relevance of these studies to humans,” said the study authors.
Leveraging one of the few cohort studies examining PD and exposure to PQ, the researchers performed a longer follow-up for a mortality study of workers involved in the manufacture of PQ in 4 plants at Widnes, England, from 1961 to 1995. In a previous 2011 analysis that followed these workers until 2009, no exposure-related health effects were observed, including PD.
Conducting an extended follow-up of 7.5 years, health data of 926 male and 42 female workers derived from the UK National Health Service digital database were included in the retrospective cohort analysis. Mortality rates for males were compared with national and local rates, including rates for PD as a mentioned cause of death.
“The primary objective of the study was to assess whether there is any evidence of increased PD mortality as the underlying cause of death or as a mentioned cause of death,” noted the study authors. “A secondary objective was to provide updated information on mortality from major causes of death to confirm the absence of other exposure-related effects.”
Of the study participants, 117 workers were indicated to have had high PQ exposure, with 202 workers linked with medium exposure. The average age of male workers at first exposure was 32.8 years, accounting for 33,805 person-years of follow-up.
By the end of the follow-up, 394 male and 21 female workers had died. There were 4 death certificates for male workers that mentioned PD, in which 2 deaths indicated PD as the underlying cause. Notably, at least 6 certificates of male workers would have been expected to have mentioned PD (standardized mortality ratio [SMR], 0.67; 95% CI, 0.18-1.72). No relationship was identified with duration or level of exposure, and none of the death certificates of the 21 deceased female workers mentioned PD.
Furthermore, in comparison with local rates, workers exhibited reduced mortality rates for major causes of death, including lung cancer (SMR, 0.74; 95% CI, 0.52-1.02).
“The study provided no evidence of an increased risk of PD, or increased mortalities from other causes among PQ production workers whose exposure to PQ on a daily basis was at least comparable to that of a PQ sprayer or mixer/loader,” concluded the study authors.
Tomenson JA, Campbell C. Mortality from Parkinson’s disease and other causes among a workforce manufacturing paraquat: an updated retrospective cohort study. J Occup Med Toxicol. Published online May 27, 2021. doi:10.1186/s12995-021-00309-z