Work Exposure to Paraquat Doesn’t Raise Risk of Dying of Parkinson’s, Study Finds

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by Steve Bryson PhD |

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Workplace exposure to the herbicide paraquat during its manufacture does not lead to a greater risk of dying of Parkinson’s disease or of developing the disorder, an analysis of death certificates in the U.K. found.

Certificates, examined through December 2017, covered more than 950 people who had worked at four sites where paraquat was manufactured between 1961 and 1995, and were exposed to the herbicide.

The study, “Mortality from Parkinson’s disease and other causes among a workforce manufacturing paraquat: an updated retrospective cohort study,” was published in the Journal of Occupational Medicine and Toxicology.

Parkinson’s disease is characterized by the damage and death of nerve cells (neurons) that produce the chemical signaling molecule dopamine. The loss of these neurons lowers dopamine levels, affecting brain activity and causing disease symptoms.

While the exact underlying mechanisms that trigger neuron death are unclear, it is thought that a combination of genetic and environmental factors contributes to Parkinson’s development.

Over the past four decades, several studies have investigated an association between Parkinson’s and rural living, including the intake of well-water, farming, and pesticide use.

In particular, interest has focused on the herbicide paraquat because its chemical structure is similar to another chemical, called MPTP, which has been shown to cause Parkinson’s-like symptoms in humans, non-human primates, and rodents.

While several case-controlled studies have found an association between paraquat exposure and Parkinson’s, a recent analysis noted that objective measures of paraquat exposure were inadequate, and further studies were needed.

A mortality study of all workers (926 men and 42 women) at plants that manufactured paraquat between 1961 and 1995 in the town of Widnes, an industrial town in Cheshire, England, is one of the few that examined Parkinson’s and paraquat exposure in groups of people, instead of individual cases. A recently published report that followed these workers up to 2009 found no exposure-related health effects, including Parkinson’s disease.

This study updates that original investigation with follow-up extended through 2017. It was funded by Syngenta Crop Protection in Switzerland, which markets paraquat, and one of its two researchers is a company employee.

“The primary objective of the study was to assess whether there is any evidence of increased [Parkinson’s disease] mortality as the underlying cause of death or as a mentioned cause of death,” the team wrote.

Health data on the site workers and cause of death information was collected from the U.K.’s National Health Service (NHS) Digital database.

The observed number of deaths from causes was compared with the expected number of deaths, calculated based on national (England and Wales) mortality rates as well as local age and period-specific mortality rates. A standardized mortality ratio (SMR) was calculated as the ratio of the number of observed deaths to the expected deaths in the general population.

Mortality rates were also calculated for Parkinson’s disease using all certified causes of death listed on the death certificate, and rates for Parkinson’s as the underlying cause of death.

Jobs with high paraquat exposure included 117 workers, while another 202 jobs involved medium exposure to the herbicide. The average age of male workers at first exposure was 32.8, and contributed to 33,805 person-years of follow-up.

Analysis found local mortality rates around Widnes due to Parkinson’s similar to those of England and Wales.

Among men exposed through work to paraquat, there were two deaths due to Parkinson’s disease compared to an expected 3.3 deaths in the general population. Moreover, in the death certificates of two other workers that mentioned Parkinson’s, one died of pneumonia and the other of Alzheimer’s disease. No relationship was found between the duration or level of paraquat exposure.

One worker exposed for more than five years had a death certificate that mentioned Parkinson’s, compared with 2.38 expected mentions. Two workers with high or medium paraquat exposure had a death certificate that mentioned Parkinson’s (2.44 expected mentions). One of these was exposed to paraquat for more than five years (1.14 expected mentions).

Of the 21 deceased women working at the sites, none of the death certificates mentioned Parkinson’s. Death due to all other neurological disorders was low, and there was no mention of secondary Parkinson’s or other movement disorders on the death certificates of male and female employees.

A further assessment of other causes of death found deaths due to heart disease in workers were significantly lower than expected, compared to England and Wales and to local rates. Mortality due to lung diseases were slightly lower in workers, and significantly lower compared to local rates. A similar pattern was seen for lung cancer mortality. Deaths from all causes were lower during the first 15 years after the start of paraquat exposure.

Death due to kidney disease was of interest due to the acute kidney toxicity of paraquat; however, no deaths were observed compared to the 2.5 expected.

Mortality patterns were also examined for 319 workers who had ever held a job with high or medium exposure to paraquat. Most of these people started on paraquat production in the 1960s, and 204 had died by December 2017.

Compared with local mortality rates, all-cause mortality was lower than expected in workers. While deaths due to cancer were higher than expected in exposed workers, there was no evidence of any trends with a duration of exposure of more than one year, one to five years, or more than five years.

Two deaths due to Parkinson’s occurred in this group of workers. Still, Parkinson’s was only mentioned on the death certificate, and there were fewer Parkinson’s mentions than expected in the general population.

Researchers noted the study had some limitations, namely that Parkinson’s cases were only identified if Parkinson’s was a certified cause of death listed on the workers’ death certificate.

“However, although it is widely regarded that most patients with PD [Parkinson’s disease] die of its complications and not the disease, the limited information available suggests that PD is coded as the underlying cause of death of many patients and is mentioned on the death certificates of the majority of patients,” the researchers wrote.

Overall, the researchers believe there is “no evidence of an increased incidence of PD among PQ [paraquat] production workers based on mentions of PD on the death certificates of workers who had died. In addition, there was no evidence of adverse mortality due to other causes.”