Literature review and meta-analysis of environmental toxins associated with increased risk of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder and leading cause of death worldwide, whose pathogenesis has been linked to toxic environmental exposures. We used the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (i) to compile, and group by exposure setting (non-specified general; residential; occupational), environmental factors reported to modulate the risk of developing PD and (ii) to map and geospatially analyze global regions of both research activity and paucity. Among the broader environmental settings, occupational exposures had the highest average odds ratio value at 3.82, followed by general (non-specified or mixed) exposures at 3.07, and residential exposures at 2.36. Occupational exposure to industrial toxins was the highest ranked subset of exposures with an odds ratio of 10.74. Among the studies meeting the inclusion criteria, 75 % were conducted in Europe or the Western United States. The number of individuals partaking per study ranged from a high of 55,585 (Taiwan) to a low of 233 (Faroe Islands), with a mean of n = 14,462. The top three environmental factors associated with high odds ratios for increased risk of developing PD were (i) exposure to dyes (25.33), (ii) methylene chloride (16.5) and specifically in adult men (iii) consumption of fatty whale meat (10.57), which is known to harbor a broad spectrum of so called persistent, bioaccumulative, toxic (PBT) pollutants. Geospatially, the highest odds ratio values were identified in European countries, whereas notable data gaps were revealed for South America, Australia, Africa, and the majority of Asia with the exception of Taiwan. Whereas occupational exposures to industrial chemicals, such as harmful dyes and methylene chloride, ranked highest in risk values, available data suggest notable opportunities for reducing PD cases globally by limiting harmful environmental exposures to a spectrum of toxic chemicals, particularly via the food intake route. Thus, current efforts in improving environmental quality globally by limiting toxic emission may deliver the added benefit of helping to reign in PD. Agents of concern in this respect include pesticides (e.g., paraquat, demeton, monocrotophos), particulate matter associated with air pollution, and a spectrum of organic and inorganic neurotoxins including heavy metals.