title: Is pesticide use related to Parkinson disease? Some clues to heterogeneity in study results.
abstract: BACKGROUND
  Previous systematic reviews have indicated that pesticide exposure is possibly associated with Parkinson disease (PD). However, considerable heterogeneity has been observed in study results.
  
  
  OBJECTIVE
  We aimed at providing an update of the literature published on PD and exposure to pesticides by performing a systematic review and meta-analysis. In addition, we investigated whether methodological differences between studies could explain the heterogeneity in study results.
  
  
  METHODS
  We identified studies through a systematic literature search. We calculated summary risk ratios (sRRs) for pesticide exposure and subcategories using random effects meta-analyses and investigated sources of heterogeneity by meta-regression and stratified analyses.
  
  
  RESULTS
  Thirty-nine case-control studies, four cohort studies, and three cross-sectional studies were identified. An sRR of 1.62 [95% confidence interval (CI): 1.40, 1.88] for pesticide exposure (ever vs. never) was found. Summary estimates for subclasses of pesticides indicated a positive association with herbicides and insecticides, but not with fungicides. Heterogeneity in individual study results was not related to study design, source of control population, adjustment of results for potential confounders, or geographical area. However, results were suggestive for heterogeneity related to differences in the exposure assessment. Job title-based exposure assignment resulted in a higher sRR (2.5; 95% CI: 1.5, 4.1) than did assignment based on self-reported exposure (e.g., for self-reported ever/never exposure, sRR = 1.5; 95% CI: 1.3, 1.8).
  
  
  CONCLUSIONS
  This review affirms the evidence that exposure to herbicides and insecticides increase the risk of PD. Future studies should focus on more objective and improved methods of pesticide exposure assessment.
authors.affiliation: Institute for Risk Assessment Sciences, Division of Environmental Epidemiology, Utrecht University, The Netherlands. M.vanderMark@uu.nl
authors.forename: Marianne
authors.initials: M
authors.lastname: van der Mark
authors.forename: Maartje
authors.initials: M
authors.lastname: Brouwer
authors.forename: Hans
authors.initials: H
authors.lastname: Kromhout
authors.forename: Peter
authors.initials: P
authors.lastname: Nijssen
authors.forename: Anke
authors.initials: A
authors.lastname: Huss
authors.forename: Roel
authors.initials: R
authors.lastname: Vermeulen
mesh_terms: D004781:Environmental Exposure; D004785:Environmental Pollutants / Q000633:toxicity*; D006801:Humans; D010300:Parkinson Disease / Q000453:epidemiology* / Q000209:etiology*; D010302:Parkinson Disease, Secondary / Q000139:chemically induced / Q000453:epidemiology; D010575:Pesticides / Q000145:classification / Q000633:toxicity*; D012107:Research Design
chemical_list: D004785:Environmental Pollutants; D010575:Pesticides