title: Associations of welding and manganese exposure with Parkinson disease: review and meta-analysis.
abstract: OBJECTIVE
  To examine associations of welding and manganese exposure with Parkinson disease (PD) using meta-analyses of data from cohort, case-control, and mortality studies.
  
  
  METHODS
  Epidemiologic studies related to welding or manganese exposure and PD were identified in a PubMed search, article references, published reviews, and abstracts. Inclusion criteria were 1) cohort, case-control, or mortality study with relative risk (RR), odds ratio (OR), or mortality OR (MOR) and 95 confidence intervals (95% CI); 2) RR, OR, and MOR matched or adjusted for age and sex; 3) valid study design and analysis. When participants of a study were a subgroup of those in a larger study, only results of the larger study were included to assure independence of datasets. Pooled RR/OR estimates and 95% CIs were obtained using random effects models; heterogeneity of study effects were evaluated using the Q statistic and I(2) index in fixed effect models.
  
  
  RESULTS
  Thirteen studies met inclusion criteria for the welding meta-analysis and 3 studies for the manganese exposure meta-analysis. The pooled RR for the association between welding and PD for all study designs was 0.86 (95% CI 0.80-0.92), with absence of between-study heterogeneity (I(2) = 0.0). Effect measures for cohort, case-control, and mortality studies were similar (0.91, 0.82, 0.87). For the association between manganese exposure and PD, the pooled OR was 0.76 (95% CI 0.41-1.42).
  
  
  CONCLUSIONS
  Welding and manganese exposure are not associated with increased PD risk. Possible explanations for the inverse association between welding and PD include confounding by smoking, healthy worker effect, and hormesis.
authors.affiliation: Department of Epidemiology and Biostatistics, University of South Florida, Tampa, USA. jmortime@health.usf.edu
authors.forename: James A
authors.initials: JA
authors.lastname: Mortimer
authors.forename: Amy R
authors.initials: AR
authors.lastname: Borenstein
authors.forename: Lorene M
authors.initials: LM
authors.lastname: Nelson
mesh_terms: D006801:Humans; D008345:Manganese / Q000633:toxicity*; D016273:Occupational Exposure; D010300:Parkinson Disease / Q000209:etiology*; D014896:Welding
chemical_list: D008345:Manganese