title: Systematic review and meta-analysis of hydrocarbon exposure and the risk of Parkinson's disease.
abstract: BACKGROUND
  There is no consensus on the association between exposure to hydrocarbons and the risk of Parkinson's disease (PD). We conducted a systematic review and meta-analysis to summarise the epidemiological evidence and included a new large case-control study.
  
  
  METHODS
  Data were extracted following a predefined protocol. Risk estimates regarding the association between hydrocarbon exposure and PD were consolidated to produce a summary odds ratio (OR), 95% confidence intervals (CI), and p-value. In our case-control study, 1463 PD patients and 685 controls were recruited from clinical trials and completed a structured questionnaire describing their previous working exposure to hydrocarbons and other demographic measures. The association between exposure to hydrocarbons and risk of PD was evaluated using logistic regression.
  
  
  RESULTS
  The systematic search identified 13 case-control studies matching the inclusion criteria. The meta-analysis included 3020 PD cases and 6494 controls. The summary OR was 1.32 (95% CI 1.08-1.62, p = 0.006) for hydrocarbon exposure (ever versus never). In the PD GEN study, occupational exposure to hydrocarbons significantly increased the risk of PD (OR = 1.61; 95% CI 1.10-2.36, p = 0.014), and risk dose-dependently increased for subjects exposed greater than 10 years compared to subjects never exposed (OR = 2.19; 95% CI 1.13-4.26, p = 0.021). The addition of PD GEN data increased the total numbers to 4483 PD cases and 7179 controls and strengthened the significant association (summary OR = 1.36; 95% CI 1.13-1.63, p = 0.001).
  
  
  CONCLUSIONS
  This systematic review supports a positive association between hydrocarbon exposure and PD. Data from prospective studies are required to reinforce the relationship between hydrocarbon exposure and PD.
authors.affiliation: Birmingham Clinical Trials Unit, School of Cancer Sciences, Robert Aitken Institute, University of Birmingham, Birmingham, UK.
authors.forename: Oliver
authors.initials: O
authors.lastname: Palin
authors.affiliation: School of Clinical and Experimental Medicine, College of Medicine and Dental Sciences, University of Birmingham, Edgbaston, Birmingham, UK.
authors.forename: Clare
authors.initials: C
authors.lastname: Herd
authors.affiliation: School of Clinical and Experimental Medicine, College of Medicine and Dental Sciences, University of Birmingham, Edgbaston, Birmingham, UK; Department of Neurosciences, Queen Elizabeth Hospital, University Hospitals Birmingham NHS Foundation Trust, Edgbaston, Birmingham, UK.
authors.forename: Karen E
authors.initials: KE
authors.lastname: Morrison
authors.affiliation: Public Health, Epidemiology and Biostatistics, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK.
authors.forename: Alison C
authors.initials: AC
authors.lastname: Jagielski
authors.affiliation: Cancer Research UK Clinical Trials Unit, School of Cancer Sciences, University of Birmingham, Birmingham, UK.
authors.forename: Keith
authors.initials: K
authors.lastname: Wheatley
authors.affiliation: Public Health, Epidemiology and Biostatistics, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK; Institute of Public Health, Social and Preventive Medicine, Mannheim Medical Faculty, University of Heidelberg, 68135 Mannheim, Germany.
authors.forename: G Neil
authors.initials: GN
authors.lastname: Thomas
authors.affiliation: School of Clinical and Experimental Medicine, College of Medicine and Dental Sciences, University of Birmingham, Edgbaston, Birmingham, UK; Department of Neurology, Sandwell and West Birmingham Hospitals NHS Trust, Birmingham, UK. Electronic address: carlclarke@nhs.net.
authors.forename: Carl E
authors.initials: CE
authors.lastname: Clarke
mesh_terms: D016022:Case-Control Studies; D005260:Female; D006801:Humans; D006838:Hydrocarbons / Q000009:adverse effects*; D008297:Male; D016273:Occupational Exposure / Q000009:adverse effects*; D010300:Parkinson Disease / Q000453:epidemiology*
chemical_list: D006838:Hydrocarbons
keywords: Case-control; Epidemiology; Hydrocarbons; Occupational exposure; Parkinson's disease