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Meta-analyses in environmental and occupational health
  1. John P A Ioannidis1,2,3,4
  1. 1Department of Medicine, Stanford University School of Medicine, Stanford, California, USA
  2. 2Department of Health Research and Policy, Stanford University School of Medicine, Stanford, California, USA
  3. 3Department of Statistics, Stanford University School of Humanities and Sciences, Stanford, California, USA
  4. 4Meta-Research Innovation Center at Stanford (METRICS), Stanford University, Stanford, California, USA
  1. Correspondence to Professor John P A Ioannidis, Stanford University School of Medicine, Stanford, CA 94305, USA; jioannid{at}


Objectives Meta-analyses are considered generally as the highest level of evidence, but concerns have been voiced about their massive, low-quality production. This paper aimed to evaluate the landscape of meta-analyses in the field of occupational and environmental health and medicine.

Methods Using relevant search terms, all meta-analyses were searched for, but those published in 2015 were assessed for their origin, whether they included randomised trials and individual-level data and whether they had authors from the industry or consultancy firms.

Results PubMed searches (last update February 2017) identified 1251 eligible meta-analyses in this field. There was a rapid increase over time (n=16 published in 1995 vs n=163 published in 2015). Of the 163 eligible meta-analyses published in 2015, 49 were from China, followed at a distance by the USA (n=19). Only 16 considered randomised (intervention) trials and 13 included individual-level data. Only 1 of the 150 meta-analyses had industry authors and none had consultancy firm authors. As an example of conflicting findings, 12 overlapping meta-analyses addressed mobile phones and brain cancer risk and they differed substantially in number of studies included, eligibility criteria and conclusions.

Conclusions There has been a major increase in the publication of meta-analyses in occupational and environmental health over time, with the majority of these studies focusing on observational data, while a commendable fraction used individual-level data. Authorship is still limited largely to academic and non-profit authors. With massive production of meta-analyses, redundancy needs to be anticipated and efforts should be made to safeguard quality and protect from bias.

  • meta-analysis

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  • Contributors JPAI conceived this editorial, collected and interpreted data, and wrote the paper.

  • Funding This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient consent Not required.

  • Provenance and peer review Commissioned; internally peer reviewed.

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