The effect of occupational exposure to welding fumes on trachea, bronchus, and lung cancer: A supplementary analysis of regular occupational exposure and of occasional occupational exposure based on the systematic review and meta-analysis from the WHO/ILO Joint estimates of the work-related burden of disease and Injury.
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BACKGROUND: The World Health Organization (WHO) and the International Labour Organization (ILO) are the producers of the WHO/ILO Joint Estimates of the Work-related Burden of Disease and Injury (WHO/ILO Joint Estimates). The WHO International Agency for Research on Cancer has classified welding fumes as carcinogenic to humans (Group 1). A previous systematic review and meta-analysis from the WHO/ILO Joint Estimates reported that there was "sufficient evidence of harmfulness" that compared with no (or low) occupational exposure to welding fumes, any (or high) occupational exposure to welding fumes increased the risk of developing trachea, bronchus, and lung cancer. It concluded that WHO/ILO Joint Estimates could be produced of the attributable burden of trachea, bronchus, and lung cancer. However, occupational exposure to welding fumes must be considered in greater detail, as there may be differences in risk between those with regular occupational exposure to welding fumes and those with occasional occupational exposure, the latter of which has previously been estimated to be highly prevalent. Regular and occasional occupational exposure to welding fumes have not previously been considered in a systematic review. Here, we present a supplementary analysis to our previous systematic review and meta-analysis, providing parameters for estimating the number of deaths and disability-adjusted life years from trachea, bronchus, and lung cancer attributable to regular and to occasional occupational exposure to welding fumes, to inform the development of WHO/ILO Joint Estimates of this burden of disease. OBJECTIVES: We sought to systematically review and meta-analyse estimates of the effect of regular occupational exposure to welding fumes and of occasional occupational exposure to welding fumes, compared with no (or very rare) occupational exposure to welding fumes, on trachea, bronchus, and lung cancer (three outcomes: prevalence, incidence, and mortality). DATA SOURCES: We developed and published a protocol for our previous systematic review, applying the Navigation Guide as an organizing systematic review framework where feasible. We searched electronic databases for potentially relevant records from published and unpublished studies, including Medline, EMBASE, Web of Science, CENTRAL and CISDOC, up until 27 May 2024. We also searched grey literature databases, internet search engines, and organizational websites; hand-searched reference lists of previous systematic reviews; and consulted additional experts. STUDY ELIGIBILITY AND CRITERIA: We included studies of working-age workers (≥15 years) in the formal and informal economy in any Member State of WHO and/or ILO but excluded studies of children (<15 years) and of unpaid domestic workers. We included randomized controlled trials, cohort studies, case-control studies, and other non-randomized intervention studies with an estimate of the effect of regular and/or occasional occupational exposure to welding fumes, compared with no (or very rare) occupational exposure to welding fumes, on trachea, bronchus, and lung cancer (prevalence, incidence, and mortality). STUDY APPRAISAL AND SYNTHESIS METHODS: At least two review authors independently: screened titles and abstracts against the eligibility criteria, screened full texts of potentially eligible records, and extracted data from the included studies. We combined effect estimates using random-effects meta-analysis. Two or more review authors assessed the risk of bias, quality of evidence, and strength of evidence, using the Navigation Guide's tools and approaches adapted to the WHO/ILO Joint Estimates. We conducted subgroup analyses and sensitivity analyses. RESULTS: Twenty-eight records from 19 studies (17 case control studies and two cohort studies) met the inclusion criteria, comprising over 2.4 million participants (unclear number of females, but N < 4388) and conducted in 17 countries in three WHO Regions (Region of the Americas, European Region, and Western Pacific Region). Across included studies, we judged risk of bias as generally probably low/low, but the risks of selection bias as probably high for several studies and of exposure assessment bias as high for one study. Our search identified no evidence on the outcome of having trachea, bronchus, and lung cancer (prevalence). Compared with no (or very rare) occupational exposure to welding fumes, regular occupational exposure to welding fumes increased the risk of developing trachea, bronchus, and lung cancer (incidence) by an estimated 39% (RR 1.39, 95% confidence interval [CI] 1.15-1.67, 15 studies, 29,785 participants, I2 23%; high quality of evidence), and occasional occupational exposure to welding fumes increased the risk of developing trachea, bronchus, and lung cancer (incidence) by an estimated 16% (RR 1.16, 95% CI 1.06-1.27, 16 studies, 32,838 participants, I2 15%; moderate quality of evidence). Compared with no (or very rare) occupational exposure to welding fumes, regular occupational exposure to welding fumes increased the risk of dying from trachea, bronchus, and lung cancer (mortality) by an estimated 25% (RR 1.25, 95% CI 0.88-1.77, 1 study, 3583 participants; low quality of evidence), and occasional occupational exposure to welding fumes increased the risk of dying from trachea, bronchus, and lung cancer (mortality) by an estimated 31% (RR 1.31, 95% CI 1.07-1.59, 1 study, 4215 participants, low quality of evidence). Subgroup analyses found no evidence for differences by WHO Region and sex. Sensitivity analyses supported the main analyses' findings. CONCLUSIONS: Overall, for incidence of trachea, bronchus, and lung cancer, we judged the existing body of evidence for human data as "sufficient evidence of harmfulness" for both regular and occasional occupational exposure to welding fumes; a positive relationship is observed between exposure and outcome where chance, bias, and confounding can be ruled out with reasonable confidence. For mortality of trachea, bronchus, and lung cancer, we judged the existing body of evidence for human data as "inadequate evidence of harmfulness"; the available evidence is insufficient to assess effects of the exposure. The summary effect estimates presented in this systematic review could be used as input data for the WHO/ILO Joint Estimates to produce estimates of proportions of the working-age population with regular and occasional occupational exposure to welding fumes and the attributable burden of trachea, bronchus, and lung cancer.