Firefighting and Cancer: A Meta-analysis of Cohort Studies in the Context of Cancer Hazard Identification

Our plain-English summary of Firefighting and Cancer: A Meta-analysis of Cohort Studies in the Context of Cancer Hazard Identification, by N.L. DeBono, R.D. Daniels, L.E. Beane Freeman, et al (Safety and Health at Work, 2023).

The study

• Authors: N.L. DeBono, R.D. Daniels, L.E. Beane Freeman, J.M. Graber, J. Hansen, L.R. Teras, T. Driscoll, K. Kjaerheim, P.A. Demers, D.C. Glass, D. Kriebel, T.L. Kirkham, R. Wedekind, A.M. Filho, L. Stayner, M.K. Schubauer-Berigan
• Journal: Safety and Health at Work
• Published: 2023;14(2):141-152
• Type of article: Review
• Cited by: 45 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objective: We performed a meta-analysis of epidemiological results for the association between occupational exposure as a firefighter and cancer as part of the broader evidence synthesis work of the IARCMonographs program.

Methods: A systematic literature search was conducted to identify cohort studies of firefighters followed for cancer incidence and mortality. Studies were evaluated for the influence of key biases on results. Random-effects meta-analysis models were used to estimate the association between ever-employment and duration of employment as a firefighter and risk of 12 selected cancers. The impact of bias was explored in sensitivity analyses.

Results: Among the 16 included cancer incidence studies, the estimated meta-rate ratio, 95% confidence interval (CI), and heterogeneity statistic (I2) for ever-employment as a career firefighter compared mostly to general populations were 1.58 (1.14-2.20, 8%) for mesothelioma, 1.16 (1.08-1.26, 0%) for bladder cancer, 1.21 (1.12-1.32, 81%) for prostate cancer, 1.37 (1.03-1.82, 56%) for testicular cancer, 1.19 (1.07-1.32, 37%) for colon cancer, 1.36 (1.15-1.62, 83%) for melanoma, 1.12 (1.01-1.25, 0%) for non-Hodgkin lymphoma, 1.28 (1.02-1.61, 40%) for thyroid cancer, and 1.09 (0.92-1.29, 55%) for kidney cancer. Ever-employment as a firefighter was not positively associated with lung, nervous system, or stomach cancer. Results for mesothelioma and bladder cancer exhibited low heterogeneity and were largely robust across sensitivity analyses.

Conclusions: There is epidemiological evidence to support a causal relationship between occupational exposure as a firefighter and certain cancers. Challenges persist in the body of evidence related to the quality of exposure assessment, confounding, and medical surveillance bias.

© 2023 Occupational Safety and Health Research Institute.

In plain English

This meta-analysis pooled earlier cohort studies of firefighters and cancer. It supported the International Agency for Research on Cancer's evaluation of firefighting. The authors searched the literature to June 13, 2022, and combined 35 cohort studies from the USA, Canada, Northern and Western Europe, Australia, New Zealand and the Republic of Korea. Most compared male career firefighters with general populations. For new mesothelioma cases, the combined rate ratio was 1.58 (95% confidence interval 1.14 to 2.20) across 7 studies, with little variation between studies. For mesothelioma deaths the ratio was 1.75 (0.83 to 3.69) across 3 studies, a wide interval. New bladder cancer cases occurred at 1.16 times the comparison rate (1.08 to 1.26). New cases of prostate, testicular, colon and thyroid cancer, melanoma and non-Hodgkin lymphoma were also more frequent. New lung cancer cases were less frequent (0.85, 0.75 to 0.96). The authors noted that firefighters may disturb asbestos-containing building materials. They concluded that the mesothelioma and bladder findings could plausibly relate to the job, while the others were inconsistent or possibly explained by bias. No included study controlled directly for smoking.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Systematic review and meta-analysis of cohort studies (random-effects models), with bias assessment and sensitivity analyses, done for IARC Monographs volume 132
• Who was studied: 35 cohort studies of firefighters from the USA, Canada, Northern and Western Europe, Australia, New Zealand and the Republic of Korea, published up to June 13, 2022. Results limited to females or to part-time/volunteer firefighters were excluded, and the main comparisons were mostly with general populations.
• Exposure: Occupational exposure as a firefighter, measured as ever-employment and duration of employment. Asbestos is named among possible exposures from disturbing building materials; no fiber types are specified.
• Main results:
  • Mesothelioma incidence in ever-employed career firefighters: meta-rate ratio 1.58 (95% CI 1.14 to 2.20), heterogeneity (I2) 8%, from 7 studies
  • Removing one Danish mesothelioma study (4 cases, many part-time and volunteer firefighters) raised the mesothelioma meta-rate ratio from 1.58 to 1.70 and reduced heterogeneity to 0%
  • Mesothelioma mortality: meta-rate ratio 1.75 (95% CI 0.83 to 3.69), from 3 studies. The authors noted the confidence intervals were wide for most mortality outcomes.
  • Mesothelioma incidence appeared inversely associated with duration of employment, but the estimates were statistically imprecise
  • Bladder cancer incidence: meta-rate ratio 1.16 (95% CI 1.08 to 1.26), I2 0%
  • Other incidence results: testicular cancer 1.37 (1.03 to 1.82, I2 56%); colon cancer 1.19 (1.07 to 1.32, I2 37%); prostate cancer 1.21 (1.12 to 1.32, I2 81%); melanoma 1.36 (1.15 to 1.62); non-Hodgkin lymphoma 1.12 (1.01 to 1.25); thyroid cancer 1.28 (1.02 to 1.61)
  • Lung cancer incidence was lower in firefighters: meta-rate ratio 0.85 (95% CI 0.75 to 0.96), from 14 studies
  • The authors judged that asbestos exposure outside firefighting was unlikely to explain the mesothelioma results
• The authors' conclusion: The authors concluded that the positive associations for mesothelioma and bladder cancer could plausibly be related to exposures in the occupation, with the bladder cancer association modest in size. For prostate, testis and colon cancer, melanoma and non-Hodgkin lymphoma, they judged the findings inconsistent, or more likely to be partly or fully explained by bias.
• Limitations the authors state:
  • Cumulative exposure measures (such as exposed days or fire responses) could not be meta-analyzed, so inferences rest mainly on ever-employment and duration of employment
  • The reviewed studies were limited by exposure assessment problems, medical surveillance bias, lack of adjustment for confounders such as tobacco, healthy worker biases and short follow-up
  • No included study controlled directly for smoking
  • There was no cause-of-death code for mesothelioma before 1999 (ICD-10), which may have obscured mesothelioma risk in older mortality studies
  • The duration-of-employment results for mesothelioma were based on only three studies and were highly imprecise
  • No studies reported risk specific to wildland firefighters

Topics

Medical terms in this study, explained in our Lexicon: Meta-Analysis, Occupational Exposure, rate ratio, confidence interval, Mesothelioma, Colorectal Cancer, Stomach Cancer, Confounding.

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