Mortality risk among workers with exposure to dioxins

Our plain-English summary of Mortality risk among workers with exposure to dioxins, by J.J. Collins, K.M. Bodner, L.L. Aylward, et al (Occupational Medicine, 2016).

The study

• Authors: J.J. Collins, K.M. Bodner, L.L. Aylward, T.J. Bender, S. Anteau, M. Wilken, C.M. Bodnar
• Journal: Occupational Medicine
• Published: 2016;66(9):706-712
• Cited by: 6 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Background: In several studies, dioxin exposure has been associated with increased risk from several causes of death.

Aims: To compare the mortality experience of workers exposed to dioxins during trichlorophenol (TCP) and pentachlorophenol (PCP) production to that of the general population and to examine mortality risk by estimated exposure levels.

Methods: A retrospective cohort study which followed up workers' vital status from 1940 to 2011, with serum surveys to support estimation of historical dioxin exposure levels.

Results: Among the 2192 study subjects, there were nine deaths in TCP workers from acute non-lymphatic leukaemia [standardized mortality ratio (SMR) = 2.88, 95% confidence interval (CI) 1.32-5.47], four mesothelioma deaths (SMR = 5.12, 95% CI 1.39-13.10) and four soft tissue sarcoma (STS) deaths (SMR = 3.08, 95% CI 0.84-7.87). In PCP workers, there were eight deaths from non-Hodgkin's lymphoma (SMR = 1.92, 95% CI 0.83-3.79), 150 from ischaemic heart disease (SMR = 1.20, 95% CI 1.01-7.89) and five from stomach ulcers (SMR = 3.38, 95% CI 1.10-7.89). There were no trends of increased mortality with increased dioxin exposure except for STS and 2,3,7,8-tetrachlorodibenzo-p-dioxin levels. This finding for STS should be interpreted with caution due to the small number of deaths and the uncertainty in diagnosis and nosology.

Conclusions: While some causes of death were greater than expected, this study provides little evidence of increased risk when dioxin exposures are considered.

© The Author 2016. Published by Oxford University Press on behalf of the Society of Occupational Medicine. All rights reserved. For Permissions, please email: [email protected].

In plain English

This retrospective cohort study followed 2192 workers at a Midland, Michigan chemical plant. All had potential dioxin exposure in trichlorophenol (TCP) or pentachlorophenol (PCP) departments, and deaths were tracked through 2011. Dioxin levels were estimated from 431 blood samples and job histories. Deaths were compared with US rates (standardized mortality ratio, SMR). There were 1198 deaths overall (SMR 0.95, 95% CI 0.90 to 1.01). Among 1615 TCP workers, 4 died of mesothelioma (SMR 5.12, 95% CI 1.39 to 13.10) and 9 of acute non-lymphatic leukaemia (SMR 2.88, 95% CI 1.32 to 5.47). Among 773 PCP workers, one mesothelioma death gave an SMR of 3.56 (95% CI 0.09 to 19.84). Mesothelioma risk was not linked to dioxin level. Only soft tissue sarcoma showed a trend with TCDD. The authors urged caution because it rested on 4 deaths with uncertain diagnoses, and a prior tissue review found one of them, in the highest TCDD category, was not soft tissue sarcoma. The authors concluded there was little evidence that dioxin levels were related to the excess deaths. They suggested other plant exposures, possibly asbestos insulation on the autoclaves, may explain the mesothelioma excess.

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: retrospective cohort study (mortality comparison with the US population plus exposure-response analysis)
• Who was studied: 2192 workers with potential dioxin exposure in the TCP and/or PCP departments of a chemical plant in Midland, Michigan, from 1937 to 1982, followed to 31 December 2011: 1419 TCP only, 577 PCP only, 196 both
• Exposure: Dioxins (TCDD, summed HxCDD, HpCDD, OCDD and toxic equivalency) from TCP and PCP production. Historical levels were estimated from serum samples and a pharmacokinetic model applied to work histories. Asbestos was not a measured exposure.
• Main results:
  • All workers: 1198 deaths (SMR 0.95, 95% CI 0.90 to 1.01), of which 326 were cancers (SMR 0.98, 95% CI 0.88 to 1.10)
  • TCP workers: more non-lymphatic leukaemia deaths (SMR 2.88, 95% CI 1.32 to 5.47; 9 deaths) and mesothelioma deaths (SMR 5.12, 95% CI 1.39 to 13.10; 4 deaths) than expected
  • All TCP/PCP workers combined: 5 mesothelioma deaths, SMR 5.07 (95% CI 1.65 to 11.84). Per the table footnote, this comparison is limited to the ICD-10 period.
  • PCP workers: the raised SMRs for soft tissue sarcoma (1.73) and mesothelioma (3.56, 95% CI 0.09 to 19.84) were each based on one death
  • PCP workers: ischaemic heart disease deaths were raised (150 deaths, SMR 1.20, 95% CI 1.01 to 1.41)
  • Ischaemic heart disease, mesothelioma and acute non-lymphatic leukaemia exceeded expected numbers, but their risk was not associated with level of dioxin exposure
  • Soft tissue sarcoma (4 deaths) was the only cause showing an increasing trend with cumulative TCDD exposure (hazard ratio 1.06, 95% CI 1.02 to 1.09)
  • Of the four soft tissue sarcoma deaths, a previous tissue review found one was not soft tissue sarcoma, and that death was in the highest TCDD category. The analysis did not correct for this.
  • The authors urged caution about the soft tissue sarcoma finding because of the small number of deaths and the uncertainty in diagnosis and coding
  • The authors suggested that the mesothelioma excess may reflect other plant exposures, possibly asbestos insulation on the production autoclaves
• The authors' conclusion: The authors concluded that while some causes of death were greater than expected, there was little evidence that dioxin exposure levels were related to the increase
• Limitations the authors state:
  • The relatively small size (fewer than 2200 workers) limits study of rare causes of death
  • Exposure estimates extrapolated back several decades from present-day serum levels are likely subject to some misclassification
  • Soft tissue sarcoma diagnoses on death certificates are uncertain; one of the four deaths was previously found not to be soft tissue sarcoma, and reclassifying one or two deaths could change trends

Topics

Medical terms in this study, explained in our Lexicon: Cohort Study, standardized mortality ratio (SMR), confidence interval, Mesothelioma, Sarcoma.

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