A cancer incidence and mortality study of Dow Chemical Canada Inc. manufacturing sites

Our plain-English summary of A cancer incidence and mortality study of Dow Chemical Canada Inc. manufacturing sites, by C. Burns, K. Harrison, B. Jammer, et al (Occupational Medicine, 2005).

The study

• Authors: C. Burns, K. Harrison, B. Jammer, D. Zuccarini, B. Lafrance
• Journal: Occupational Medicine
• Published: 2005;55(8):618-24
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Background: Previously, the mortality was reported in a cohort of male workers at an Ontario chemical plant.

Aims: To expand the cohort and to investigate the mortality and cancer incidence risk among chemical manufacturing sites.

Methods: We followed 5277 men and 1301 women from 1950 to 1999.

Results: Employees experienced lower mortality and cancer incidence rates than the general population for several major causes of death, including heart disease, respiratory cancer and many other cancers. There were no cases of angiosarcoma of the liver. We observed a lower mortality rate of prostate cancer [standardized mortality ratio = 0.79, 95% confidence interval (CI) 0.43-1.32], but a higher incidence rate of prostate cancer [standardized incidence ratio (SIR) = 1.22, 95% CI 1.00-1.48]. Among the Sarnia employees, the incidence of pleural neoplasms was increased (5 observed versus 1.48 expected, SIR = 3.37, 95% CI 1.09-7.86). These cancers were included in the 12 deaths with malignant mesothelioma at Sarnia.

Conclusion: Consistent with the earlier report, lower mortality rates were observed for the major classifications of disease and malignant neoplasms. The higher incidence rates of prostate cancer are not readily explainable but may reflect increased screening among current employees and recent retirees. Past asbestos exposure prior to 1980 is probably a contributor to the deaths due to malignant mesothelioma but is not reflected in lung cancer mortality. We find little indication of any other increased rates of mortality or cancer within the overall workforce of these chemical plants.

In plain English

This cohort study followed 6,578 people (5,277 men and 1,301 women) who worked at least 12 continuous months between 1950 and 1996 at Dow Chemical Canada plants in Sarnia, Ontario, and Fort Saskatchewan, Alberta. Their deaths (1950 to 1999) and new cancers (1969 to 1999) were compared with rates for all of Canada. Overall, workers had fewer deaths and cancers than expected. Among men, six cancers of the pleura (the lining around the lungs) were found against 1.67 expected (standardized incidence ratio 3.59, 95% confidence interval 1.31 to 7.81). Five of them were at Sarnia. The authors identified 13 deaths from malignant mesothelioma, 12 among Sarnia employees, but national comparison rates could not be computed. Lung cancer in men was below expected. Prostate cancer cases in men exceeded expectation (104 versus 85.25 expected), though prostate cancer deaths did not. No liver angiosarcoma was found. The authors concluded that past asbestos exposure before 1980 probably contributed to the mesothelioma deaths. They noted that they lacked data on smoking and on jobs held before and after Dow employment, and that the study did not evaluate specific chemical exposures.

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 (historical) cohort study of mortality and cancer incidence, using record linkage with Statistics Canada death and cancer databases and comparison with national rates (standardized mortality and incidence ratios)
• Who was studied: 6,578 employees (5,277 men, 1,301 women) who worked at least 12 continuous months during 1950 to 1996 at Dow Chemical Canada Inc. sites in Sarnia, Ontario, or Fort Saskatchewan, Alberta. Mortality was followed from 1950 to 1 July 1999 and cancer incidence from 1969 to 1999.
• Exposure: Employment at chemical manufacturing plants (products included styrene, chlorine, vinyl chloride monomer, chlorinated solvents, epoxy resins and plastics). The study did not address specific chemical exposures. Asbestos exposure was discussed as a past exposure, with control measures introduced in the later 1970s. No fiber type is named.
• Main results:
  • Men: 558 deaths, against 913.47 expected; all-cause standardized mortality ratio (SMR) 0.61 (95% CI 0.56 to 0.66). No cause of death was statistically significantly higher than expected.
  • Men, cancer incidence: 6 cancers of the pleura, against 1.67 expected (SIR 3.59, 95% CI 1.31 to 7.81). Five of the six occurred among Sarnia employees (5 observed versus 1.48 expected, SIR 3.37, 95% CI 1.09 to 7.86).
  • Men: five of the six pleural cancers occurred in men who worked at least 15 years (SIR 5.87, 95% CI 1.89 to 13.70). All six occurred among men hired 1950 to 1970.
  • 13 deaths from malignant mesothelioma were identified, 12 of them among Sarnia employees. All six incident pleural cancers were among these deaths. Expected numbers could not be computed because older ICD revisions lack a specific code.
  • Men: respiratory cancer deaths were below expected (48 observed versus 91.09 expected, SMR 0.53, 95% CI 0.39 to 0.70). Bronchus and lung cancer incidence was 40 observed versus 99.37 expected (SIR 0.40, 95% CI 0.29 to 0.55).
  • Men, prostate cancer: 14 deaths versus 17.78 expected (SMR 0.79, 95% CI 0.43 to 1.32), but 104 incident cases versus 85.25 expected (SIR 1.22, 95% CI 1.00 to 1.48).
  • No cases of angiosarcoma of the liver were found on review of selected death certificates.
  • Women: 39 deaths among 1,301 women. There were 42 cancers overall (SIR 0.53, 95% CI 0.38 to 0.71).
• The authors' conclusion: The authors concluded that the workforce showed a strong healthy worker effect, with lower death and cancer rates than the general population. Past asbestos exposure before 1980 was probably a contributor to the malignant mesothelioma deaths but was not reflected in lung cancer mortality. They suggested the lack of excess lung cancer indicates occupational asbestos exposure at the site was not pervasive. They also suggested the higher prostate cancer incidence may reflect increased screening.
• Limitations the authors state:
  • Specific chemical exposures were not addressed.
  • There were no data on smoking, lifestyle or other potential confounders.
  • There was no information on jobs and avocations held before and after Dow employment.
  • Comparison rates for mesothelioma could not be computed because asbestos-related deaths are not specifically coded in older ICD revisions.
  • The very low death rates may reflect a younger cohort with less follow-up.
  • Small numbers and wide confidence intervals for some causes make causal inferences difficult.
  • There are no data confirming that screening explains the higher prostate cancer incidence.

Topics

Medical terms in this study, explained in our Lexicon: Incidence, standardized mortality ratio (SMR), confidence interval, Standardized Incidence Ratio (SIR), Neoplasm, Malignant Mesothelioma, Asbestos Exposure, Lung Cancer.

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