Updated mortality among diverse operating segments of a petroleum company

Our plain-English summary of Updated mortality among diverse operating segments of a petroleum company, by R.J. Lewis, A.R. Schnatter, A.M. Katz, et al (Occupational and Environmental Medicine, 2000).

The study

• Authors: R.J. Lewis, A.R. Schnatter, A.M. Katz, F.S. Thompson, N. Murray, G. Jorgensen, G. Thériault
• Journal: Occupational and Environmental Medicine
• Published: 2000;57(9):595-604
• Cited by: 34 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: To update mortality for 34 560 employees from diverse operating segments of a Canadian petroleum company; and to investigate potential relations with occupational factors.

Methods: Employees from 1964-83 were linked to the Canadian mortality data base to provide 11 years additional follow up. There were 6760 deaths and 750 683 person-years of follow up compared with 3909 and 428 190, respectively, in the earlier study. Analyses used standardised mortality ratios (SMRs) to compare worker cause specific mortality with the Canadian general population. Duration of employment and latency analyses were performed for causes previously found to be increased in this and other petroleum cohorts, as well as any findings of interest.

Results: For the period 1964-94, employees experienced significantly low overall mortality (SMR=0.86 men, SMR=0.80 women). Kidney cancer, which has been increased in some studies of petroleum workers, was not increased. Acute non-lymphocytic leukaemia in exposed operating segments was consistent with the expected or only slightly, non-significantly increased. The most notable finding was increased deaths from mesothelioma among refinery and petrochemical workers (SMR 8.68; 95% confidence interval (95% CI) 5.51 to 13.03), most of whom were long term employees in jobs with presumed exposure to asbestos (mechanical and pipefitters). Deaths from multiple myeloma among marketing and distribution workers, which were previously increased, remained increased (SMR 2.08; 95% CI 0.95 to 3.95) in the update period 1984-94; however, there was no clear pattern by duration of employment or latency. Aortic aneurysms, which also were previously significantly increased among marketing and distribution workers approached the expected in the update period (SMR 1.18; 95% CI 0. 65-1.98). Analyses by duration of employment showed suggestive trends for aortic aneurysms, but earlier studies of this cohort have not found a relation between aortic aneurysms and exposure to hydrocarbons.

Conclusion: The additional 2851 deaths and 322 493 person-years of follow up strengthened the assessment of mortality patterns relative to occupational factors. With the exception of mesothelioma, no clear work related increases in disease were identified.

In plain English

This updated mortality study followed 34,560 people who worked at least a year between 1964 and 1983 for a Canadian petroleum company, or were retirees alive in 1964, to the end of 1994. Deaths were compared with Canadian general population rates using standardised mortality ratios (SMRs; 1.00 means as expected). For mesothelioma, expected numbers came from cancer incidence rates instead, which the authors said may slightly inflate those SMRs. Individual exposure data were not available. Overall death rates were below expected (men SMR 0.86, 95% CI 0.84 to 0.88; women SMR 0.80, 95% CI 0.73 to 0.88). All 24 mesothelioma deaths were in men (SMR 3.51; 95% CI 2.25 to 5.22). Twenty-three had worked in refinery or petrochemical operations, where the SMR was 8.68 (95% CI 5.51 to 13.03). The cases had worked a mean of 34 years, mostly in mechanical and pipefitter jobs with potential asbestos exposure. No fiber type or tumour site is named. Lung cancer was not increased among refinery and petrochemical workers. The authors concluded the mesothelioma excess likely reflects past asbestos exposure, and otherwise identified no clear work-related increases.

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 mortality study (an update of an earlier study) comparing deaths with expected numbers from Canadian general population rates using standardised mortality ratios (SMRs). For mesothelioma, expected deaths were calculated from general population incidence rates because national mortality rates were lacking.
• Who was studied: 34,560 employees of a Canadian petroleum company (26,322 men, 8,238 women) who worked 1 year or more between 1 January 1964 and 31 December 1983, plus retired workers alive on 1 January 1964. Follow-up ran to 31 December 1994, with 6760 deaths and 750,683 person-years. The workers were grouped into operating segments such as refinery, petrochemical, marketing and distribution, marine, exploration/drilling/production/pipeline, and office.
• Exposure: No individual exposure data. Operating segment was used as a broad exposure grouping (hydrocarbons, finished petroleum products, crude oil). For mesothelioma, refinery and petrochemical segments were considered potentially exposed to asbestos, mainly in mechanical and pipefitter jobs. No asbestos fiber type is named.
• Main results:
  • All-cause mortality was significantly below expected in men (SMR 0.86; 95% CI 0.84 to 0.88) and women (SMR 0.80; 95% CI 0.73 to 0.88).
  • There were 24 mesothelioma deaths among men and none among women. Men in the total cohort: 24 observed versus 6.83 expected, SMR 3.51 (95% CI 2.25 to 5.22).
  • Men ever employed in the combined refinery and petrochemical segment: 23 mesothelioma deaths observed versus 2.65 expected, SMR 8.68 (95% CI 5.51 to 13.03). Twenty-three of the 24 cases had worked in refining or petrochemicals. The one other case, classed as exploration, drilling or production, had incomplete work history that mentioned mechanical foreman.
  • The 24 mesothelioma cases had a mean duration of employment of 34 years (range 12 to 44) and an average year of first employment of 1942 (range 1927 to 1978).
  • Refinery and petrochemical mesothelioma SMR was 7.50 (95% CI 2.75 to 16.32, 6 observed) for 1964 to 1983, and 9.14 (95% CI 5.32 to 14.63, 17 observed) for 1984 to 1994.
  • Despite the mesothelioma excess, lung cancer was not increased among refinery and petrochemical workers. Lung cancer SMRs were 0.85 (95% CI 0.77 to 0.93) for all men and 0.96 (95% CI 0.83 to 1.09) for male refinery workers.
  • Aortic aneurysm deaths were significantly increased in men overall (SMR 1.27; 95% CI 1.04 to 1.53). Multiple myeloma in marketing and distribution men remained raised in 1984 to 1994 (SMR 2.08; 95% CI 0.95 to 3.95).
• The authors' conclusion: The authors concluded that the increased mesothelioma deaths among refinery and petrochemical workers likely reflect past asbestos exposures in the cohort. With the exception of mesothelioma, they identified no clear increases in work-related diseases.
• Limitations the authors state:
  • No data on the exposures of specific employees. Some results rested on small numbers, which limited the statistical power to detect increases.
  • Incomplete work histories meant some operating segments had to be extrapolated, possibly misclassifying workers' segment.
  • No data on potential confounders such as smoking and socioeconomic status.
  • The manual review of death certificates for mesothelioma could have biased the SMR upward. The authors said this should be offset because the comparison rates were generated the same way.
  • Expected mesothelioma deaths were calculated from cancer incidence rates rather than death rates, which may have slightly inflated the SMR.

Topics

Medical terms in this study, explained in our Lexicon: Latency, Mesothelioma, confidence interval, Pipefitter, Myeloma.

Related studies

• Mortality and cancer morbidity in a cohort of Canadian petroleum workers — R.J. Lewis, A.R. Schnatter, et al. (2003)
• Mortality Update of a Cohort of Canadian Petroleum Workers — A.R. Schnatter, N.C. Wojcik, et al. (2019)
• Update of mortality and cancer incidence in the Australian petroleum industry cohort — R.T. Gun, N. Pratt, et al. (2006)
• An updated study of mortality among workers at a petroleum manufacturing plant — Y. Honda, E. Delzell, et al. (1995)
• Updated epidemiological study of workers at two California petroleum refineries, 1950-95 — K.P. Satin, W.J. Bailey, et al. (2002)
• Update of a prospective study of mortality and cancer incidence in the Australian petroleum industry — R.T. Gun, N.L. Pratt, et al. (2004)

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