Long-term follow-up mortality study of petroleum refinery and chemical plant employees

Our plain-English summary of Long-term follow-up mortality study of petroleum refinery and chemical plant employees, by S.P. Tsai, E.L. Gilstrap, S.R. Cowles, et al (American Journal of Industrial Medicine, 1996).

The study

• Authors: S.P. Tsai, E.L. Gilstrap, S.R. Cowles, P.J. Snyder, C.E. Ross
• Journal: American Journal of Industrial Medicine
• Published: 1996;29(1):75-87
• Cited by: 32 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

A previous report presented the 1948-1983 mortality patterns of the Shell Deer Park Manufacturing Complex employees who were employed for at least 3 months from 1948 through 1972. The present study updates the earlier investigation by extending the vital status follow-up through 1989 and by expanding the cohort to include employees hired after 1972. As in the previous study, the overall mortality and cancer mortality for both refinery and chemical employees were quite favorable compared to residents in the local population. Among refinery workers, cancers for which a suspicion of work-relatedness was raised in the previous study, i.e. leukemia and cancers of the central nervous system and biliary passage/liver, no supportive evidence was found in this update. For both refinery and chemical plant employees, the mortality rate due to cancers of all lymphopoietic tissue increased with increasing duration of employment; this finding was also noted by the original study. This was also evident for lymphoreticulosarcoma in refinery employees and for leukemia in chemical plant employees. However, elevations of cancers of all lymphatic and hematopoietic tissue are primarily confined to employees who started work at the complex before 1946. By contrast, deaths from cancer of all lymphatic and hematopoietic tissue for employees hired after 1945 were 22% lower than the comparison population. Seven deaths with mesothelioma mentioned on the death certificates were identified, with 3.2 deaths expected, resulting in a statistically nonsignificant SMR of 219.

In plain English

This study, by authors from Shell Oil Company's Corporate Medical Department, updated an earlier mortality study of employees at Shell's Deer Park refinery and chemical complex in Texas. It followed 9,720 employees who worked at least 3 months between 1948 and 1989 and compared their death rates with those of Harris County residents. Among men, overall deaths (SMR 73, 95% CI 70-77) and cancer deaths were significantly lower than expected. None of more than 60 causes of death examined showed a statistically significant increase. Deaths from lymphatic and blood-forming cancers rose with longer employment, but these elevations were mainly among employees hired before 1946. Seven deaths had mesothelioma mentioned on the death certificate, against 3.2 expected from cancer incidence rates. This gave an SMR of 219 (95% CI 88-452), which was not statistically significant. Six of the seven had held maintenance positions; the authors said most of their longest jobs were likely to have involved some asbestos exposure. The authors concluded that overall mortality was favorable and that suspicions of work-related cancers raised in the earlier study were not supported.

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 (update of an earlier study). Death rates were compared with Harris County, Texas, using standardized mortality ratios (SMRs).
• Who was studied: 9,720 employees with potential plant exposure who worked at least 3 months at the Shell Deer Park Manufacturing Complex (refinery and chemical plant), Texas, followed 1948 to 1989. The cohort included 9,007 men and 713 women and contributed 246,656 person-years.
• Exposure: Employment at the refinery or chemical plant, analyzed by work area, duration of employment and hire date. Asbestos was listed among potentially hazardous substances at the facility. No fiber type was named.
• Main results:
  • All-cause mortality among male employees was significantly lower than in Harris County: refinery SMR 79 (95% CI 75-84), chemical plant SMR 62 (95% CI 57-68), total SMR 73 (95% CI 70-77).
  • All-cancer mortality among male employees was also significantly lower than in Harris County: SMR 86 (77-96) for refinery employees and 78 (66-92) for chemical plant employees.
  • Among male employees, none of more than 60 causes of death examined showed a statistically significant increase.
  • Among male employees, lung cancer (SMR 75, 64-87) and nonmalignant respiratory disease (SMR 68, 55-83) were significantly lower than in the comparison population.
  • Among male refinery employees, mortality from cancers of the lymphopoietic tissue increased with increasing duration of employment, particularly for lymphoreticulosarcoma. Findings for male chemical plant employees were similar.
  • Elevations of cancers of all lymphatic and hematopoietic tissue were primarily confined to employees who started work before 1946.
  • Seven deaths with mesothelioma mentioned on the death certificate were found, with 3.2 expected (estimated from SEER incidence rates for 1973-1988). This gave an elevated but statistically nonsignificant SMR of 219 (88-452).
  • Five of the seven mesothelioma decedents were hired before 1946.
  • The mesothelioma SMR for employees hired after 1945 was 106 (2 observed, 1.88 expected, 13-384). For those hired before 1946 it was 382 (5 observed, 1.31 expected, 124-889).
  • Six of the seven mesothelioma cases had worked in maintenance positions and one in production. The authors stated that most of the seven cases' longest job assignments were likely to have had some occupational asbestos exposure.
  • Among the 713 female employees there were 15 deaths, giving an all-cause SMR of 49 (28-81).
• The authors' conclusion: The authors concluded that overall mortality was favorable compared with local residents. They found no supportive evidence for the possible work-related cancers suggested in the original study.
• Limitations the authors state:
  • Expected mesothelioma deaths could not be estimated directly because general-population mortality rates for mesothelioma were not available. Incidence rates were used instead.
  • The authors attributed the lower overall mortality partly to selection of healthy workers and other factors (the healthy worker effect).
  • Numbers of deaths among women were small, giving unstable estimates.
  • Employees with unknown race were counted as white in the analyses.
  • For cancers with long survival, mortality data could underestimate actual incidence.

Topics

Medical terms in this study, explained in our Lexicon: Mesothelioma, Death Certificate.

Related studies

• Mortality among maintenance employees potentially exposed to asbestos in a refinery and petrochemical plant — S.P. Tsai, L.C. Waddell, et al. (1996)
• An updated mortality study of workers at a petroleum refinery in Beaumont, Texas — G.K. Raabe, K.W. Collingwood, et al. (1998)
• Asbestos exposure and cancer mortality among petroleum refinery workers: a Poisson regression analysis of updated data — F. Montanaro, M. Ceppi, et al. (2004)
• Mortality and cancer morbidity in a cohort of Canadian petroleum workers — R.J. Lewis, A.R. Schnatter, et al. (2003)
• An updated study of mortality among workers at a petroleum manufacturing plant — Y. Honda, E. Delzell, et al. (1995)
• Update of mortality and cancer incidence in the Australian petroleum industry cohort — R.T. Gun, N. Pratt, et al. (2006)

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