Updated epidemiological study of workers at two California petroleum refineries, 1950-95

Our plain-English summary of Updated epidemiological study of workers at two California petroleum refineries, 1950-95, by K.P. Satin, W.J. Bailey, K.L. Newton, et al (Occupational and Environmental Medicine, 2002).

The study

• Authors: K.P. Satin, W.J. Bailey, K.L. Newton, A.Y. Ross, O. Wong
• Journal: Occupational and Environmental Medicine
• Published: 2002;59(4):248-56
• Cited by: 23 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: To further assess the potential role of occupational exposures on mortality, a second update of a cohort study of workers at two petroleum refineries in California was undertaken.

Methods: Mortality analyses were based on standardised mortality ratios (SMRs) and 95% confidence intervals (95% CIs) using the general population of California as a reference. Additional analyses of lymphatic and haematopoietic cancer deaths and diseases related to asbestos were undertaken.

Results: The update consisted of 18,512 employees, who contributed 456,425 person-years of observation between 1950 and 1995. Both overall mortality and total cancer mortality were significantly lower than expected, as were several site specific cancers and non-malignant diseases. In particular, no significant increases were reported for leukaemia cell types or non-Hodgkin's lymphoma. Mortality excess from multiple myeloma was marginally significant. The excess was confined to employees enrolled before 1949. Furthermore, there was no significant upward trend based on duration of employment, which argues against a causal interpretation relative to employment or exposures at the refineries. No increase was found for diseases related to asbestos: pulmonary fibrosis; lung cancer; or malignant mesothelioma. There was no significant increase in mortality from any other cancers or non-malignant diseases.

Conclusion: This second update provides additional reassurance that employment at these two refineries is not associated with increased risk of mortality.

In plain English

This second update of a cohort mortality study followed 18 512 employees (92% men) of Chevron's Richmond and El Segundo refineries in California who worked between 1950 and 1995. Because asbestos had been used there, asbestos-related diseases were analysed separately. No fiber type was named. Among men, deaths from all causes were lower than expected from California rates. One death was coded as asbestosis. Pulmonary fibrosis or pneumoconiosis caused 22 deaths against 28.4 expected. Lung cancer deaths were below expected overall but significantly raised in men employed under 5 years. Three pleural mesotheliomas gave a standardised mortality ratio (SMR) of 39.6 (95% CI 8.2 to 115.8). Counting three more mesotheliomas of unstated site as pleural gave 79.2 (95% CI 29.0 to 172.5). One peritoneal mesothelioma was recorded. Among men at both refineries, multiple myeloma was significantly raised (29 deaths against 18.22 expected; SMR 159.2, 95% CI 106.7 to 228.9). The authors said this excess was confined to employees enrolled before 1949. The authors concluded the update gave additional reassurance that refinery employment was not associated with increased mortality. They had no smoking data or specific chemical measurements.

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: Historical (retrospective) cohort mortality study; second update of an earlier study, using standardised mortality ratios (SMRs)
• Who was studied: 18 512 employees (88% white, 12% non-white; 92% men) of the Chevron Richmond and El Segundo refineries, California. They had completed 1 year of work and worked at least 1 day between 1950 and 1995, contributing 456 425 person-years of observation.
• Exposure: Employment at two petroleum refineries where asbestos had been used historically (fiber type not named). Asbestos measurements from 1973 to 1986, taken during asbestos removal and maintenance work, showed average 8-hour time-weighted concentrations of 0.001 to 0.09 fiber/ml. There were no measurements before the early 1970s and no chemical-specific exposure data.
• Main results:
  • Men, both refineries combined: deaths from all causes were significantly lower than expected (SMR 84.1, 95% CI 81.8 to 86.6).
  • Entire cohort: only one death was coded as asbestosis on the death certificate.
  • Pulmonary fibrosis or pneumoconiosis (analysed because asbestosis may be coded this way): 22 deaths compared with 28.4 expected (SMR 77.5, 95% CI 48.6 to 117.4).
  • The authors reported that lung cancer mortality among men was significantly decreased at both refineries. Table 1 gives a combined SMR of 84.1 (95% CI 75.5 to 93.4) from 349 deaths.
  • Lung cancer showed an inverse trend with duration of employment at both refineries (chi-square trend 36.5, p less than 0.0001). Men employed more than 30 years had the lowest lung cancer risk.
  • Lung cancer was significantly increased among men employed less than 5 years: at Richmond (SMR 144.1) and at both refineries combined (SMR 138.7, Table 2).
  • Malignant pleural mesothelioma: three deaths gave an SMR of 39.6 (95% CI 8.2 to 115.8), with expected numbers based on US SEER rates.
  • Three further deaths were listed as malignant mesothelioma without a stated site. If these were assumed to be pleural, the SMR would have been 79.2 (95% CI 29.0 to 172.5).
  • One death from malignant peritoneal mesothelioma was recorded against about 0.84 expected (SMR 119.1, 95% CI 1.56 to 662.3). The authors described this as no increased risk.
  • Of the mesothelioma deaths with specific diagnostic information, one was a former shipyard worker. The other two had no available work history from before joining Chevron.
  • The authors stated they found no increase in malignant or non-malignant diseases related to asbestos.
  • Multiple myeloma mortality, men, both refineries combined: SMR 159.2 (95% CI 106.7 to 228.9), 29 observed against 18.22 expected.
  • The authors stated that the multiple myeloma increase was confined to employees enrolled before 1949 and showed no significant upward trend with duration of employment.
• The authors' conclusion: The authors concluded that this second update provides additional reassurance that employment at the two refineries is not associated with increased risk of mortality.
• Limitations the authors state:
  • There were no exposure measurements of specific chemicals.
  • There were no data on smoking or other potential confounding factors.
  • Diagnostic information on death certificates may be inaccurate.
  • Work history records were incomplete, so mortality could not be analysed by specific job or work location.
  • Because many comparisons were made, some significant increases or deficits would be expected by chance alone.
  • No asbestos monitoring data were available for the refineries before the early 1970s.

Topics

Medical terms in this study, explained in our Lexicon: Occupational Exposure, Cohort Study, confidence interval, Myeloma, Pulmonary Fibrosis, Lung Cancer, Malignant Mesothelioma.

Related studies

• Updated mortality among diverse operating segments of a petroleum company — R.J. Lewis, A.R. Schnatter, et al. (2000)
• Mortality and cancer morbidity in a cohort of Canadian petroleum workers — R.J. Lewis, A.R. Schnatter, et al. (2003)
• 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)
• Update of a prospective study of mortality and cancer incidence in the Australian petroleum industry — R.T. Gun, N.L. Pratt, et al. (2004)
• Asbestos exposure and cancer mortality among petroleum refinery workers: a Poisson regression analysis of updated data — F. Montanaro, M. Ceppi, et al. (2004)

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