A case-control study relating railroad worker mortality to diesel exhaust exposure using a threshold regression model

Our plain-English summary of A case-control study relating railroad worker mortality to diesel exhaust exposure using a threshold regression model, by M.L. Lee, G.A. Whitmore, F. Laden, et al (Journal of Statistical Planning and Inference, 2009).

The study

• Authors: M.L. Lee, G.A. Whitmore, F. Laden, J.E. Hart, E. Garshick
• Journal: Journal of Statistical Planning and Inference
• Published: 2009;139(5):1633-1642
• Cited by: 12 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

A case-control study of lung cancer mortality in U.S. railroad workers in jobs with and without diesel exhaust exposure is reanalyzed using a new threshold regression methodology. The study included 1256 workers who died of lung cancer and 2385 controls who died primarily of circulatory system diseases. Diesel exhaust exposure was assessed using railroad job history from the US Railroad Retirement Board and an industrial hygiene survey. Smoking habits were available from next-of-kin and potential asbestos exposure was assessed by job history review. The new analysis reassesses lung cancer mortality and examines circulatory system disease mortality. Jobs with regular exposure to diesel exhaust had a survival pattern characterized by an initial delay in mortality, followed by a rapid deterioration of health prior to death. The pattern is seen in subjects dying of lung cancer, circulatory system diseases, and other causes. The unique pattern is illustrated using a new type of Kaplan-Meier survival plot in which the time scale represents a measure of disease progression rather than calendar time. The disease progression scale accounts for a healthy-worker effect when describing the effects of cumulative exposures on mortality.

In plain English

This paper reanalyzed an earlier case-control study of U.S. railroad workers with threshold regression, a method treating health as declining until it reaches the point of death. The study included 1256 workers who died of lung cancer and 2385 controls who died primarily of circulatory system diseases. All were railroad workers in the Railroad Retirement Board system who died in 1981 to 1982. Diesel exhaust exposure was equated with train-operations work from 1959 on (engineers, firemen, conductors, brakemen and hostlers), which the authors called the engineer-brakeman category. Smoking and potential asbestos exposure, based on steam-era job category, were also included. The model coefficients for ever working in this category were much larger than those for smoking and asbestos. These pointed to higher starting health but faster decline. The two offset each other, and mean survival comparisons were mixed. These workers showed high survival followed by a sharp drop near 30 'retirement-equivalent years', the authors' disease-progression unit. Asbestos effects were statistically significant except for deaths from other causes. The authors concluded that the reanalysis supported the original findings and extended them, especially for cardiovascular disease.

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: Statistical reanalysis of an existing case-control study using a threshold regression (first-hitting-time) model
• Who was studied: 3641 U.S. railroad workers in the Railroad Retirement Board system: 1256 who died of lung cancer between March 1, 1981 and February 28, 1982, and 2385 controls drawn from the same population, who died primarily of circulatory system diseases
• Exposure: Diesel exhaust, equated with employment in the train operations category from 1959. This category covered train operations personnel such as engineers, firemen, conductors, brakemen and hostlers, and the authors called it the engineer-brakeman job category. Smoking and potential asbestos exposure (steam-era jobs) were also included as indicator variables
• Main results:
  • The model coefficients for the engineer-brakeman (train operations) category were much larger in absolute size than those for the other indicator variables (smoking and asbestos), for all causes of death
  • Workers in the engineer-brakeman category had a higher estimated initial health status but a steeper rate of health decline than other workers
  • The two engineer-brakeman effects offset each other in the formula for mean survival, and the estimated mean survival comparisons between this category and other workers were mixed
  • For the engineer-brakeman category, the modified Kaplan-Meier plots showed high survival for a long initial period and then a sharp decline near 30 retirement-equivalent years
  • For lung cancer deaths among non-smokers never exposed to asbestos, estimated mean survival was 39.9 retirement-equivalent years outside the engineer-brakeman category and 30.1 within it
  • For cardiovascular deaths in the same non-smoking, non-asbestos group, the corresponding means were 25.1 years (outside the engineer-brakeman category) and 26.2 years (within it)
  • Exposure beyond 10 years in the engineer-brakeman category had positive coefficients (faster disease progression) for all three causes of death, significant only for lung cancer and cardiovascular disease
  • All indicator effects (smoking, engineer, asbestos) were statistically significant except asbestos for deaths from other causes (P-values 0.101 and 0.067)
  • Among 665 workers who had ever worked in a railroad shop, the authors found some increased risk of death from lung cancer and cardiovascular disease
• The authors' conclusion: The authors concluded that the reanalysis supported the original case-control findings and extended them, especially regarding cardiovascular disease risks from diesel exposure
• Limitations the authors state:
  • Inferences apply only to a reference population like the Railroad Retirement Board workers who died in 1981 to 1982, excluding the causes of death excluded from the controls
  • The model does not account for, or estimate, the population proportions of cases and controls
  • Detailed job history was unavailable before 1959. Steam-era job category, used to classify potential asbestos exposure, was based on the last job held on or before 1959
  • The shop job category included non-diesel shop workers, so the degree of diesel exposure in that group was uncertain
  • The non-smoking category also included workers with unknown smoking status

Topics

Medical terms in this study, explained in our Lexicon: Case-Control Study, Lung Cancer, Industrial Hygiene, Habit, Asbestos Exposure, Disease Progression, Cumulative Exposure.

Related studies

• A case-control study of lung cancer and diesel exhaust exposure in railroad workers — E. Garshick, M.B. Schenker, et al. (1987)
• A retrospective cohort study of lung cancer and diesel exhaust exposure in railroad workers — E. Garshick, M.B. Schenker, et al. (1988)
• Occupational exposure to diesel motor exhaust and risk of lung cancer by histological subtype: a population-based case-control study in Swedish men — A. Ilar, N. Plato, et al. (2017)
• Analysis of the effect of occupational exposure to asbestos based on threshold regression modeling of case-control data — A. Chambaz, D. Choudat, et al. (2014)
• A population-based case-control study of mesothelioma deaths among U.S. railroad workers — M.B. Schenker, E. Garshick, et al. (1986)
• The diesel exhaust in miners study: I. Overview of the exposure assessment process — P.A. Stewart, J.B. Coble, et al. (2010)

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