RE: Mesothelioma and lung tumors attributable to asbestos among petroleum workers. Am. J. Ind. Med. 2000. 37:275-282

Comparison bias and dilution effect in occupational cohort studies

Health effects of occupational exposures are frequently evaluated by comparing the mortality of a whole cohort of workers with that of the general population. This study design may be affected by two major biases: a dilution effect (DE), due to the inclusion of unexposed subjects in the study cohort, and a comparison bias (CB), due to the different distribution of risk factors in the reference population. A theoretical model of the joint effect of DE and CB is proposed. Their impact was evaluated in two actual cohorts, selecting specific causes of death based on a priori hypotheses of an association. A linear relationship between the risk estimates and the two biases was found after applying either direct or indirect standardization to adjust for confounding. In the two cohorts, higher risks in exposed workers emerged only after adjusting for DE and CB. Cohort studies without an internal referent group may provide unreliable results.

Business bias: how epidemiologic studies may underestimate or fail to detect increased risks of cancer and other diseases

In spite of claiming primary prevention as their aim, studies of potential occupational and environmental health hazards that are funded either directly or indirectly by industry are likely to have negative results. The authors present three common scenarios in which faulty design of epidemiologic studies skews results, and list 15 study design flaws that lead to results that are dangerously misleading with regard to both the evaluation and the improvement of public health.

Asbestos Exposure and Cancer Mortality among Petroleum Refinery Workers: A Poisson Regression Analysis of Updated Data

The authors investigated the relationship between asbestos exposure and respiratory cancer mortality among maintenance workers and other blue-collar workers at an Italian oil refinery. The cohort contained 931 men, 29,511 person-years, and 489 deaths. Poisson regression analysis using white-collar workers as an internal referent group provided relative risk estimates (RRs) for main causes of death, adjusted for age, age at hiring, calendar period, length of exposure, and latency. Among maintenance workers, RRs for all tumors (RR = 1.50), digestive system cancers (RR = 1.41), lung cancers (RR = 1.53), and nonmalignant respiratory diseases (RR = 1.71) were significantly increased (p < 0.05); no significant excess was found for all causes and among maintenance (RR = 1.12) and other blue-collar workers (RR = 1.01). Results confirm the increased risk of death from respiratory diseases and cancer among maintenance workers exposed to asbestos, whereas other smoking-related diseases (circulatory system) were not statistically different among groups.