Cardiovascular disease among professional drivers and subway staff in Stockholm

Abstract

Professional drivers are at an increased risk of myocardial infarction (MI), but the underlying causes are not clear. The aims of this thesis were to broaden our understanding of the causal factors involved in this elevated risk of MI in professional drivers, and to study time trends in the incidence of MI in professional drivers. The aim was also to study the risk of cardiovascular disease (CVD) in subway employees exposed to particulate matter. High levels of airborne particles have been detected in the subway system of Stockholm, as well as in several other large cities. This has raised concern about negative effects on health, in view of the well-known association between particulate air pollution in large cities and the risk of CVD.

The thesis is based on three source materials. The first is a population-based case-control study (Stockholm Heart Epidemiology Program - SHEEP) comprising cases of first MI and controls recruited 1992-1993 in Stockholm County. We used 1,067 cases and 1,482 controls to investigate possible causes of the increased risk of MI in professional drivers, and to study the risk in bus, taxi and truck drivers (Paper 1). The second is a register-based case-control study of first MI cases recruited in Stockholm County during the period 1976-1996. To investigate time trends in the incidence of MI in bus, taxi and truck drivers, we included 20,364 incident first MI cases and 136,342 controls (Paper 11). In the same source material we included 22,311 cases and 131,496 controls to study whether there is an increased incidence of MI in subway drivers exposed to subway-derived particles (Paper 111). The third is a crosssectional clinical study involving employees in the Stockholm subway with 79 participants recruited 2004-2005 for analysis of hematological risk markers of CVD (Paper IV).

Professional drivers were at an increased risk of MI with an odds ratio (OR) of 2.14 (95% Cl 1.34-3.41) for bus drivers, 1.88 (1.19-2.98) for taxi drivers, and 1.66 (1.22-2.26) for truck drivers. Adjustment for confounders such as socioeconomic status, tobacco smoking, alcohol drinking, physical inactivity during leisure time, overweight, diabetes, and hypertension gave reduced ORs, although significantly increased risks remained for bus and taxi drivers. An exposure-response pattern (by duration of work) was found for bus and taxi drivers (Paper I). During the 20-year period 1977-1996, there was a greater decline in incidence of MI for professional drivers than for other manual workers. Despite this favorable trend, the relative risk (RR) of MI remained elevated for taxi and truck drivers during the entire period relative to other manual workers (Paper II). The RR of MI in subway drivers was not increased: the RR was 0.92 (95% Cl 0.68-1.25) compared to other manual workers (Paper III). Subway employees working at the platforms, who were highly exposed to particles, had significantly higher plasma concentrations of plasminogen activator inhibitor-1 than employees with low exposure. Higher levels of high-sensitivity Creactive protein, interleukin-6 and fibrinogen, although not significantly higher, were also found in the highly exposed group, which suggests that there may be a long-term inflammatory effect of particle exposure. These differences remained after adjusting for BMI (Paper IV).

In conclusion, the increased risk of MI in bus and taxi drivers could only be partially explained by unfavorable lifestyle factors, suggesting that the work environment may be a contributory cause, but for truck drivers, individual risk factors appeared to explain most of the elevated risk. During 1977-1996, the increased incidence of MT in professional drivers gradually approached the incidence for other manual workers, and bus drivers showed the most favorable trend. Stockholm subway drivers moderately exposed to airborne particles did not show an increased incidence of NIL There were indications of an inflammatory response in platform workers who were highly exposed to particles, which suggests that there may be an increased risk of future CVD in individuals who are highly exposed to airborne particles in subway systems.