Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-007820, 2007 Oct; :1-95
Background. Low levels of habitual physical activity have been identified as a major risk factor for cardiovascular diseases. However, the evidence for this observation is primarily based on leisure time physical activity. Only few studies examined specifically the effect of occupational physical activity (OPA) and their findings have been inconsistent. In fact, several recent prospective studies linked higher levels of OPA with accelerated progression of atherosclerosis, cardiovascular morbidity, and increases in mortality. Work time determines the intensity of OPA and of other occupational exposures. Long work hours have been associated with myocardial infarction and mortality from coronary heart disease in some studies but not in others. Scarcity of prospective studies, confounding, health-based selection, and lack of repeat exposure measures in longitudinal studies may have been in part responsible for these inconsistent findings. Objectives. The primary aim of this study was to investigate the effect of work time and different types and levels of OPA on chronic circulatory diseases while addressing key methodological issues by a prospective design, use of repeat measurements of exposures, adjustment for a comprehensive set of confounders, and dealing with the thorny issue of selection bias (the so-called healthy worker effect) by using subclinical outcome measures, i.e. change in carotid artery intima media thickness (IMT) and systolic blood pressure, instead of symptomatic cardiovascular disease or mortality outcomes. A secondary aim of this study was to investigate whether these occupational risk factors are more strongly associated with change in IMT among men with pre-existing ischemic heart disease (IHD) or carotid artery stenosis (CAS) compared to men without these conditions as would be expected according to the hemodynamic theory of atherosclerosis. Methods. This population-based prospective study of ultrasonographically assessed carotid intima media thickness (IMT) uses repeat measures of work time and OPA during baseline, 4-year, and 11-year examinations of 621 Finnish men who were 42-60 years old when they enrolled in the Kuopio Ischemic Heart Disease Risk Factor Study (KIHD). The KIHD study measured the most comprehensive set of biological, behavioral, social, and psychological cardiovascular risk factors of any study, allowing for the control of virtually all known possible confounders. The association between different measures of work time, energy expenditure, and work postures with 11-year change in maximum IMT was evaluated in regression models adjusting for 21 potential confounders including among other factors cholesterol, body mass index, leisure time physical activity, smoking, socio-economic status, and job stress. Analyses were also performed by baseline cardiovascular health status. Key Findings. About one third of middle-aged men worked more than the standard 40- hour workweek. Work time was positively associated with accelerated progression of carotid atherosclerosis. Men with preexisting ischemic heart disease or carotid artery stenosis appear to be especially vulnerable to the effects of long work hours. Regardless of the specific occupational conditions that may constitute the pathways for the observed relationships between long work time and progression of atherosclerosis, findings suggest that reducing weekly and yearly work time could have significant cardiovascular and public health benefits, especially in the aging working population. The findings of this study do not support the notion that heavy physical labor has ceased to be a potential health hazard in the so-called modern service economy. To the contrary, the study shows that 30% of aging men, including over 50% of those with ischemic heart disease, were still exposed to excessive physical job demands based on current recommended maximum levels for relative aerobic strain. This study demonstrates for the first time that the observed high levels of energy expenditure and relative aerobic strain at work are associated with accelerated progression of carotid atherosclerosis, even after controlling for virtually all known cardiovascular risk factors, including blood pressure, smoking, cholesterol, body mass index, leisure time physical activity, aerobic fitness, socioeconomic status, and psychosocial job factors among others. Older workers, workers with pre-existing ischemic heart disease, and workers with carotid stenosis appear especially vulnerable to the atherogenic effects of increasing levels of energy expenditure. Prolonged upright work postures are associated with an accelerated progression of atherosclerosis. Prolonged time walking on uneven ground or climbing stairs were the most relevant components of an upright work posture that by themselves were significantly associated with progression of atherosclerosis. The association of a standing work posture with progression of atherosclerosis varied by exposure assessment method used. A prolonged upright work posture was also significantly associated with long-term increases in systolic blood pressure, a major risk factor for cardiovascular diseases. Findings are consistent with the hemodynamic theory of atherosclerosis and known occupational risk factors for cardiovascular disease. Implications. Job design, work organization, occupational health practice, and regulatory statutes dealing with working time and rest schedules need to assure that workers are protected from weekend work, long work hours, prolonged upright work postures, and excessive physical job demands. Protective measures are especially needed for older workers with age- or disease-related reductions in cardiorespiratory fitness, narrowing of carotid arteries, or manifest ischemic heart disease. Additional research is needed to determine the overall cardiovascular disease burden and societal costs associated with excessive work hours, upright working positions, high levels of energy expenditure, and other occupational cardiovascular risk factors in order to quantify the potential benefits of worksite-based strategies for the prevention of circulatory diseases that are still the number one cause of death in industrialized countries.