[The "healthy worker effect" or the "exposed general population effect"?].
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Environmental Exposure↗
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The "Healthy Worker Effect": science or prejudice?
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Healthy Worker Effect↗
[The association between physical workload and low back pain clouded by the "healthy worker" effect].
INTRODUCTION: Both physical and psychosocial workplace factors are considered risk factors for low back pain (LBP). However, today, no consensus has been reached regarding the exact role of these factors in the genesis of LBP. MATERIAL AND METHODS: Questionnaire data were collected at baseline for 1,397 (and after five years for 1,163) men and women aged 31-50 years at baseline. LBP (any LBP within the past year; LBP < or = 30 days in total during the past year; LBP > 30 days in total during tha past year) was analysed in relation to physical workload (sedentary, light physical, and heavy physical work) using logistic regression and controlling for age, gender, and social group. The proportions af workers changing between the workload groups over the five-year period were analysed in relation to LBP status. RESULTS: A baseline no statistically significant differences in LBP outcomes were found for workers exposed to sedentary, light physical, or heavy physical work. This was true for both genders and all age and social groups. At follow-up these was a statistically significant dose-response association between any LBP and long-standing LBP during the past year and increasing physical workload at baseline, also after controlling for age, gender and social group. Subjects with a heavy physical workload at baseline changed significantly more often to sedentary work, if they had experienced LBP for more than 30 days out of the past year. DISCUSSION: Having a sedentary job might have a protective or neutral effect in relation to LBP, whereas having a heavy physical job constitutes a significant risk factor. Because og migration between exposure groups (the "healthy worker" effect), longitudinal studies are necessary for investigating the associations between physical workload and LBP.
Adult↗
Healthy worker effect in a longitudinal study of one-second forced expiratory volume (FEV1) and chronic exposure to granite dust.
BACKGROUND: Low level effects of granite dust on one-second forced expiratory volume (FEV1) are estimated in 618 Vermont granite workers followed for 5 years with annual pulmonary function tests. Reduced pulmonary function has already been reported for the subset of subjects lost to follow-up (dropouts) suggesting possible bias in analyses based only on survivors. METHOD: Healthy worker selection bias is directly assessed by comparing the dose-response associations between survivors who remained in the study for the full 5-year observation period and the dropouts. RESULTS: The 353 survivors had an FEV1 of 96% of predicted at baseline and were losing FEV1 at an average rate of 44 ml/yr. No association was found in this group between the rate of FEV1 decline and lifetime dust exposure. However, the 265 workers with incomplete follow-up, 'dropouts', had a lower FEV1 at baseline (94%) and were losing FEV1 at an average rate of 69 ml/yr. The dose-response parameter in this group was estimated to be 4 ml/yr loss per mg/m3-year and was statistically significant. CONCLUSIONS: These results provide an illustration of bias due to the healthy worker effect and an example of the failure to detect a true work-related health effect in a study based only on a 'survivor' population.
Adult↗
The association between physical workload and low back pain clouded by the "healthy worker" effect: population-based cross-sectional and 5-year prospective questionnaire study.
STUDY DESIGN: A population-based cross-sectional and 5-year prospective questionnaire study. OBJECTIVE: To investigate self-reported physical workload as a risk factor for low back pain. SUMMARY OF BACKGROUND DATA: Both physical and psychosocial workplace factors are considered risk factors for low back pain. However, today no consensus has been reached regarding the exact role of these factors in the genesis of low back pain. METHODS: Questionnaire data were collected at baseline for 1397 (and after 5 years for 1163) men and women aged 31--50 years at baseline. Low back pain ("any low back pain within the past year," "low back pain < or = 30 days in total during the past year," "low back pain > 30 days in total during the past year") was analyzed in relation to physical workload (sedentary, light physical, and heavy physical work) using logistic regression and controlling for age, gender, and social group. The proportions of workers changing between the workload groups over the 5-year period were analyzed in relation to low back pain status. RESULTS: At baseline no statistically significant differences in low back pain outcomes were found for workers exposed to sedentary, light physical, or heavy physical work. This was true for all age, gender, and social groups. At follow-up there was a statistically significant dose-response association between any low back pain and longstanding low back pain within the past year and increasing physical workload at baseline also after controlling for age, gender, and social group. Subjects with heavy physical workload at baseline changed statistically significantly more often to sedentary work if they experienced low back pain for more than 30 days out of the past year. CONCLUSIONS: Having a sedentary job might have a protective or neutral effect in relation to low back pain, whereas having a heavy physical job constitutes a significant risk factor. Because of migration between exposure groups (the "healthy-worker" effect), longitudinal studies are necessary for investigating the associations between physical workload and low back pain.
Cross-Sectional Studies↗
Healthy worker effect in a cross-sectional study of lead workers.
To study the role lead may play in the development of renal disease, we performed a cross-sectional study of workers at a lead smelting plant. Renal function was defined based on calculated creatinine clearance using the prevalence of values under the 3rd percentile to compare groups. The prevalence of calculated creatinine clearance values under the 3rd percentile in these workers (n = 1782) as a whole was 2.81%, a result comparable to that which has to be expected for the general population. Closer analysis, however, showed significantly lower prevalence of calculated creatinine clearance under the 3rd percentile in certain subgroups of workers. These subgroups were workers between the ages of 30 and 39, workers over the age of 50, and Belgian workers who had worked in the plant for longer than 10 years. We conclude that these observations once more clearly demonstrate a "healthy worker effect" on the measurement of renal function in this work force, a major problem in epidemiologic cross-sectional studies.
Actuarial Analysis↗
High mortality in lower salaried Norwegian men: the healthy worker effect?
STUDY OBJECTIVE: This study aimed to examine whether the high mortality in lower salaried men in Norway was related to an inflow into this group of unskilled workers with high mortality. DESIGN: Individual information on occupation was derived from the 1970 and the 1980 censuses and linked to mortality data for the period 1980-5 by the official, individual identification numbers. PARTICIPANTS: The study population included Norwegian men aged 20-64 years in 1980. All men enumerated in the 1980 census (and who were registered in the 1970 census) within the two occupational groups, unskilled workers and lower salaried employees, were included. MEASUREMENTS AND MAIN RESULTS: The subjects were allocated to the two occupational groups according to the official Norwegian classification of socioeconomic status. Mortality was measured by standardised mortality ratios (SMR). The findings suggest that a fairly large number of unskilled workers with high mortality move into the lower salaried employee group. This transition produces a higher SMR in lower salaried employees and, simultaneously, an artificially but modestly lower SMR in unskilled workers. The difference in SMR between the two groups is thereby exaggerated by 44%. The difference among the elderly was smaller. CONCLUSION: Occupational mobility consistent with "the healthy worker effect" increases the SMR of lower salaried men and, at the same time, reduces slightly the SMR of unskilled workers.
Adult↗
Cause-deleted proportional mortality analysis and the healthy worker effect.
This paper evaluates the relationships between various indices of proportional mortality, used in occupational and other settings where denominator data are unavailable. In particular, we examine the necessary conditions for validity of the proportional mortality ratio (PMR) and mortality odds ratio (MOR) as estimates of the standardized mortality ratio (SMR), in general and for the situation where one considers only a subset of deaths, with certain causes of death deleted from the data. We may exclude data in this way to avoid the inflationary effect on the PMRs for some causes of death (e.g. cancer) that results from a 'healthy worker effect' for other causes of death (e.g. heart disease). We find that cause-deleted PMRs and MORs may be unbiased estimates of their corresponding SMR, and these indices appear most useful for specific hazards of employment or relatively infrequent causes of death.
Biometry↗
Social inequality in health--the role of the healthy worker effect.
Current occupational status is frequently used as an indicator of position in the socioeconomic structure in analyses of health inequalities. People outside the work-force are thereby ignored. One may hypothesize that due to a 'healthy worker effect' economically active people are a positively selected health group and that health-related exits from the labour marked are concentrated among low socioeconomic groups. One may therefore expect that this system of classification will show artificially small differences in health between socioeconomic groups. Analyses of material from a nation-wide representative survey and a complete set of mortality statistics confirm these expectations. When previously employed are included among currently employed, the differentials in health between the extreme groups in the occupational hierarchy widen. The system of classification has only a moderate impact on the main conclusion regarding men. For women, however, the conclusion depends more heavily on how they are classified. If one relies only on current occupation, small or inconsistent patterns emerge. Inclusion of previously employed has the effect of showing that low status women suffer from poorer health than high status women. This applies to several measures of health, i.e. morbidity, restricted activity and mortality.
Adult↗
A strategy to reduce healthy worker effect in a cross-sectional study of asthma and metalworking fluids.
This report describes the reanalysis of a cross-sectional study of asthma in a large cohort of autoworkers with exposure to metalworking fluids (MWF). There is strong evidence from case reports, clinical studies, and medical surveillance data that exposure to MWF can cause asthma, yet no association was found in the original analysis. The central hypothesis of the reanalysis was that the absence of an association between asthma and MWF exposure was the result of bias caused by the self-selection of asthmatics out of exposed jobs. We addressed the potential job transfer bias by redefining exposure and disease status at the time of asthma onset, rather than at the time of the health survey. This permitted us to treat the cross-sectional study as if it were a historical cohort study, despite the fact that the population was a biased sample of the full cohort. This approach resulted in a significantly elevated incidence rate ratio of 3.2 (95% CI: 1.2-8.3) for synthetic MWF estimated in a Cox proportional hazards model. Although the cross-sectional design makes it impossible to document or control for differential selection out of the workforce, the approach described here provides a strategy for reducing the healthy-worker effect due to job transfer bias in cross-sectional studies.
Adult↗
Healthy worker effect and changes in respiratory symptoms and lung function in hairdressing apprentices.
AIMS: To compare the prevalence and incidence of respiratory symptoms and lung function values between hairdressing apprentices and office apprentices. METHODS: A total of 322 hairdressing apprentices and 277 office apprentices (controls) were studied. Two cross sectional surveys were conducted in 1994 and 1996/97 with longitudinal follow up for a subgroup of apprentices (191 hairdressing apprentices and 189 office apprentices). RESULTS: In the initial phase, the prevalence of respiratory symptoms was significantly lower among hairdressing apprentices than among office apprentices. Lung function test results showed significantly higher values for hairdressing apprentices. Non-specific bronchial reactivity was similar in the two groups. In the final phase, results for respiratory symptoms were similar. The incidence of respiratory symptoms was not significantly different between hairdressing apprentices and office apprentices. Subjects who dropped out had lower values for FVC and FEV1 in the initial phase than those who completed the final phase. There was a significant deterioration of FEV1 and FEF25-75% in hairdressing apprentices compared to office apprentices. There was a link between atopy and the incidence of most of the respiratory symptoms (day/night cough, wheezing, dyspnoea, mucosal hyperresponsiveness) and between smoking and the incidence of bronchial hyperreactivity. There was no significant correlation between change in lung function tests and specific hairdressing activities reported at the end of the apprenticeship or with environmental working conditions in hairdressing salons. CONCLUSIONS: Although a healthy worker effect can be suspected, results showed a significant deterioration of baseline values of lung function tests in the hairdressing apprentice group. However, no clear link was shown between change in lung function tests and specific parameters of occupational activities.
Adolescent↗
Smoking policies and the healthy worker effect.
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Absenteeism↗
A review of the healthy worker effect in occupational epidemiology.
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Bias↗
[The general population as a reference group. A discussion of the concept of the healthy worker effect].
In cohort studies concerning occupational diseases the general population is a commonly used reference entity in countries with national cancer registries or census bureaus. However, former and present employees regularly experience lower disease incidence than the general population, even in occupations with exposures known to cause adverse health outcomes. This phenomenon is often referred to as the "healthy worker effect". The concept is discussed in the present paper.
Cohort Studies↗
Healthy worker effect and pregnancy: role of adverse obstetric history and social characteristics.
Data from a survey conducted in 1981 on a national sample of 5508 births in France were used to analyse the role of a history of previous adverse pregnancy outcome (spontaneous abortion, perinatal death or adverse fetal condition) in the selection mechanisms of women with regard to occupational activity, and the impact on the relation between work and preterm delivery. The study, carried out separately in each parity group, showed that occupational activity was associated with a more favourable outcome for women of parity one: the preterm delivery rate was significantly higher among women who had never worked than among those who continued working during pregnancy. However, the hypothesis of a "healthy worker effect" linked to a history of adverse obstetric outcome was not confirmed in this study. A selection effect of women from a history of spontaneous abortions was observed, but these were not linked to preterm delivery. Among multiparous women, a history of perinatal death or adverse fetal condition did not seem to modify women's behaviour towards their work. Selection mechanisms of women towards occupational activity according to sociodemographic factors were also analysed and showed that the higher percentage of younger women among those who had never worked explained the higher rate of preterm delivery in that group.
Age Factors↗
The "healthy worker effect"--fact or artifact?
Workers in five chemical plants with diverse characteristics were studied by attempting to locate all white male employees who worked at least 12 months over a 25 to 30 year period. Former employees were contacted directly, rather than using questionnaires or Social Security Administration files. The data were analyzed by comparing observed mortality to expected mortality based on age-sex-race specific death rates in the U.S. population. The results indicate that the mortality experience of the chemical workers studied is essentially that of the general population. It is suggested that the "healthy worker effect" may be due to methods frequently employed in searching for death claims which understate the true experience of the employee group studied, or if present, does not affect mortality rates for neoplastic or cardiovascular disease.
Cardiovascular Diseases↗
Longitudinal study of grain elevator and control workers with demonstration of healthy worker effect.
Changes in the respiratory symptoms and pulmonary function of 441 grain elevator workers and a control group of 180 civic workers during a three-year period were compared. The grain handlers who left their jobs during the interval between the first and second surveys were found to have shown a significantly higher prevalence of cough and shortness of breath at the beginning of the study period than those who remained employed; this difference was not observed among the civic workers. The grain and civic workers who were examined on both occasions showed similar longitudinal changes in their symptoms and pulmonary function. Nevertheless, the results of this study are biased by the healthy worker effect among the grain handlers.
Age Factors↗
Healthy worker effect in morbidity studies.
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Bias↗