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PubMed1983

Anatomy of the healthy worker effect: a critical review.

The healthy worker effect is the composite result from factors such as (1) selection of the work force, (2) changes in lifestyle accompanying employment, and (3) methodological characteristics of the standardized mortality ratio (SMR). While the choice of the general population as the comparison population is the underlying reason for this effect, no more feasible, widely accepted or "better" alternative is yet available. Much of the current understanding of the healthy worker effect has been limited to an examination of selection of the work force. The purpose of this report is to illustrate, using examples from a large cohort study, the importance of many factors other than selection. They include employment-associated benefits such as economic gain, medical insurance and lifestyle changes, the proportion of active workers, the calculation method, data completeness, length of follow-up and certain characteristics of SMR methodology. It is shown that some of the healthy worker effect is characteristic of SMR methodology and that the strength of the healthy worker effect depends on the proportion of active workers in the cohort. Furthermore, the disappearance of the healthy worker effect may be due to factors such as aging of the cohort and can be totally unrelated to the true increase in the mortality risk. In other words, its disappearance, in many instances, may be an artifact of SMR methodology.

Adult↗
PubMed1999

Gender differences in the healthy worker effect among synthetic vitreous fiber workers.

The aim of this study was to determine whether the healthy worker effect and its component parts operate similarly for women and men. A cohort of workers from 14 synthetic vitreous fiber factories in seven countries, employed for at least 1 year between 1933 and 1977 and followed up to the early 1990s, included 375 deaths and 53,608 person-years among females and 2,568 deaths and 210,073 person-years among males. Standardized mortality ratios for all-cause and circulatory diseases were adjusted for country, age, calendar time, and gender. In addition, internal comparisons were adjusted for time since hire and employment status. The analyses addressed the following: 1) the healthy hire effect, 2) the time since hire effect, and 3) the healthy worker survivor effect. In this cohort, an overall healthy worker effect was not present in either gender. The healthy hire effect, based on standardized mortality ratios for years 1-4 since hire, was observed in males (standardized mortality ratio (SMR) = 0.8; 95% confidence interval (CI): 0.7, 1.0) but was less in females (SMR = 0.9; 95% CI: 0.5, 1.6). The relative risks increased slightly with time since hire in males but not in females. Higher mortality ratios were seen among those leaving employment than among those who remained actively employed; however, this effect was substantially greater for women (relative risk (RR) = 3.4; 95% CI: 1.8, 6.3) than men (RR = 1.8; 95% CI: 1.5, 2.1). The gender difference for active versus inactive status was stronger up to age 60 (men: RR = 1.7; 95% CI: 1.4, 2.0; women: RR = 3.6; 95% CI: 1.8, 7.1) than above that age. In conclusion, it appears that there is a stronger selection of healthy men than women into the workforce, while health-related selection out of the workforce is stronger for women than men.

Adult↗
PubMed1987

The healthy worker effect. Selection of workers and work forces.

The favorable health status of employed populations in comparison to that of the general population is often called the "healthy worker effect," and most researchers feel that it results from the selection of healthy individuals during the hiring process. However, other mechanisms may also contribute to the healthy worker effect. In particular, the healthy worker effect may reflect the selection, for epidemiologic study, of relatively advantaged workforces in terms of socioeconomic status and health. Evidence of differential health by socioeconomic status within the workforce is reviewed, and its possible role in the healthy worker effect is discussed.

Employment↗
PubMed1992

Definition, sources, magnitude, effect modifiers, and strategies of reduction of the healthy worker effect.

This article summarizes, compares, and contrasts the definition, sources, magnitude, effect modifiers, and strategies of reduction of the healthy worker effect (HWE), based on the opinion expressed in the papers of nine contributors who responded to the request of the Industrial Disease Standards Panel (IDSP), Ontario, Canada. It provides an insight into the complex issues relating to the HWE. In addition, the catalog of 15 strategies to reduce the HWE is deemed to be useful for investigators in occupational epidemiology.

Age Factors↗
PubMed1995

Reversal of the Healthy-worker Effect.

The well-known healthy-worker effect (HWE) may become reversed at advanced ages. The HWE deficit of deaths at younger ages must be compensated for by extra deaths at advanced ages. A hypothesized distribution of these compensating deaths illustrates how they may increase age-specific mortality rates. This can probably produce spurious associations with occupational exposures when cohort follow-up is long. This point is illustrated for lung cancer by data from a reported study of beryllium workers.

Journal Article↗
PubMed1988

Components and modifiers of the healthy worker effect: evidence from three occupational cohorts and implications for industrial compensation.

The authors examined the components and modifiers of the healthy worker effect using mortality data from three occupational cohorts: the employees of Atomic Energy of Canada Limited followed between 1950 and 1981, a 10% sample of the Canadian labor force followed between 1965 and 1979, and workers at the Eldorado Resources Limited Beaverlodge uranium mine followed between 1950 and 1980. Two important components of the healthy worker effect have been identified in these cohorts, namely, initial selection of and continuing employment of healthy individuals. There is less evidence for a contribution from the existence of differential risk factors among employed individuals as compared with the general population. The healthy worker effect is, however, substantially modified by time since employment, sex, age, specific cause of death, and specific occupation. Because of this variation, it is inappropriate to account for the healthy worker effect by a single parameter, and all of the above factors must be taken into account in any appropriate analysis. When the only available comparison group for an occupational cohort is the general population, the healthy worker effect is unlikely to have any substantial influence on the process of assessing causality for any observed association or attributing cause in an individual case. This would be particularly true for cancer, and even more so for lung cancer, a disease often associated with industrial compensation cases.

Adolescent↗
PubMed1998

Evaluation of chronic respiratory effects in the potato processing industry: indications of a healthy worker effect?

OBJECTIVE: To investigate the occurrence of chronic respiratory effects of exposure to organic dust in the potato processing industry. METHODS: Self reported chronic respiratory symptoms and spirometric lung function were assessed in a cross sectional study among 135 potato processing workers. A comprehensive study of current exposure to dust, endotoxin, and potato antigens had been performed previously. Workers were grouped into low and high exposure categories for each of the three exposure indices. Relations between exposure concentrations and respiratory health variables were investigated either by calculating prevalence rate ratios or by performing linear regression analyses. Atopy was assessed by measuring total immunoglobulin E (IgE) and specific IgE to five common aeroallergens in serum samples of workers. RESULTS: Evident relations between current exposure indices and respiratory health in the entire group were not found. Workers employed < or = 5 years showed a two-fold higher prevalence of respiratory symptoms, lower lung function, and higher endotoxin exposure than workers employed for > 5 years. Also, atopy was more prevalent in workers employed < or = 5 years. After stratification for duration of employment, negative effects of endotoxin on lung function among workers employed < or = 5 years were suggested. CONCLUSIONS: This study does not show chronic respiratory effects of exposure to organic dust in the potato processing industry, despite the fact that the levels of exposure to endotoxin found in this industry have been reported to be associated with decreases in lung function in other occupational settings. A likely explanation for not detecting apparent effects might be that many symptomatic workers drop out of this industry a few years after starting the job, suggesting a healthy worker effect.

Adult↗
PubMed2000

A technique to re-assess epidemiologic evidence in light of the healthy worker effect: the case of firefighting and heart disease.

The healthy worker effect (HWE) is a bias that is believed to have strongly affected the validity of previous cohort mortality studies on the relationship between firefighting and heart disease. There is a strong healthy hired effect (a component of the HWE) among firefighters, owing particularly to the recruitment of nondiabetic candidates. This is shown in previous studies in which the reported standardized mortality ratios for diabetes are much less than unity, generally around 0.3 to 0.5. Because diabetes is known to increase the risk of heart disease, a deficit of diabetes among firefighters is expected to lead to a deficit of heart injury and disease. This would make the cohort mortality studies incapable of detecting any increase in risk of heart injury and disease among firefighters. There is also a strong healthy worker survivor effect (another component of the HWE) among firefighters. In addition, heart disease is a classic example of the HWE because heart disease is chronic and its risk factors can be identified in the selection process. It is believed that (1) a major problem of previous studies on firefighting and heart disease is their failure to recognize the importance of the HWE when interpreting their results, and (2) a technique to re-assess results in light of the HWE is urgently needed. This article addresses the generally accepted principles relating to the HWE, including its definition and sources, and proposes a technique for re-assessing the literature in light of the HWE. The technique was applied to carefully re-assess 23 studies that provided direct evidence for the relationship between firefighting and heart disease. Before the re-assessment, 7 of the 23 studies showed positive evidence and 16 showed no evidence. After the re-assessment, 11 studies showed positive evidence and 12 showed no evidence. Based on the results of the re-assessment of the 23 studies, we concluded that (1) there is strong evidence of an increased risk of death overall from heart disease among firefighters; (2) there is insufficient evidence, even after considering the HWE, that there is an increased risk of death from aortic aneurysm among firefighters; and (3) there is insufficient evidence, even after considering the HWE, for a relationship between firefighting and any heart disease subtype, such as acute myocardial infarction.

Adult↗
PubMed1987

The healthy worker effect in actively working communications workers.

The healthy worker effect is examined in a large population of actively working communications workers. The mortality experience of 338,306 white male workers at the American Telephone and Telegraph Company grouped by age, duration of service, and general cause categories is compared with the mortality experience of US white males in 1976. Observed were statistically significant and often substantial deficits in mortality among these active workers in almost all categories. For cancers and cardiovascular diseases, mortality ratios tended to decrease across age in the subgroups with longer service durations; these ratios consistently increased across duration of service in older ages. In subgroups with long service durations, the secondary selection of the healthy worker effect in this active worker population appears to increase with increasing age.

Adult↗
PubMed1996

The influence of study characteristics on the healthy worker effect: a multiple regression analysis.

The so-called 'healthy worker effect' (HWE) describes a reduced mortality rate in occupational populations. From 85 occupational cohorts of workers exposed to organic solvents, we have previously found a low weighted standardized mortality ratio (SMR) for all causes (SMR = 89.5, 95 per cent confidence interval [Cl] = 89.0-90.2). Characteristics of the cohorts were examined for association with the HWE. A multiple regression analysis revealed that, for overall deaths, the closer the comparison group, the smaller the HWE (p = 0.001); the more hazardous the potential exposures, the higher the SMR (p = 0.02); the higher the social class, the greater the HWE (p = 0.02); and the higher the rates of loss to follow up, the greater the HWE (p = 0.04). These results have shown that, in addition to time-related modifiers, the comparison group, type of occupational exposure, social class and rates of loss to follow-up significantly influence the size of the so-called HWE. Differences in the HWE related to gender and race were probably attributable to different rates of loss to follow-up.

Cohort Studies↗
PubMed1998

Characteristics of the healthy worker effect: a comparison of male and female occupational cohorts.

The healthy worker effect (HWE) poses a serious methodological problem to investigators of occupational cohorts in that it may mask mortality excesses that result from occupational exposures. This problem is further complicated by the fact that the strength of the HWE generally varies according to sociodemographic, employment, and time-related factors. While the HWE has been well documented among numerous cohorts of male workers, little is known about its expression among female occupational workers. Follow-up mortality data on 44,154 employees from the Hanford nuclear facility for the period of 1944-1986 were examined using standardized mortality ratio (SMR) analysis to assess whether modifiers of the HWE were expressed differently in females than in males. Results of this analysis show that while the HWE was modified by race, age at hire, occupational class, and length of follow-up in both male and female cohorts, different patterns of modification emerged across the two subgroups. Learning about how gender differentiates expression of the HWE will help investigators more precisely assess the confounding effect of the HWE in studies of working cohorts. Therefore, this study's findings are relevant for designing and interpreting future occupational cohort studies.

Adult↗
PubMed1985

The 'healthy worker effect' on morbidity rates.

Studies relating mortality and occupation are difficult to interpret because of the "healthy worker effect." That effect is primarily due to the fact that individuals entering the labor force are in reasonably good health. A similar phenomenon might be expected in a comparison of morbidity among members of an occupational group with that of the general population. A comparison of morbidity among different population and occupation groups is made possible through the Household Interview Survey (HIS) of the National Center for Health Statistics. Data from the HIS for the years 1969 through 1974 were combined to form a sample classified according to race, sex, age, occupation, and occurrence of various health conditions. Among subjects between 25 and 59 years of age, age-specific morbidity ratios and standardized morbidity ratios were computed for blue-collar and professional/managerial employees and for unemployed who were either seeking or not seeking employment. There appears to be a "healthy worker morbidity effect" for chronic, but not for acute, conditions just as there is for mortality. The healthy worker effect for chronic morbidity maintains itself and, in fact, appears to increase throughout the 35-year period of stable employment. The age-specific morbidity ratio may have considerable theoretical importance for evaluating the related mortality experience of employed populations.

Adult↗
PubMed1996

[Methodology for controlling healthy worker effect on coal miners' mortality].

This paper reports how to control the healthy worker effect (HWE) in a mortality study of coal miners. We used four methods, namely (1) age-specific corrective coefficient, (2) proportional mortality ratio (PMR), (3) corrective standarized mortality ratio (CSMR), and (4) a control group composed of factory workers. The results showed that all these methods could control HWE, and the total mortality in the coal miner group was significantly higher than that of the general population group (P < 0.05). However, the best way for controlling HWE awaits further studies.

Adult↗
PubMed1989

Occupational cohort studies: the influence of design characteristics on the healthy worker effect.

A cross-sectional analysis of the relation between the estimated healthy worker effect (HWE), as measured by a total Standardized Mortality Ratio (SMR) less than 100, and relevant design characteristics in 270 published retrospective occupational cohort studies is presented. The majority of the reviewed studies showed a HWE, varying in SMR from 50 to 99 (mean: 84). The estimated HWE seemed to influence the final outcomes of the studies to a great extent. A tendency for a positive relation between the study size in terms of the number of exposed workers, total number of person-years of follow-up, and the HWE emerged. Studies with a comparatively short follow-up period had an increased chance of resulting in a HWE. Cross-sectional cohorts did not show a stronger HWE than open cohorts. Studies of chemical exposures revealed a fivefold excess of having a HWE compared with other studies.

Cohort Studies↗
PubMed1999

A review of the healthy worker effect in occupational epidemiology.

This review article aims to anatomize sources of the healthy worker effect (HWE) and to summarize advantages and limitations of several approaches frequently proposed to eliminate the HWE. Although the HWE is frequently addressed in the context of selection bias, our review suggests that the selection of occupational cohorts with advantageous health status would preferably be addressed as a source of confounding biases. The authors also conclude that the exclusion of unhealthy workers at employment and the study of active workers are the two main sources of HWE, and that the use of the general population as a comparison group in occupational epidemiology should be avoided if possible. The authors encourage investigators to make distinctions between the underlying factors related to the use of the general population as the comparison group in occupational epidemiology.

Bias↗
PubMed1984

Mobility of diesel versus non-diesel coal miners: some evidence on the healthy worker effect.

Workers who are particularly susceptible to the effects of their occupational exposure, from the perspective of the healthy worker effect, soon leave the workplace. The result of this mobility, called survival bias, is that cross sectional studies based on the survivors underestimate the true risk of occupational exposures. Two questions are addressed in this empirical study of the "survival bias" component of the "healthy worker" effect. Do miners with respiratory impairment or symptoms disproportionately leave jobs that have a potentially harmful respiratory exposure? And does the presence of an additional potentially harmful respiratory exposure, in this case diesel emissions, accelerate the rate of mobility for miners with respiratory impairment or symptoms? No confirmation was found for the survival effect in a study of 738 diesel and 420 non-diesel US underground coal miners. No additional increment in mobility was associated with exposure to both coal mine dust and diesel emissions.

Adolescent↗
PubMed1988

Mortality odds ratio, proportionate mortality ratio, and healthy worker effect.

The standardized proportionate mortality ratio (PMRi) and the mortality odds ratio (MORi) are two statistics used to approximate the cause specific standardized mortality ratio (SMRi) when death data are available but the population at risk is not known. When there is a healthy worker effect, the MORi will always overestimate the SMRi and will always be greater than the PMRi. The PMRi is influenced by the relative frequency of the cause of death. For rare causes, such as brain cancer or leukemia, the PMRi will overestimate the SMRi to essentially the same degree as the MORi. For more common conditions, such as lung cancer, the PMRi will overestimate or underestimate the SMRi depending on the magnitude of the healthy worker effect. When the SMRi = 1 and there is a healthy worker effect, both the PMRi and MORi are in excess of one (1) regardless of the disease rate. As the SMRi increases it is more likely to be bounded by the PMRi (lower) and the MORi (upper). We therefore recommend that each statistic be derived when death certificates are the only source of data used to assess risk due to occupational exposures.

Environmental Exposure↗