A structural approach to address the healthy-worker survivor effect in occupational cohorts: an application in the trucking industry cohort

Physical functioning trajectories over statutory retirement: a finnish occupational cohort study

BACKGROUND

The association of workload and performance with physical functioning is recognised among the ageing public sector workforce. The characteristics of working conditions and social- and health-related factors associated with physical functioning after statutory retirement are still unknown. Also, previous studies on changes in physical functioning have not used a person-oriented approach. We examined physical functioning trajectories over statutory retirement and how social- and health-related factors are associated with them. Our aim was to identify distinct developmental trajectories of physical functioning over statutory retirement and to examine how social- (age, gender, marital status, education) and health-related (physical workload, self-reported sleep problems, alcohol consumption, smoking, fruit and vegetable (F&V) consumption, leisure-time physical activity (LTPA), and body mass index (BMI)) factors before retirement were associated with the identified trajectories.

METHODS

We used data from the Helsinki Health Study cohort. Participants consisted of 2736 employees of the City of Helsinki, Finland who retired during the follow-up. Growth mixture modelling was used to identify physical functioning trajectories and multinominal regression analyses to examine associations of social- and health-related factors with them.

RESULTS

Three distinct developmental patterns in physical functioning before and after retirement were found among ageing and retired employees. Lower educational level, sleep problems, physical inactivity, and obesity were associated with the trajectory groups of 'fast decreasing' and 'slowly increasing', compared to the 'stable high' trajectory.

CONCLUSION

The results suggest that poor social- and health-related factors are key risk factors associated with declining and lower-level physical functioning over the retirement period. Supporting healthy lifestyles among older employees might maintaining good physical functioning until retirement and beyond.

Healthy worker hire and survivor effects in a cohort of medical radiation workers

BACKGROUND

The healthy worker effect may distort the association between exposure and health effects in workers. However, few studies have investigated both the healthy worker hire and survival effects simultaneously, and they are limited to mortality studies in male workers.

METHODS

We utilized a data set comprising South Korean diagnostic medical radiation workers registered in the National Dose Registry between 1996 and 2011, and merged it with mortality and cancer incidence data. Standardized mortality ratios (SMRs) and standardized incidence ratios (SIRs) were computed for comparison with the general population. To account for time-varying confounders influenced by prior occupational radiation exposure, we applied g-estimation using structural nested accelerated failure time models and compared the outcomes with those from Weibull regression.

RESULTS

A total of 1831 deaths and 3759 first primary cancer cases were identified among 93 918 workers. Both male (SMR = 0.44; 95% CI: 0.42, 0.46) and female workers (SMR = 0.53; 95% CI: 0.46, 0.60) showed lower mortality rates compared with national rates. In the SIR analysis, male workers exhibited reduced risks of solid cancer whereas female workers had increased risks. The g-estimation-derived hazard ratios (HRs) from radiation exposure exceeded those from Weibull regression estimates for all-cause death (HR = 2.55; 95% CI: 1.97, 3.23) and all-cancer incidence (HR = 1.96; 95% CI: 1.52, 2.55) in male workers whereas female workers showed the opposite results.

CONCLUSIONS

Comprehensive consideration of the healthy worker effect by sex is essential for estimating the unbiased impact of occupational exposure on health outcomes, notably in studies focusing on male mortality.

A comparison of meta-analysis results with and without adjustment for the healthy worker effect: cancer mortality among workers in the semiconductor industry

OBJECTIVES

This study compared the results of meta-analysis with and without adjustment for the healthy worker effect on the association between working in the semiconductor industry and cancer mortality.

METHODS

Six studies that reported standardized mortality ratios (SMRs) for cancers were selected for meta-analysis. Using a random-effects model, the SMR results from each study were combined for all cancers and leukemias to estimate the summary SMRs (95% confidence interval, CI). To adjust for the healthy worker effect, the relative standardized mortality ratio (rSMR=SMRx/SMRnot x) were calculated using observed and expected counts for the specific cause of interest (i.e., all cancers and leukemias) and the observed and expected counts for all other causes of mortality. Then, the rSMR results were combined to estimate the summary rSMRs (95% CIs).

RESULTS

The SMRs for all causes of mortality among semiconductor industry workers ranged from 0.25 to 0.80, which reflects a significant healthy worker effect. A remarkable difference was found between the summary SMRs and the summary rSMRs. The summary SMR for all cancers was 0.70 (95% CI, 0.63 to 0.79) whereas the summary rSMR was 1.38 (95% CI, 1.20 to 1.59). The summary SMR for leukemia was 0.88 (95% CI, 0.72 to 1.07), and the summary rSMR was 1.88 (95% CI, 1.20 to 2.95).

CONCLUSIONS

Our results suggest that adjustment for the healthy worker effect (i.e., rSMR) may be useful in meta-analyses of cohort studies reporting SMRs.

Lung Cancer Mortality and Styrene Exposure in the Reinforced-Plastics Boatbuilding Industry: Evaluation of Healthy Worker Survivor Bias

The evidence for styrene's being a human lung carcinogen has been inconclusive. Occupational cohorts within the reinforced-plastics industry are an ideal population in which to study this association because of their relatively high levels of exposure to styrene and lack of concomitant exposures to other known carcinogens. However, healthy worker survivor bias (HWSB), where healthier workers stay employed longer and thus have higher exposure potential, is a likely source of confounding bias for exposure-response associations, in part due to styrene's acute effects. Through December 31, 2016, we studied a cohort of 5,163 boatbuilders exposed to styrene in Washington State who were employed between 1959 and 1978; prior regression analyses had demonstrated little evidence for an exposure-response relationship between styrene exposure and lung cancer mortality. Based on estimates of necessary components of HWSB, we found evidence for a potentially large HWSB. Using g-estimation of a structural nested model to account for HWSB, we estimated that 1 year of styrene exposure at more than 30 parts per million accelerated time to lung cancer death by 2.29 years (95% confidence interval: 1.53, 2.94). Our results suggest possibly strong HWSB in our small cohort and indicate that large, influential studies of styrene-exposed workers may suffer from similar biases, warranting a reassessment of the evidence of long-term health effects of styrene exposure.

Associations between depressive complaints and indicators of labour participation among older Dutch employees: a prospective cohort study

PURPOSE

European policy measures have led to an increased net labour participation of older employees. Yet, via different routes (for instance disability schemes) employees still often leave the labour market early. Mental health may be an important factor hindering labour participation. Aims of this study are twofold: first, to examine the relationship between mental health-particularly depressive complaints-and indicators of labour participation among older employees over a 2-year follow-up period and second, to explore the impact of different work contexts when studying this relation.

METHODS

A subsample of older employees (aged > 45 years; n = 1253) from the Maastricht Cohort Study was studied. Depressive complaints were assessed using the Hospital Anxiety and Depression scale. Logistic and Cox regression analyses covered 2 years of follow-up and were also stratified for relevant work-related factors.

RESULTS

Employees with mild depressive complaints showed statistically significantly higher risks for poor mental workability (HR 2.60, 95% CI 1.14-5.92) and high psychological disengagement levels (HR 2.35, 95% CI 1.21-4.57) over time compared to employees without depressive complaints. Within various work contexts, for instance in which employees perform physically demanding work or have high psychological job demands, significantly stronger associations were found between depressive complaints and poor mental workability over time.

CONCLUSIONS

This study shows strong longitudinal associations between depressive complaints and indicators of labour participation, also within different work contexts over time. Results provide valuable input for developing preventive measure aiming to enhance sustainable labour participation of older employees.

Adaptive treatment strategies for chronic conditions: shared-parameter G-estimation with an application to rheumatoid arthritis

Most estimation algorithms for adaptive treatment strategies assume that treatment rules at each decision point are independent from one another in the sense that they do not possess any common parameters. This is often unrealistic, as the same decisions may be made repeatedly over time. Sharing treatment-decision parameters across decision points offers several advantages, including estimation of fewer parameters and the clinical ease of a single, time-invariant decision to implement. We propose a new computational approach to estimation of shared-parameter G-estimation, which is efficient and shares the double robustness of the "unshared" sequential G-estimation. We use this approach to analyze data from the Scottish Early Rheumatoid Arthritis (SERA) Inception Cohort.

Assessment of the healthy worker survivor effect in the relationship between psychosocial work-related factors and hypertension

Objectives

The healthy worker survivor effect (HWSE) usually leads to underestimation of the effects of harmful occupational exposures. HWSE is characterised by the concomitance of three associations: (1) job status–subsequent exposure, (2) job status–disease and (3) previous exposure–job status. No study has reported the coexistence of these associations in the relationship between psychosocial work-related factors and health. We assessed if HWSE is present when measuring the effects of cumulative exposure to psychosocial work-related factors on the prevalence of hypertension in white-collar workers.

Methods

Data were obtained from two timepoints (1991–1993 at baseline and 1999–2001 at follow-up) of a prospective cohort study. At baseline, the population was composed of 9188 white-collar employees (women: 49.9%) in Quebec City. Job strain as psychosocial work-related factor and blood pressure were measured using validated methods. Job status (retirees vs employees) at follow-up was self-reported. Multiple multilevel robust Poisson regressions were used to estimate prevalence ratios of hypertension and risk ratios of retirement separately by gender. We performed multiple imputations to control selection bias due to missing values.

Results

Retirement eliminated the subsequent exposure to job strain de facto and was associated with the reduction in the prevalence of hypertension in younger (−33%) and older (−11%) men and in older women (−39%). Job strain was associated with job status in younger men and in women of any age.

Conclusion

Data showed the presence of HWSE in younger men and older women given the coexistence of the three structural associations.

Occupational silica exposure and mortality from lung cancer and nonmalignant respiratory disease: G-estimation of structural nested accelerated failure time models

BACKGROUND

Occupational exposure to crystalline silica is known to increase risks of both lung cancer and noninfectious nonmalignant respiratory diseases (NMRD). However, associations between silica exposure and survival times have not been described.

METHODS

In a longitudinal cohort of diatomaceous earth workers exposed to crystalline silica (primarily cristobalite) and followed from 1942 to 2011, we applied g-estimation of structural nested accelerated failure time models to adjust for time-varying confounding that could result in healthy worker survivor bias. A continuous measure of exposure was used in analyses estimating the hypothetical effect of banning exposure to silica on survival time. Since a ban is infeasible, sensitivity analyses examined the hypothetical effects of enforcing various Occupational Exposure Limits.

RESULTS

The estimated median number of years of life lost per worker (for all natural causes) due to silica exposure was 0.48 (95% confidence interval = 0.02, 1.01). For NMRD deaths, the corresponding estimate was 3.22 (0.82, 7.75) and for lung cancer deaths, 2.21 (0.97, 3.56). Cause-specific estimates were sensitive to the use of weights to adjust for competing events. Lung cancer mortality, which tended to occur at younger ages, was an important competing event for NMRD mortality. Sensitivity analyses supported the main results, but with larger estimates, and suggested that a strict limit would be nearly as effective as a complete ban on silica exposure.

CONCLUSIONS

Workplace exposure to crystalline silica in this industry appears to shorten survival times significantly, particularly for those who die of lung cancer or NMRD. More stringent exposure limits are probably warranted.

Estimating the Impact of Changes to Occupational Standards for Silica Exposure on Lung Cancer Mortality

BACKGROUND

Respiratory exposure to silica is associated with the risk of death owing to malignant and nonmalignant disease. 2.3 million US workers are exposed to silica. Occupational exposure limits for silica are derived from a number of lines of evidence, including observational studies. Observational studies may be subject to healthy worker survivor bias, which could result in underestimates of silica's impact on worker mortality and, in turn, bias risk estimates for occupational exposure limits.

METHODS

Using data on 65,999 workers pooled across multiple industries, we estimate the impacts of several hypothetical occupational exposure limits on silica exposure on lung cancer and all-cause mortality. We use the parametric g-formula, which can account for healthy worker survivor bias.

RESULTS

Assuming we could eliminate occupational exposure, we estimate that there would be 20.7 fewer deaths per 1,000 workers in our pooled study by age 80 (95% confidence interval = 14.5, 26.8), including 3.91 fewer deaths owing to lung cancer (95% CI = 1.53, 6.30). Less restrictive interventions demonstrated smaller but still substantial risk reductions.

CONCLUSIONS

Our results suggest that occupational exposure limits for silica can be further strengthened to reduce silica-associated mortality and illustrate how current risk analysis for occupational limits can be improved.

A Note on G-Estimation of Causal Risk Ratios

G-estimation is a flexible, semiparametric approach for estimating exposure effects in epidemiologic studies. It has several underappreciated advantages over other propensity score-based methods popular in epidemiology, which we review in this article. However, it is rarely used in practice, due to a lack of off-the-shelf software. To rectify this, we show a simple trick for obtaining G-estimators of causal risk ratios using existing generalized estimating equations software. We extend the procedure to more complex settings with time-varying confounders.

The Impact of Physical Work Demands on Need for Recovery, Employment Status, Retirement Intentions, and Ability to Extend Working Careers: A Longitudinal Study Among Older Workers

OBJECTIVE

Prospectively investigating whether different approaches of physical work demands are associated with need for recovery (NFR), employment status, retirement intentions, and ability to prolong working life among older employees from the industry and health care sector.

METHODS

A subsample from the Maastricht Cohort Study was studied (n = 1126). Poisson, Cox, and logistic regression analyses were performed to investigate outcomes.

RESULTS

Perceiving physical work demands as strenuous was associated with higher NFR. Continuous physical strain was associated with being out of employment 4 years later. Employees with the highest amount of physical work demands perceived they were less able to prolong working life, although no significant associations between physical work demands and retirement intentions were found.

CONCLUSIONS

Overall, physical work demands were associated with adverse outcomes, with divergent insights for the different approaches of physical work demands.

Attenuation of exposure-response rate ratios at higher exposures: a simulation study focusing on frailty and measurement error

BACKGROUND

Positive exposure-response trends for rate ratios (RRs) often diminish at higher exposures. Depletion of susceptibles with higher exposure and increased measurement error at higher exposures are possible reasons.

METHODS

We conducted simulations to investigate attenuation under various assumptions about susceptibility to exposure effects and measurement error, considering a hypothetical occupational cohort, using an excess relative risk model. We simulated an occupational cohort in which entry occurred over time. The metric of interest was cumulative exposure, which had a strong linear effect (RR = 4 for mean exposure), maximizing potential depletion. Measurement error of both classical and Berkson types was also simulated, increasing with increasing exposure. We conducted 100 simulations per scenario, each with 25,000 subjects enrolled from years 1940 to 2010, followed through 2010.

RESULTS

With less than 100% susceptibility to exposure (a requirement for depletion), there was only modest evidence of depletion of susceptibles with increasing cumulative exposure distributed normally. There was correspondingly little attenuation of RRs, and linear exposure-response models fit well. Adding classical measurement error to cumulative exposure, increasing with increasing exposure, resulted in some modest attenuation. Using log normal instead of normally distributed cumulative exposure also resulted in some attenuation.

CONCLUSIONS

Strong attenuation of relatively strong linear exposure-response trends using cumulative exposure, with relatively common disease and heterogeneous susceptibility, does not appear likely due to depletion of susceptibles. Strong attenuation seems more likely to be due to other mechanisms.

Evolving methods for inference in the presence of healthy worker survivor bias

Healthy worker survivor bias may occur in occupational studies due to the tendency for unhealthy individuals to leave work earlier, and consequently accrue less exposure, compared with their healthier counterparts. If occupational data are not analyzed using appropriate methods, this bias can result in attenuation or even reversal of the estimated effects of exposures on health outcomes. Recent advances in computing power, coupled with state-of-the-art statistical methods, have greatly increased the ability of analysts to control healthy worker survivor bias. However, these methods have not been widely adopted by occupational epidemiologists. We update the seminal review by Arrighi and Hertz-Picciotto (Epidemiology.1994; 5: 186-196) of the sources and methods to control healthy worker survivor bias. In our update, we discuss methodologic advances since the publication of that review, notably with a consideration of how directed acyclic graphs can inform the choice of appropriate analytic methods. We summarize and discuss methods for addressing this bias, including recent work applying g-methods to account for employment status as a time-varying covariate affected by prior exposure. In the presence of healthy worker survivor bias, g-methods have advantages for estimating less biased parameters that have direct policy implications and are clearly communicated to decision-makers.

Early-Life State-of-Residence Characteristics and Later Life Hypertension, Diabetes, and Ischemic Heart Disease

OBJECTIVES

We examined how state characteristics in early life are associated with individual chronic disease later in life.

METHODS

We assessed early-life state of residence using the first 3 digits of social security numbers from blue- and white-collar workers from a US manufacturing company. Longitudinal data were available from 1997 to 2012, with 305 936 person-years of observation. Disease was assessed using medical claims. We modeled associations using pooled logistic regression with inverse probability of censoring weights.

RESULTS

We found small but statistically significant associations between early-state-of-residence characteristics and later life hypertension, diabetes, and ischemic heart disease. The most consistent associations were with income inequality, percentage non-White, and education. These associations were similar after statistically controlling for individual socioeconomic and demographic characteristics and current state characteristics.

CONCLUSIONS

Characteristics of the state in which an individual lives early in life are associated with prevalence of chronic disease later in life, with a strength of association equivalent to genetic associations found for these same health outcomes.