Controlling the healthy worker survivor effect: an example of arsenic exposure and respiratory cancer

Disparities in job characteristics by race and sex in a Southern aluminum smelting facility

BACKGROUND

Former workers at a Southern aluminum smelting facility raised concerns that the most hazardous jobs were assigned to Black workers, but the role of workplace segregation had not been quantified or examined in the company town. Prior studies discuss race and gender disparities in working conditions, but few have documented them in the aluminum industry.

METHODS

We obtained workers' company records for 1985-2007 and characterized four job metrics: prestige (sociologic rankings), worker-defined danger (worker assessments), annual wage (1985 dollars), and estimated total particulate matter (TPM) exposure (job exposure matrix). Characteristics of job at hire and trajectories were compared by race and sex using linear binomial models.

RESULTS

Non-White males had the highest percentage of workers in low prestige and high danger jobs at hire and up to 20 years after. After 20 years tenure, 100% of White workers were in higher prestige and lower danger jobs. Most female workers, regardless of race, entered and remained in low-wage jobs, while 50% of all male workers maintained their initial higher-wage jobs. Non-White females had the highest prevalence of workers in low-wage jobs at hire and after 20 years-increasing from 63% (95% CI: 59-67) to 100% (95% CI: 78-100). All female workers were less likely to be in high TPM exposure jobs. Non-White males were most likely to be hired into high TPM exposure jobs, and this exposure prevalence increased as time accrued, while staying constant for other race-sex groups.

CONCLUSIONS

There is evidence of job segregation by race and sex in this cohort of aluminum smelting workers. Documentation of disparities in occupational hazards is important for informing health interventions and research.

Exposure to Metalworking Fluids and Cancer Incidence in the United Auto Workers-General Motors Cohort

In previous studies, investigators have reported increased risks of specific cancers associated with exposure to metalworking fluids (MWFs). In this report we broadly examine the incidence of 14 types of cancer, with a focus on digestive, respiratory, and hormonal cancers, in the United Auto Workers-General Motors (UAW-GM) cohort, a cohort of workers exposed to MWFs (1973-2015). The cohort included 39,132 workers followed for cancer incidence. Cox models yielded estimates of adjusted hazard ratios, with categorical variables for lagged cumulative exposure to 3 types of MWF (straight, soluble, and synthetic). We fitted penalized splines to examine the shape of the exposure-response relationships. There were 7,809 incident cancer cases of interest. Oil-based straight and soluble MWFs were each modestly associated with all cancers combined. Exposure-response patterns were consistent with prior reports from this cohort, and results for splined exposures generally reflected their categorically modeled counterparts. We found significantly increased incidence of stomach and kidney cancer with higher levels of straight MWF exposure and increased rectal and prostate cancer with increasing water-based synthetic MWF exposure. Only non-Hodgkin lymphoma and prostate cancer were associated with soluble MWF. All results for colon and lung cancers were null. Our results provide updated evidence for associations between MWF exposure and incidence of several types of cancer.

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.

Risk of major chronic diseases in transport, rescue and security industries: a longitudinal register-based study

OBJECTIVE

To investigate the risk of hospitalisation for major chronic diseases across representative transport, rescue and security industries.

METHODS

We performed a register-based study of 624 571 workers from six industries in Denmark between 2000 and 2005, followed up hospitalisation for chronic diseases up to 17 years, and compared with a 20% random sample of the economically active population.

RESULTS

HR from the Cox regression models showed that seafarers had higher risk of lung cancer (men: 1.54, 95% CI 1.31 to 1.81; women: 1.63, 95% CI 1.13 to 2.36), and male seafarers had higher risk of diabetes (1.32, 95% CI 1.21 to 1.43) and oral cancer (1.51, 95% CI 1.21 to 1.88). Men and women in land transport had increased risk of diabetes (men: 1.68, 95% CI 1.63 to 1.73; women 1.55, 95% CI 1.40 to 1.71) and chronic respiratory disease (men: 1.21, 95% CI 1.16 to 1.25; women 1.42, 95% CI 1.32 to 1.53). Among women, a higher risk of gastrointestinal cancer was observed in aviation (1.53, 95% CI 1.23 to 1.89) and police force (1.29, 95% CI 1.01 to 1.65), oral cancer in defence forces (1.83, 95% CI 1.20 to 2.79), and chronic respiratory disease in rescue service (1.47, 95% CI 1.21 to 1.77), while men in defence forces, police force and rescue service had mainly lower risk of these chronic diseases.

CONCLUSIONS

We observed considerable health disparities from chronic diseases across transport, rescue and security industries, with workers in seafaring and land transport generally bearing the greatest relative burden.

Long-term effect of arsenic exposure: Results from an occupational cohort study

BACKGROUND

In 1976 in Manfredonia (Italy), arsenic was released into the atmosphere due to an accident in a petrochemical plant. We aimed to analyze the mortality of workers involved in the factory for the site cleaning activities.

METHODS

The cohort consisted of 1467 workers grouped into contract, fertilizer, and plastic workers. The outcome of interest was mortality for specific causes. Standardized mortality ratios (SMR) and 95% confidence intervals (95%CI) were computed.

RESULTS

For all workers and all causes of death combined, the SMR was less than 1.0. Mortality ratios were increased for malignant neoplasms of the pleura, bone and melanoma of the skin. Contract workers, the group mostly exposed to arsenic, showed statistically significant SMRs for several malignancies, in particular for lung cancer (SMR = 1.26; 95%CI: 1.05-1.54).

CONCLUSIONS

Overall, the results reported here on mortality among persons occupationally exposed to arsenic are consistent with the literature and biologically plausible.

Reassessing the Link between Airborne Arsenic Exposure among Anaconda Copper Smelter Workers and Multiple Causes of Death Using the Parametric g-Formula

BACKGROUND

Prior studies have indicated associations between ingestion of inorganic arsenic and ischemic heart disease, nonmalignant respiratory disease, and lung, skin, bladder, and kidney cancers. In contrast, inhaled arsenic has been consistently associated only with lung cancer. Evidence for health effects of inhaled arsenic derives mainly from occupational studies that are subject to unique biases that may attenuate or obscure such associations.

OBJECTIVES

We estimated the excess mortality from respiratory cancers, heart disease, and other causes resulting from occupational arsenic exposure while controlling for confounding using the parametric g-formula.

METHODS

Using a cohort of 8,014 male copper smelter workers who were hired between 1938 and 1955 and followed through 1990, we estimated the impacts of hypothetical workplace interventions on arsenic exposure on the risk of mortality from all causes, heart disease, and lung cancer using the parametric g-formula.

RESULTS

We estimate that eliminating arsenic exposure at work would have prevented 22 deaths by age 70 per 1,000 workers [95% confidence interval (CI): 10, 35]. Of those 22 excess deaths, we estimate that 7.2 (95% CI: -1.2, 15) would be due to heart disease, 4.0 (95% CI: -0.8, 8.2) due to respiratory cancers, and 11 (95% CI: 0.0, 23) due to other causes.

CONCLUSIONS

Our analyses suggest that the excess deaths from causes other than respiratory cancers comprise the majority of the excess deaths caused by inhaled arsenic exposure. Healthy worker survivor bias may have masked such associations in previous analyses. These results emphasize the need for consideration of all exposure routes for upcoming risk assessment by the U.S. Environmental Protection Agency.

Assessment of the healthy worker survivor effect in cancer studies of the United Autoworkers-General Motors cohort

OBJECTIVE

The healthy worker survivor effect (HWSE) can affect the validity of occupational studies when data are analysed incorrectly. HWSE depends on three underlying conditions: (1) leaving work predicts future exposure, (2) leaving work is associated with disease outcome and (3) prior exposure increases probability of leaving work. If all these conditions are satisfied, then employment status is a time-varying confounder affected by prior exposure, and standard regression will produce bias. We assessed these conditions for cancer outcomes in a cohort of autoworkers exposed to metalworking fluids (MWF).

METHODS

The cohort includes 31 485 workers followed for cancer incidence from 1985 to 1994. As occupational exposures to straight, soluble and synthetic MWFs are necessarily zero after leaving work, condition (1) is satisfied. Cox models for cancer incidence and for employment termination were used to assess conditions (2) and (3), respectively. Employment termination by select ages was examined to better gauge the presence of condition (2).

RESULTS

The HR for leaving work as a predictor of all cancers combined and prostate cancer was null, but elevated for lung and colorectal cancers among men. Condition (2) was more clearly satisfied for all cancer outcomes when leaving work occurred by age 50. Higher exposures to all three MWF types were associated with increased rates of leaving work (condition (3)), with the exception of straight MWF among women.

CONCLUSIONS

We found evidence for the structural conditions underlying HWSE in a cohort of autoworkers. G-methods should be applied to reduce HWSE bias in studies of all cancers presently examined.

Healthy Worker Effect Phenomenon: Revisited with Emphasis on Statistical Methods - A Review

Known since 1885 but studied systematically only in the past four decades, the healthy worker effect (HWE) is a special form of selection bias common to occupational cohort studies. The phenomenon has been under debate for many years with respect to its impact, conceptual approach (confounding, selection bias, or both), and ways to resolve or account for its effect. The effect is not uniform across age groups, gender, race, and types of occupations and nor is it constant over time. Hence, assessing HWE and accounting for it in statistical analyses is complicated and requires sophisticated methods. Here, we review the HWE, factors affecting it, and methods developed so far to deal with it.

Adjustment of lifetime risks of space radiation-induced cancer by the healthy worker effect and cancer misclassification

Background. The healthy worker effect (HWE) is a source of bias in occupational studies of mortality among workers caused by use of comparative disease rates based on public data, which include mortality of unhealthy members of the public who are screened out of the workplace. For the US astronaut corp, the HWE is assumed to be strong due to the rigorous medical selection and surveillance. This investigation focused on the effect of correcting for HWE on projected lifetime risk estimates for radiation-induced cancer mortality and incidence.

Methods. We performed radiation-induced cancer risk assessment using Poisson regression of cancer mortality and incidence rates among Hiroshima and Nagasaki atomic bomb survivors. Regression coefficients were used for generating risk coefficients for the excess absolute, transfer, and excess relative models. Excess lifetime risks (ELR) for radiation exposure and baseline lifetime risks (BLR) were adjusted for the HWE using standardized mortality ratios (SMR) for aviators and nuclear workers who were occupationally exposed to ionizing radiation. We also adjusted lifetime risks by cancer mortality misclassification among atomic bomb survivors.

Results. For all cancers combined (“Nonleukemia”), the effect of adjusting the all-cause hazard rate by the simulated quantiles of the all-cause SMR resulted in a mean difference (not percent difference) in ELR of 0.65% and mean difference of 4% for mortality BLR, and mean change of 6.2% in BLR for incidence. The effect of adjusting the excess (radiation-induced) cancer rate or baseline cancer hazard rate by simulated quantiles of cancer-specific SMRs resulted in a mean difference of −1.2% in the all-cancer mortality ELR and mean difference of −6.4% in the mortality BLR. Whereas for incidence, the effect of adjusting by cancer-specific SMRs resulted in a mean change of −14.4% for the all-cancer BLR. Only cancer mortality risks were adjusted by simulated quantiles for misclassification, and results indicate a mean change of 1.1% for all-cancer mortality ELR, while the mean change in the all-cancer PC was approximately 4% for males and 6% for females.

Conclusions. The typical life table approach for projecting lifetime risk of radiation-induced cancer mortality and incidence for astronauts and radiation workers can be improved by adjusting for HWE while simulating the uncertainty of input rates, input excess risk coefficients, and bias correction factors during multiple Monte Carlo realizations of the life table.

Real world effectiveness of primary implantable cardioverter defibrillators implanted during hospital admissions for exacerbation of heart failure or other acute co-morbidities: cohort study of older patients with heart failure

OBJECTIVES

To examine the effectiveness of primary implantable cardioverter defibrillators (ICDs) in elderly patients receiving the device during a hospital admission for exacerbation of heart failure or other acute co-morbidities, with an emphasis on adjustment for early mortality and other factors reflecting healthy candidate bias rather than the effect of the ICD.

DESIGN

Retrospective cohort study.

SETTING

Linked data from the Centers for Medicare and Medicaid Services and American College of Cardiology-National Cardiovascular Data Registry ICD registry, nationwide heart failure registry, and Medicare claims data 2004-09.

POPULATION

23,111 patients aged ≥ 66 who were admitted to hospital for exacerbation of heart failure or other acute co-morbidities and eligible for primary ICDs.

MAIN OUTCOME MEASURES

All cause mortality and sudden cardiac death. Latency analyses with Cox regression were used to derive crude hazard ratios and hazard ratios adjusted for high dimension propensity score for outcomes after 180 days from index implantation or discharge.

RESULTS

Patients who received an ICD during a hospital admission had lower crude mortality risk than patients who did not receive an ICD (40% v 60% at three years); however, with conditioning on 180 day survival and with adjustment for high dimension propensity score, the apparent benefit with ICD was no longer evident for sudden cardiac death (adjusted hazard ratio 0.95, 95% confidence interval 0.78 to 1.17) and had a diminished impact on total mortality (0.91, 0.82 to 1.00). There were trends towards a benefit with ICD in reducing mortality or sudden cardiac death in patients who had had a myocardial infarction more than 40 days previously, left bundle branch block, or low serum B type natriuretic peptide; however, these trends did not reach significance.

CONCLUSION

After adjustment for healthy candidate bias and confounding, the benefits of primary ICD therapy seen in pivotal trials were not apparent in patients aged 66 or over who received ICDs during a hospital admission for exacerbation of heart failure or other acute co-morbidities. Future research is warranted to further identify subgroups of elderly patients who are more likely to benefit from ICDs. Recognition of those patients whose dominant risk factors are from decompensated heart failure and non-cardiac co-morbidities will allow better focus on ICDs in those patients for whom the device offers the most benefit and provides meaningful prolonging of life.

Occupational physical demand and risk of hip fracture in older women

OBJECTIVES

Hip fractures are leading causes of disability, morbidity and mortality among older women. Since physical activity helps maintain physical functioning and bone mineral density, occupational physical demand may influence fracture risk. This study investigates the association of occupational physical demand with hip fracture incidence among women.

METHODS

The Women's Health Initiative Observational Study is a multiethnic cohort of 93,676 postmenopausal women, 50-79 years of age at enrolment, enrolled from 1994 to 1998 at 40 geographically diverse clinical centres throughout the USA. Outcomes including hip fractures were assessed annually and up to 3 jobs held since age 18 years were reported by each woman. Occupational physical demand levels were assigned for each job through linkage of occupational titles with Standard Occupational Codes and the Occupational Information Network. Average, cumulative and peak physical demand scores both before and after menopause and throughout women's work life were estimated.

RESULTS

Women were followed through 2010 for an average of 11.5 years; 1834 hip fractures occurred during this time. We did not observe an overall association of occupational physical demand with subsequent risk of hip fracture after adjusting for age, race/ethnicity, birth region and education.

CONCLUSIONS

Previous research on occupations and hip fracture risk in women is inconclusive. This study was able to take critical risk periods into account and control for confounding factors in a large cohort of older women to show that overall occupational physical demand neither increases nor decreases risk of hip fracture later in life.

Healthy worker survivor bias in the Colorado Plateau uranium miners cohort

Cohort mortality studies of underground miners have been used to estimate the number of lung cancer deaths attributable to radon exposure. However, previous studies of the radon-lung cancer association among underground miners may have been subject to healthy worker survivor bias, a type of time-varying confounding by employment status. We examined radon-mortality associations in a study of 4,124 male uranium miners from the Colorado Plateau who were followed from 1950 through 2005. We estimated the time ratio (relative change in median survival time) per 100 working level months (radon exposure averaging 130,000 mega-electron volts of potential α energy per liter of air, per working month) using G-estimation of structural nested models. After controlling for healthy worker survivor bias, the time ratio for lung cancer per 100 working level months was 1.168 (95% confidence interval: 1.152, 1.174). In an unadjusted model, the estimate was 1.102 (95% confidence interval: 1.099, 1.112)-39% lower. Controlling for this bias, we estimated that among 617 lung cancer deaths, 6,071 person-years of life were lost due to occupational radon exposure during follow-up. Our analysis suggests that healthy worker survivor bias in miner cohort studies can be substantial, warranting reexamination of current estimates of radon's estimated impact on lung cancer mortality.

The Relationship Between Caregiving and Mortality After Accounting for Time-Varying Caregiver Status and Addressing the Healthy Caregiver Hypothesis

BACKGROUND

Previous studies have shown inconsistent associations between caregiving and mortality. This may be due to analyzing caregiver status at baseline only, and that better health is probably related to taking on caregiving responsibilities and continuing in that role. The latter is termed The Healthy Caregiver Hypothesis, similar to the Healthy Worker Effect in occupational epidemiology. We applied common approaches from occupational epidemiology to evaluate the association between caregiving and mortality, including treating caregiving as time-varying and lagging exposure up to 5 years.

METHODS

Caregiving status among 1,068 women (baseline mean age = 81.0 years; 35% caregivers) participating in the Caregiver-Study of Osteoporotic Fractures study was assessed at five interviews conducted between 1999 and 2009. Mortality was determined through January 2012. Cox proportional hazards models were used to estimate adjusted hazard ratios and 95% confidence intervals adjusted for sociodemographics, perceived stress, and functional limitations.

RESULTS

A total of 483 participants died during follow-up (38.8% and 48.7% of baseline caregivers and noncaregivers, respectively). Using baseline caregiving status, the association with mortality was 0.77, 0.62-0.95. Models of time-varying caregiving status showed a more pronounced reduction in mortality in current caregivers (hazard ratios = 0.54, 0.38-0.75), which diminished with longer lag periods (3-year lag hazard ratio = 0.68, 0.52-0.88, 5-year lag hazard ratios = 0.76, 0.60-0.95).

CONCLUSIONS

Overall, caregivers had lower mortality rates than noncaregivers in all analyses. These associations were sensitive to the lagged period, indicating that the timing of leaving caregiving does influence this relationship and should be considered in future investigations.

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

BACKGROUND

Occupational cohort studies are often challenged by the Healthy Worker Survivor Effect, which may bias standard methods of analysis. G-estimation of structural failure time models is an approach for reducing this type of bias. Accelerated failure time models have recently been applied in an occupational cohort but cumulative failure time models have not.

METHODS

We used g-estimation of a cumulative failure time model to assess the effect of working as a long-haul driver on ischaemic heart disease mortality in a cohort of 30 448 men employed in the unionised US trucking industry in 1985. Exposure was defined by job title and based on work records. We also applied g-estimation of an accelerated failure time model as a sensitivity analysis and approximated HRs from both models to compare them.

RESULTS

The risk ratio (RR) obtained from the cumulative failure time model, comparing the observed risk under no intervention to the risk had nobody ever been exposed as a long-haul driver, was 1.09 (95% CI 1.02 to 1.16). The RR comparing the risk had everyone been exposed as long-haul driver for 8 years to the risk had nobody ever been exposed was 1.20 (95% CI 1.04 to 1.46). After HR approximations, accelerated failure time model results were similar.

CONCLUSIONS

The cumulative failure time model can effectively control time-varying confounding by Healthy Worker Survivor Effect, and provides an easily interpretable effect estimate. RRs estimated from the cumulative failure time model indicate an elevated ischaemic heart disease mortality risk for long-haul drivers in the US trucking industry.

Causal inference in occupational epidemiology: accounting for the healthy worker effect by using structural nested models

In a recent issue of the Journal, Kirkeleit et al. (Am J Epidemiol. 2013;177(11):1218-1224) provided empirical evidence for the potential of the healthy worker effect in a large cohort of Norwegian workers across a range of occupations. In this commentary, we provide some historical context, define the healthy worker effect by using causal diagrams, and use simulated data to illustrate how structural nested models can be used to estimate exposure effects while accounting for the healthy worker survivor effect in 4 simple steps. We provide technical details and annotated SAS software (SAS Institute, Inc., Cary, North Carolina) code corresponding to the example analysis in the Web Appendices, available at http://aje.oxfordjournals.org/.

Lung cancer and elemental carbon exposure in trucking industry workers

BACKGROUND

Diesel exhaust has been considered to be a probable lung carcinogen based on studies of occupationally exposed workers. Efforts to define lung cancer risk in these studies have been limited in part by lack of quantitative exposure estimates.

OBJECTIVE

We conducted a retrospective cohort study to assess lung cancer mortality risk among U.S. trucking industry workers. Elemental carbon (EC) was used as a surrogate of exposure to engine exhaust from diesel vehicles, traffic, and loading dock operations.

METHODS

Work records were available for 31,135 male workers employed in the unionized U.S. trucking industry in 1985. A statistical model based on a national exposure assessment was used to estimate historical work-related exposures to EC. Lung cancer mortality was ascertained through the year 2000, and associations with cumulative and average EC were estimated using proportional hazards models.

RESULTS

Duration of employment was inversely associated with lung cancer risk consistent with a healthy worker survivor effect and a cohort composed of prevalent hires. After adjusting for employment duration, we noted a suggestion of a linear exposure-response relationship. For each 1,000-µg/m³ months of cumulative EC, based on a 5-year exposure lag, the hazard ratio (HR) was 1.07 [95% confidence interval (CI): 0.99, 1.15] with a similar association for a 10-year exposure lag [HR = 1.09 (95% CI: 0.99, 1.20)]. Average exposure was not associated with relative risk.

CONCLUSIONS

Lung cancer mortality in trucking industry workers increased in association with cumulative exposure to EC after adjusting for negative confounding by employment duration.

An updated inhalation unit risk factor for arsenic and inorganic arsenic compounds based on a combined analysis of epidemiology studies

The United States Environmental Protection Agency (USEPA) developed an inhalation unit risk factor (URF) of 4.3E-03 per μg/m(3) for arsenic in 1984 for excess lung cancer mortality based on epidemiological studies of workers at two smelters: the Asarco smelter in Tacoma, Washington and the Anaconda smelter in Montana. Since the USEPA assessment, new studies have been published and exposure estimates were updated at the Asarco and Anaconda smelters and additional years of follow-up evaluated. The Texas Commission on Environmental Quality (TCEQ) has developed an inhalation URF for lung cancer mortality from exposures to arsenic and inorganic arsenic compounds based on a newer epidemiology study of Swedish workers and the updates of the Asarco and Anaconda epidemiology studies. Using a combined analysis approach, the TCEQ weighted the individual URFs from these three epidemiology cohort studies, to calculate a final inhalation URF of 1.5E-04 per μg/m(3). In addition, the TCEQ also conducted a sensitivity analysis, in which they calculated a URF based on a type of meta-analysis, and these results compared well with the results of the combined analysis. The no significant concentration level (i.e., air concentration at 1 in 100,000 excess lung cancer mortality) is 0.067μg/m(3). This value will be used to evaluate ambient air monitoring data so the general public in Texas is protected against adverse health effects from chronic exposure to arsenic.

A comparison of standard methods with g-estimation of accelerated failure-time models to address the healthy-worker survivor effect: application in a cohort of autoworkers exposed to metalworking fluids

BACKGROUND

Studies of autoworkers exposed to straight metalworking fluids report excess risks of several cancers. These studies, however, have not addressed the healthy-worker survivor effect. Most methods proposed to address this bias do not consider that it may be caused by time-varying confounders affected by prior exposure. G-estimation of accelerated failure-time models was developed to handle this issue but has never been applied to account for the healthy-worker survivor effect.

METHODS

We compare results from Cox models and g-estimation in 38,747 autoworkers exposed to straight metalworking fluids. Exposure was defined based on job records and air samples. We examine relationships between duration of exposure and mortality from all causes, cancers, ischemic heart disease, and chronic obstructive pulmonary disease (COPD).

RESULTS

In standard models, hazard ratios were elevated for cancers of the larynx, prostate, and rectum, but below or approximately equal to 1.0 for all other outcomes considered. Adjustment for the healthy-worker survivor effect using time off work, employment status, time since hire, and restriction to inactive workers after 15 years of follow-up did not substantially change the hazard ratios. However, g-estimation yielded higher hazard ratios than standard Cox models for most outcomes. Exposure was related to increased risks of mortality from all causes combined, heart disease, COPD, and all cancers, as well as lung and prostate cancers.

CONCLUSIONS

G-estimation may provide a better control for the healthy-worker survivor effect than standard methods.

A comparison of methods to estimate the hazard ratio under conditions of time-varying confounding and nonpositivity

In occupational epidemiologic studies, the healthy worker survivor effect refers to a process that leads to bias in the estimates of an association between cumulative exposure and a health outcome. In these settings, work status acts both as an intermediate and confounding variable and may violate the positivity assumption (the presence of exposed and unexposed observations in all strata of the confounder). Using Monte Carlo simulation, we assessed the degree to which crude, work-status adjusted, and weighted (marginal structural) Cox proportional hazards models are biased in the presence of time-varying confounding and nonpositivity. We simulated the data representing time-varying occupational exposure, work status, and mortality. Bias, coverage, and root mean squared error (MSE) were calculated relative to the true marginal exposure effect in a range of scenarios. For a base-case scenario, using crude, adjusted, and weighted Cox models, respectively, the hazard ratio was biased downward 19%, 9%, and 6%; 95% confidence interval coverage was 48%, 85%, and 91%; and root MSE was 0.20, 0.13, and 0.11. Although marginal structural models were less biased in most scenarios studied, neither standard nor marginal structural Cox proportional hazards models fully resolve the bias encountered under conditions of time-varying confounding and nonpositivity.

Update of the NIOSH life table analysis system: a person-years analysis program for the windows computing environment

BACKGROUND

Person-years analysis is a fundamental tool of occupational epidemiology. A life table analysis system (LTAS), previously developed by the National Institute for Occupational Safety and Health, was limited by its platform and analysis and reporting capabilities. We describe the updating of LTAS for the Windows operating system (LTAS.NET) with improved properties.

SOFTWARE DEVELOPMENT PROCESS

A group of epidemiologists, programmers, and statisticians developed software, platform, and computing requirements. Statistical methods include the use of (indirectly) standardized mortality ratios, (directly) standardized rate ratios, confidence intervals, and P values based on the normal approximation and exact Poisson methods, and a trend estimator for linear exposure-response associations.

SOFTWARE FEATURES

We show examples using LTAS.NET to stratify and analyze multiple fixed and time-dependent variables. Data import, stratification, and reporting options are highly flexible. Users may export stratified data for Poisson regression modeling.

CONCLUSIONS

LTAS.NET incorporates improvements that will facilitate more complex person-years analysis of occupational cohort data.

Quantification of the healthy worker effect: a nationwide cohort study among electricians in Denmark

Background

The healthy worker effect (HWE) is a well-known phenomenon. In this study we used the extensive registration of all Danish citizens to describe the magnitude of HWE among all Danish electricians and evaluated strategies for minimizing HWE bias of the association between occupation and mortality.

Methods

All Danish male citizens aged 26-56 years in the period 1984-1992 were followed for three years in several registers. We evaluated HWE bias among electricians because they were unexposed to detrimental occupational exposures. We compared electricians to three reference groups (general population, construction industry and carpenters/brick layers) and utilized analytical methods for minimizing HWE bias (lag time analyses, age-stratified analyses, marginal structural model and restriction to employed, newly employed or long-term workers).

Results

The mortality rate was higher among electricians, who the year following active employment received incapacity benefits or were on long-term sick leave. Electricians receiving incapacity benefits, on long-term sick leave, unemployed, or with increased comorbidity index had lower odds of re-employment. Electricians had lower mortality rate (rate ratio,0.60;95%CI,0.52-0.69) compared to the general population, while electricians leaving employment had increased mortality (1.90;1.50-2.40). Adjusting for several social events slightly attenuated the estimates, while the marginal structural model did not minimize bias. Electricians had the same mortality as the construction industry and carpenters/brick layers. Mortality was comparable to the general population after three or more years of lag time.

Conclusions

In this nationwide study, employment as electricians had marked effect on mortality. Appropriate reference selection and lag time analyses minimized the HWE bias.

Long-term outcomes of end-stage renal disease patients admitted to the ICU

BACKGROUND

End-stage renal disease (ESRD) patients admitted to the intensive care unit (ICU) have poor survival and high rates of readmission; however, little evidence exists on long-term outcomes. We set out to investigate the long-term (6 and 12 months) survival of ESRD patients admitted to the ICU and whether differential survival could be explained by dialysis modality and vascular access.

METHODS

We compared the admission characteristics, outcomes and readmission rates of 619 ESRD [95 peritoneal dialysis (PD), 334 hemodialysis with a catheter (HD CVC), 190 hemodialysis with an AV fistula (HD AVF)] patients admitted to 11 ICU's in Winnipeg, Manitoba, Canada. Parametric and nonparametric tests were used as appropriate to determine differences in baseline characteristics. Multivariable Cox and logistic regression was used to assess outcomes between the groups.

RESULTS

The 6- and 12-month crude survival was 62 and 52%, respectively. In a univariate model, modality and vascular access were associated with an increased hazard ratio (HR) of death [PD HR 1.60 95% confidence interval (CI) 1.20-2.13, HD CVC HR 1.55 95% CI 1.25-1.93] compared to patients on HD with an AVF. In three different multivariate adjusted models, this association persisted with HRs for death of 1.63-1.75 for PD and 1.50-1.58 for HD CVC.

CONCLUSIONS

Overall long-term survival of ESRD patients after admission to the ICU is poor. Being on PD or being dialyzed with a catheter was independently associated with an increased mortality.

Pack wounds of donkeys and mules in the Northern High Atlas and lowlands of Morocco

REASONS FOR PERFORMING STUDY

Wounds are one of the primary welfare concerns of working equids and are often related to harness or load-bearing packs.

OBJECTIVES

To identify and quantify factors associated with the presence of pack wounds in donkeys and mules working in the wood trade in the Midelt (Northern High Atlas) region of Morocco, and to compare wounds of these donkeys and mules to those working in other trades.

METHODS

The study population comprised equids working in the Midelt region involved in the wood trade. Comparison populations were comprised of equids working in other trades, in the same region and in Khemmiset region (lowlands of Central Morocco). Wounds were numbered and scored according to their severity. Questionnaires were used to gather data from owners, and objective data regarding the animal, berdaa (back-pack) and load were collected. Data were analysed using multilevel uniand multivariable regression models to allow for clustering of wound sites within an equid.

RESULTS

Analysis showed an overall pack wound prevalence of 54% (n=80/147). The most common site was the withers. Equids working in the wood trade were at increased risk of suffering pack wounds. Equids in the lowland study population were at increased risk of developing pack wounds, and donkeys were at increased risk of suffering pack wounds compared to mules both overall and within the wood trade. Other factors affecting the prevalence of wounds included the cleanliness of the berdaa and the frequency and distances of work imposed on the equid.

CONCLUSION

Pack wounds in donkeys and mules are common in the 2 study regions of Morocco and appear to be associated with work factors including the trade the equids work in, the frequency and distances of work imposed and cleanliness of the pack. Donkeys are more susceptible than mules to developing pack wounds.

POTENTIAL RELEVANCE

These results allow recommendations to be made to owners of equids that may decrease the prevalence of wounds and, therefore, improve welfare and working capability of these equids. Research is required to identify reasons for regional difference in prevalence of pack wounds in equids not working in the wood trade.

Reducing healthy worker survivor bias by restricting date of hire in a cohort study of Vermont granite workers

OBJECTIVE

To explore the healthy worker survivor effect (HWSE) in a study of Vermont granite workers by distinguishing "prevalent" from "incident" hires based on date of hire before or after the start of follow-up.

METHODS

Records of workers between 1950 and 1982 were obtained from a medical surveillance programme. Proportional hazards models were used to model the association between silica exposure and lung cancer mortality, with penalised splines used to smooth the exposure-response relationship. A sensitivity analysis compared results between the original cohort and subcohorts defined by restricting date of hire to include varying proportions of prevalent hires.

RESULTS

Restricting to incident hires reduced the 213 cases by 74% and decreased the exposure range. The maximum mortality rate ratio (MRR) was close to twofold in all subcohorts. However, the exposure at which the maximum MRR was achieved decreased from 4.0 to 0.6 mg-year/m3 as the proportion of prevalent hires decreased from 50% in the original cohort to 0% in the subcohort of incident hires.

CONCLUSION

Despite loss in power and restricted exposure range, decreasing the relative proportion of prevalent to incident hires reduced HWSE bias, resulting in stronger evidence for a dose-response between silica exposure and lung cancer mortality.

Rectal cancer and exposure to metalworking fluids in the automobile manufacturing industry

BACKGROUND

Rectal cancer has been previously associated with exposure to metalworking fluids in a cohort mortality study of autoworkers.

OBJECTIVE

To better specify the exposure-response relationship with straight metalworking fluids (mineral oils) by applying non-parametric regression methods that avoid linearity constraints and arbitrary exposure cut points and by lagging exposure to account for cancer latency, in a nested case-control analysis.

METHODS

In addition to the classical Poisson regression with categorical exposure, survival models with penalised splines were used to estimate the exposure-response relationship between cumulative exposure to straight metalworking fluid and mortality from rectal cancer. Exposures to water-based metalworking fluids were treated as potential confounders, and all exposures were lagged by 5, 10, 15 and 20 years to account for cancer latency. The influence of the highest exposures was dealt with by a log transformation and outlier removal. The sensitivity of the penalised splines to alternative criteria for model selection and to the placement of knots was also examined.

RESULTS

The hazard ratio for mortality from rectal cancer increased essentially linearly with cumulative exposure to straight metalworking fluid (with narrow confidence bands) up to a maximum of 2.2 at the 99th centile of exposure and then decreased (with wide confidence bands). Lagging exposure up to 15 years increased the initial steepness of the curve and raised the maximum hazard ratio to 3.2.

CONCLUSIONS

Non-parametric smoothing of lagged exposures has shown stronger evidence for a causal association between straight metalworking fluid and rectal cancer than was previously described using standard analytical methods. This analysis suggests an exposure-response trend that is close to linear and statistically significant over most of the exposure range and that increases further with lagged exposures. Smoothing should be regularly applied to environmental studies with quantitative exposure estimates to refine characterisation of the dose-response relationship.

Arsenic in the aetiology of cancer

Arsenic, one of the most significant hazards in the environment affecting millions of people around the world, is associated with several diseases including cancers of skin, lung, urinary bladder, kidney and liver. Groundwater contamination by arsenic is the main route of exposure. Inhalation of airborne arsenic or arsenic-contaminated dust is a common health problem in many ore mines. This review deals with the questions raised in the epidemiological studies such as the dose-response relationship, putative confounders and synergistic effects, and methods evaluating arsenic exposure. Furthermore, it describes the metabolic pathways of arsenic, and its biological modes of action. The role of arsenic in the development of cancer is elucidated in the context of combined epidemiological and biological studies. However, further analyses by means of molecular epidemiology are needed to improve the understanding of cancer aetiology induced by arsenic.

Occupational acrylonitrile exposure and lung cancer: a meta-analysis

The present work summarizes the currently available published studies on lung cancer and occupational acrylonitrile exposure. Meta-analytic methods were used to estimate the overall risk. To adjust for the healthy worker effect, rate ratio estimates based on regression analyses and ratios of standard mortality ratios were aggregated. Overall effect estimates were 0.95 (95% CI 0.86 to 1.06) and 1.25 (95% CI 1.10 to 1.43) before and after adjustment for the healthy worker effect, respectively. Therefore, a 25% increase in lung cancer risk attributable to occupational acrylonitrile exposure is suggested. Possible contribution of smoking confounding the increased risk cannot be fully excluded.

Arsenic exposure and cardiovascular disease: a systematic review of the epidemiologic evidence

Arsenic exposure is a likely cause of blackfoot disease and a potential risk factor for atherosclerosis. The authors performed a systematic review of the epidemiologic evidence on the association between arsenic and cardiovascular outcomes. The search period was January 1966 through April 2005. Thirteen studies conducted in general populations (eight in high-arsenic areas in Taiwan, five in other countries) and 16 studies conducted in occupational populations were identified. Exposure was assessed ecologically in most studies. In Taiwan, relative risks comparing the highest arsenic exposure category with the lowest ranged from 1.59 to 4.90 for coronary disease, from 1.19 to 2.69 for stroke, and from 1.66 to 4.28 for peripheral arterial disease. In other general populations, relative risks ranged from 0.84 to 1.54 for coronary disease, from 0.69 to 1.53 for stroke, and from 0.61 to 1.58 for peripheral arterial disease. In occupational populations, relative risks ranged from 0.40 to 2.14 for coronary disease mortality and from 0.30 to 1.33 for stroke mortality. Methodologic limitations, however, limited interpretation of the moderate-to-strong associations between high arsenic exposure and cardiovascular outcomes in Taiwan. In other populations or in occupational settings, the evidence was inconclusive. Because of the high prevalence of arsenic exposure, carefully performed studies of arsenic and cardiovascular outcomes should be a research priority.

Smoothing in occupational cohort studies: an illustration based on penalised splines

AIMS

To illustrate the contribution of smoothing methods to modelling exposure-response data, Cox models with penalised splines were used to reanalyse lung cancer risk in a cohort of workers exposed to silica in California's diatomaceous earth industry. To encourage application of this approach, computer code is provided.

METHODS

Relying on graphic plots of hazard ratios as smooth functions of exposure, the sensitivity of the curve to amount of smoothing, length of the exposure lag, and the influence of the highest exposures was evaluated. Trimming and data transformations were used to down-weight influential observations.

RESULTS

The estimated hazard ratio increased steeply with cumulative silica exposure before flattening and then declining over the sparser regions of exposure. The curve was sensitive to changes in degrees of freedom, but insensitive to the number or location of knots. As the length of lag increased, so did the maximum hazard ratio, but the shape was similar. Deleting the two highest exposed subjects eliminated the top half of the range and allowed the hazard ratio to continue to rise. The shape of the splines suggested a parametric model with log hazard as a linear function of log transformed exposure would fit well.

CONCLUSIONS

This flexible statistical approach reduces the dependence on a priori assumptions, while pointing to a suitable parametric model if one exists. In the absence of an appropriate parametric form, however, splines can provide exposure-response information useful for aetiological research and public health intervention.

Skin cancer induced by arsenic in the water

BACKGROUND

Arsenic (As) is a well-recognized poison. Exposure may be of an acute nature, leading to high concentrations and acute arsenic poisoning. Chronic exposure may lead to benign skin changes, skin cancer, and internal malignancy.

OBJECTIVE

Our purpose was to study the nature, incidence, and sequelae of skin disorders in a group of Argentinean patients suffering from chronic arsenicism due to the intake of contaminated well water.

METHODS

All patients who presented with chronic arsenicism at the Dermatology Department of Hospital Posadas (Buenos Aires, Argentina) during a 10-year period (1988-1998) were included in this study. The patient group compromised 9 women and 14 men, the age range was 37-72 years. Diagnosis was based on the clinical triad (keratoderma, leucoderma and epiteliomatosis). We performed clinical, laboratory, and histopathologic studies to confirm diagnosis. We screened for possible internal diseases.

RESULTS

All patients included in this study had cutaneous lesions associated with long-term arsenic exposure. The mean age of the patients was 58.2 years. The estimated mean time of the beginning of the lesions was of 3.7 years. All patients were Argentinean from endemic areas of our country where the arsenic levels are higher than those accepted by the World Health Organization.

CONCLUSION

This study allows us to conclude that the relationship between arsenic and cancer is frequent and it describes the principal characteristics of this entity in our group of patients.

Performance of different exposure assessment approaches in a study of bitumen fume exposure and lung cancer mortality

BACKGROUND

We compared performance of different exposure assessment approaches in a cohort study of cancer risk among European asphalt workers.

METHODS

Three bitumen fume exposure indices (duration of exposure (years), average exposure (mg/m3) and cumulative exposure (mg/m3*years)) and two latency models (with and without a 15 year lag) were considered for an association between lung cancer mortality and bitumen fume.

RESULTS

There was no association between lung cancer risk and either duration or cumulative exposure. However, there was the suggestion of an increase in lung cancer risk accompanying rise in average exposure. Only models with average bitumen fume exposure (with or without lag) improved model fit, albeit to the same extent.

CONCLUSIONS

Constructing quantitative indices of exposure intensity was justified because they produced the greatest improvement in fit of models that explored possible relationship between bitumen fume exposure and lung cancer risk. The identified associations require further investigation.