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PubMed2001

[Selection effects in epidemiological cohorts: nature, causes and consequences].

Selection effects in cohort studies occur when the population observed over time differs from the target population due to the inclusion or follow-up of the subjects. Selection effects may bias the estimation of the frequency of the disease and of the exposure of interest, or the estimation of the association between disease and exposure. In industrial cohorts, where subjects are not volunteers, the reasons why they were hired and they stay working in a company are often linked with lifestyle factors or health problems that can be associated with the diseases or the exposures of interest. This phenomenon is known as the "healthy worker effect". When the cohort includes volunteers, other selection effects associated with the reasons why the individuals participate in the study and stay in the cohort may occur. The time course of both exposure of interest and health status during follow-up may also be associated with the probability of staying in the cohort. We describe the main components of the various selection effects associated with inclusion of the subjects at baseline, with their follow-up during the observation of the cohort, and we examine the main components of the selection effects and their consequences.

Adult↗
PubMed1995

Airway responsiveness and job selection: a study in coal miners and non-mining controls.

BACKGROUND: It has been suggested that health related job selection is a major cause of the healthy worker effect, and may result in inaccurate estimates of health risks of exposures in the working environment. Improved understanding of self selection, including the role of airway hyperresponsiveness, should improve accuracy in estimating occupational risks. METHODS: We evaluated symptoms of the respiratory tract, lung function, occupational and smoking histories, and airway responsiveness from a cross sectional survey of 478 underground bituminous coal miners and non-mining controls. Workers with abnormal spirometry were excluded from methacholine testing. RESULTS: Methacholine responsiveness (> or = 15% decline in forced expiratory volume in one second) was associated in both miners and controls with reduced ventilatory lung function and an increased risk of respiratory symptoms. Miners with the longest duration of work at the coal face had a low prevalence of methacholine responsiveness, compared with miners who had never worked at the coal face (12% v 39%, P < 0.01). Throughout their mining careers, miners who responded to methacholine were consistently less likely to have worked in dusty jobs than miners who did not respond to methacholine. CONCLUSIONS: These results provide evidence that workers who are employed in dusty jobs are less likely than their unexposed coworkers to show increased non-specific airway responsiveness, presumably as a result of health related job selection. Surveys of workers in which responsiveness data are unavailable may underestimate the effects of dust exposure on respiratory health.

Adult↗
PubMed1995

Mortality amongst Paris fire-fighters.

This paper is the first mortality cohort study undertaken in France to examine the association between fire-fighting and cause of death. The cohort investigated in this study consisted of 830 male members of the Brigade des sapeurs-pompiers de Paris (BSPP). These professional had served for a minimum of 5 years on 1 January 1977. They were monitored for a 14 year period, finishing 1 January 1991. When compared to the average French male, the Paris fire-fighters were found to have a far lower overall mortality (SMR = 0.52 [0.35-0.75]). None of the cause specific SMRs were significantly different from unity. However a greater number of deaths than expected was observed for genito-urinary cancer (SMR = 3.29), digestive cancer (SMR = 1.14), respiratory cancer (SMR = 1.12) and 'cerebrovascular disease' (SMR = 1.16). The low overall SMR observed was consistent with the healthy worker effect. As for cause specific SMRs, they will be confirmed or invalidated by a further analysis as the follow-up of this cohort is being carried on.

Adult↗
PubMed1994

Mortality risk among elderly workers.

We analyzed population-based data for respondents aged 70 and older from the Longitudinal Study on Aging. We compared mortality risk among 358 baseline-year working with 4,373 nonworking respondents. Including other demographic and health status predictors in a multiple logistic regression model, employment remained a significant predictor of survival (mortality odds ratio (OR) = 0.4). Diabetes mellitus was the only factor that displayed a mortality risk that was significantly interactive with work (OR = 3.5). These data suggest that a healthy worker effect persists in older age groups, but that within the working stratum, patterns of risk may differ as compared to those among the nonworking.

Aged↗
PubMed1997

A meta-analysis of epidemiologic studies of lung cancer in welders.

OBJECTIVE: A meta-analysis of epidemiologic studies was carried out on lung cancer risk among shipyard, mild steel, and stainless steel welders, and the role of asbestos exposure and smoking was considered. METHODS: The meta-analysis consisted of calculating combined relative risks (RR) and their variances through a logarithm transformation of published RR values and a weighing using the inverted variance of each RR. RESULTS: The literature provided 18 case-referent and 31 cohort studies. The combined RR values were 1.38 [observed 1028, 95% confidence interval (95% CI) 1.29-1.48] for "all or unspecified welding categories", 1.30 (observed 305, 95% CI 1.14-1.48) for shipyard welders, and 1.35 (observed 173, 95% CI 1.15-1.58) for nonshipyard welders. Similar combined RR values (RR) were observed for mild steel welders (combined RR 1.50, observed 137, 95% CI 1.18-1.91) and stainless steel welders (combined RR 1.50 observed 114, 95% CI 1.10-2.05). No significant heterogeneity was discerned between studies of any welding or study design category. A marked healthy worker effect may also lead to an underestimation of the standardized mortality ratio for lung cancer among stainless steel welders. Furthermore, welders of any category are likely to be exposed to asbestos. Welders also seem to smoke more than the general male population, and therefore the hypothesis of tobacco overconsumption among welders could not be discarded. CONCLUSIONS: A 30-40% increase in the RR of lung cancer among welders cannot be explained by hexavalent chromium and nickel exposure among stainless steel welders. The combination of the carcinogenic effects of asbestos exposure and smoking may account for part of the lung cancer excess observed.

Asbestos↗
PubMed1996

Coal mine dust exposure and spirometry in experienced miners.

In a previous study of new miners from the National Study of Coal Workers' Pneumoconiosis (NSCWP), researchers examined changes in spirometry values associated with coal mine dust exposure (Br J Ind Med 1993; 50:929-937). An unusual pattern of dust-related effects was observed: initial sharp decrements in FVC and FEV1 were followed by partial recovery. In the current study, similar methods were used to analyze data from experienced miners. Each of 1,915 male subjects contributed data from two of the NSCWP field surveys: either Round 1 (1969-71) and Round 2 (1972-75) and Round 4 (1985-88). From the cross-sectional analysis at Round 1 or Round 2 (R1/R2), changes of +0.6 ml FVC and -0.5 ml FEV1 were associated with each mg/m3-yr of cumulative coal mine dust exposure, but were not statistically significant (p > 0.05). From the analysis of longitudinal change in spirometry from R1/R2 to Round 4 (R4), annual declines in FVC (-0.10 ml/yr per mg/m3-yr, p = 0.003) and FEV1 (-0.07 ml/yr per mg/m3-yr, p = 0.006) were associated with pre-R1/R2 exposure. Both the pattern and the magnitude of the exposure-response relationship were different for experienced versus new miners. Possible reasons for these contrasts include differences in cumulative exposure between the two groups and the healthy worker effect among experienced miners.

Coal↗
PubMed1997

[Survival after 60 years of age among elderly retired miners].

This study analyses long term effects of working conditions at an advanced age and post retirement. A longitudinal study was done among 63,000 retired miners between 60 to 65 years old. The standardised mortality ratio (SMR) in the cohort was above that expected on the basis of concurrent mortality in the reference population (SMR = 111; 95% confidence interval (CI): 108-114). The excess mortality was observed in occupational groups with exposure to underground tasks (10 years of underground tasks: RR = 2; 95% CI: .1.7-2.5), but there was sign of a healthy worker effect after 25 underground years. The effects of modern technologies and of medical progress have changed working conditions and care has reduced mortality for retired manual workers: exposure to underground tasks before 1950: RR = 1.7; CI 95%: 1.4-2.0; after 1950: RR = 1.2; CI 95%: 1.1-1.2.

Aged↗
PubMed1989

A study of mortality in workers engaged in the mining, smelting, and refining of nickel. I: Methodology and mortality by major cause groups.

Following the publication of the NIOSH nickel criteria document in 1977, the Joint Occupational Health Committee of the International Nickel Company (INCO) commissioned a mortality study of the company's Ontario workforce. This paper describes the detailed methodology and primary mortality results of the ensuing study; subsequent papers will describe more detailed findings of cause-specific mortality. An historical prospective mortality study of approximately 54,000 INCO workers has been conducted. Men with six months or more of service were followed for mortality during a 35-year period by computerized record linkage to the Canadian National Mortality Data Base. From a company-provided list of men known to have died and through independent follow-up of a random sample of 1,000 subjects of unknown status, we estimate a mortality ascertainment rate of 95%. Cause-specific standardized mortality ratios calculated with respect to Ontario provincial mortality rates indicate an excess of accidental deaths in men working in the Sudbury area and an excess of cancer deaths at the company's Port Colborne nickel refinery. A strong healthy worker effect was found for both all-disease mortality ad cancer mortality. The lower than expected mortality persisted for about 15 years beyond initial hiring.

Adolescent↗
PubMed1995

A cautionary note about the cross-national and clinical validity of cut-off points for the Maslach Burnout Inventory.

In the present study, burnout scores of three samples, as measured with the Maslach Burnout Inventory, were compared: (1) the normative American sample from the test-manual (N = 10,067), (2) the normative Dutch sample (N = 3,892), and (3) a Dutch outpatient sample (N = 142). Generally, the highest burnout scores were found for the outpatient sample, followed by the American and Dutch normative samples, respectively. Slightly different patterns were noted for each of the three components. Probably sampling bias, i.e., the healthy worker effect, or cultural value patterns, i.e., femininity versus masculinity, might be responsible for the results. It is concluded that extreme caution is required when cut-off points are used to classify individuals by burnout scores; only nation-specific and clinically derived cut-off points should be employed.

Adult↗
PubMed1993

The mortality of US nuclear submariners, 1969-1982.

A mortality study of 76,160 men who served on US nuclear submarines is reported. Indirect standardization was used to compare mortality rates to those of the US male population. Multiplicative models were developed to explore patterns of mortality within the cohort. Mortality rates for leukemia, acute myocardial infarction, and for motor vehicle accidents were equivalent to those of US males; rates for other causes were lower, generally consistent with the "healthy worker effect." Motor vehicle accident mortality dropped during the study period, perhaps reflecting efforts to control the problem. Suicide rates were depressed during the period of active duty. There was a suggestion that cancer mortality was associated with submarine type; however, the age distribution casts doubt that the excess was occupationally induced.

Adolescent↗
PubMed1988

Mixed solvent exposure and organic brain damage. A study of painters.

In recent years, the possible role of occupational solvent exposure as a cause of chronic effects on the brain has been a matter of increasing concern. The present study was undertaken with the purpose of further investigating this problem, trying to minimize confounding owing to effects of recent solvent exposure and a 'healthy worker' effect. This was accomplished by studying a historical cohort of painters. The material consists of a random sample of 85 painters, and as a non-exposed control group of 85 bricklayers, selected in the same way. Parts of the study are based on this sample supplemented by a sample of 9 painters and 14 bricklayers, selected among subjects with a high degree of complaints on mental impairment. On the basis of a detailed interview on previous solvent exposure, the painters were divided into three groups with low, medium and high solvent exposure, respectively. As signs of organic brain damage we used the degree of dementia and performance in psychometric tests as assessed at a neuropsychological examination; the degree of dyscoordination as assessed from clinical neurological tests; and the degree of cerebral atrophy as assessed from CT-scans of the brain. CT-scans were performed only on a specially selected subsample. The degree of dementia, dyscoordination, and cerebral atrophy increased significantly with the degree of solvent exposure. A similar, but non-significant association was found for the relation between performance in psychometric tests and solvent exposure. The degree of dementia, of dyscoordination, and of cerebral atrophy were positively associated with each other, suggesting that they reflect different aspects of a common underlying factor, an organic brain damage. A review of the literature compared with the results of the present study suggests that a number of studies may have been biased towards negative findings owing to the inclusion in the study material of large proportions of subjects with a solvent exposure that is too little to increase the risk of an organic brain damage. Conflicting results between different studies on performance in psychometric tests may also arise from insufficient adjustment for primary intellectual level.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed2004

[Uranium exposure and cancer risk: a review of epidemiological studies].

OBJECTIVE: At the end of 2000, certain diseases including leukemia were reported among soldiers who participated in the Balkan and in the Gulf wars. Depleted uranium used during these conflicts was considered as a possible cause. Its radiotoxicity is close to that of natural uranium. This paper reviews the epidemiological knowledge of uranium, the means of exposure and the associated risk of cancer. METHODS: The only available epidemiological data concerns nuclear workers exposed to uranium. A review of the international literature is proposed by distinguishing between uranium miners and other workers of the nuclear industry. French studies are described in details. RESULTS: In ionizing radiation epidemiology, contamination by uranium is often cited as a risk factor, but the dose-effect relationship is rarely studied. Retrospective assessment of individual exposure is generally insufficient. Moreover, it is difficult to distinguish between uranium radiotoxicity, its chemical toxicity and the radiotoxicity of its progeny. A causal relation between lung cancer and radon exposure, a gas derived from the decay of uranium, has been demonstrated in epidemiological studies of miners. Among other nuclear workers exposed to uranium, there is a mortality deficit from all causes (healthy worker effect). No cancer site appears systematically in excess compared to the national population; very few studies describe a dose-response relationship. CONCLUSION: Only studies with a precise reconstruction of doses and sufficient numbers of workers will allow a better assessment of risks associated with uranium exposure at levels encountered in industry or during conflicts using depleted uranium weapons.

Air Pollutants, Occupational↗
PubMed2000

Acute respiratory symptoms in workers exposed to vanadium-rich fuel-oil ash.

BACKGROUND: Occupational exposure to fuel-oil ash, with its high vanadium content, may cause respiratory illness. It is unclear, however, what early acute health effects may occur on the pathway from normal to compromised respiratory function. METHODS: Using a repeated measures design, we studied prospectively 18 boilermakers overhauling an oil-fired boiler and 11 utility worker controls. Subjects completed a respiratory symptom diary five times per day by using a 0-3 scale where 0=symptom not present, 1=mild symptom, 2=moderate symptom, and 3=severe symptom. Daily symptom severity was calculated by using the highest reported score each day for upper and lower respiratory symptoms. Daily symptom frequency was calculated by summing all upper or lower airway symptom reports, then dividing by number of reporting times. Respiratory symptom frequency and severity were analyzed for dose-response relationships with estimated vanadium and PM(10) doses to the lung and upper airway by using robust regression. RESULTS: During the overhaul, 72% of boilermakers reported lower airway symptoms, and 67% reported upper airway symptoms. These percentages were 27 and 36 for controls. Boilermakers had more frequent and more severe upper and lower respiratory symptoms compared to utility workers, and this difference was greatest during interior boiler work. A statistically significant dose-response pattern for frequency and severity of both upper and lower respiratory symptoms was seen with vanadium and PM(10) in the three lower exposure quartiles. However, there was a reversal in the dose-response trend in the highest exposure quartile, reflecting a possible healthy worker effect. CONCLUSIONS: Boilermakers experience more frequent and more severe respiratory symptoms than utility workers. This is most statistically significant during boiler work and is associated with increasing dose estimates of lung and nasal vanadium and PM(10).

Adult↗
PubMed1997

Mortality among a cohort of electric utility workers, 1960-1991.

Overall mortality trends among an electric utility workforce are examined. The study cohort (n = 40,335) included all workers with at least 1 year of work experience from 1960-1991; 3,753 deaths were observed in this cohort. Standardized mortality ratios (SMRs) and internal cohort analyses were used to assess mortality trends for the entire cohort and for specific occupational groups. Most SMRs were < or = 1.0 and were generally lower for noncancer (cardiovascular, COPD, and injuries) than for cancer mortality. Compared to office staff, rate ratios (RR) were higher for respiratory cancers for field staff [(RR = 2.3, 95% CI, 1.0-5.0) linecrew (RR = 2.2 95% CI, 1.5-3.1), and power plant occupations (RR = 2.4, 95% CI, 1.6-3.6)]. Nonmanagement occupations had rate ratios for motor vehicle injuries and all types of injuries, within a range of 2.5-4.7, with all lower CIs > 1.0. The healthy worker effect is an important factor in explaining the difference between SMR and internal cohort analyses results. The SMR results indicate that this workforce has lower rates for overall mortality, cardiovascular disease, cancer and nonintentional injury. A consistent finding in the internal cohort analyses that merits further research was higher mortality rates for respiratory cancer and injuries among nonoffice staff.

Accidents, Occupational↗
PubMed1994

Mortality among Icelandic seamen.

BACKGROUND: The aim of this cohort study was to evaluate the mortality of Icelandic seamen with particular reference to cancer deaths. SUBJECTS: In all 27,884 seamen, both fishermen and sailors of the merchant fleet, who had been members of a pension fund between 1958 and 1986 were followed-up. RESULTS: Most standardized mortality ratios (SMR) were greater than 1:1.26 for all causes, 1.13 for all malignant neoplasms, 1.80 for liver cancer, 1.19 for stomach cancer, 1.56 for lung cancer, and 1.21 for kidney cancer. The highest SMR was found for unknown causes, 3.16. There was no 'healthy worker effect'. For most causes of death latency time and mortality were strongly correlated. When analysing the relation between duration of employment and mortality a correlation was found for all-causes and for stomach cancer; however, this was not statistically significant. CONCLUSION: Seamen have an unusual mortality pattern and the excess for unknown causes is a rare finding indicating a peculiar situation for this group. The excess of stomach and lung cancer was high but it was not convincingly related to duration of employment and thus a relationship to occupation cannot be confirmed.

Adult↗
PubMed1995

Mortality patterns among women in the motor vehicle manufacturing industry.

This retrospective follow-up study evaluated the mortality experience of 11,271 women employed in a large motor vehicle manufacturing company in 1973. Company computerized personnel records were used to identify subjects and to obtain their employment histories. Follow-up through December 31, 1985, was conducted using company, state, and national data sources. The mortality rates of the cohort were compared with the rates of the U.S. general population of white or black women, adjusting for age and calendar time and using the standardized mortality ratio (SMR) as the measure of association. The overall mortality rate of the cohort was lower than the U.S. rate by 15% among whites (SMR = 85, 95% confidence interval (CI) = 78-93) and by 40% among blacks (SMR = 59, CI = 49-70). Among whites, the low mortality rate was due primarily to deficits of noncancer deaths, although a 32% deficit also was observed for breast cancer (SMR = 68, CI = 48-92). Black female workers had 40% fewer deaths than expected overall, and their all cancer SMR also was below expectation (SMR = 82, CI = 60-109). Despite evidence of a strong healthy worker effect among women in the motor vehicle manufacturing industry, the study found excesses of lung cancer among white women in assembly jobs (SMR = 158, CI = 107-226) and of pancreatic cancer among white assembly workers in upholstery and trim operations (SMR = 302, CI = 97-704).

Adult↗
PubMed2003

[Mortality among active workers at EDG-GDF: social and occupational disparities and evolution].

BACKGROUND: Two studies, conducted in the eighties and the nineties, reported that mortality was lower in the French national electric and gas company (EDF-GDF) the utility workers than in the general French population. The purpose of our study was to compare the mortality of these 140,000 utility workers to that of the French population of the same age for the period from 1997 to 2001. Secondly we aimed to assess the disparities of mortality among the workforce according to demographic and socioprofessional criteria. Finally, we analyzed the evolution of this mortality over the last twenty years. METHODS: We undertook a cross-sectional study using the indirect standardization method. RESULTS: Risk of death was lower in the utility workers than in the French population (SMR=54% for men, and 58% for women). These differences had increased over the last twenty years for each main cause of death (cancers, cardio-vascular diseases and accidents). The reported excess of brain cancer death risk raises etiologic questions. Moreover, we noticed important social disparities in the workforce, increasing with time. CONCLUSION: The well-known "healthy worker effect" seems to be particularly important in the EDF-GDF company. The workers have a high level of living conditions and good working conditions, although internal social disparities persist. Further studies in other large corporations are greatly needed for comparisons.

Accidents↗
PubMed1995

PMR study of mortality among Alabama workers and farmers.

Computerized records of all Alabama deaths occurring to persons 16 years of age and over for the 5-year period 1984-1988 were obtained from the Alabama Department of Public Health. Using proportionate mortality ratio (PMR) methodology and death certificate occupation, cause-specific mortality patterns were examined for all Alabama decedents (N = 182,178), for all Alabama workers (N = 125,369), and for the occupational group of "farm operators and managers" (N = 11,691). In comparison with the U.S. general population, little difference was found between cause-specific PMR results for the total Alabama population and those for all workers, suggesting the absence of a generalized "healthy worker effect." In comparison with the U.S. population, PMR results for farmers suggested lowered mortality from all malignant neoplasms and all heart disease, and elevated mortality from all external causes of death. In comparison with the Alabama population, PMR results for farmers continued to suggest lowered mortality from all malignant neoplasms, specifically for cancers of the respiratory, digestive and lymphopoietic systems. However, significantly elevated PMRs for external causes of death appeared only among nonwhite female farmers. Further investigation of these deaths suggested that this finding was more likely attributable to an increased risk of housefire deaths associated with rural lifestyle than with occupational factors.

Accidents, Home↗