© 2005 American Public Health Association DOI: 10.2105/AJPH.2004.048835
Maria Melchior, Isabelle Niedhammer, and Marcel Gold-berg are with the National Institute of Health and Medical Research, Saint-Maurice, France. Nancy Krieger, Ichiro Kawachi, and Lisa F. Berkman are with the Department of Society, Human Development and Health, Harvard School of Public Health, Boston, Mass. Correspondence: Reprint requests should be sent to Maria Melchior, INSERM, U687-IFR69, HNSM, 14, rue du Val dOsne, 94415 Saint-Maurice, France (e-mail: maria.melchior{at}st-maurice.inserm.fr).
Objectives. To estimate the contribution of stress-related and physical work factors to occupational class disparities in sickness absence from work. Methods. Our sample consisted of 8847 men and 2886 women participating in the French GAZEL cohort study. Occupational class and medically certified sickness absence data (19952001) were obtained from the participants employer. Work characteristics (physical and stress-related) were self-reported. We calculated rate ratios with Poisson regression models; fractions of sickness absence attributable to work factors were estimated with the Miettinen formula. Results. Sickness absence was distributed along an occupational gradient. Work characteristics accounted for 19% (women) and 21% (men) of all absences. Physical work conditions accounted for 42% and 13% of absences for musculoskeletal reasons, and work stress accounted for 48% and 40% of psychiatric absences. Overall, about 20% of the occupational class gradient in sickness absence could have been associated with deleterious work conditions. Conclusion. Work conditions contribute to sickness absence, particularly among manual workers and clerks. Policies that decrease ergonomic constraints and work stress also could reduce the burden of ill health and sickness absence among the lowest strata of working populations.
Among working populations, occupational hazards and job stress may contribute to occupational class disparities in health.1 Specifically, adverse work conditions may influence the risk of musculoskeletal disorders, psychiatric symptoms, and injury that occur frequently among middle-aged populations and may constitute some of the leading reasons for taking sick leave.2,3 There is some evidence that job stress contributes to occupational class differences in both health and sickness-related absence from work (sickness absence)4,5; however, little research has examined the contribution of other work conditions. To date, only 2 studies have investigated the joint contribution of physical and psychosocial work characteristics to occupational class health disparities; however, both were cross-sectional and used self-reported health as an outcome.6,7 In a previous analysis of the GAZEL cohort study, we showed that job stress is most prevalent among manual workers and office clerks and predicts the occurrence of sickness absence.8 In this study, we examined the contribution of both stress-related and physical work exposures to the occupational class gradient in overall and cause-specific sick leave.
Study Population The GAZEL cohort study began in 1989, when 44 922 employees of Frances national gas and electricity company, Electricité de France-Gaz de France (EDF-GDF), were asked to participate in a long-term observational study. Forty-five percent of those eligible14 752 men and 5317 womenaccepted. At baseline, men were aged 40 to 50, and women were aged 35 to 50 years old. Women represented only 20% of company employees and therefore were oversampled.9 EDF-GDF employees hold a civil servantlike status that entails job stability and opportunities for occupational mobility. Typically, employees are hired when they are in their 20s and stay with the company until retirement (usually around 55 years of age). Retirees pensions are paid by the company. Because of these characteristics, study follow-up is very thorough: since baseline (1989), less than 1% of participants were lost to follow-up (39 left the company, and 19 withdrew from the study). GAZEL participants are followed with an annual mailed survey, which is usually completed by 75% of the cohort.9 Additionally, participants records are linked to validated occupational and health data collected by the company, including medically certified sickness absence. In this study, we analyzed data from GAZEL participants who responded to the 1995 survey (11183 men and 4095 women; 75% of the original cohort), which included measures of job stress. We excluded respondents who had retired (n=2304) or who had incomplete work exposure data, which left a sample of 8847 men and 2886 women. The study population was healthier than GAZEL participants who did not complete the 1995 questionnaire.8 Person-time of follow-up with regard to the outcomesickness absencewas accrued from the date of completion of the 1995 GAZEL survey until the date of retirement, death, withdrawal from EDF-GDF or the GAZEL cohort, or December 31, 2001 (whichever occurred first).
Measures
Other explanatory variables were collected from GAZEL surveys. Physical work characteristics, including postural constraints (7 items), occupational hazards (5 items), night work (yes or no), and outdoor work activities (never, sometimes, > 50% of the time) were measured in 1990 (Table 1
Job stress measures, which were based on the work of Karasek and Johnson,10,11 were obtained in 1995: control over the content and the execution of work-related tasks (6 items); psychological demands, evaluated work load, and time pressures (5 items); and social support received from colleagues (5 items). Each summary scale showed satisfactory factorial validity and adequate internal consistency reliability, with Cronbach coefficients of 0.65 for decision latitude, 0.69 for psychological demands, and 0.52 for work social support (our measure of work social support showed lower reliability than in other studies because it included fewer items).12 After verifying that the association between quartiles of psychosocial work factors and sickness absence was graded (data not shown), we dichotomized each scale at its median value.13 To examine the contribution of work factors to the occupational gradient, we modeled job stress factors as continuous variables.14
Demographic and behavioral characteristics measured in 1995 included age (4549, 5054, and 5556 years for men, and 4244, 4549, 5054, and 5556 years for women), marital status (married/living with a partner, single, or divorced), current smoking (none vs at least 1 cigarette per day), alcohol consumption in drinks per week (none, light [113 for men, 16 for women], intermediate [1427 for men, 720 for women], or heavy [
Sickness Absence Data
Statistical Analysis The contribution of work factors to the occupational class gradient in sickness absence was evaluated in 4 steps. First, with managers as the reference category, we calculated rate ratios across occupational groups and adjusted for age, demographic characteristics, and health behaviors (Model 1). Next, we successively added physical work factors (Model 2) and job stress (Model 3) before including all work and adjustment variables in a single statistical model (Model 4). The contribution of work factors to the occupational gradient was measured by fitting a linear term for occupational class and comparing Models 4 and 1 (we show Models 1 and 4; more detailed data are available upon request). Additionally, we estimated the fraction of sickness absence attributable to work factors with the Miettinen formula (attributable fraction = [RR1] / RR [no. exposed cases / no. cases]).17 For each work exposure statistically significant in Model 4, the attributable fraction was calculated controlling only for adjustment variables; the contribution of all work factors was calculated controlling for all work factors and adjustment variables.
We verified the consistency of the results in a subsample restricted to participants who worked in the same occupation in both 1990 and 1995 (n = 8830) by using different lengths of sickness absence ( All analyses were conducted separately for men and women with SAS, version 8.2 (SAS Institute Inc, Cary, NC); log-linear Poisson regression models were fitted with the PROC GENMOD procedure.18
The study population included 8847 men and 2886 women, on average, aged 50 years (range = 4756) and 48 years (range = 4256), respectively (Table 1 Over the 6 years of follow-up (mean = 4.8, SD = 2.0 for men; mean = 5.8, SD = 1.72 for women), there were 18 818 absences among men and 15 803 female absences. On average, men experienced 47 absences per 100 person-years of observation, and women experienced 95; corresponding median numbers of sick leave days were 8 and 36, respectively. Most absences (58%) lasted 7 days or less, 26% lasted 8 to 21 days, and 17% lasted more than 21 days. Respiratory illness (14%) and musculoskeletal disorders (14%) were the leading causes of absence, followed by psychiatric reasons (7%) and injury (6%).
Manual workers and office clerks more frequently reported job stress and physical work exposures (Table 1
As expected, sickness absence occurred along an occupational gradient (Figure 1
Adjustment for all work factors reduced the occupational class gradient in all-cause sickness absence by 16% for men and 25% for women; musculoskeletal absences were reduced by 27% and 25%, and psychiatric absences were reduced by 10% and 40%. Work factors contribution was greatest with regard to men and womens gradient in absences of 8 to 21 days (21% and 27%, respectively) and was less for short and long absences (13 days and 26 days; 9% and 20%, respectively). Associations between occupational class and work factors and sickness absence(s) were weaker among healthier participants than among the full sample. All-cause absenteeism rates among manual workers and clerks were 2 times higher than among managers, which is 30% lower than among the entire study population, and work factors contribution to men and womens occupational gradients were 35% and 70% lower.
Across occupational groups, 21% of mens and 19% of womens all-cause sickness absences were attributable to work factors (Table 3
Main Results Overall, 19% to 21% of all-cause sickness absences and 16% to 25% of the occupational class gradient in absenteeism were related to adverse work conditions. Adding to previous research,19 we found that occupational gradients in cause-specific sickness absences were associated with physical and psychosocial work exposures. Occupational class differences in sickness absences due to injury were associated with physical work exposures, and work stress contributed to psychiatric sickness absence gradients. Sickness absence due to musculoskeletal reasons reflected both physical and stress-related work exposures.
Limitations A second limitation is that physical work exposures were obtained 5 years before the beginning of follow-up, which may have resulted in measurement error. About 80% of participants held the same job in both 1990 and 1995, and results among this subgroup were comparable to the full sample, which is reassuring. However, it is possible that levels of exposure change over time, even within occupational groups, which may have reduced the precision of our estimates. Third, to calculate attributable fractions, we dichotomized work-stress variables at their median value. Although this is the standard method used in the field, we recognize that it makes comparisons with other studies difficult. To our knowledge, work stress in GAZEL was as frequent as in other studies,22 and our results are valid among other populations. However, identifying meaningful work-stress exposure thresholds, which would make comparisons across study populations more straightforward, is an important goal for future research. Finally, we studied employees of a large public sector company who were healthier than the general population of France,23 and we probably underestimated both occupational class differences in sickness absence and the role of work factors. At the same time, GAZEL participants jobs are not at stake if they take sick leave, and they may be less reluctant to be absent from work when ill than men and women who are in more unstable job situations.24 Other large cohorts, such as the Whitehall II study, faced similar issues.4 In our study, sickness absence rates were somewhat lower than among the White-hall II cohort, but not by very much (47 and 95 absences per 100 person-years for GAZEL men and women, respectively, compared with 70 and 120 absences, respectively, in the Whitehall II study). Therefore, our findings can be compared with previous reports from that cohort.4 Yet, more broadly, it is important to recognize that sickness absence patterns vary across work sectors, workplaces, time periods, countries, and study populations, and their association with occupational factors needs to be studied among other working populations.
Gender Differences
Work Factors and Sickness Absence The health effects of work stress may be due to direct and indirect mechanisms. The specific pathways of musculoskeletal problems are not yet well understood, but increased muscle tension and the inability to take necessary breaks from work are the most likely explanations.25 Low job control and insufficient support from colleagues and supervisors may directly undermine psychological well-being and thus increase the risk for depression5 while simultaneously affecting health behaviors (e.g., cigarette smoking, alcohol consumption, and behaviors that result in being overweight).26,27 Manual workers and clerks are simultaneously exposed to a variety of deleterious work factors, some of which can be interrelated. For example, occupational hazards may cause stress. In our study, work variables showed at most modest collinearity, and we chose to include them jointly in our statistical models. We assumed that the effects of physical and stress-related factors were additive; however, we acknowledge that other approaches are possible (e.g., multiplicative models). An important question when interpreting our results is whether the association between work factors and sickness absence can be considered causal. Indeed, it may be that individuals whose work conditions are the worst and who are employed in subordinate jobs are also exposed to nonwork situations associated with sickness absence (e.g., comorbidity or a lack of personal social support). More broadly, thought needs to be given to the complex associations between work exposures, nonwork characteristics, and sickness absence, including their patterning along occupational class lines.
Sickness Absence as a Measure of Health Despite potential biases that may have affected reports of sickness absence, this indicator bears public health relevance because it reflects individuals general physical, psychological, and social well-being8,2830 and collective workplace factors (e.g., it is lower in workplaces that have equitable policies).31 Organizations and employees are embedded within a broader social, political, and economic context, and sickness absence also reflects the generosity of sick leave provisions and macroeconomic trends (e.g., downsizing of firms and contingent job insecurity).32,33 The population effects of these macrolevel factors, which we did not take into account because we restricted our study to middle-aged employees of a single company with high levels of job security, deserve further research attention.
Health Selection into Occupational Groups
Conclusions
The authors wish to thank EDF-GDF and the men and women who participate in the GAZEL study. We particularly thank the Service des Etudes Médicales, Service Général de Médecine de Contrôle, who collected the sickness absence data. We are grateful to the GAZEL cohort study team, particularly Sébastien Bonenfant, who is responsible for data management. Additionally, we thank Alice Guégen for statistical advice, and David Ellwood, Annette Leclerc, Doris V. Báez-Feliciano, and an anonymous reviewer for their comments on previous versions of the article.
Human Participant Protection
Peer Reviewed
Contributors Accepted for publication October 12, 2004.
1. Goldberg M, Melchior M, Leclerc A, Lert F. Epidémiologie et déterminants sociaux des inégalités de santé. Rev Epidemiol Sante Publique. 2003;51: 381401.[Web of Science][Medline]
2. Hoogendoorn WE, Bongers PM, de Vet HC, Ariens GA, van Mechelen W, Bouter LM. High physical work load and low job satisfaction increase the risk of sickness absence due to low back pain: results of a prospective cohort study. Occup Environ Med. 2002; 59:323328.
3. Paterniti S, Niedhammer I, Lang T, Consoli SM. Psychosocial factors at work, personality traits and depressive symptoms. Longitudinal results from the GAZEL Study. Br J Psychiatry. 2002;181:111117.
4. North F, Syme SL, Feeney A, Shipley M, Marmot MG. Psychosocial work environment and sickness absence among British civil servants: the Whitehall II study. Am J Public Health. 1996;86: 332340. 5. Stansfeld SA, Head J, Marmot MG. Explaining social class differences in depression and well-being. Soc Psychiatry Psychiatr Epidemiol. 1998;33:19.[CrossRef][Web of Science][Medline]
6. Schrijvers CT, van der Mheen HD, Stronks K, Mackenbach JP. Socioeconomic inequalities in health in the working population: the contribution of working conditions. Int J Epidemiol. 1998;27:10111018. 7. Borrell C, Muntaner C, Benach J, Artazcoz L. Social class and self-reported health status among men and women: what is the role of work organisation, household material standards, and household labour? Soc Sci Med. 2004;58:18691887.
8. Melchior M, Niedhammer I, Berkman LF, Gold-berg M. Psychosocial work factors, social relations, and sickness absence: a 6-year prospective study of the GAZEL cohort. J Epidemiol Community Health. 2003; 57:285293. 9. Goldberg M, Leclerc A, Chastang J, Morcet J, Marne M, Luce D. Mise en place dune cohorte épidémiologique à Electricité de FranceGaz de France: principales caractéristiques de léchantillon. Rev Epidemiol Sante Publique. 1990;38:378380. 10. Karasek R, Theorell T. Healthy Work: Stress, Productivity and the Reconstruction of Working Life. New York, NY: Basic Books; 1990. 11. Johnson J, Hall E, Theorell T. Combined effects of job strain and social isolation on cardiovascular disease morbidity and mortality in a random sample of the Swedish male working population. Scand J Work Environ Health. 1989;15:271279.[Web of Science][Medline]
12. Stansfeld SA, Bosma H, Hemingway H, Marmot MG. Psychosocial work characteristics and social support as predictors of SF-36 health functioning: the Whitehall II study. Psychosom Med. 1998;60: 247255. 13. Landsbergis P, Theorell T. Measurement of psychosocial workplace exposure variables. Occup Med. 2000;15:163188. 14. International Classification of Diseases. Manual of the International Statistical Classification of Diseases, Injuries and Causes of Death. Geneva, Switzerland: World Health Organization; 1977. 15. International Classification of Diseases: 10th Revision. Geneva, Switzerland: World Health Organization; 1992.
16. Niedhammer I, Bugel I, Goldberg M, Leclerc A, Guegen A. Psychosocial factors at work and sickness absence in the Gazel cohort: a prospective study. Occup Environ Med. 1998;55:735741.
17. Hanley J. A heuristic approach to the formulas for population attributable fraction. J Epidemiol Community Health. 2001;55:508514. 18. SAS Institute. SAS/STAT Software: Changes and Enhancements Through Release 6.12. Cary, NC: SAS Institute; 1997. 19. North F, Syme SL, Feeney A, Head J, Shipley MJ, Marmot MG. Explaining socioeconomic differences in sickness absence: the Whitehall II study. Br Med J. 1993;306:361365. 20. Niedhammer I. Psychometric properties of the French version of the Karasek Job Content Questionnaire: a study of the scales of decision latitude, psychological demands, social support and physical demands. Int Arch Occup Environ Health. 2002;75:129144.[CrossRef][Web of Science][Medline] 21. Césard M, Dussert F. Le travail ouvrier sous contrainte. In: INSEE, ed. Données Sociales. Paris, France: INSEE; 1993:202211. 22. Karasek R, Brisson C, Kawakami N, Houtman I, Bongers P, Amick B. The Job Content Questionnaire (JCQ): an instrument for internationally comparative assessments of psychosocial job characteristics. J Occup Health Psychol. 1998;3:322355.[CrossRef][Medline]
23. Goldberg M, Chastang J-F, Leclerc A, Zins M, Bonenfant S, Bugel I. Socioeconomic, demographic, occupational and health factors associated with participation in a long-term epidemiological survey. A prospective study of the French Gazel cohort and its target population. Am J Epidemiol. 2001;154:373384.
24. Aronsson G, Gustafsson K, Dallner M. Sick but yet at work. An empirical study of sickness presenteeism. J Epidemiol Community Health. 2000;54:502509.
25. Leclerc A, Chastang J-F, Niedhammer I, Landre M-F, Roquelaure Y. Study Group on Repetitive Work. Incidence of shoulder pain in repetitive work. Occup Environ Med. 2004;61:3944. 26. Sorensen G, Stoddard A, Hammond SK, Hebert JR, Avrunin JS, Ockene JK. Double jeopardy: workplace hazards and behavioral risks for craftspersons and laborers. Am J Health Promotion. 1996;10:355363.[Web of Science][Medline] 27. Brisson C, Larocque B, Moisan J, Vezina M, Dagenais G. Psychosocial factors at work, smoking, sedentary behavior, and body mass index: a prevalence study among 6995 white collar workers. J Occup Environ Med. 2000;42:4046.[Web of Science][Medline] 28. Kivimäki M, Head J, Ferrie JE, Shipley MJ, Vahtera J, Marmot MG. Sickness absence as a global measure of health: evidence from mortality in the Whitehall II prospective cohort study. Br Med J. 2003; 327:16. 29. Kristensen TS. Sickness absence and work strain among Danish slaughterhouse workers: an analysis of absence from work regarded as coping behaviour. Soc Sci Med. 1991;32:1527.
30. Rael E, Stansfeld S, Shipley M, Head J, Feeney A, Marmot M. Sickness absence in the Whitehall II study, London: the role of social support and material problems. J Epidemiol Community Health. 1995;49:474481.
31. Kivimäki M, Elovainio M, Vahtera J, Ferrie JE. Organisational justice and health of employees: prospective cohort study. Occup Environ Med. 2003;60:2733.
32. Kivimäki M, Vahtera J, Ferrie J, Hemingway H, Pentti J. Organisational downsizing and musculoskeletal problems in employees: a prospective study. Occup Environ Med. 2001;58:811817. 33. Knutsson A, Goine H. Occupation and unemployment rates as predictors of long-term sickness absence in two Swedish counties. Soc Sci Med. 1998;47:2531.
34. Ribet C, Zins M, Guéguen A, et al. Occupational mobility and cardiovascular risk factors in working men: selection, causality, or both? Results from the GAZEL study. J Epidemiol Community Health. 2003; 57:901906.
35. Power C, Matthews S, Manor O. Inequalities in self-rated health in the 1958 birth cohort: lifetime social circumstances or social mobility? Br Med J. 1996; 313:449453. 36. Hallqvist J, Lynch JW, Bartley M, Lang T, Blane D. Can we disentangle life course processes of accumulation, critical period and social mobility? An analysis of disadvantaged socio-economic positions and myocardial infarction in the Stockholm Heart Epidemiology Program (SHEEP). Soc Sci Med. 2004; 8:15551562. This article has been cited by other articles:
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||