Significant social differences were observed for premature and total mortality in this 12.5-year follow-up study among the French working population. Manual workers were at increased risk of total and premature mortality compared to managers/professionals with HRs reaching almost 2. Occupational factors played a substantial role in explaining social differences in mortality. Their contributions were 31–74%, and were more pronounced for men and for premature mortality. The contribution of behavioural factors was very low.
Manual workers were the occupational group that displayed a significant excess of mortality compared to managers/professionals. Other previous studies showed social disparities in mortality in France and in other countries, using various SES markers [12
]. Our study also underlined social inequalities in occupational exposures, with the lowest occupational groups, especially manual workers, being more likely to be exposed to negative working conditions. Other previous studies reported the accumulation of unfavourable working conditions in the lowest occupational categories [22
]. One major exception was psychological demands, that displayed a strong inverse social gradient, managers/professionals being more likely to be exposed, something already reported [22
Although there were a number of studies describing social inequalities in mortality in various populations, studies that tried to explain these inequalities were less numerous. This study is one of the seldom studies evaluating the contribution of occupational factors to social inequalities in all-cause mortality, and suggested that these factors may play a substantial role.
Other studies explored the contribution of occupational factors to social inequalities in several measures of morbidity. Our study is in agreement with some previous studies underlying the role of physical and biomechanical exposure [22
], job insecurity [21
], and low social support [25
] in explaining social inequalities in health outcomes such as self-reported health. Furthermore, the occupational factors, that were the most prevalent, may play a substantial role in explaining social differences in mortality, i.e. standing/walking, awkward posture, and manual materials handling among the biomechanical factors, and noise, and hot and cold temperatures among the physical factors, supporting previous results on the explanation of social inequalities in morbidity outcomes in France [25
]. Our results are also in agreement with another study showing that the role of occupational factors in explaining social inequalities in health was not modified very much when behavioural factors were taken into account [22
]. The issue of independent (direct) and indirect (through behavioural factors) effects of occupational factors is consequently less important in our study as we did not observe any major role of behavioural factors. Consequently, the contribution of occupational factors remained almost the same with or without adjusting for behavioural factors.
Behavioural factors did not play an important role in explaining social inequalities in mortality in our study. Other authors demonstrated that behavioural factors may explain only a modest proportion of social inequalities in mortality [10
]. Several hypotheses may be assumed to explain this. Behavioural factors were evaluated only at baseline, and as the follow-up was long, people might change their behaviours, which is likely to lead to misclassification and dilution of their effects. Behavioural factors were based on self-reported data, that may lead to an underreporting bias of the most negative health behaviours. For example, the heaviest drinkers may be underrepresented in our sample, because of both selection and underreporting bias. Evaluation of alcohol consumption was done using the CAGE instrument that may be adequate to measure alcohol-related problems, but may neglect some specific ones that may be more strongly related to SES, such as binge drinking.
Gender differences were also of interest in our study, this was why genders were studied separately. Women were more likely to be service workers/clerks, and men manual workers. The prevalence of occupational and behavioural factors was found to be different between genders. Men were more likely to be exposed to physical exposure, and women to job insecurity. Men were more likely to be smokers and overweight, and to have alcohol-related problems. The risk of mortality was also higher for men than for women. These results confirm the different patterns of occupation and occupational exposures between genders, related to the strong sexual division of labour, as well as the differences in health behaviours and mortality between genders. Similar social inequalities in mortality were observed for men and women, but the contribution of occupational factors was found to be higher in men than in women. This result is in agreement with other studies [25
]. Strong gender differences were observed for the associations between behavioural factors and mortality; smoking and alcohol abuse were found to be strong predictors of premature and total mortality for men, but not for women. Nevertheless, the contribution of behavioural factors was very modest and appeared to be similar in explaining social inequalities in mortality in both genders.
Limitations of our study may be mentioned. A selection bias may have occurred, as the response rate was about 44%. However, this response rate is similar to those of other studies using postal self-administered questionnaires in France [34
]. Furthermore, the gender and age distributions of the initial sample were close to those of the census population. Nevertheless, previous studies showed that non-respondents may be more likely to have lower SES, poorer health-related and behavioural factors [34
]. Consequently, it is likely that such a bias may lead to underestimate social inequalities in health. A limitation was related to sample size especially for women, and led to more uncertainty in the estimation of HRs and explained fractions for this group. Another limitation was that behavioural and occupational factors were not based on lifetime exposures. Other authors demonstrated that this may lead to underestimate the contributions of behavioural and occupational factors to social inequalities in health [35
]. The contribution of these factors may also be underestimated because some behavioural and occupational factors were not explored, such as diet or physical activity, as well as chemical/biological exposures, decision latitude at work, reward, or workplace violence. Thus, inclusion of more mediators might result in different estimates of the contributions of mediators. Finally, the generalisation of our results to other populations should be made with caution because of cultural and socioeconomic differences between countries.
Strengths of the study also deserve to be mentioned. The sample was derived from the general population, making generalisation possible for the population in the nord-east of France. Sample size allowed us to study men and women separately, which may be crucial in occupational epidemiology [37
]. The study was based on a 12.5-year follow-up, i.e. a rather long period. Mortality was measured using national database (an exhaustive and independent source of data). Mortality is also an objective outcome measure, consequently no reporting bias may be suspected. Occupational groups were used in this study as a marker of social position, and are a well-known measure of social position in the working population. Although results may differ somewhat using other measures of social position (such as education or income) [5
], relatively similar conclusions have been provided by others [24
]. We performed additional analyses that included the presence of chronic disease at baseline in our models to make sure that no previous chronic disease may introduce a confounding effect in our results. These results confirmed the robustness of our findings. We also performed the analyses for premature mortality before 65 and found similar results, but statistical power was lower because of a smaller number of premature deaths.
To conclude, occupational factors may play a substantial role in explaining social inequalities in mortality, especially premature mortality. Preventive actions focusing on these factors and specific social groups may be useful to reduce social inequalities in mortality. More research is needed to better understand the role of these factors, over the life course, on social inequalities in various health outcomes.