Design
Between 2013 and 2018, 2,546,802 subjects were surveyed in a repeated-cross sectional survey conducted with a representative sample of the French population. The questionnaire [36] was administered face to face (for the first and the final measurement) and by telephone (for all other measurements). Since we cannot determine the number of people contacted people who fulfilled our inclusion criteria in this dataset, the response rate cannot be calculated. Nevertheless, the INSEE indicates a response rate of 80%, and we have no reason to believe that our response rate is different.
Initial Population and Sample
A sample of workers characterised by different workplace locations was extracted from the labour survey. Our sample fulfilled the following inclusion criteria: aged between 20 and 60 years, employed workers according to the International Labour Office (ILO) definition and residing in the French territory within the French cross-border departments (see Figure 1: The French cross-border regions.). The French state is decomposed into three administrative districts: the ‘Communes’ (town), the ‘Départements’ (county) and the ‘Régions’ (states) (UK regions) (there are 34,970 Communes, 101 Départements (called departments from now on) and 13 Régions). All departments in which at least 50 CBWs lived and in which CBWs represent at least 1% of the workforce were included in the analyses. Four commute destinations were retained, namely Germany (DE), Belgium (BE), Switzerland (CH) and Luxembourg (LU), since these countries attracted 92% of the French’s CBWs [37]. The final sample was composed of 34,362 workers.
Data Collection
Since 2013, the survey integrated information about workers’ health, and the module assessing this is composed of four questions (see health variables). Between 2013 and 2018, the number of CBWs per year (e.g. 490 for 2015) leads to combine the labour survey individual folders for years and the health questions. The labour survey benefits from the approval of the Institutional Review Board called Comité du Label de la Statistique Publique, which depends on the Conseil National de l’Information Statistique (CNIL). The questionnaire of the mother study has been previously published and only the relevant questions were kept.
Exposure Variables: ‘CBW status’
Two parameters were considered to describe the worker’s situation, his commuting status and his country of destination. A worker can decide either to work in France or to commute abroad.
Health Outcomes
Four questions assessing health outcomes were included in the labour survey and one question not directly linked to health issues was used to generate a variable relating to leisure activity participation.
Perceived Health Status
- Low perceived health: This subjective scale included one item: ‘How do you rate your overall health?’ and responses options were: (1) very good, (2) good, (3) fair, (4) poor, and (5) very poor, (6) refusal and (7) do not know.
For the statistical analyses, the score was dichotomised: (3), (4) and (5) were coded ‘low self-perceived health’ following the recommended cut-off scores [24; 25]. (6) and (7) were coded as missing values.
Physical Health Factors
- Activity limitation: ‘Have you experienced restrictions in performing activities that people typically do because of a health problem for at least six months?’. Response options were: (1) yes heavily restricted, (2) yes, limited but not strongly, (3) no, not limited at all, (4) refusal and (5) do not know.
Responses were coded: (1) and (2) ‘limitations due to health reasons’ and response (3) was coded as ‘no limitations due to health reasons’. (4) and (5) were coded as missing values.
- Chronic diseases: ‘Do you have an illness or health problem that is chronic or of a lasting nature?’ Responses options were: (1) yes, (2) no, (3) refusal and (4) do not know.
Responses were coded: 1) ‘having a chronic disease’ vs. 2) ‘no chronic disease’. Responses 3) and 4) were coded as missing values.
- Disability: ‘Is your disability or loss of autonomy recognised by the administration?’ Response options were: (1) yes, (2) demand under review, (3) no, (4) refusal and (5) do not know.
Responses were coded: (1) and (2) ‘handicapped’ vs. (3) ’not handicapped’. Responses (4) and (5) were coded as missing values.
- No leisure activities: ‘During the past three months, did you take sports lessons or courses related to cultural or leisure activities?’ Response options were (1) yes and (2) no.
Responses were coded: (2) ‘no physical or cultural activities’ vs. (1) ‘physical or cultural activities’.
Covariates
Consolidated variables are those composed by the respondents’ answers to several questions.
Demographic background
- (Var.1) Sex: (2 categories: men women)
- (Var.2) Age: (4 categories: 20-29, 30-39, 40-49, 50-60)
- (Var.3) Education: (3 categories: up to secondary school, up to Bachelor’s degree, Master’s degree & above)
- (Var.4) Occupational category: (5 categories: white collars, intermediate professions, employees, blue collars)
- (Var.5) Father’s occupational category: assessed at the end of the respondent’s own schooling. (6 categories: farmers; artisans, merchants, company directors; white collars; intermediate professions; employees, blue collars)
- (Var.6) Born abroad: (2 categories: born in France born abroad) Respondents were asked ‘Where were you born?’. Responses options were 1) in France and 2) not in France.
- (Var.7) Cohabiting: (2 categories: living together with someone living alone) Respondents were asked ‘Are you living together with someone in one household?’. Response options were (1) yes and (2) no.
- (Var.8) Children: (2 categories: has a child(ren) does not have child(ren)) Respondents were asked ‘Do you have children in the household or in alternate care?’. Response options were (1) yes and (2) no.
- (Var.9) Departments: (11 categories: departments of residence (see Figure 1: The French cross-border regions)
- (Var.10) Urban area: (2 categories: place of residence located in an urban area with fewer than 200,000 inhabitants with 200,000 inhabitants or more)
Sex, age, education, occupational category, father’s occupational category, departments, and urban area are consolidated variables.
Labour status
- (Var.11) Permanency of the job: (3 categories: permanent contract, temporary contract, interim contract)
- (Var.12) Sector: (9 categories: agriculture; industry-construction; trade-transport-accommodation and catering; information and communication; finance and insurance; real estate; scientific activities and administrative services; public administration; other services)
- (Var.13) Number of persons working at the local unit: (4 categories: 1 to 9 workers, 10 to 49 workers, 50 to 499 workers, 500 workers and more). Respondents were asked ‘How many employees are approximately on the site which employs you?’.
- (Var.14) Wage: (3 categories: up to 2,000 net € per month, premiums included, between 2,001 and 4,000€, 4,001€ and higher) Nominal wage.
- (Var.15) Full-time/part-time employment: (2 categories: full-time employment part-time employment)
- (Var.16) Overtime: (2 categories: overtime no overtime) Respondents were asked ‘How many extra hours do you usually work per week in your professional job?’.
Permanency of the job, sector, number of persons working at the local unit, wage and full-time/part-time employment are consolidated variables.
Statistical Analysis
To distinguish CBW from NCBW, a dichotomous variable was generated as well as four binary variables, one for each country of destination. Example, the variable for Germany was coded as: 1) is working in Germany and 0) is working in France but lives in the same department, as CBW working in Germany. To preselect the covariates, chi-square tests were used for qualitative variables and Student’s t-tests for the quantitative ones, including those which were significant (p<0.10) for at least one of the health outcomes. The covariates were selected based on the previously described theoretical frameworks and introduced in three steps. A logistic model was estimated for each health variables and covariates were introduced in order to predict the probability of CBWs to report a health issue compared to NCBWs.
In a first step, only the commuting status was introduced in the model (unadjusted model). In a second step, the demographic background of the individuals was introduced in the adjusted model. In a third step, the labour status of the individuals was introduced in the fully adjusted model.
The variable cohabiting was preferred to the marital status i.e., the legal recognition of cohabitation, because it allows us to capture all the workers that beneficiated from a lower mortality rate [38] and not only those having the legal recognition of their situations. All the outcome variables were coded with the aim to model the probability of being in ill health according to the cross-border worker status. An odds-ratio greater than one can also be understood as verification of ill health for CBWs compared to NCBWs, whereas an odds-ratio smaller than one indicated a better health in the group of CBWs compared to the reference group of NCBWs.
The Health Index (5 items) was an additional measure obtained with the score of four physical health variables and the perceived health score: ‘the higher the score, the healthier the worker’. For each item, one point was assigned if the answer indicated a high health state and no point if the answer indicated a poor health. For example, the absence of disability was considered as an indication of a good health state. For the perceived health, one point was assigned if the respondents estimated their health as ‘very good’ or ‘good’, whereas a zero score was assigned if the answer was ‘fair’, ‘poor’ or ‘very poor’.
Binary logistic regression modelling was used to determine associations between the commuting status and each of the five health outcomes. Odds ratios were estimated with a 5% risk of error i.e., 95% confidence intervals (CI)). Covariates were added to the model to provide adjusted and fully adjusted associations for CBWs as a whole as well as well as for CBWs of the different country destinations. All statistical analyses were performed using the software STATA 16.0.