Mortality following unemployment in Canada, 1991–2001

The study is based on a cohort consisting of a 15% sample of the non-institutionalized population of Canada aged 25 years and over at the time of cohort inception in 1991. The cohort was followed for mortality until the end of 2001.

The Canadian census mortality follow-up study is broadly representative of the non-institutional population of Canada in 1991 [1]. We selected cohort members from among the approximately 20% of households who completed a “long-form” census questionnaire in 1991. To ascertain date and cause of death over the follow-up period (from 4 June 1991 to 31 December 2001), we linked records for long-form census respondents to mortality records. For the specific purpose of this study, reporting on the association between unemployment and the risk of mortality, we restricted the study sample to individuals aged 30 to 69 at baseline who had worked one or more weeks in the period 1 January 1990 through 3 June 1991. This represented 1,488,800 persons who were employed on census day (826,600 men and 662,200 women), and 110,800 who were unemployed (61,400 men and 49,400 women).

In this report, we describe the relationship between unemployment and mortality by occupational categories, where occupations were grouped into five ordinally-ranked occupational skill levels (professional, managerial, skilled/technical/supervisory, semi-skilled, unskilled) based on the 1990 National Occupational Classification [28]. We also describe the relationship between unemployment and mortality according to baseline age, marital status, immigration status, educational attainment, and industry.

The underlying cause of death is coded to the World Health Organization’s International Classification of Diseases, Ninth Revision (ICD-9) [29] for deaths occurring in the period 1991 through 1999, and to the Tenth Revision (ICD-10) [30] for deaths occurring in 2000 or 2001. We classified causes of death as malignant neoplasms (stomach, lung, intestine and rectum, other cancers), circulatory diseases (acute myocardial infarction, other ischaemic heart disease, cerebro-vascular disease, other circulatory diseases), respiratory diseases, alcohol-related diseases, accidents and violence (suicide, homicide, traffic accidents, falls, other accidents and violence), and all other causes. Details of our groupings of causes of death are provided in Appendix.

For each member of the cohort, we calculated person-days of follow-up from the beginning of the study (census day, 4 June 1991) to the date of death, emigration (ascertained from the name file and known for 1991 only), or end of the study (31 December 2001). Person-days of follow-up were then divided by 365.25 to get person-years at risk.

Age-standardized mortality rates were calculated by the direct method, with 1991 mid-year population estimates used as the standard population. We used cox proportional hazards models to estimate mortality hazard ratios comparing the unemployed to the employed. Models were estimated for all-cause and cause-specific mortality. Men and women were modelled separately. Models were also estimated for two consecutive periods of follow-up time (1991–1996, and 1997–2001) and for four age groups (30–39, 40–49, 50–59, 60–69 years at baseline). All models were estimated first with age-adjustment only (age in years), and then adjusted for age, marital status, immigration status, occupational skill level, educational attainment, and industry.

The Canadian census mortality follow-up study was approved by the Statistics Canada Policy Committee and was reviewed and approved by the Health Sciences I Research Ethics Board of the University of Toronto. At the time analyses reported in this paper were conducted, access to Canadian census mortality follow-up study database was restricted to employees or deemed employees of Statistics Canada.

There are a number of important strengths of the Canadian census mortality follow-up study, including a very large sample broadly representative of the non-institutional adult population of Canada in 1991 and the exclusion of persons not in the labour force [13]. There are also a number of important limitations to acknowledge that are characteristic of census follow-up studies. The study did not have access to information on the duration of the unemployment episode nor information on the number of episodes of unemployment in the follow-up period. Both duration and frequency are relevant for understanding the relationship between exposure burden and health consequences and would be important to address in future research. The census questionnaire did not collect information on behavioral risk factors that would be relevant for understanding the degree to which health status may be a risk factor for subsequent unemployment and separately, a risk factor for mortality [13, 20, 32‐34]. We note, however, that long-duration follow-up studies that do adjust for baseline health status only partially account for the elevated risk of mortality following unemployment. For example, an important recent meta-analysis of 42 studies estimating the association between unemployment and all-cause mortality assessed the magnitude of hazard ratios in studies that controlled for health status and studies that did not have information allowing control for health status [13]. The authors concluded that while baseline health status accounts for a portion of the association between unemployment and mortality, pre-existing health status is not the common cause of both unemployment and mortality. In interpreting the measures of mortality risk reported in our study, readers should note that the hazard ratios may be inflated relative to estimates fully adjusted for baseline health status. As a final limitation, we note that Canada’s adoption in 2000 of ICD-10 for the classification of causes of death may have influenced the comparability of results over time for certain causes [35]. However, we believe the potential influence of the change in disease classification standards to be small.