Disease Assimilation: The Loss of the Healthy Immigrant Advantage over Time with Exposure to Fine Particulate Matter

Abstract

Background. Immigrants make up 20 percent of the Canadian population, yet recent immigrants (< 20 years in Canada) are often excluded from air pollution mortality analyses to reduce possible confounding. Thus, little is known about the mortality impacts of fine particulate air pollution (PM2.5) on this population and how it changes over time.Methods. We used the 2001 Canadian Census Health and Environment Cohort (CanCHEC), a longitudinal cohort of 3.5 million subjects of which 764,000 were immigrants linked to annual mobility and PM2.5 concentration from 1994 to 2011. Exposures were assigned as a 3 and 8-year mean prior to the follow-up year. Income tax files were used to account for residential mobility among respondents using postal codes, with probabilistic imputation used for missing postal codes in the tax data. We used Cox survival models to determine hazard ratios (HRs) for cause-specific mortality, and assessed year of immigration and country of origin.Results. In partially adjusted models (individual socioeconomic variables, airshed, and population centre size) stratified by age and sex, preliminary analyses of only immigrants compared the full cohort with immigrants as a covariate showed slightly lower HR estimates for cardiovascular (CVD) mortality: HR=1.07 (95% CI: 1.03 to 1.12) vs. HR=1.08 (95% CI: 1.06 to 1.10); and ischemic heart disease (IHD) mortality: (HR=1.09; 95% CI: 1.02 to 1.15) vs. (HR=1.11; 95% CI: 1.08 to 1.14) per 5 μg/m3 increase in PM2.5 concentration respectively. The HR estimates of CVD, IHD and other chronic diseases increased with increased time in Canada among immigrants. Interaction models with country of origin and sensitivity analyses are on-going.Conclusions. Preliminary analyses suggest that air pollution is an equal opportunity risk factor; however, the relationship between immigration, health and air pollution is complex, partly due to unknown previous exposures along with a diverse and dynamic country of origin population.