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Identifying the shape of the association between long-term exposure to low levels of ambient air pollution and the risk of mortality: An extension of the Canadian Census Health and Environment Cohort using innovative data linkage and exposure methodology

Principal Investigator: 

University of British Columbia, Canada

This study will investigate mortality effects of low levels of air pollution in Canada using Canadian Census data from about 6 million people. The shape of the exposure-response function will be characterized using newly developed flexible non-linear exposure-response functions. See also this Program Summary of HEI's research program on low levels of air pollution. 

Funded under
Abstracts for the 2018 HEI Annual Conference. Please scroll down to view the two abstracts and posters. 

MAPLE: Mortality-Air Pollution Associations in Low Exposure Environments

Michael Brauer1, Jeffrey R. Brook2, Yen Li Chu1, Tanya Christidis3, Dan L. Crouse4,5, Anders Erickson1, Perry Hystad6, Chi Li7, Randall V. Martin7,8, Jun Meng7, Amanda J. Pappin3, Lauren L. Pinault3, Michael Tjepkema3, Aaron van Donkelaar7, Scott Weichenthal9, Richard T. Burnett1

1University of British Columbia, Vancouver, BC, Canada;2University of Toronto, Toronto, ON, Canada; 3Statistics Canada, Ottawa, ON, Canada; 4University of New Brunswick, Fredericton, NB, Canada; 5New Brunswick Institute for Research, Data, and Training, Fredericton, NB, Canada; 6Oregon State University, Corvallis, OR, USA; 7Dalhousie University, Halifax, NS, Canada; 8Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA; 9McGill University, Montreal, QC, Canada;10Health Canada, Ottawa, ON, Canada 

Background. Fine particulate matter (PM2.5) is associated with mortality, though uncertainty exists regarding the shape of the concentration response function at low levels. Since nearly the entire population of Canada lives in areas with ambient concentrations below 12 µg/m3, it is ideal for the study of low PM2.5 concentration mortality impacts.

Objectives. To apply novel satellite-based estimates of PM2.5 exposure to several large population-based cohorts, and characterize the shape of the relationship between PM2.5 exposure with mortality.

Methods. We developed annual satellite-based PM2.5 exposure estimates at 1 km resolution across Canada for 1998-2016. Estimates were back-casted to 1981 using remote sensing, chemical transport models and historical ground monitoring data. Estimates for NO2 and ozone were also developed for the same period. Exposures were estimated for ~8.5 million subjects in three Canadian Census Health and Environment Cohort (CanCHEC) cohorts, and 540,900 participants in the Canadian Community Health Survey (CCHS). Subjects were linked to annual tax records to establish residential histories, and to mortality until 2016. Sensitivity and sub-analyses evaluated (i) adjustment for individual, contextual, and geographic risk factors, (ii) adjustment for gaseous co-pollutants, (iii) exposure time window, and (iv) exposure spatial scale. Models were adjusted for a range of individual and contextual covariates. Estimates were evaluated across strata of age, sex and immigrant status and the shape of the association between PM2.5 and mortality was examined with the Shape Constrained Health Impact Function (SCHIF).

Results. The mean 3-year average PM2.5 level was 7.4 μg/m3 and 5.9 µg/m3 over all person-years of follow-up in pooled CanCHEC and CCHS analysis, respectively. We estimated mortality hazard ratios (HR) of 1.04 (95% CI: 1.04-1.05) and 1.11 (95% CI 1.04-1.18) per 10 µg/m3 change in outdoor PM2.5 for the CanCHEC and CCHS analyses, respectively. In CanCHEC the SCHIF model predicted a sublinear concentration-mortality curve with little increase in risk below 5 mg/m3, and monotonically increased HR predictions above this concentration. In CCHS we estimated a supra-linear relationship extending to concentrations below 2 µg/m3. In both cohorts, HRs were larger for females, younger ages and non-immigrants and attenuated by inclusion of gaseous pollutants.

Conclusions. In large population-based cohorts exposed to low levels of air pollution with extended follow-up, we found evidence of associations between PM2.5 and mortality for concentrations as low as 5 µg/m3 suggesting health benefits of reductions in air pollution where higher concentrations exist.


Poster by Brauer et al., 2019 Annual Conference

MAPLE: Mortality–Air Pollution Associations in Low Exposure Environments

Evaluating the Sensitivity of PM2.5-Mortality Association to the Spatial and Temporal Scale of Exposure Assessment and the Inclusion of Immigrant Populations

Dan L Crouse1, Anders C Erickson2, (Presenter), Tanya Christidis3, Lauren Pinault3, Aaron van Donkelaar4, Chi Li4, Jun Meng4, Randall V. Martin4, Michael Tjepkema3, Perry Hystad5, Rick Burnett6, Amanda Pappin3, Michael Brauer2, Scott Weichenthal7

1University of New Brunswick, Fredericton, NB, Canada;2University of British Columbia, Vancouver, BC, Canada; 3Statistics Canada, Ottawa, ON, Canada; 4Dalhousie University, Halifax, NS, Canada; 5Oregon State University, Corvallis, OR, USA; 6Health Canada, Ottawa, ON, Canada; 7McGill University, Montreal, QC

Background. Few studies have examined the sensitivity of PM2.5-mortality associations to the spatial and temporal scale of exposure assessment or the inclusion of immigrant populations. There are important issues, particularly at low PM2.5 concentrations where the shape of the concentration-response curve remains uncertain.

Objectives. This study had two objectives: (1) To evaluate the impact of the spatial and temporal scale of exposure assessment on associations between long-term exposure to PM2.5 and mortality at low mass concentrations; (2) To evaluate how the healthy immigrant advantage impacts the PM2.5-mortality association in Canada.

Methods. We used the 2001 Canadian Census Health and Environment Cohort (2.4 million people with 10-year follow-up), and fit standard Cox proportional hazards models to examine the associations between ambient PM2.5 exposure and non-accidental and cause-specific mortality. Satellite-based estimates of PM2.5 exposures were assigned annually to residential locations. Three different temporal moving averages (1, 3, and 8-years) and three spatial scales (1-, 5-, and 10-km2) of exposure assignment were compared. In addition, we examined different spatial scales based on age, employment status, urban/rural location, and adjustment for O3, NO2, or their combined oxidant capacity (Ox). Separately, models were examined by stratifying or adjusting for immigrant status (n=764,000), year immigrated (>30 years, 21-30 years, 11-20 years, ≤ 10 years), or excluding the most recent immigrants. Effect modification by age at immigration, country of birth, and neighborhood-level ethnic concentration was also explored.

Results. Longer moving averages and smaller spatial scales resulted in improved model fit and stronger associations between PM2.5 and mortality. Respiratory and lung cancer mortality were more sensitive to the spatial scale of exposure assessment than cardiovascular outcomes. All of the best fitting models included adjustment for oxidant gases; this attenuated associations between PM2.5 and cardiovascular mortality and strengthened associations between PM2.5 and lung cancer. A healthy immigrant effect was observed for all mortality outcomes with a clear gradient of reduced mortality risk with shorter duration in Canada. Stratified analyses indicated that PM2.5 was positively associated with mortality in the two most recent immigrant groups; including all immigrants in the overall analysis increased PM2.5 hazard ratios compared to models excluding most recent immigrants.

Interpretations and Conclusions. The unique characteristics and settlement patterns of immigrants influence PM2.5-mortality relationships in Canada, as does the spatial and temporal pattern of exposure assessment. Nevertheless, our findings provide further support for an important relationship between long-term exposure to PM2.5 and mortality at low mass concentrations.


Poster by Bauer and Erickson et al., 2019 Annual Conference


Pinault LL, Weichenthal S, Crouse DL, Brauer M, Erickson A, Donkelaar AV, Martin RV, Hystad P, Chen H, Finès P, Brook JR, Tjepkema M, Burnett RT. Associations between fine particulate matter and mortality in the 2001 Canadian Census Health and Environment Cohort. Environ Res. 2017 Nov;159:406-415. doi: 10.1016/j.envres.2017.08.037. Epub 2017 Sep 18.

Pinault L, Brauer M, Crouse DL, Weichenthal S, Erickson A, van Donkelaar A, Martin RV, Charbonneau S, Hystad P, Brook JR, Tjepkema M, Christidis T, Ménard R, Robichaud A, Burnett RT. Diabetes status and susceptibility to the effects of PM2.5 exposure on cardiovascular mortality in a national Canadian cohort. Epidemiology. 2018 Aug 1. doi: 10.1097/EDE.0000000000000908. Video link