Ambient Air Pollution and Subclinical Cardiovascular Disease Measured by Magnetic Resonance Imaging in the Canadian Alliance for Healthy Hearts and Minds Study

BackgroundLong-term exposure to air pollution, even at levels below regulatory standards, has been associated with higher risk of cardiovascular-related mortality. Less is known about the association of air pollution and initial development of CVD in low-exposure settings in generally healthy human populations. ObjectiveIn the Canadian Alliance for Healthy Hearts and Minds Cohort Study (CAHHM), we aimed to investigate the association between low-level exposure to key air pollutants and subclinical carotid atherosclerosis in adults without known clinical CVD. To date, the association of ambient air pollution and atherosclerosis measured by magnetic resonance imaging (MRI) has not been studied. MethodsWe studied 6,645 Canadian adults recruited between 2014-2018 from the provinces of British Columbia, Alberta, Ontario, Quebec, and Nova Scotia, for whom average long-term exposures to nitrogen dioxide (NO2), ozone (O3), and fine particulate matter (PM2.5) were estimated for five years prior to the start of CAHHM recruitment, and who underwent MRI to assess carotid vessel wall volume (CWV). Linear mixed models were used to quantify associations between each air pollutant and CWV adjusting for individual-level and community-level risk factors for CVD. Secondary analyses included region-specific stratification and modeling the effect of one pollutant on CWV within low, medium, and high levels of a second pollutant to test for interactions. ResultsHigher PM2.5 was nominally associated with lower CWV (quintile 5: 893.3 mm3, quintile 1: 908.8 mm3; p-trend =0.05), but this was not robust in region-stratified analysis. Higher NO2 was associated with lower CWV (quintile 5: 889.5 mm3, quintile 1: 918.6 mm3; p-trend <.0001). Higher O3 was associated with higher CWV (quintile 5: 925.4 mm3, quintile 1: 899.7 mm3; p-trend =0.02). NO2 emerged as a consistent effect modifier of both PM2.5 and O3. ConclusionIn a cohort of generally healthy adults living in Canada, a country with relatively low levels of air pollution, exposure to NO2 was negatively associated, and O3 was positively associated with CWV as a measure of subclinical atherosclerosis by MRI, while associations to PM2.5 were inconsistent. The reasons for these associations warrant further study.