Temporal and spatial effect of air pollution on hospital admissions for myocardial infarction: a case-crossover study

In studies showing associations between ambient air pollution and myocardial infarction (MI), data have been lacking on the inherent spatial variability of air pollution. The aim of this study was to determine whether the long-term spatial distribution of air pollution influences short-term temporal associations between air pollution and admission to hospital for MI.

We identified adults living in Calgary who were admitted to hospital for an MI between 2004 and 2012. We evaluated associations between short-term exposure to air pollution (ozone [O₃], nitrogen dioxide [NO₂], sulfur dioxide [SO₂], carbon monoxide [CO], particulate matter < 10 μm in diameter [PM₁₀] and particulate matter < 2.5 μm in diameter [PM_2.5]), and hospital admissions for MI using a time-stratified, case-crossover study design. Air Quality Health Index (AQHI) scores were calculated from a composition of O₃, NO₂ and PM_2.5. Conditional logistic regression models were stratified by low, medium and high levels of neighbourhood NO₂ concentrations derived from land use regression models; results of these analyses are presented as odds ratios (ORs) with 95% confidence intervals (CIs).

From 2004 to 2012, 6142 MIs were recorded in Calgary. Individuals living in neighbourhoods with higher long-term air pollution concentrations were more likely to be admitted to hospital for MI after short-term elevations in air pollution (e.g., 5-day average NO₂: OR 1.20, 95% CI 1.03-1.40, per interquartile range [IQR]) as compared with regions with lower air pollution (e.g., 5-day average NO₂: OR 0.90, 95% CI 0.78-1.04, per IQR). In high NO₂ tertiles, the AQHI score was associated with MI (e.g., 5-day average OR 1.13, 95% CI 1.02-1.24, per IQR; 3-day average OR 1.13, 95% CI 1.04-1.23, per IQR).

Our results show that the effect of air pollution on hospital admissions for MI was stronger in areas with higher NO₂ concentrations than that in areas with lower NO₂ concentrations. Individuals living in neighbourhoods with higher traffic-related pollution should be advised of the health risks and be attentive to special air quality warnings.