Air pollution in relation to very short-term risk of ST-segment elevation myocardial infarction: Case-crossover analysis of SWEDEHEART

Objective: Studies have related air pollution to myocardial infarction (MI) events over days or weeks, with few data on very short-term risks. We studied risk of ST-segment elevation MI (STEMI) within hours of exposure to air pollution while adjusting for weather.

Methods: We performed a case-crossover study of STEMI cases in Stockholm, Sweden (Jan 2000-June 2014) based on SWEDEHEART. Exposures during hazard periods up to 24 h prior to admission were compared to bidirectionally sampled control periods. Risks attributable to sulphur dioxide (SO₂), nitrogen dioxide (NO₂), ozone and particulate pollutants (PM_2.5, PM₁₀) were studied in conditional logistic regression models for interquartile range increments.

Results: Risk of STEMI (n = 14,601) was associated with NO₂ (strongest at 15-h lag) and with PM_2.5 (strongest at 20-h lag), in single-pollutant models adjusting for air temperature and humidity (NO₂: odds ratio (OR; 95% confidence interval) 1.065 (1.031-1.101); PM_2.5: 1.026 (1.001-1.054)). After adjusting models for atmospheric pressure (significantly associated with STEMI risk at 14-24-h lags), NO₂ remained highly statistically significant (1.057 (1.022-1.094)) but not PM_2.5 (1.024 (0.997-1.052)). No associations were seen for SO₂, ozone or PM₁₀.

Conclusion: Risk of STEMI rises within hours of exposure to air pollutants, with strongest impact of NO₂. These findings are complementary to earlier reports which have not acknowledged widely the importance of very short-term fluctuations in air pollution.