The effects of ambient temperature and the role of air pollution on the risk of myocardial infarction

Background: High and low ambient temperature, and increased pollution levels have been associated with increases in both overall and cardiovascular mortality, but systematic reviews suggested that associations with myocardial infarction (MI) specifically are unclear. Methods: Using data from the Myocardial Ischaemia National Audit Project (MINAP) registry, which aims to record all hospital admissions for acute coronary events in England and Wales, daily numbers of MI admissions in 15 large conurbations in England and Wales during 2003-6 were related first to daily mean temperature, and then daily mean levels of five pollutants (particulate matter with diameter < 10μm or PM10, ozone, CO, NO2 and SO2). Poisson-based regression models were used, allowing for delayed effects and adjusted for a number of potential time-varying confounding factors. In a subsequent analysis the effects of each pollutant were investigated at an hourly temporal resolution, using a case crossover study design. Results: 84010 MI events were recorded in the 15 conurbations during the study period. Ambient temperature was inversely associated with MI risk in a broadly linear relationship, with each 1°C decrease in temperature associated with a 2% (95% CI 1.1 to 2.9) increase in MI risk over the current and subsequent 28 days. Elderly individuals up to age 85 years and those with previous coronary heart disease appeared to be most vulnerable to the effect. No detrimental effect of higher temperatures was observed. There was little evidence that daily pollutant levels were associated with MI risk. In hourly analyses, 10μg/m3 increases in PM10 and NO2 levels were respectively associated with 1.0% (0.0 to 2.0) and 2.0% (0.8 to 3.3) increases in MI risk 1-6 hours later, but this was followed by a period of reduced risk at longer lags. Conclusions: Lower temperatures appear to be associated with an increased risk of MI; adaptive measures and public health interventions may have a role in mitigating this effect. A transiently increased risk of MI a few hours after exposure to higher levels of PM10 and NO2 appears to be followed by risk reductions at longer lags and may reflect events being triggered a few hours earlier than they would have otherwise occurred.