• Krishnan Bhaskaran, Ben Armstrong, Shakoor Hajat, Andy Haines, Paul Wilkinson, and Liam Smeeth.
• Department of Non-communicable Diseases Epidemiology, London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK.
• BMJ. 2012 Jan 1;345:e8050.

ObjectiveTo quantify the association between exposure to higher temperatures and the risk of myocardial infarction at an hourly temporal resolution.DesignCase-crossover study.SettingEngland and Wales Myocardial Ischaemia National Audit Project (MINAP) database.Participants24,861 hospital admissions for myocardial infarction occurring in 11 conurbations during the warmest months (June to August) of the years 2003-09.Main Outcome MeasureOdds ratio of myocardial infarction for a 1 °C increase in temperature.ResultsStrong evidence was found for an effect of heat acting 1-6 hours after exposure to temperatures above an estimated threshold of 20 °C (95% confidence interval 16 °C to 25 °C). For each 1 °C increase in temperature above this threshold, the risk of myocardial infarction increased by 1.9% (0.5% to 3.3%, P=0.009). Later reductions in risk seemed to offset early increases in risk: the cumulative effect of a 1 °C rise in temperature above the threshold was 0.2% (-2.1% to 2.5%) by the end of the third day after exposure.ConclusionsHigher ambient temperatures above a threshold of 20 °C seem to be associated with a transiently increased risk of myocardial infarction 1-6 hours after exposure. Reductions in risk at longer lags are consistent with heat triggering myocardial infarctions early in highly vulnerable people who would otherwise have had a myocardial infarction some time later ("short term displacement"). Policies aimed at reducing the health effects of hot weather should include consideration of effects operating at sub-daily timescales.

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