London, Aug 28 (IANS) The risk of suffering a heart attack is more likely to peak in the winter season and decline in summers because air temperature acts as an external trigger for the life threatening disease, an analysis has shown. The findings revealed that the average number of heart attacks per day was significantly higher during colder temperatures as compared to warmer. When the daily temperature dropped to less than zero degree Celsius, the average rate of heart attacks a day peaked to four, as compared to when it was above 10 degrees. Furthermore, the occurrence of heart attacks increased with higher wind velocities, limited sunshine duration and higher air humidity. "There is seasonal variation in the occurrence of heart attack, with incidence declining in summer and peaking in winter," said lead author Moman A. Mohammad, from the Lund University in Sweden. "Our results consistently showed a higher occurrence of heart attacks in sub-zero temperatures. The findings were the same across a large range of patient subgroups, and at national as well as regional levels, suggesting that air temperature is a trigger for heart attacks," Mohammad added. The body responds to cold by constricting superficial blood vessels, which decreases thermal conduction in the skin and subsequently increases arterial blood pressure. Other responses are shivering and increased heart rate, which raise the metabolic rate and in turn increase body temperature, the researchers explained. "In the majority of healthy people these mechanisms are well tolerated. But in people with atherosclerotic plaques in their coronary arteries they may trigger a heart attack," Mohammad noted. Respiratory tract infections and influenza are also known risk factors for heart attack that have a clear seasonal variation. "In addition, seasonal-dependent behaviours such as reduced physical activity and dietary changes could also play a role in the increased occurrence of heart attack during colder weather," Mohammad said. The results were presented at the European Society of Cardiology Congress in Barcelona. For the study, the team investigated the association between heart attack incidence and weather conditions such as air temperature, sunshine duration, precipitation, and air pressure in more than 2,80,000 patients.