A fall in the average temperature outside can increase heart attack risk.
They found that each 1 degree C reduction in temperature on a single day is associated with around 200 extra heart attacks.
Previous studies have shown that ambient outdoor temperature is linked to mortality risk in the short term, with both hot and cold days having an effect, but the effect of temperature on the risk of myocardial infarctions (heart attacks) is unclear.
Researchers from the London School of Hygiene and Tropical Medicine carried out a study to examine the short term relation between ambient temperature and risk of heart attack.
They analysed data on 84,010 hospital admissions for heart attack recorded in the Myocardial Ischaemia National Audit Project (MINAP) during 2003-2006, and daily temperatures from the British Atmospheric Data Centre, focusing on 15 geographical areas in England and Wales.
The results were adjusted to take into account factors such as air pollution, influenza activity, seasonality and long term trends.
The researchers found that a 1 degree C reduction in average daily temperature was associated with a cumulative 2 percent increase in risk of heart attack for 28 days. The highest risk was within two weeks of exposure.
The heightened risk may seem small, but the UK has an estimated 146,000 heart attacks a year and 11,600 events in a 29 day period, so even a small increase in risk translates to substantial numbers of extra heart attacks, around 200 for each 1 degree C reduction in temperature nationwide on a single day.
Older people between the ages of 75 and 84 and those with previous coronary heart disease seemed to be more vulnerable to the effects of temperature reductions, while people who had been taking aspirin long-term were less vulnerable.
The researchers found no increase in the risk of heart attacks at higher temperatures, possibly because temperature in the UK is rarely very high in global terms.
In conclusion, they say "our study shows a convincing short term increase in risk of myocardial infarction associated with lower ambient temperature, predominantly operating in the two weeks after exposure."
They call for further studies to help shed light on the role of adaptive measures such as clothing and home heating, and further clarify which groups are likely to be the most vulnerable.
The study has been published on bmj.com.