High humidity, even in a relatively mild climate, boosts the risk of a heart attack among the elderly, reveals research published ahead of print in Heart.
The researchers analysed all reported deaths in Athens for the whole of 2001 and looked at daily weather reports from the National Meteorological Society on temperature, pressure levels, and humidity for the same year.
The total number of heart attack deaths during the year numbered 3126, of which 1953 were in men.
There were sharp seasonal variations in the timing of the deaths, with the overall proportion of deaths a third higher in winter than in summer.
Deaths among those aged 70 and above accounted almost entirely for this variation.
In this age group deaths from heart attack were 3.5 times higher in June and seven times higher in December than rates in other age groups.
The lowest recorded temperature on three days in December reached 1 degree Celsius (34 degrees Fahrenheit), with an average of 6 degrees Celsius, and the highest, on two days in August reached 39 degrees Celsius (102 degrees Fahrenheit), with an average of 34 degrees Celsius.
The average daily temperature for the preceding week was the most significant factor influencing the daily death rate
But average monthly humidity was the single most important factor influencing average monthly death rates from heart attack in the over 70s. Maximum humidity level reached 91% and the minimum reached 26%.
The authors point out that even in a relatively mild Mediterranean climate, such as that enjoyed by Athens, changes in temperature and humidity have a significant impact on the chances of dying from a heart attack.
The December peak of deaths has often been attributed to the "Merry Christmas Coronary" phenomenon, otherwise known as a combination of overindulgence in food and alcohol and emotional stress, say the authors.
But in Greece, Easter is celebrated with even more gusto than Christmas, yet there is no equivalent peak in heart attack deaths.
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