High humidity raises the risk of heart attack among the elderly, even when temperatures are relatively low, according to a study published in the medical journal Heart.

Researchers looked at all deaths in Athens, Greece for the year 2001 and daily weather reports from the Greek National Meteorological Society on temperature, air pressure levels and humidity for the same year.

There were sharp seasonal variations in the timing of the deaths, with the proportion of deaths a third higher in winter than in summer. Surprisingly, deaths of those 70 and older accounted almost entirely for this variation.

Among those 70 or older, deaths from heart attack were 3.5 times higher than other age groups in June, but seven times higher in December.

Average monthly humidity was the single most important factor influencing average monthly death rates from heart attack in the over 70s. The maximum recorded humidity level for the year, 91%, was recorded in December.

The authors conclude that even in a relatively mild Mediterranean climate, such as that of Athens, changes in humidity have a significant impact on the chances of an elderly person dying from a heart attack.

The December peak of deaths is traditionally attributed by Greeks to the "Merry Christmas Coronary" phenomenon, that is, a combination of overindulgence in food and alcohol and emotional stress. But in Greece, Easter is celebrated with even more gusto than Christmas, and there is no equivalent peak in heart attack deaths.