High humidity can increase the risk of a heart attack in the elderly even in relatively mild climates, according to research published on Thursday.

Scientists who studied deaths from heart attacks in Athens, Greece in 2001 found that humidity was one of the most important factors influencing average monthly death rates from heart attacks for people over 70 years old.

"It was linearly associated with heart attack deaths," said Dr Georgios Giannopoulos, of the University of Athens. "It means that higher humidity values were related to higher death rates," he told Reuters.

The research published in the journal Heart is thought to be the first study to identify a relationship between humidity and heart attacks, which are a leading cause of death in developed countries.

The scientists analysed the link between weather conditions including temperature, pressure levels and humidity with the 3,126 heart attack deaths that occurred in Athens in 2001.

They found that in the elderly age group the average daily temperature during the previous week was also a significant factor in daily death rates.

The researchers believe consistent high temperatures could have a cumulative effect on the body which raises the risk of heart attack.

"We know that increased humidity, especially combined with temperature, increases physiological stress on the body especially the circulatory system," Giannopoulos said.

A heart attack occurs when coronary arteries which carry blood to the heart are restricted, reducing supply. Each year in the United States alone more than a million people suffer heart attacks, many of them fatal.

Smoking, high blood pressure, raised cholesterol, obesity, lack of exercise and diabetes raise the risk of a heart attack.

A family history of heart disease and age also increases the likelihood. Heart attacks are more common in men over 45 years old and women over 55.

The researchers also found a peak in heart attack deaths in December, which they attributed to the "Merry Christmas Syndrome" or over-indulging in food, alcohol and emotional stress during the holiday season.