The winter cold thickens the blood and increases strain on the heart. Average blood pressure is around 5 per cent higher in winter than in summer and that can make the difference between life and death.
A study of 11,000 people who had heart attacks between 1988 and 1997 found those who succumbed in winter were 19 per cent less likely to survive than those who were struck down in summer. There was no difference in the number of resuscitation attempts made during both seasons, but the winter victims were more likely to die before they got to hospital.
Evidence from Scandinavia, where the winters are more severe but the population is better prepared with warmer houses and better outdoor clothing, suggests that it is not the cold itself but our inability to cope with it that accounts for Britain's high death rate. In these northern countries there is less of a differential between summer and winter deaths.
Dr Jill Pell and colleagues from the department of public health medicine at the Greater Glasgow Health Board, who report their findings in the journal Heart, say that the lack of sunshine in winter means cholesterol levels are higher and vitamin D levels are lower, which increase the blood's tendency to clot.
The best defence, particularly for the elderly who are most vulnerable, is better heating indoors and more protective clothes outdoors, especially for the face. Breathing in very cold air can trigger a reaction leading to a heart attack but the air can be warmed by judicious use of coat collar and scarf.