By Clare Murphy
Health reporter, BBC News
The cold can make you more susceptible to germs
Global warming dominates the headlines, but in the UK the cold of winter is much more hazardous to health - especially for the elderly and the sick.
For every degree the temperature drops below 18C, deaths in the UK go up by nearly 1.5%.
This year, any new year's resolutions that involve strenuous outdoor activity - at least for those with existing health problems - could be best put on ice.
Heart attacks and strokes rise as temperatures fall. This is because when confronted with the cold, the blood vessels in the skin contract to conserve heat by preventing blood from flowing to the surface. The composition of the blood also changes.
The heart has to work harder to pump blood through narrower vessels, while the change in concentration means it is more liable to clot, with all the ensuing health problems.
The British Heart Foundation says: "There is growing evidence to suggest that heart attacks are linked with extreme weather conditions, especially cold weather.
"If you have a heart problem and are outside in cold weather, you should avoid sudden exertion - for example, shovelling snow or pushing a car.
"In very cold weather it may be best to stay indoors."
A winter coat
A further hazard is infection. Going outside and feeling cold at the bus stop will not cause you a cold - but the low temperatures conspire to make people more susceptible to any viruses that might be circulating on the bus.
Cold air affects the way in which the respiratory tract protects us from disease, producing a thicker, sluggish mucus that is less effective at clearing out unwanted intruders like viruses.
There is also evidence to suggest influenza itself is able to put on its optimum performance when it is cold. Scientists have found it develops a hard, rubbery shell when temperatures are low - a robust coating that can even resist detergents.
Once it has entered the respiratory tract this protective winter coat melts away so it can infect its new host.
This theory goes some way to explain why flu viruses appear so much more virulent in the winter months, but it cannot explain all of them.
The Spanish flu that killed up to 50 million people between 1918 and 1919 started out in the spring and summer, while swine flu took hold in Mexico in temperatures higher than those the UK usually enjoys in the summer.
Old and cold
It is the elderly who often suffer the most as temperatures drop.
Physiologically, their blood vessels are more susceptible to clotting, finances could make them less likely to turn up the heating and more likely to use public transport, while socially they could have fewer people around them to spot problems.
"One of the effects of ageing is that your body is less able to regulate its temperature, so you become less able to judge if you are warm or cold," says Andrew Harrop, head of policy at Age Concern and Help the Aged.
"Failing to keep adequately warm can expose older people to the threat of cold-related illnesses, which contribute to the thousands of excess winter deaths each year."
Last year's low temperatures saw the highest number of "excess deaths" - the number of those who perished over and above what is normal for the time of year - for nearly a decade.
The 40,000 "excess deaths" in England and Wales represented a rise of nearly 50% from the previous year. In the South East, where people were perhaps least prepared for a cold snap, deaths nearly doubled.
Don your cap
Studies of weather-related mortality have found that with heat-related deaths there tends to be a significant level of death "displacement" - put bluntly those who were likely to die anyway simply died sooner, so the period after a heat wave tends to see fewer than average deaths.
"But this doesn't appear to be the case with winter deaths," says Dr Gavin Donaldson, a specialist in respiratory medicine at University College London.
"There has been much focus on hot weather and global warming, but in this country we see many more days of cooler temperatures. It really does need highlighting that winter can take its toll in this way, particularly on the elderly.
"But we could all be better prepared. In the summer we don't have any qualms about telling people to keep hydrated. Maybe, patronising as it may sound, it is time to start telling people to wrap up warmly with hats and gloves, and if you are going on a long journey take a thermos flask."
Keeping a house at between 18C and 21C is key. Hypothermia is only really a risk when temperatures at home are as low as 5C, but spending more than two hours at 12C raises blood pressure and so increases the risk of heart disease and stroke.
"Remembering the needs of friends, relatives and neighbours who could be at risk is essential," said Yvonne Doyle, director of public health for the South East Coast region.
"If they can't heat all the rooms they use, they should heat the living room during the day and the bedroom just before they go to sleep. The elderly, and those who are ill, are particularly vulnerable during cold weather."