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The idiosyncracies of the British climate are well enough known and constantly discussed - perhaps more than ever in recent weeks, when the spring weather hasn't behaved at all as it should. An absence of westerly winds, the meteorological trademark of our islands, has created eccentric conditions and a weather forecaster's nightmare. Two weeks ago, icy winds were still blasting British shores; trees and shrubs blossomed late; there was unseasonal ground frost and a marked absence of rain. Yet despite the May chill, there is every chance that this summer will be one of the hottest and driest on record.

Whenever there are extremes of temperature, tens of thousands of people - particularly the elderly - die as a result. Scientists are only now beginning to discover that the body reacts in surprising ways to both heatwaves and extreme cold. Their findings overturn a few cherished myths about the conditions that kill us and our defences against them.

We usually imagine, for example, that old people die of heat stroke (in which the body simply overheats). In fact, heat stroke in this country is extremely rare, and is mostly suffered by young people doing heavy exercise in too much clothing, such as during army manoeuvres. The majority of heat-related deaths are caused instead by a lethal assault on the blood's chemistry. At London University's Queen Mary and Westfield College, Professor William Keatinge and his team have found that blood thickens in hot weather because water is lost by sweating. This leads to higher levels of red blood cells, clotting factors and cholesterol - all starting within half an hour of exposure to the sun.

This dangerous cocktail of thick, sticky blood being pumped around the body triggers a sharp jump in coronaries and strokes because blood vessels are more likely to get blocked. Surprisingly, even people who drink plenty of water are susceptible to these blood changes.

Once the temperature tops 20 degrees Celsius, the deadly effects of heat make very striking impacts on national death statistics. In last summer's heatwaves, over 500 people died in the northern US, and thousands in Europe. Of the estimated 3,000 heat-related deaths in western Europe in the hot summer of 1994, only a third were already seriously ill.

Not all heatwaves are lethal, however. During two recent examples in London, only the first was dangerous. By the time the second one struck, most people were acclimatised to the heat. In the tropics, people adapt biologically; they have a larger volume of blood, and larger, more efficient sweat glands.

The key threshold temperature that triggers heat deaths varies from place to place. Laurence Kalkstein at the University of Delaware has found that, when New York City's temperature hits 92 degrees Fahrenheit, its normal 200 deaths a day can jump to 300. In Phoenix, Arizona, the population is more used to heat, and it takes a much higher temperature of 112 degrees Fahrenheit to accelerate the death rate. In a study conducted in the St Louis area, Kalkstein found that the most dangerous weather pattern also featured high humidity, which makes days more oppressive and unbearable. In the US generally, heat is a massive killer - even deadlier than cold. "Heat related mortality may very well be the most important direct weather impact on deaths, exceeding tornadoes, hurricanes, and other disasters," comments Kalkstein.

The big question being tackled by researchers is what effects global warming might have on the population's health. Ian Langford and Graham Bentham at the University of East Anglia in Norwich recently calculated that the higher temperatures predicted in 50 years' time might cut winter deaths by 9,000 each year in Britain. But will this fall in winter deaths be balanced out by the extra mortality expected from heatwaves?

"Deaths from heat will be dramatic and very visible," speculates Bentham, "such as in older people suffering in hospital. But I expect the fall in winter deaths will be much bigger than the summer increase." Encouraging, but if our climate continues to grow hotter it may force us to think about air conditioning for hospitals and the elderly to avoid a jump in summertime deaths.

Extreme cold, too, leads to a surge in mortality. No other feature of the weather has such a dramatic impact on our health, and the statistics show that temperature and deaths go hand-in-hand so tightly that you can predict fairly accurately how many extra deaths to expect as the thermometer falls. For every degree Celsius drop in temperature in the autumn and winter, the number of deaths in Britain rises by 300. The total number of cold-related deaths in Britain is 60,000 per year.

If your nose is blocked, your throat sore and your head throbbing, you may already have fallen victim to the weather in a modest way. The human nose is upset by cold air, dropping its guard and letting the common cold virus strike.

When the mortality figures are calculated for the unusual winter we have just experienced, they are expected to show a steep rise in deaths from cold. One cold spell just after the New Year killed over 1,000 more people in a week than the same period the previous year, when it was much milder. Yet it's a myth that these deaths are caused by hypothermia (the body temperature dropping too low). Like heatstroke, hypothermia is actually very rare in this country, only causing about 300 deaths per year, even in bitterly cold winters. Aside from epidemics of 'flu, the vast majority of winter-related deaths are caused by heart attacks, thromboses, aneurysms (burst arteries), strokes and chest diseases.

We know that cold weather punishes the blood circulation system by making blood vessels tighten and blood pressure rise. Professor Keatinge at Queen Mary and Westfield College shows that the cold weather inflicts a sometimes catastrophic blow to the blood. Within half an hour of the body getting cold, blood loses water and thickens. The levels of red blood cells, cholesterol and clotting agents increase, making a blood clot much more likely. Just one cold day can make a devastating attack on the blood. Keatinge and his team have found that even slightly cooling young volunteers for a few hours - just enough to start them shivering - turned their blood thicker and raised the level of their red blood cells and cholesterol. But it's the elderly, not the young, who are in grave danger from the cold.

Profesor Keatinge upset a few other applecarts when he made a survey of healthy senior citizens living a comfortable life in well-heated residential homes. Despite 24-hour heating, they suffered just as high winter mortality as elderly people in the general population. The trouble was that they often kept windows open at night and enjoyed walks in the cold. "For elderly people," says Keatinge, "we don't agree with the traditional British belief that open windows and plenty of fresh air are good for you." He suggests that keeping warm both inside and outside is the answer.

Keatinge is now undertaking a survey of winter-related deaths across Europe. Contrary to what you might expect, countries with ferociously cold climates have lower winter death rates than those in warmer Mediterranean climates. Athens, for instance, has more winter mortality in proportion to its population than Finland, and Britain doesn't compare too well with northern countries either. But exactly why is a mystery - it could be that people living in northern countries are better prepared for cold weather, and have better heating, insulation and clothing, or northerners have bodies better adapted to cold. We should know more when Keatinge's survey is completed later this year.

! 'Weird Weather' by Paul Simons is published by Little Brown at pounds 15.99.