Tomorrow promises to be the hottest day in Britain since records began. But with the sunshine comes a health warning: extreme temperatures have severe effects on every part of the body. Jeremy Laurance explains

The mercury rises inexorably. Britain prepares to sweat it out. The Meteorological Office has confirmed that the threshold for the Government's Heatwave Plan - 31C - has been reached in the South-east and is very likely to be reached in other parts of England and Wales over the next few days.

Official advice is being dispensed: stay out of the sun; keep your home as cool as possible (shutting windows during the day may help, opening them at night when it is cooler); eat cold food and drink plenty of water. But when it gets this hot, our bodies are sure to suffer.

Stifling temperatures cause nothing more than discomfort to most but for some they can be lethal. The heatwave of August 2003 showed that very warm weather can be a killer. Official figures showed there were more than 2,000 excess deaths in England. In continental Europe, more than 27,000 people died because of the heat.

The biggest danger is of heat stroke or sunstroke, an advanced state of hyperthermia (not to be confused with hypothermia, caused by excessive cold). This is a serious condition in which the body produces or absorbs more heat than it can dissipate.

The heat-regulating mechanisms become overwhelmed and unable to deal effectively with the heat. As a result the body's temperature climbs uncontrollably. It is a medical emergency that requires immediate hospital treatment.

The most important method of temperature regulation is through perspiration - sweating. The sweat evaporating from the skin cools the body, ridding it of excess heat. Athletes produce vast quantities of sweat - several kilograms a day in the case of the cyclists competing in the Tour de France, entering its final stages this week - which have to be put back in the evening through the consumption of vast quantities of liquids.

A similar challenge faces the non-sporting population in a heatwave - keeping up the liquid intake. This can be a problem for children, the elderly and the sick and for others unused to consuming large quantities of liquid. Athletes can develop stomach cramps, as has happened to some riders on the Tour, because of the huge amount of liquid they have to absorb. If sufficient liquid is not consumed the body becomes dehydrated and eventually unable to produce sweat. Once this method of heat reduction is closed, the core body temperature begins to rise rapidly.

The first sign of heatstroke is heat exhaustion, a state marked by mental confusion, muscle cramps and often nausea or vomiting. At this stage the victim may still be sweating profusely but unless rapid action is taken to cool the body down its temperature may rise, leading to full-blown heatstroke.

Normal body temperature is 36.8C (98.4F). A small variation up or down can have serious consequences. A temperature of 39-40C is a serious fever and above 40C (104F) is life threatening. A child not normally prone to epilepsy may start fitting at this level. At 45C (113F) death is imminent.

As body temperature rises, victims become more confused and may seem drunk. They may suffer dizzy spells or faint as blood pressure drops, because of dehydration. A fall in blood pressure triggers an increase in heart rate and breathing, as the body tries to maintain the body's supply of oxygen. The blood vessels closest to the skin dilate to increase the dissipation of heat and the skin reddens. Extremities - hands and feet - swell under pressure of the dilated blood vessels.

However, if heat exhaustion worsens and blood pressure continues to fall, the blood vessels contract and the victim becomes pale.

As in a fever produced by illness, victims of hyperthermia caused by overheating may be gasping from the heat one moment and trembling and chilled the next. Acute dehydration can lead to temporary blindness and eventually results in unconsciousness and coma.

Although the vulnerable and frail are at greatest risk, the young and fit may also succumb. There have been a number of deaths among young soldiers, put through punishing exercise regimes, who have suffered hyperthermia and died. The reason is thought to be that they carried out their exercise in full battle dress, which prevented their sweat from evaporating and thus cooling their bodies down. Although they had plenty to drink, and were not dehydrated, their sweat was not doing its job.

The lesson of these tragedies is that strenuous exercise in hot weather should only be taken in lightweight, loose clothing that allows sweat to evaporate.

A victim of heat exhaustion should be moved into a cool area indoors, or into the shade, their clothing loosened or removed and bathed in cool water to bring their temperature down. They should be given water to drink, but not alcohol, coffee or tea.

Avoiding heatstroke is mostly a matter of common sense. A hat will keep the sun off, light, loose fitting clothing will allow sweat to evaporate, and staying out of the sun in the middle of the day will minimise the risk of overheating as well as burning. Strenuous exercise should be avoided in the hottest weather, except by those trained and acclimatised.

The single most important measure is to drink plenty of fluids to replace those lost by sweating. Thirst is not a reliable guide to dehydration; a better indicator is the colour of urine. It should be pale yellow or straw coloured - dark yellow indicates dehydration.

Water is the best drink. Alcohol, coffee and tea should be avoided as they are diuretic (stimulating the production of urine) and hence dehydrating.

Isotonic beverages are also good for restoring electrolytes lost through sweat. A good substitute for a commercial brand is a two-litre bottle of cola or other carbonated drink with half a teaspoon of salt added.

The Department of the Environment issued a smog warning yesterday as the hot weather is expected to lead to a sharp increase in ozone levels.

Ozone is produced by the action of sunlight on vehicle pollution - and is a particular threat to asthma sufferers, contributing to the 70,000 cases of serious or life-threatening attacks each year and the 1,500 deaths.

The hot weather also carries a risk of skin cancer as people strip off to lie in the sun. Every year about 50,000 people in Britain develop skin cancer - disfiguring lesions on the skin - although most can be successfully treated.

About 6,000 people develop melanoma, the most serious form of skin cancer, and about 1,500 of these die. Deaths from melanoma have doubled every 10 to 20 years in recent decades, reflecting Britons' love affair with the sun.

Severe sunburn - especially before the age of 15 - is the biggest risk for melanoma. The highest incidence occurs among Scots, whose fair skin puts them at risk of sunburn even under northern skies.

A burning issue: how excessive heat affects the body

1 Head: Symptoms of heat exhaustion include mental confusion, headache and sometimes aggressive behaviour. Victims may seem drunk, suffer dizzy spells or faint as blood pressure falls due to dehydration.

2 Armpit: The main mechanism by which the body controls temperature is sweating. The cooling effect of the sweat evaporating from the skin rids the body of excess heat. Several litres may be lost by athletes on a hot day.

3 Mouth: Drinking plenty of fluids is the single most important measure to protect against heat exhaustion. Water, soft drinks or isotonic drinks are best - alcohol, tea and coffee should be avoided.

4 Skin: The skin absorbs heat and is vulnerable to burning by the sun which can cause cancer. Staying in the shade, wearing light clothing and avoiding going out in the hottest part of the day reduce the risk of heat stress.

5 Feet and hands: Blood vessels dilate close to the surface of the skin and at the extremities to promote heat loss. This causes the skin to redden and hands and feet to swell, making it difficult to remove rings from the fingers.

6 Genitals: The most accurate indicator of dehydration is the colour of urine. It should be pale yellow or straw coloured. Dark yellow means drink more - but not alcohol, coffee or tea which are diuretic (urine producing) and increase dehydration.