Travel broadens the mind and brings untold benefits to the human spirit, but in doing so it often rains a multitude of physiological insults upon the human body. Dehydration is one of the most fundamental of these, but there have been some important recent developments in our understanding of its mechanisms and how to avoid it.

Dehydration

In a temperate climate, most people need a daily fluid intake of two litres of water to remain in balance. In a hot, humid climate, and with increased physical activity, ten litres a day (one seventh of body weight) and sometimes more may be needed.

It takes about three weeks for people who normally live in a temperate climate to acclimatise to a hot one: for most trips by British holiday-makers, there is therefore no chance of acclimatising fully. During acclimatisation, sweat glands develop the ability to produce more sweat, to respond more quickly and to lose less salt; stomach and intestines also adapt to become better able to absorb salt and water more efficiently. Without acclimatisation, newcomers to hot climates have difficulty conserving water and salt and are at a significant risk of developing heat-related illnesses. Excessive physical exertion increases the risk; avoid this until acclimatisation is complete.

Acclimatisation is usually much more difficult in hot and humid climates than hot and dry ones. In humid climates, sweat does not evaporate easily and temperature and humidity tend to remain high through the night. This is a continuous stress, whereas in dry climates they both tend to fall at night, allowing the sweating mechanism to rest.

Small, thin people tend to acclimatise most easily to the heat - because their body surface area is relatively higher in relation to their volume, giving a relatively greater area from which to sweat and lose heat. Unfit, overweight people acclimatise more slowly, and do badly in the heat. People with high blood pressure and heart disease may be at risk from complications.

To remain in balance under such conditions, the body needs a greatly increased intake of salt and water. The trouble is that thirst and taste give an extremely poor indication of exactly how much is required. Many people have a reduced appetite on first arrival in a warm climate, which may reduce salt intake even further.

Deficiency of salt, water, or both, is called heat exhaustion. Lethargy, fatigue and headache are typical features, eventually leading to coma and death. Many sufferers do not even feel thirsty, and may have no idea that they are suffering from this problem. They feel 'hung over'. In fact, most symptoms of a typical, bad hangover are the direct result of dehydration. They feel irritable, and simply want to be left alone.

Prevention is by far the best approach. Perhaps the best method is the British Army's pre-salted water regime. Salt is added to all fluids —tea, coffee, soup, fruit juices, water. The required amount is one quarter of a level teaspoon (approximately one gram) per pint - which results in a solution that is just below the taste threshold. (Don't use salt tablets: they are poorly absorbed, irritate the stomach and may cause vomiting.) Plenty of pre-salted fluid should be the rule for anyone spending much time in the tropics.

The only reliable guide to how much you need to drink in a tropical climate is the colour of your urine. Always drink enough to ensure that it is consistently pale in colour, and don't just wait until you feel thirsty before drinking.

Heat exhaustion should not be confused with heatstroke (formerly called sun-stroke). Although dehydration is almost always a factor, the main problem is a failure of the body's heat control mechanisms. Sweating diminishes and the body temperature rises, headache and delirium also occur. Prompt treatment is essential. Once the body temperature begins to rise, death may occur within four hours. The priority is to lower body temperature. Remove clothing, and cover the victim with a wet bed-sheet, while arranging transfer to hospital. There are well- documented cases of travellers who have been left in their hotel rooms to die, simply because their condition was mistaken for a drunken stupor.

Infectious diseases that cause a fever can sometimes be mistaken for heatstroke, again with potentially fatal results. Malaria and meningitis are especially important in this context, because in both cases deterioration is rapid if treatment is not given.

Prickly heat

Prickly heat is the most common heat-related skin disorder - a sweat rash occurring on the sweatier parts of the body and consisting of tiny blisters on sore, reddened, mildly inflamed skin. You can prevent it with frequent showers and by keeping the skin clean and dry. Treat with calamine lotion.

Brown without burning

The effects of the sun on skin include sunburn, thickening and - in the longer term - drying, loss of elasticity, wrinkling, loosening, discolouration, premature ageing and skin cancer. People with fair, blond or red hair are most at risk, even after they've turned grey. Acute sunburn is a miserable way to begin a holiday. It results in a blotchy uneven tan and is all the more miserable for children whose skin is easily damaged.

If you're not bothered about a tan, cover up and use a high- protection factor sunscreen. But if vanity gets the better of you, tan very slowly. The protection factor numbers on skin preparations provide a rough measure of how much longer you can stay out in the sun without burning. If your skin normally burns in strong sunlight after 20 minutes' exposure, for example, a sunscreen with a protection factor of four will allow you to stay out for four times as long (80 minutes). After that, you would have to cover up; you would have had your maximum dose of sunlight, and more factor four would not protect you. If you wanted to stay out in the sun for 160 minutes, you would burn if you used anything less than a protection factor of eight. Protection factor numbers refer mainly to UVB protection - protection against the rays that cause acute sunburn. Protection against UVA- the rays mainly responsible for ageing and skin cancer effects - is usually shown by means of a 'star rating' system. Clearly, using a sunscreen that only protected against UVB rays but allowed people to spend more time in the sun, might actually increase the possibility of long-term damage. Check that any product you use gives protection against both.

Apart from cosmetic acceptability and protection factor, there is, however, little else to choose between the different brands of sunscreen. Whichever brand you pick, you should re-apply it frequently, especially when swimming or sweating. Most of the leading manufacturers now produce waterproof sunscreens which are particularly useful for children.

Some parts of the body are especially vulnerable and need extra care - the face, particularly the nose and forehead, neck and ears; parts of the body that are normally covered; the tops of the collar bones, bald patches on the scalp; and feet. Avoid sunbathing in the hottest part of the day and be guided by the habits of those more accustomed to hot climates, who take a relaxing indoor siesta instead.

If you burn, calamine lotion will soothe affected areas and mild painkillers are often helpful. More extensive or severe burns should be treated with a mild antiseptic and kept clean and dry. Stay out of the sun or use a total block sunscreen until the skin has healed.

The eyes, too, can be affected by the sun. The conjunctiva and retina are sensitive to ultra-violet light and are easily damaged. Pain usually begins several hours after exposure, when the delicate cells of the conjunctiva swell and become painful and inflamed. In the long term, excessive exposure to the elements causes a pterygium - an unsightly yellow patch on the white of the eye that may need to be removed. Good quality sunglasses provide effective protection.

Skin cancer and the sun

In the last few years, the number of cases of skin cancer in the UK has risen. The number of people taking holidays abroad has also risen, so the Royal College of Physicians in London commissioned a special study to determine whether or not there was a link. The results of this work have important implications for travellers: it found that the risks of skin cancer and skin damage from strong sunlight relate not just to long-term exposure, but also to the number of episodes of acute sunburn.

Skin cancers grow slowly and tend to destroy the area of skin in their immediate vicinity. Since they usually occur on exposed areas - President Reagan's nose was one well-publicised site - they can inflict much cosmetic damage. Some types - melanomas especially - become able to spread through the body. The Royal College of Physicians advises examining every pigmented patch and mole on your skin as follows:

If the answer is 'yes' to three or more of these questions, seek medical advice. The treatment is simple if the cancer is detected at an early stage.