As the body is already working beyond its normal range, the additional load placed on it by even minor infections can rapidly become serious. People have died from illnesses that would hardly be noticed at lower levels. Minor cuts and grazes are inevitable, and will normally be tolerable, though slow to heal. However, deeper wounds or a persistent, hacking cough must be treated at lower altitude.

Cold
At high altitudes, the air is colder than at sea level, roughly by 6.5 degrees Celsius (11.7 degrees Fahrenheit) for every 1000 m of height. Bad weather, especially wind, can obviously make matters much worse, and since these are relative changes, the temperatures at the summits of the more polar mountains are dangerously low, even in good weather.

Apart from requiring the climber to wear suitable clothing, the cold has a significant effect on the metabolism, in a number of ways. Perhaps most importantly, the additional energy required to keep the body warm can only come from "burning" food, and this requires oxygen, which is of course in short supply. The cold air makes breathing painful, and the efficiency of the chemical processes to do with the metabolism of oxygen is impaired. The dreadful combination of extreme altitude, cold, and resulting fatigue is a common factor in deaths in high mountains.

The dangers of frostbite and hypothermia are serious, but are not covered here.

Dehydration
Adequate hydration is essential, to allow the body to regulate its chemical balance in response to the change in altitude. However, the air at high altitude is always very dry; above 5,000 m there is virtually no water vapour present. This has the effect of stripping water from the lungs, and making sweat evaporate very rapidly. These factors, combined with the effort of climbing, lead to extreme dehydration unless a special effort is made to drink more fluid than usual.

Especially during prolonged exposure to high altitude, the importance of fluid throughput cannot be overstated. To quote, "unless you're pissing gin-clear and lots of it, you're not adequately hydrated" [3]. The rate of fluid output (micturation) is a good guide, as unless you are having to get up several times a night to eliminate it, you are definitely not drinking enough. This can be a great inconvenience, but there is really no alternative.

Unfortunately, fetching enough ice or snow, melting it and making even cold drinks, never mind hot ones, take a lot of fuel, effort, and especially time. You may be luck to have a Porter to attend to your water needs so this may or may not be relevant.

In Europe, generally climbing from refuges, we have found that an energy drink such as Isostar, made up with very hot water in a plastic juice bottle, keeps warm for several hours in an insulating sleeve of the sort generally used to keep a wine bottle cool.

Hot, weak black tea is an excellent drink, and a good way of getting sugar into the system [3], but some feel that coffee should be avoided (although it is invariably offered for breakfast in Alpine refuges) as it is a diuretic (but then, so is tea). Remember that Diamox (acetazolamide) taken to assist with acclimatization increases the need for drinking. At least 3 litres (nearly 7 pints) per day of water or watery drinks should be consumed every 24 hrs, and 4 litres is a better target, even if this is rarely achieved outside the lower camps.

A serious effect of dehydration is that the blood becomes thickened, with additional risk of strain on the heart and circulatory system, clotting and general impairment of performance. Aspirin can be taken to offset this.

Diet and Nutrition
It's very difficult to maintain appetite at high altitude. The senses of smell and taste are greatly inhibited, and the general feeling of lethargy and nausea that can accompany mild altitude sickness compound to put one off eating. It's also a chore to prepare meals, as to save weight, high altitude rations are generally dehydrated, and fetching enough ice or snow, melting it and heating the food take a lot of fuel, effort, and especially time. It is vitally important to motivate oneself (and one's companions) to eat a high calorie diet, whether one is hungry or not. Again you may have a Porter who cooks the food for you. Is so you are very lucky, be courteous and polite to your Porter.

Specific food cravings occur, and this can be turned to advantage by careful advance selection of the high-altitude rations. Sir John (now the late Lord) Hunt tells a wonderful tale in his "Ascent of Everest" of his delight at finding a tin of tuna fish left on the South Col by the Swiss expedition. Snack foods not needing cooking are great, too: biscuits, crackers, sweets, chocolate, etc... We find chewy cereal bars are an especially good pick-me-up, if you can carry enough.

Since the high-altitude experience is generally brief, there is no need to worry too much about a balanced diet. Go for readily digested, starchy carbohydrates such as pasta, mashed potato and thick soups. Fatty foods are difficult to digest, and should be avoided. There may be some advantage in taking vitamin and mineral supplements, particularly potassium and iron, to maintain the nerves and blood, and of course salt is vital as so much is lost through sweat evaporation in the very dry air.

Constipation and diarrhoea tend to alternate, and this can be dangerous as well as distressing. Some medicinal assistance may help to regulate the system. On many routes, water pollution is a very serious issue, and great care should be taken to avoid faecal contamination of drinking water. Sterilization by boiling is impossible, as water boils at too low a temperature, so chemical treatment with iodine may be required. The objectionable taste of iodine can be neutralised by dissolving Vitamin C (ascorbic acid) in the water after purification. We find that effervescent tablets work best.

Menstruation and Contraception
Menstruation (periods) may be upset during the acclimatization process, and while this seems unlikely to cause harm, it could be inconvenient. Lady climbers may wish to take additional "requisites" or make medicinal arrangements beforehand. Oral contraceptives may be less effective at high altitude, and there may also be an increased risk of thrombosis, as with any oestrogen-based medicine. These matters should be discussed with one's doctor.

Sleep and sleeping pills
Sleep can be elusive above 4,000m. Quite apart from the altitude (which makes it difficult to develop a relaxed involuntary breathing pattern) the cold, noise and general discomfort are impediments to a good night's rest. There is also likely to be worry about one's situation, and the likelihood of an early start. However, taking a sleeping pill is extremely dangerous, as the body is already lethargic, and it is easy to slip into a coma.

Summary: how to prevent and manage AMS

• prepare well by becoming fitter (and giving up smoking)
• take suitable supplies and pack your gear for easy retrieval
• avoid over-exertion: if possible ascend slowly enough that you can still breathe through your nose
• avoid sleeping pills and alcohol on the mountain
• eat small amounts of food often, even if you don't feel hungry; avoid excessive salt
• drink plenty of fluids (four to five litres per day), especially water
  if you plan to use Diamox, experiment with dosage well ahead of time, under medical supervision
• do not deny any symptoms you may experience and keep the group leader or guide informed.
  

Other health issues
Your decision to try to walk to extreme altitude carries risks as well as benefits. Before you commit yourself, talk to your doctor (general practitioner or physician).

Take this chance to check the latest information on which vaccinations are required, and over what timetable. Ensure that you store your records safely: you may be refused entry without proof of yellow fever protection, for example. Take advice about anti-malarial

drugs and insect repellents (eg something with a high percentage of DEET), and follow it carefully. Malaria is a life-threatening disease which is easy to prevent but difficult to treat. Although for much of the time you will be too high to be at risk, you need protection if only for the beginning and end of your trip; a single infected bite is all it takes.

If you haven't taken anti-malaria before, discuss with your doctor whether you need to take an experimental dose ahead of time. Some can cause side­ effects, including nausea and other problems which could be confused with AMS symptoms and generally won't help your attempt. You might want to ask his or her views on Diamox at the same time.

Remember to visit your dentist well before departure. Your feet are about to become the most important part of your body, so consider seeing a chiropodist, and obtain blister prevention and treatment. If you are a blood donor, make your last donation at least eight to ten weeks before you leave.

Upset digestion is not uncommon, so consider what remedies to take, including anti-diarrhoea medicine. Some of those who have to turn back do so because of diarrhoea and consequent dehydration. The nature of the latrines and absence of running water in most campsites makes it crucial to keep yourself clean: take a good supply of wet wipes, preferably medicated.

Finally, the sun's rays are far stronger at altitude, because the thinner air screens out less of the harmful radiation. Since the equatorial sun is already much stronger than most tourists are used to, the risk of sunburn is doubly severe. (On this trip you may risk sunburn and hypothermia within a matter of hours.) Bring a sun hat, cover-up clothing and cream with the highest Sun Protection Factor you can find, minimum of SPF 25 for your face and SPF 33 for your lips, which are especially at risk.