Brian Jones suffered a cold early on in the flight, while Bertrand Piccard was reported to be suffering from exhaustion on Wednesday.

As a psychiatrist, Bertrand treated himself with a course of self-hypnosis.

And that is the key to surviving such a gruelling trip - having the tools to cope with health problems on board.

After all, a round-the-world balloon trip is a hazardous business. Crew members have to spend 21 days together in a confined space, and if a health problem arises they cannot call a GP.

The balloonists need to be aware of the main risks and be able to deal with any problems that occur.

Exhaustion

The crew's bodies are put under immense strain over the 21 or so days of a major flight.

This means they must get proper rest and avoid stressful situations as much as possible - no easy task given the nature of their challenge.

Exhaustion can severely impair performance, and is caused by insomnia, illness, stress and overwork.

A spokesman for the team described how the crew coped with their ills: "Brian had a cold early on in the flight and Bertrand was suffering from exhaustion on Wednesday afternoon - we were really worried as he was quite breathless.

"He did some self-hypnosis, as he is a psychiatrist, and had a couple of soups and some oxygen and he got a new lease of life.

Self-hypnosis can be used relax the mind in times of extreme stress.

This is said to leave the mind open to therapeutic suggestions and strengthen will-power.

Exposure to cold

Although the cabin is heated to 15°C, temperatures outside are well below freezing - and the higher they go, the colder it gets.

Should the cabin's heating systems fail, the balloonists will face two main threats - hypothermia and frostbite.

Hypothermia symptoms take effect in stages, starting with bouts of shivering and an inability to think straight.

It progresses until the sufferer loses consciousness, their breathing comes close to stopping and the pulse is extremely weak.

It can be prevented by wearing many layers of clothing, drinking plenty of fluids and hot drinks (but not alcohol) and keeping well nourished.

Maintaining movement to keep circulation up is also advised.

The Breitling Orbiter crew has prepared for the cold. They wear insulating synthetic fibre clothes all the time they are awake, and have isothermal survival suits ready for emergencies.

Oxygen starvation and altitude sickness

In the event of cabin decompression, the crew will have more than the cold to contend with - they will be severely deprived of oxygen due to the lower concentrations at altitude.

However, the body requires the same amount of oxygen regardless of how much is available, so the breathing rate increases to compensate - even when the body is at rest.

Altitude itself also has negative effects on health.

If the human body is left exposed at high altitude without acclimatisation, the effects can be devastating.

Sometimes the combination of high altitude and lower atmospheric pressure causes fluid to leak from the capillaries.

From take off to touch down the crew spend all their time in a cramped cabin

This can cause fluid build-up in both the lungs and the brain.

In the lungs this can result in high altitude pulmonary oedema, where the fluid prevents effective oxygen exchange.

As the condition worsens the level of oxygen in the bloodstream decreases.

This can lead to cyanosis (where the skin turns blue), impaired cerebral function, and death.

Fluid build-up in the brain causes high altitude cerebral oedema.

Symptoms include headache, loss of co-ordination, weakness, and decreasing levels of consciousness including, disorientation, loss of memory, hallucinations, psychotic behavior, and coma.

It occurs after a week or more at high altitude, and can lead to death if not treated promptly.

Balloonists will prepare for such eventualities.

The crew of the recent Cable and Wireless attempt to circumnavigate the globe in a balloon got training from the RAF.

Andy Elson and Colin Prescot were put in a pressure chamber where the oxygen levels were reduced to simulate conditions at 25,000ft.

They were assessed for signs of hypoxia every 30 seconds for three minutes.

The assessment consisted of them performing simple tasks such as writing their name, standing up and sitting down five times, drawing a house and continually subtracting seven from 1,000.

Their performance declined as the oxygen levels in their blood decreased, but the conclusion was that they acquitted themselves well under the challenging conditions.

Malnutrition

Malnutrition is a serious condition when it occurs in conjunction with other problems.

However, it is unlikely to affect the Breitling Orbiter team.

They have stocked up enough food for 30 days, even though their flight is expected to last only 21.

For the first week they have fresh foods - bread, margarine, honey, cheese, cereals and powdered milk, reheated vegetables and pre-cooked steaks.

After that they survive on dried foods.