A study has revealed that many of the effects of long-distance flight which are usually put down to jet lag and fatigue, may in fact be altitude sickness.
In experiments carried out by aircraft manufacturer Boeing, the symptoms most people experience at altitudes of more than 10,000ft (3,000m) such as headache, nausea and dizziness, fatigue and a general feeling of malaise, are the symptoms of acute mountain sickness.
A team of doctors led by Michael Muhm of Boeing, recruited 500 healthy volunteers and asked them to spend several hours in a low-pressure chamber designed to simulate the pressure at various altitudes up to 8,000ft.
The volunteers were placed, a dozen at a time, in a pressure chamber, in coach seats, for 20 hours; they were given airline food but no alcohol and were able to watch five movies if they wished.
Most aircraft are designed to have the cabin pressurised to a minimum of 565mm of mercury, equivalent to an altitude of about 8,000ft, when flying at their maximum height; pressure at ground level is 760mm.
Although flying at lower altitudes makes the cabin pressure higher it also requires more fuel and makes the aluminum planes wear out faster.
The research team found that after three hours of exposure to cabin pressures equivalent to 7,000 to 8,000 feet, the simulated fliers were more likely than others to report backaches, headaches, shortness of breath, light-headedness and impaired coordination, and women and younger people appeared to suffer the most.
Symptoms of acute mountain sickness were experienced by 7.4 per cent of the volunteers.
Jeanne Yu, Boeing's director of environmental performance, says the Muhm study has prompted the company to set the cabin pressure on its new 787 planes, to be brought out next week, for 6,000 feet.
Jeanne Yu says that is possible in these planes, because the fuselage is made up of a composite structure instead of aluminum.
The most common complaints were of backache, headache, light-headedness, shortness of breath and impaired coordination.
The research is published in the New England Journal of Medicine.