Until now, such symptoms had been attributed to jet lag, dehydration, air contamination or being stuck in a cramped seat for hours.

Researchers report that true altitude sickness - with its nausea, vomiting and sleep disturbances - was no more likely in volunteers in simulated airplane cabins where the pressure was equivalent to 8,000 feet above sea level than it was when the pressure was closer to sea level.

But 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.

Women and younger people were the most likely to experience symptoms.

"On the basis of our findings, we conclude that maintaining a cabin altitude of 6,000 feet or lower (equivalent to a barometric pressure of 609 mm Hg or higher) on long-duration commercial flights will reduce the discomfort among passengers," wrote Michael Muhm and colleagues at the Boeing Company, which financed the study.

Typically the pressure in the cabin is equivalent to 5,500 to 7,500 feet above sea level and designing an aircraft to withstand a cabin pressure equivalent to a lower altitude would add to the cost of the plane.

Source: New England Journal of Medicine