WEDNESDAY, July 4 (HealthDay News) -- The discomfort some passengers feel on airplanes could be due to the air pressure settings inside cabins, a new study finds.

Most aircraft cabins are pressurized to 8,000 feet above sea level, an altitude that lowers the amount of oxygen in the blood by about 4 percentage points, researchers say.

This decrease in oxygen saturation isn't enough to bring on acute mountain sickness, but pressurizing the cabin to 6,000 feet could help some passengers feel better when flying, concludes a study in the July 5 issue of the New England Journal of Medicine.

"We found that the altitudes did not affect the occurrence of acute mountain sickness syndrome, but it did affect discomfort," said the study's lead author, Dr. J. Michael Muhm, senior occupation physician for Boeing Commercial Airplanes in Seattle, which funded the study. "There was no difference in the likelihood of discomfort at ground level and 6,000 feet, but the likelihood increased between 6,000 and 8,000 feet."

He added, "We concluded that passenger and crew comfort would be enhanced" if the cabin was pressurized to 6,000 feet during long-duration flights.

Most commercial aircraft are pressurized to 6,000 to 8,000 feet, not sea level.

"In order to pressurize at ground level, we would have to increase the weight of the aircraft tremendously, because the material as it exists right now couldn't tolerate pressure at ground level," explained Dr. Claude Thibeault, medical director of the International Air Transport Association in Montreal. "We would have to increase fuel, decrease passengers. So, it's an operational sort of factor."

And 8,000 feet, the maximum allowed, is also physiologically acceptable for "normal" people, he added.

"The average Joe in good condition could tolerate 8,000 feet without health effects," Thibeault said. But he noted, "They didn't say without discomfort."

Short-haul flights are usually pressurized at 5,000 to 6,000 feet while long-haul flights are closer to 8,000 feet, according to Thibeault.

But acute mountain sickness, which can involve headache, nausea and vomiting, can occur at altitudes of 6,500 feet and higher.

The authors of the study wanted to see, among other things, if airplane travelers were prone to mountain sickness.

To that end, 502 adult volunteers participated in a 20-hour simulated flight to determine the effect of barometric pressures equivalent to altitudes of 650, 4,000, 6,000, 7,000 and 8,000 feet above sea level on arterial oxygen saturation and the occurrence of acute mountain sickness and discomfort.

Mean oxygen saturation went down as the altitude increased, eventually reaching a maximum decrease of 4.4 percentage points at 8,000 feet.

Acute mountain sickness occurred in 7.4 percent of participants, but the incidence did not vary between the different altitudes.

Discomfort rose with increasing altitude and was greater at 7,000 to 8,000 feet than at all the other altitudes combined. Discomfort became apparent 3 to 9 hours into the "flight."

Older people (over 60) were less likely to report discomfort than younger people, and men seemed more affected than women.

"There were no health effects, but the discomfort was there," Thibeault said. "What they were trying to do in this study is isolate one factor, which is altitude, so you can't blame these effects on other factors."

Next week, Boeing plans to launch its newest aircraft, the 787, with cabins pressurized to 6,000 feet.

"The 787 is one of our first airplanes with the fuselage made out of composites rather than aluminum, a structure that allows us to pressurize to 6,000 feet," said Jeanne Yu, director of environmental performance for Boeing. "The findings of this study, as well as the development of materials technology, enable us to fly the 787."

But she added, changing cabin pressures in existing aircraft is not that simple because of their aluminum structures.

"Airplanes are designed to withstand the continuous pressurization/depressurization cycles that occur in takeoffs and landings throughout their service life. To lower the maximum cruise cabin altitude, the pressure difference must be increased between the interior and exterior of the airplane. This change would fatigue (wear out) the structure more rapidly and impact the airplane's service life in an uncertain way, perhaps even compromising the design integrity of the airplane," she said.

More information

For more on staying healthy when you fly, head to the Academy of Family Physicians.

SOURCES: J. Michael Muhm, M.D., senior occupational physician, Boeing, Seattle; Jeanne Yu, director, environmental performance, Boeing, Seattle; Claude Thibeault, M.D., medical advisor, International Air Transport Association, Montreal; July 5, 2007, New England Journal of Medicine

Copyright © 2007 ScoutNews, LLC. All rights reserved.