Have you ever wondered how well-pressurized airlines keep the cabin of the average commercial flight? I have. So, in my gadget days, I once took my altimeter on a flight and learned that on my particular flight the cabin was pressurized to the equivalent of an altitude of 7200 ft (2195 m) above sea level. At the time, I was living at about 8000 ft (2438 m) so I never gave thought to the fact that a prolonged flight might produce symptoms of acute mountain sickness in otherwise unacclimatized individuals predisposed to the disorder.
Now, in research supported by Boeing and published in last week’s New England Journal of Medicine, researchers at Oklahoma State University simulated long flights at moderate altitudes and revealed that some people might experience discomfort that precedes acute mountain sickness that might otherwise be attributed to jet lag. At altitudes simulating 7000 to 8000 ft, participants experienced discomfort (altitude-related malaise, muscular discomfort, and fatigue) due to a 4% reduction in arterial blood oxygen saturation. So what does this mean for the average traveler? From the paper’s introduction,
A commonly encountered but generally unrecognized exposure to moderate altitude (6500 to 8000 ft [1981 to 2438 m]) occurs during commercial flight. Although the cabins of commercial aircraft are pressurized to protect occupants from the very low barometric pressures at flight altitudes, sea-level pressure (760 mm Hg) is not maintained. Instead, aircraft are designed to maintain cabin pressure at a level no lower than 565 mm Hg (equivalent to an altitude of 8000 ft) when the airplane is at its maximum operating altitude. Higher levels of pressurization decrease the energy available for other aircraft systems, reduce the operational lifetime of aluminum airframes, and necessitate increased structural weight, resulting in decreased fuel efficiency.
Mild forms of acute mountain sickness result from reduced air pressure and hypoxia. According to another excellent NEJM review by Hackett and Roach, “Acute mountain sickness is a syndrome of nonspecific symptoms and is therefore subjective. The Lake Louise Consensus Group defined acute mountain sickness as the presence of headache in an unacclimatized person who has recently arrived at an altitude above 2500 m plus the presence of one or more of the following: gastrointestinal symptoms (anorexia, nausea, or vomiting), insomnia, dizziness, and lassitude or fatigue.”
Treatments (preferably prophylactically) include the steroid, dexamethasone, or the carbonic anhydrase-inhibiting diuretic, acetazolamide (Diamox). This disorder is not merely some laboratory curiosity or restricted to extreme mountain climbers: quoting MedlinePlus,
“Approximately 20% of people will develop mild symptoms at altitudes between 6,300 to 9,700 feet, but pulmonary and cerebral edema are extremely rare at these heights. However, above 14,000 feet, a majority of people will experience at least mild symptoms. Some people who stay at this height can develop pulmonary or cerebral edema.”
Altitude sickness is a public health problem that causes significant discomfort to the individual and has an economic impact on areas dependent on high-altitude tourism, such as ski resorts. But even the relatively low altitudes achieved in pressurized airline cabins are enough to cause mild symptoms in unacclimatized individuals.
We conducted a prospective, single-blind, controlled study to determine the effect of barometric pressures equivalent to terrestrial altitudes of up to 8000 ft on oxygen saturation and the occurrence of acute mountain sickness and discomfort in volunteers selected to represent commercial airline passengers during a simulated 20-hour flight. Secondary objectives were to determine the effect of exercise at these altitudes on discomfort and the effect of altitude on sensory and psychomotor performance.
A total of 502 study subjects were randomized to 5 groups to be placed in a hypobaric chamber for 20 hr at simulated altitudes of 650 ft, 4000 ft, 6000 ft, 7000 ft, or 8000 ft. Arterial oxygen saturation was monitored every two or three hours by pulse oximetry. Just for fun, “During hours 1 through 9 of every test session, five randomly selected participants between the ages of 21 and 60 years exercised by walking on a horizontal treadmill at a rate of 3.0 mi (4.8 km) each hour for 10 minutes per hour.” Symptoms were monitored using a modified Environmental Symptoms Questionnaire (ESQ-IV).
The primary conclusions were that the incidence of acute mountain sickness did not vary by altitude, however:
We found that ascent from ground level to 8000 ft by healthy unacclimatized adults lowered oxygen saturation by approximately 4 percentage points. This degree of hypoxemia did not affect the occurrence of acute mountain sickness, other adverse health outcomes, or impairment of sensory or psychomotor performance, but it was associated with an increased prevalence of discomfort after 3 to 9 hours. Exercise reduced muscular discomfort but did not significantly affect other ESQ-IV factors.
It seems that 7000 to 8000 ft is the cutoff for malaise and headaches among a good percentage of unacclimatized travelers. Therefore, it is of note that commercial airline pressurization flirts with this range. In fact, the authors close with the following,
…we did find evidence that the level of hypoxemia manifested at 7000 to 8000 ft played an important role in the development of discomfort. On the basis of our findings, we conclude that maintaining a cabin altitude of 6000 ft or lower (equivalent to a barometric pressure of 609 mm Hg or higher) on long-duration commercial flights will reduce the occurrence of discomfort among passengers.
Since the study was supported by Boeing, it will be interesting to learn if airlines respond by pressurizing cabins to a lower relative altitude. Moreover, this study raises the question as to whether it is advisable to bring personal oxygen canisters (e.g., O2 To Go, Oxia) for use on cross-country or trans-continental flights.
You can be certain that the companies that sell these products will pay close attention to the results of this study.