Investigation to Determine the Effects of Aircraft Cabin Altitudes on Passenger Comfort and Discomfort

Altitude affects human health and well being through its effect on tissue oxygenation by altering the partial pressure of oxygen in the gas that enters the lungs. The partial pressure of any component of a mixed gas is equal to the total pressure of the gas multiplied by the fraction of the gas that is made up by the component--approximately 78% for nitrogen and 21% for oxygen in air at all altitudes. As altitude increases, total air pressure decreases and consequently, the partial pressures of all component gases, including oxygen, decrease. As air is inhaled, it becomes saturated with water vapor from body tissues, further reducing its partial pressure of oxygen. Tissue oxygenation, measured in terms of partial pressure of oxygen in arterial blood (paO2), is directly related to the partial pressure of oxygen in the gas entering the lungs.

The United States Federal Aviation Agency requires that commercial aircraft be designed so that the barometric pressure in the cabin at maximum cruise altitude exceeds that found in the external atmosphere at 8,000 feet (565 mm Hg).1 The scientific basis for this limit is unclear. Although there is a large body of knowledge concerning the effects of altitude on humans, most of it involves healthy young people at altitudes higher than 8,000 feet. An investigation of the effects of altitude on commercial airline passengers performed by McFarland in 1937 found "...older persons up to 72 years of age respond to moderate altitudes, i.e., up to 16,000 feet, without unusual difficulties." However, the same author went on to conclude, "The human factors analysis presented here would suggest that the comfort and well-being of airline passengers would be significantly benefited by as near sea level conditions as possible. In any event, cabin altitudes of 3,000 - 5,000 feet should not be exceeded." 2, 3 In 1986, the Committee on Airliner Cabin Air Quality of the National Research Council (NRC) concluded, "Similarly, the decrease in oxygen partial pressure (pO2) that occurs at 8,000 feet is safe for normal people, but possibly hazardous for patients with COPD." 4 In a report published in 2001, the NRC concluded that research into the effect of cabin pressure on susceptible persons should be given high priority.5

Acute Mountain Sickness (AMS), a syndrome characterized by headache, anorexia, nausea, vomiting, lassitude and sleep disturbance, the onset of which closely follows ascent to altitude6 has been reported in 12 to 42% of visitors to altitudes ranging between 6,300 and 10,000 feet in the mountains of Colorado.7-12 The time course of these symptoms was not reported in sufficient detail to allow characterization of their onset during the first several hours after arrival at altitude.

Hypoxia-related medical problems during flight are rare..13-18 However, an unpublished statistical model 19 predicts that at 8,000 feet a substantial proportion of persons whose ages are similar to those of commercial aircraft passengers will experience paO2s below the levels at which use of supplemental oxygen is recommended by various medical authorities.20-24 If the recommendations for use of supplemental oxygen are valid,25, 26 it is possible that exposure to these levels of hypoxia for up to 20 hours (the maximum anticipated duration of commercial aircraft flight segments) could adversely affect the comfort and well being of passengers.

Physical exercise has been demonstrated to reduce blood oxygenation in persons with normal and abnormal pulmonary health.27-29 Some members of the aircraft crew perform moderate levels of work as part of their job30-34, and exercise is recommended by some to alleviate the discomfort experienced as a result of the prolonged inactivity encountered in flight. It is unknown if moderate exercise has a beneficial or deleterious effect on the comfort and well being of persons at altitude.