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airliner passenger cabin
While commercial airliner cabins are pressurized to what the FAA considers a safe level, there still may not be enough oxygen for some older passengers. (credit: Boeing)

Human factors in commercial suborbital flight: Getting adequate oxygen

The previous column in this series (“Human factors in commercial suborbital flight: what do I breathe, and why?”, The Space Review, March 7, 2005) discussed the way that water vapor, and to a lesser extent carbon dioxide, displace a disproportionate amount of the oxygen in the lungs as altitude increases. Since we all need some oxygen to survive, this is a problem. How much do we need? Decompression to about 6,000 meters (20,000 feet) leaves a healthy normal young person (as in a sedentary military aviator) conscious for about six minutes. This time of useful consciousness decreases to between two and three minutes at 7,600 meters (25,000 feet), and about 15 seconds at 14,000 meters (45,000 feet). Some people will die at less than 20,000 feet, and most will eventually die at 28,000 feet while breathing air. The basic rule is to maintain the partial pressure of oxygen in arterial blood (after it is absorbed by the blood in the lungs) at 50-55 mm Hg. Keep in mind that there is an additional decrement in the partial pressure of oxygen between the gases in the lung and in the arterial blood of several mm Hg.

The cabin designer of a suborbital commercial spacecraft will have to perform a design tradeoff between minimizing cabin weight by pressurizing the cabin to the highest equivalent altitude possible while in space, and considering the demographic characteristics of the paying passengers.

The two primary ways of dealing with this problem are to either breathe supplemental oxygen or to pressurize the cabin, or both. Commercial airline cabins were required to be pressurized to a maximum equivalent altitude of 3,000 meters (10,000 feet, or 523 mm Hg) until 1957, after which they were required to maintain a maximum allowed altitude of 2,500 meters (8,000 feet, or 564 mm Hg). Recent investigations by the National Research Council have raised concerns that the current 8,000-foot limit might have an adverse affect on some passengers and crew. A recent study by J. M. Muhm at Boeing concludes that “…a substantial proportion of older passengers will [have arterial oxygen levels at 8,000 feet that fall]… below the threshold at which supplemental oxygen is recommended.”

At the commercial standard of 8,000 feet, Muhm’s estimate was that 44 percent of healthy passengers aged 65 years or more will have inadequate arterial oxygen levels while breathing air. His estimates for younger, otherwise healthy, people were that 27 percent of those 55 years and older, and 14 percent of those 45 years and older fall into this category [Ref. 1].

Therefore, the cabin designer of a suborbital commercial spacecraft will have to perform a design tradeoff between minimizing cabin weight by pressurizing the cabin to the highest equivalent altitude possible while in space, and considering the demographic characteristics of the paying passengers. The changes in oxygen exchange within the lungs in the general population as a function of altitude are not well characterized. The military has performed extensive studies of this nature in well-defined, younger populations that match their air crew demographics, but how many of these people can afford to buy a ticket to space?

Reference:

1. Predicted Arterial Oxygenation at Commercial Cabin Altitudes. J. M. Muhm, Aviation, Space, and Environmental Medicine, Vol. 75, No. 10, pp. 905-912, Oct. 2004.


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