404 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



inspiration. To gain further economy, oxygen from the tanks is mixed 

 with outside air so as to utilize the oxygen available in the atmosphere. 

 In order that this dilution be not too great at the higher altitudes, the 

 mixing occurs through an aneroid-controlled valve. It will be obvious 

 that the safe and economical design of such equipment depends upon 

 precise physiological knowledge concerning the rate of oxygen supply 

 required by the human organism, the individual variations in this 

 requirement, the additional oxygen needed at any altitude, the tolerable 

 inspiratory and expiratory pressures and the effects of varying degrees 

 of work on the respiratory demands. Our modern oxygen supply sys- 

 tems are, accordingly, the products of close cooperation between the 

 physiologist and the engineer. With this equipment, men now go 

 safely to 38,000 feet. 



There new limitations appear, for the barometric pressure at those 

 altitudes is so low that insufficient oxygen goes into the blood passing 

 through the lungs, even though pure oxygen be delivered to the mask. 

 Unless this situation is overcome, new aircraft designs and engine 

 superchargers are not usable in outreaching the range of enemy anti- 

 aircraft fire. To satisfy the human requirements at these great heights, 

 oxygen must be delivered under a pressure sufficiently high to load the 

 blood adequately. 



Two methods are now available for accomplishing this. One is to 

 design the oxygen regulator, valves, and mask so that a pressure can be 

 maintained greater than that of the ambient air. The other is to seal 

 the aircraft cabin and keep the interior at sufficient pressure by means 

 of a mechanical compressor. The first is a temporary expedient. The 

 latter is an ideal solution, for it gives the crew adequate oxygen with 

 complete freedom of movement, unencumbered by mask and tubes lead- 

 ing to a reservoir. The sealed aircraft enclosure has the further con- 

 siderable advantage of providing warm surroundings for men who 

 must quickly go from a temperate or tropic climate to regions where the 

 temperature is 40° F. below zero. With such pressurized cabins the 

 engineers have at last restored to fliers their natural environment, 

 while taking them to altitudes unsuitable for life. 



One of the most dangerous hazards of high-altitude flight is the 

 insidious character of the symptoms of beginning oxygen want. As the 

 early balloonists described them, they are lack of mental clarity or even 

 a sense of well-being that masks the danger of imminent catastrophe. 

 It is, therefore, not feasible to rely upon the judgment of an airman as 

 to when he is in need of his supplementary oxygen supply, unless he be 

 thoroughly familiar with his physiological requirements and the means 

 for satisfying those demands. 



In order to forestall such dangers, the Army Air Forces and the Navy 

 have during this war instituted altitude training programs for instruct- 



