From: Survey of Current Cardiovascular and Respiratory Examination Methods in Medical Selection and Control of Aircrew by A. Scano, NATO Agardograph No. 196, published December 1974, page 77.
“…the test can be carried out with the low oxygen mixture technique. In this case the subject is made to breathe, by means of a mouth-piece, a low oxygen mixture containing 9.5% oxygen in nitrogen. The partial pressure of the oxygen in this mixture corresponds to the pressure at the altitude of of about 6500 m a.s.l. (exactly 6185).
“This difference as compared with the simulated altitude used in the low pressure chamber was introduced as a result of experience which shows that the effects of a low oxygen mixture are less marked than those of decompression, the oxygen partial pressure being equal.
The simulated altitude of 5500 m was adopted for physiological reasons. It is well known, in fact, that at this altitude we pass from the zone of complete compensation to that of incomplete compensation.
“Also, in the case of the use of low oxygen mixture, the tests are carried out 15 minutes after the subject begins to breathe the mixture.”