The term “critical threshold” is used in many branches of science and
can have many different meanings but we cannot find a definition appropriate to
HP&L and altitude. The term “critical altitude” used has been
noted in a training video for high altitude ballooning and they used a figure
of 18,000 feet. All the usual text books point out that above that
level, or thereabouts, unconsciousness is a certainty and there is an increased
risk of decompression sickness. In Campbell and Bagshaw
they point out that a ‘continuous flow’ oxygen system is adequate at cabin
altitudes up to 22,000 feet but above that a more sophisticated system is
appropriate and, based on that and other requirements, they imply that 22,000
feet is a critical altitude. Hawkins doesn’t set a level but
stresses that it varies from person to person and smoking/non-smoking makes a
great difference but he, and other authors, imply that flight above about
18,000 to 22,000 feet becomes ‘critical’.