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’.