Ray has written on the dangers of hyperoxia and its inability to actually increase tissue oxygenation. Now it looks like there is another danger of inhaling pure O2 - raised prolactin. It also looks like hyperoxia increased prolactin more than hypoxia. So, even more reason to raise our CO2 since it is the cardinal protector against oxygen's dangerous effects.
http://www.ncbi.nlm.nih.gov/pubmed/8954292
"...Our results showed that PRL but no other hormone increased during acute exposure to hyperoxia. This PRL release was independent of exercise stress and greater than PRL augmentation during hypoxia, which was related to a higher relative exercise intensity as indicated by [LA-] and HR. Responses of plasma NH3, BCAA, free TRP and 5-HT could not explain PRL augmentation induced by the increment in blood SO2 during hyperoxia."
http://www.ncbi.nlm.nih.gov/pubmed/8954292
"...Our results showed that PRL but no other hormone increased during acute exposure to hyperoxia. This PRL release was independent of exercise stress and greater than PRL augmentation during hypoxia, which was related to a higher relative exercise intensity as indicated by [LA-] and HR. Responses of plasma NH3, BCAA, free TRP and 5-HT could not explain PRL augmentation induced by the increment in blood SO2 during hyperoxia."
