[thackney]

Thanks for posting that. I have done the same and never knew about this. I thought I was supersaturating the blood with oxygen.

After your post and some quick searching, I learned that there is little difference in the O2 level, only the CO2 level is depressed, which is what triggers the urge to breath.

People hyperventlate, reduce the CO2 levels, run out of O2, pass out without feeling the urge to breath.

http://www.scuba-doc.com/latenthypoxia.html

[Crolis]

Thanks for posting that link. It’s really interesting how human physiology adapts to water, sometimes with negative results.

[Moonman62]

From your link:

Medical researchers feel that many pool deaths, classified as drownings, are really the result of shallow-water blackout. Most occur in male adolescents and young adults attempting competitive endurance breath-holding, frequently on a dare.