Freedivers and mountaineers both tend suffer from crippling effects of brain damage early in life due to prolonged oxygen deprivation. The dementia and alzheimer's usually come about in professionals, as they spend much more time without air compared to a hobbyist. I am unsure what the exact relation is between dive profile, brain degradation, and the long, poorly documented list of other biological variables. I don't think any of the major global war criminals ever thought to study the effects of freediving on the human brain, though there's probably some study on oxygen deprivation itself.

Now that I try to think of all the countries who have performed illegal medical experiments on prisoners... I didn't expect the list to be this long in my head... I thought it would be shorter.