Below what level blood oxygen does brain damage happen?

Hypoxia. That depends on many factors: age of the person when it happened (children tolerate low oxygen better than adults), duration of the low oxygen (longer is worse), how healthy the person is (medical conditions may make the person more sensitive), and the body temperature of the person when it happened (the colder they were, the less oxygen their body and brain needed). There's no simple answer.

Related Questions

I'm considering climbing everest without oxygen, and training by sleeping and running in an altitude tent which will lower my blood 02 sat. Levels to ~70%. Can I do both without brain damage?

No guarantees. People have different susceptibilities to hypoxia depending on many factors including age, physical fitness, and health status. Also, the ability of the brain to withstand hypoxia is time dependent, so time at the summit will be critical. Read more...
Unwise. At prolonged levels of less than 90-92 percent there is a significant risk of brain injury from low oxygen levels. If you were to bring it down to the low 90's over months your body would compensate by increasing the number of red blood cells. Read more...
I agree and: Part of the danger of everest is the traffic jam in the death zone. Waiting to access the fixed ropes on a good weather day hundreds of climbers line up, wait, ; are at risk of frostbite, low oxygen, ; hypothermia. There are many dead bodies up there who made poor choices or had rotten luck. Low oxygen has injured eyes, cost thought--resulting in the wrong life ; death decision when it matters. Read more...

Can you heal or reverse brain damage cause at birth from lack of blood and oxygen to the brain with healing foods or any break through medications?

I'm sorry. Please forgive my frankness. Charlatans prey on the desperation of handicapped people to make money. Anyone offering you "healing foods" or some magic supplement is trying to scam you. However, there are whole practices of medicine that focus on helping folks with this kind of disability -- not with the hope of cure or reversing the tissue injury, but making life easier and more enjoyable. Read more...
NO. There is no scientific evidence that any foods or medications can reverse brain damage. However there are lot of claims that herbs, vitamins, medications , hyperbaric oxygen can help, but unfortunately these are not true. Read more...