Question
I know your not an expert on brains but do you know how many brain cells die per second without oxygen

Answer
Hello Helen.

Yes, I am not a brain cell expert.  I do not know how many brain cells die per second without oxygen.

A mnemonic that health professions use is:
4 minutes without oxygen, 4 days without water, 40 days without food will usually result in death.  So I guess you could say: in 4 x 60 = 240 seconds, ALL of the brain cells could be dead (?), yet some people survive longer, some do not, some only have partial damages....

People have survived longer time periods then the above without damage.  People have done damage in shorter periods.

The brain is highly dependent upon Oxygen.  Hyperbaric Therapy or Exercise with oxygen therapy can be very helpful for some brain conditions.  Amazon.com has many books on Oxygen therapy.

Keep your brain oxygenated! Breathe deeply, exercise, move!  I do not know the answer, and you may have to define it a little different for those that do.

Good Luck.  Good Health.

Victor Dolan, DC DACBSP
http://www.DocDolan.net