In general, the brain can withstand up to three to six minutes without oxygen before brain damage occurs, but this may vary from person to person. If the brain goes without oxygen longer than this, serious and often irreversible damage is likely to take place. After ten minutes, severe neurological damage has generally occurred. Very few people regain any cognitive function after the brain has gone 15 or more minutes without oxygen.

Since the brain cannot retain oxygen on its own, it relies on a steady supply provided through the bloodstream. Brain death can occur rapidly as cells begin to die off without any oxygen stores or new oxygen circulating to the brain. Almost immediately after oxygen intake has ceased, brain cells begin to perish. Before it occurs damage to the brain on some level, the brain usually only lasts a few short minutes without oxygen.

Occasionally, the brain can last longer without oxygen before brain damage becomes severe, but this usually requires very specific circumstances. It usually requires that oxygen cessation happen at the same time as the body becomes cold very rapidly, such as if a person falls into icy water and begins to drown. Young children have been known to go up to half an hour without oxygen and still survive in situations like this.