According to Medline Plus, permanent brain damage begins after four minutes without oxygen supply to the brain. If this condition continues, death follows after another four to six minutes.
This type of injury to the brain is called anoxic brain damage. The NYU Langone Medical Center explains that oxygen is carried to the brain via the bloodstream. If blood flow to the brain is blocked or slowed down, as in the case of a stroke or heart attack, then anoxic brain damage can occur. Another case of anoxic brain damage is due to a lack of oxygen in the blood even when blood flow is normal. Examples of this case include prolonged exposure to carbon monoxide and other toxins or lack of oxygen when drowning or being choked.