Konrad McDonald of Green Island School asks :-

How long does a brain last without water?

Robin Fraser, a retired pathologist at the Otago University School of Medicine in Christchurch, responded.

An adult without any food (starvation) may live for up to about 50 days, but if water is removed (dehydration), then at the most about 7 days. (This must be fresh water as a kidney cannot cope with salt water.)

A child with a higher metabolic rate and proportionally larger surface area to volume, will lose water quicker in the sweat and breath than an adult and dehydrate earlier. The brain usually becomes disorientated and eventually comatosed before death, but in fact any body organ may fail first, such as the heart from arrhythmia due to the build up of toxins or electrolyte imbalance, or total organ failure may occur from low blood pressure (hypotensive shock).

Dehydration, the main cause of brain failure from lack of water, is much more rapid in a hot, dry area such as the desert, due to sweating and water loss from the lungs, and clots may occur in the brain's veins and sinuses, which will speed up death from circulation problems.