In physiology, dehydration is a lack of total body water, with an
accompanying disruption of metabolic processes. It occurs when free water loss
exceeds free water intake, usually due to exercise, disease, or high
environmental temperature. Mild dehydration can also be caused by immersion
diuresis, which may increase risk of decompression sickness in divers.
Most people can tolerate a 3-4% decrease in total body water without
difficulty or adverse health effects. A 5-8% decrease can cause fatigue and
dizziness. Loss of over 10% of total body water can cause physical and mental
deterioration, accompanied by severe thirst. Death occurs at a loss of between
15 and 25% of the body water.