In physiology and medicine, dehydration (hypohydration) is the excessive loss of body water, with an accompanying disruption of metabolic processes. The term dehydration may be used loosely to refer to any condition where fluid volume is reduced; most commonly, it refers to hypernatremia (loss of free water and the attendant excess concentration of salt), but is also used to refer to hypovolemia (loss of blood volume, particularly plasma). Dehydration occurs when water loss exceeds water intake, usually due to exercise or disease. Most people can tolerate a three to four percent decrease in body water without difficulty. A five to eight percent decrease can cause fatigue and dizziness. Over ten percent can cause physical and mental deterioration, accompanied by severe thirst. A decrease more than fifteen to twenty-five percent of the body water is invariably fatal. Mild dehydration is characterised by thirst and general discomfort and usually resolves with oral rehydration.