How much of your body's water can you lose before it kills you?

Death comes with the loss of somewhere between 15 and 25 percent of your total body water. Long before that, the damage builds up in stages: losing just 1 to 2 percent, still called mild dehydration, has been shown to impair cognitive performance. Most people tolerate a 3 to 4 percent loss without difficulty, a 5 to 8 percent loss can cause fatigue and dizziness, and beyond 10 percent come physical and mental deterioration and severe thirst.

Dehydration means a lack of total body water that disrupts the body's metabolic processes. It happens whenever water loss outpaces intake, often through heavy sweating, illness or simply not drinking enough. It can even happen underwater: divers can become mildly dehydrated through immersion diuresis, which may raise their risk of decompression sickness.

Its hallmarks include thirst, headaches, nausea, confusion, unexplained tiredness, less urine than usual, purple fingernails and seizures. It can push blood sodium too high, a condition called hypernatremia, which is different from losing blood volume. Chronic dehydration can cause kidney stones and chronic kidney disease. Mild cases usually resolve with drinking (oral rehydration), while severe ones may need intravenous fluids.

Age changes the picture. After 50, the body's sense of thirst fades, yet one study found no difference in fluid intake between young and old people. Many older people show symptoms of dehydration, the most common being fatigue.