Loss of over 10% of total body water can cause physical and mental deterioration, accompanied by severe thirst.

[10] Dehydration contributes to morbidity in the elderly population, especially during conditions that promote insensible free water losses, such as hot weather.

Risk factors for dehydration include but are not limited to: exerting oneself in hot and humid weather, habitation at high altitudes, endurance athletics, elderly adults, infants, children and people living with chronic illnesses.

In humans, dehydration can be caused by a wide range of diseases and states that impair water homeostasis in the body.

In more extreme cases of low blood pressure, the hypothalamus releases higher amounts of ADH which also acts on V1 receptors.

Dehydration can be life-threatening when severe and lead to seizures or respiratory arrest, and also carries the risk of osmotic cerebral edema if rehydration is overly rapid.

[24] The term "dehydration" has sometimes been used incorrectly as a proxy for the separate, related condition of hypovolemia, which specifically refers to a decrease in volume of blood plasma.

[10] A urinalysis, which is a test that performs chemical and microscopic analysis of urine, may find darker color or foul odor with severe dehydration.

[35] Minimum water intake will vary individually depending on weight, energy expenditure, age, sex, physical activity, environment, diet, and genetics.

[38] In warm or humid weather, or during heavy exertion, water loss can increase markedly, because humans have a large and widely variable capacity for sweating.

Whole-body sweat losses in men can exceed 2 L/h during competitive sport, with rates of 3–4 L/h observed during short-duration, high-intensity exercise in the heat.

The most effective treatment for minor dehydration is widely considered to be drinking water and reducing fluid loss.

Plain water restores only the volume of the blood plasma, inhibiting the thirst mechanism before solute levels can be replenished.

As oral rehydration is less painful, non-invasive, inexpensive, and easier to provide, it is the treatment of choice for mild dehydration.

[45] Solutions used for intravenous rehydration may be isotonic,hypertonic, or hypotonic depending on the cause of dehydration as well as the sodium concentration in the blood.

[46] Pure water injected into the veins will cause the breakdown (lysis) of red blood cells (erythrocytes).

[47] When fresh water is unavailable (e.g. at sea or in a desert), seawater or drinks with significant alcohol concentration will worsen dehydration.

[51] Elderly people with dehydration are at higher risk of confusion, urinary tract infections, falls, and even delayed wound healing.