Estimating Dehydration Survival Limits from Plausible Mortality Thresholds

The limits of human survival during dehydration are not well defined and are infrequently based on physiological endpoints. We evaluated dehydration survivability using thresholds for hypernatremia and hypovolemia that are consistent with known mechanisms of mortality. Methods A mass balance approach was used to model changes in blood sodium concentration and blood volume in a 75 kg adult with total body water of 43.7 L. Dehydration was scaled at a 10% loss of body mass (7.5 L). Water and electrolyte losses were modeled under conditions of sweating in warm (∼32 °C) and cool (∼4 °C) environments and during gastrointestinal illness. Blood sodium and blood volume responses were calculated based on volume-weighted sodium losses and standard compartmental assumptions. Results The time to reach 10% dehydration ranged from 2.5 days (eccrine warm) to 5.9 days (eccrine cool) and 3.0 days with gastrointestinal losses. Blood sodium increased to 160 mmol·L −1 with sweating and 146 mmol·L −1 with gastrointestinal losses. Estimated blood volume reductions were 0.844 (sweating) and 1.327 L (gastrointestinal). These values reach or approach those associated with fatal hypernatremia and circulatory shock. Discussion A dehydration level of approximately 10% body mass (17% total body water) produces physiological conditions consistent with mortality from both hypernatremia and hypovolemia. Survival without water is therefore likely limited to approximately 3 to 6 days under passive conditions, depending on environmental exposure, fluid loss pathways, and individual variability. Conclusions Hypernatremia and hypovolemia provide physiologically grounded thresholds for estimating dehydration survivability. These findings refine existing survival estimates and highlight the critical importance of minimizing fluid loss in austere environments.