Dehydration is measured in several ways, so its statistics depend on the population, setting and definition used. The figures below cover low-intake dehydration, hyperosmolarity, hypernatraemia, diarrhoeal fluid loss and heat-related illness across community, hospital and global settings.
Key Dehydration Statistics
The most broadly useful figures are:
- 24% of non-hospitalised adults aged 65 or older had low-intake dehydration in studies through April 2023 across 12 upper-middle- and high-income countries.
- 34% of non-hospitalised long-term-care residents aged 65 or older had low-intake dehydration in studies through April 2023.
- 23% of hospitalised adults aged 65 or older had low-intake dehydration at or near admission in studies through July 2024.
- 5,039 of 14,398 hospitalised older adults were dehydrated at or near admission across 19 higher-quality studies reviewed through July 2024.
- 27% of 6,632 UK medical emergency admissions aged 65 or older had hyperosmolar dehydration between 1 May 2011 and 31 October 2013.
- 39% of older UK hospital patients with hyperosmolar dehydration developed acute kidney injury in admissions from 1 May 2011 to 31 October 2013.
- Hyperosmolar dehydration was associated with a 4.5-times higher hazard of acute kidney injury within 12–24 hours in that UK cohort.
- Median hospital stay was 5 days for hyperosmolar-dehydration patients versus 3 days for euhydrated patients in the same UK admissions.
- About 50% of elderly U.S. Medicare beneficiaries hospitalized with dehydration died within one year after the 1991 hospitalization.
- Nearly 1.7 billion childhood diarrhoeal disease cases occur globally each year according to the WHO fact sheet current as of 7 March 2024.
- 443,832 children under age 5 die from diarrhoeal disease each year worldwide according to the WHO estimate current as of 7 March 2024.
- 780 million people lack access to improved drinking water worldwide according to the WHO fact sheet current as of 7 March 2024.
- 119,605 heat-related illness emergency visits were recorded in the United States in 2023 through NSSP ESSENCE surveillance.
- 92% of recorded U.S. heat-related illness emergency visits occurred during May–September 2023.
- 303 heat-related emergency visits occurred per 100,000 emergency-department visits in July–August 2023 in U.S. NSSP ESSENCE data.
- WHO reports that oral rehydration can safely and effectively treat more than 90% of acute-diarrhoea dehydration cases except severe cases.
- Reduced-osmolarity oral rehydration solution lowered the need for supplemental intravenous fluids by 33% versus the previous standard solution.
Contents
- Key Dehydration Statistics
- Dehydration prevalence across ages and settings
- Older-adult dehydration data by care setting and risk
- Hospital dehydration outcomes, mortality and length of stay
- Childhood diarrhoeal dehydration and global burden
- Heat-related illness and dehydration data in U.S. emergency care
- Oral rehydration effectiveness, costs and outcomes
Dehydration Prevalence Across Ages and Settings
A systematic review of studies through April 2023 estimated low-intake dehydration in 24% of non-hospitalised adults aged 65 or older across 12 upper-middle- and high-income countries. The pooled 95% confidence interval was 7%–46%, and the evidence quality was low with high heterogeneity (Low-intake dehydration prevalence in non-hospitalised older adults).
The review included 61 studies and 22,398 older participants, but those counts describe the evidence base rather than population prevalence. Individual-study estimates ranged from 0% to 100%, reflecting different samples, settings and measures.
Direct-osmolality studies used a dehydration threshold above 300 mOsm/kg. That definition matters because low-intake dehydration, hypernatraemia and heat-related illness are related measures, not interchangeable diagnoses.
| Population or evidence measure | Result | Scope and period |
|---|---|---|
| Non-hospitalised adults aged 65+ | 24% (95% CI 7%–46%) | 12 upper-middle- and high-income countries; studies through April 2023 |
| Long-term-care residents aged 65+ | 34% (95% CI 9%–61%) | Non-hospitalised; same review |
| Community-dwelling adults aged 65+ | 19% (95% CI 0%–48%) | Non-hospitalised; same review |
| Older adults with more pre-existing illnesses | 37% vs 15% | More versus fewer illnesses; subgroup estimate |
| Older adults with renal impairment | 42% vs 23% | With versus without impairment; non-significant suggestion |
The long-term-care estimate was 34%, compared with 19% among community-dwelling older adults. Older adults with more pre-existing illnesses had an estimated prevalence of 37% versus 15% among those with fewer illnesses, while the estimate was 42% versus 23% for renal impairment; the latter was a non-significant suggestion, not a definitive association (Low-intake dehydration prevalence in non-hospitalised older adults).
Older-Adult Dehydration Data by Care Setting and Risk
A hospital-admission meta-analysis covering studies through July 2024 estimated low-intake dehydration in 23% of hospitalised adults aged 65 or older at or near admission, with a 95% confidence interval of 17%–30%. Confidence was moderate, but heterogeneity was very high at I² 98% (Prevalence of low-intake dehydration in hospitalised older adults).
Across 19 higher-quality hospital studies, 5,039 of 14,398 participants were dehydrated at or near admission, equivalent to 35% as a simple observed proportion. The pooled estimate was lower because meta-analytic weighting gives studies different influence.
Study-level admission prevalence ranged from 1% to 52%. The mean was 24%, the median 23%, and the interquartile range 11%–31%; these descriptive summaries are not adjusted pooled estimates (Prevalence of low-intake dehydration in hospitalised older adults).
A bioimpedance systematic review of evidence through May 2022 cited dehydration as high as 39% among nursing-home residents and approximately 25% among hospitalised older patients. Only four eligible acute-care detection studies were found: cohorts included 61 ICU patients, 27 acute-stroke patients, 32 ward patients and 169 geriatric-ward patients, limiting generalisability (Detecting low-intake dehydration using bioelectrical impedance analysis in older adults in acute care settings).
The same review reported that estimated glomerular filtration rate declines by more than 50% between ages 30 and 80. This is a renal-aging risk context, not a direct dehydration prevalence statistic.
Hospital Dehydration Outcomes, Mortality and Length of Stay
In a UK teaching-hospital cohort of 6,632 medical emergency admissions aged 65 or older from 1 May 2011 to 31 October 2013, 27% had hyperosmolar dehydration, defined as calculated osmolarity above 300 mOsm/L. Among those patients, 39% developed acute kidney injury, and the adjusted hazard of AKI within 12–24 hours was 4.5 times higher (HR 4.5, 95% CI 3.5–5.6) than among euhydrated patients (Hyperosmolar dehydration: A predictor of kidney injury and outcome in hospitalised older adults).
Median stay was 5 days for hyperosmolar-dehydration patients versus 3 days for euhydrated patients; the first- to third-quartile ranges were 1–12 and 1–9 days. Hyperosmolar dehydration was also associated with 60% greater 30-day mortality (HR 1.6, 95% CI 1.4–1.9), but these are retrospective associations affected by illness severity and comorbidity.
Historical U.S. Medicare data show a different measurement frame. In 1991, dehydration appeared in 6.7% of 731,695 hospitalizations, corresponding to 236.2 hospitalizations per 10,000 elderly beneficiaries; Medicare reimbursed more than $446 million when dehydration was the principal diagnosis (The burden and outcomes associated with dehydration among US elderly, 1991).
That 1991 reimbursement is nominal and not inflation-adjusted or a measure of total societal cost. Approximately 50% of elderly Medicare beneficiaries hospitalized with dehydration died within one year, a historical observational outcome in a high-risk population rather than attributable mortality.
In the HOOP prospective cohort of older adults acutely admitted to a UK teaching hospital, 7% died in hospital, and 79% of those who died were dehydrated at admission. The 79% figure is conditional on those who died and does not mean that 79% of dehydrated patients died (Hydration and outcome in older patients admitted to hospital).
Care-home residence was also associated with admission hypernatraemia, a proxy rather than an identical measure of all dehydration. Hypernatraemia prevalence was 12.0% among older hospital patients from UK care homes versus 1.3% among those from their own homes; the adjusted odds ratio was 5.32 (95% CI 3.85–7.37) after adjustment for age, sex, admission mode and dementia (Are patients admitted to hospitals from care homes dehydrated?).
A historical Torino geriatric-ward study of admissions from January 1990 through July 1995 found hypernatremic dehydration in 2.9% of 2,894 patients, or 84 people. Prevalence was 1.6% below age 65 and 5.3% above age 85, while in-hospital mortality among affected patients was 29.8%; mortality was 33.3% with serum sodium 151–153 mEq/L and 71.4% above 154 mEq/L (Hypernatremic dehydration in the elderly on admission to hospital).
Childhood Diarrhoeal Dehydration and Global Burden
The WHO estimates nearly 1.7 billion childhood diarrhoeal disease cases globally each year. Diarrhoeal disease is a major driver of dehydration, but the case and death totals include mechanisms beyond dehydration, including sepsis (Diarrhoeal disease).
WHO estimates 443,832 deaths from diarrhoeal disease each year among children under 5 and another 50,851 among children aged 5–9. Diarrhoeal disease is the third leading cause of death among children aged 1–59 months; these are disease-wide figures, not isolated dehydration mortality.
| WHO measure | Estimate | Geography or population |
|---|---|---|
| Childhood diarrhoeal disease cases | Nearly 1.7 billion/year | Worldwide |
| Deaths under age 5 | 443,832/year | Worldwide |
| Deaths ages 5–9 | 50,851/year | Worldwide |
| People without improved drinking water | 780 million | Worldwide |
| People without improved sanitation | 2.5 billion | Worldwide |
| Diarrhoeal episodes | 3 per child/year | Children under 3 in low-income countries |
About 780 million people lack improved drinking water and 2.5 billion lack improved sanitation worldwide. Children under 3 in low-income countries experience an average of three diarrhoeal episodes per year, while WHO classifies persistent diarrhoea as lasting at least 14 days.
WHO classifies severe dehydration as at least two specified danger signs, including lethargy, sunken eyes, poor drinking or a skin pinch returning in at least 2 seconds. These clinical definitions help identify risk but are not population prevalence estimates (Diarrhoeal disease).
Heat-Related Illness and Dehydration Data in U.S. Emergency Care
CDC surveillance recorded 119,605 heat-related illness emergency visits in the United States from 1 January through 31 December 2023 through NSSP ESSENCE facilities. Heat-related illness includes more than dehydration, and the surveillance system does not capture every case or every U.S. facility (Heat-Related Emergency Department Visits — United States, May–September 2023).
Ninety-two percent of recorded 2023 visits occurred during May–September. July–August averaged 303 visits per 100,000 emergency-department visits, versus 97 per 100,000 in May, June and September, a seasonal rate ratio of 3.07.
The warm-season rate was 271 per 100,000 emergency-department visits among males versus 104 among females. Rates ranged from 207 to 222 per 100,000 among adults aged 18–64 and from 120 to 173 among adults aged 65 or older.
After data-quality filtering, the analysis included as many as 823 participating emergency facilities, with a median of 111. This describes surveillance coverage rather than the national number of facilities or a dehydration-specific burden.
Oral Rehydration Effectiveness, Costs and Outcomes
WHO reports that oral rehydration can safely and effectively treat more than 90% of acute-diarrhoea dehydration cases except severe cases, when appropriate oral rehydration solution is used. This is not a cure rate for every cause or severity of dehydration (The treatment of diarrhoea).
Reduced-osmolarity oral rehydration solution lowered the need for supplemental intravenous fluids by 33% compared with the previous standard solution. In the same WHO treatment comparison, it lowered vomiting incidence by 30% and stool volume by 20%.
For childhood diarrhoeal disease, WHO says zinc supplementation reduces episode duration by 25% and is associated with a 30% reduction in stool volume. These are adjunct-treatment effects on diarrhoeal outcomes and fluid loss, not direct dehydration prevalence measures (Diarrhoeal disease).
Taken together, the treatment statistics show why measurement context matters: oral rehydration effectiveness applies chiefly to non-severe acute diarrhoeal dehydration, while hospital prevalence, hypernatraemia, hyperosmolarity and heat-related surveillance describe different populations and endpoints.