Merck Manual

Please confirm that you are a health care professional

honeypot link

Heat Exhaustion


David Tanen

, MD, David Geffen School of Medicine at UCLA

Reviewed/Revised Mar 2023

Heat exhaustion is a non–life-threatening clinical syndrome of weakness, malaise, nausea, syncope, and other nonspecific symptoms caused by heat exposure. Thermoregulation and central nervous system (CNS) function are not impaired, but patients are usually dehydrated and may have mild elevations of body temperature (< 40° C). Treatment involves rest in a cool environment and replacing fluids and electrolytes.

Symptoms and Signs of Heat Exhaustion

Symptoms of heat exhaustion are often vague, and patients may not realize that heat is the cause. Symptoms may include malaise, weakness, dizziness, headache, nausea, and sometimes vomiting. Syncope due to standing for long periods in the heat (heat syncope) may occur. On examination, patients appear tired, are usually sweaty and tachycardic, and may have orthostatic hypotension. Mental status is intact, unlike in heatstroke. Temperature is usually normal and, when elevated, usually does not exceed 40° C.

Diagnosis of Heat Exhaustion

  • Clinical evaluation

Diagnosis of heat exhaustion is clinical and requires exclusion of other possible causes of a patient's symptoms (eg, hypoglycemia, acute coronary syndrome, various infections). Laboratory testing is required only if needed to rule out such disorders. Electrolyte levels should be measured to exclude severe hyponatremia in patients who have had excessive free water intake.

Treatment of Heat Exhaustion

  • Oral or IV fluid and electrolyte replacement

Treatment of heat exhaustion involves stopping all exertion and removing patients to a cool environment, having them lie flat, and attempting oral rehydration with a solution of 0.1% sodium chloride. Patients should drink about 1 L/hour. If vomiting or nausea prevents oral rehydration, IV fluid and electrolyte replacement therapy, typically using 0.9% saline solution, is indicated. Also, if symptoms do not resolve after 30 to 60 minutes of oral rehydration, patients should be transported to an emergency department, where rehydration is usually done IV. Rate and volume of IV rehydration are guided by age, underlying disorders, and clinical response. Replacement of 1 to 2 L at 500 mL/hour is often adequate. Older patients and patients with heart disorders may require lower rates. External cooling measures (see Heatstroke: Treatment Treatment Heatstroke is hyperthermia accompanied by a systemic inflammatory response causing multiple organ dysfunction that may result in death. Symptoms include temperature > 40° C and altered mental... read more ) are usually not required. However, if patients with heat exhaustion have a core temperature of 40° C, measures may be taken to reduce it.

Key Points

  • In heat exhaustion, symptoms tend to be nonspecific, temperature is usually < 40° C, and CNS function is not impaired.

  • Diagnose heat exhaustion clinically, testing as indicated to exclude other clinically suspected disorders.

  • Have patients rest in a cool environment and try oral rehydration, transporting patients to an emergency department if these measures are unsuccessful.

Drugs Mentioned In This Article

Drug Name Select Trade
4-Way Saline, Adsorbonac, Altamist, Ayr Allergy & Sinus, Ayr Baby Saline, Ayr Saline Nasal, BD Posiflush Normal Saline, BD Posiflush Sterile Field Normal Saline, BD Posiflush SureScrub Normal Saline, Blairex Broncho Saline, Breathe Free Saline, Deep Sea , Entsol, HyperSal, Hyper-Sal, Hypertears, Little Remedies for Noses, Little Remedies Stuffy Nose, Monoject Sodium Chloride, Muro 128, NebuSal , Ocean, Ocean Complete, Ocean For Kids, Pediamist, PULMOSAL, Rhinaris, Rhinaris Lubricating, Saljet , Saljet Rinse, SaltAire, Sea Soft, Trichotine, Wound Wash, XYNASE, ZARBEE'S Soothing Saline Nasal Mist
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
quiz link

Test your knowledge

Take a Quiz!