The Difference Between Dehydration, Heat Exhaustion and Heat Stroke

By: Emory Healthcare
Date: Jun 8, 2026
Health and WellnessEmergency CareHealth and WellnessPreventive Care
a woman sitting outside wiping her brow with a towel and holding a bottle of water

Hot weather can put serious stress on your body, especially in the Southeast. Dehydration, heat exhaustion and heat stroke are all heat‑related conditions, but they are not the same. Knowing the differences can help you take the right action quickly to have a healthy, safe summer.

Dehydration vs. Heat Exhaustion vs. Heat Stroke: Key Differences

Dehydration
  • Happens when your body loses more fluids than it takes in
  • Can develop gradually
  • Often mild at first but can worsen without treatment
Heat exhaustion
  • Caused by excessive heat and fluid loss, often from heavy sweating
  • More serious than dehydration
  • Can progress quickly, especially during high heat and humidity
Heat stroke
  • A medical emergency
  • Occurs when the body can no longer regulate its temperature
  • Can cause organ damage or death if not treated immediately

What is dehydration?

One of the most important things you can do to prevent heat illness is staying hydrated. Without the right amount of fluid intake, your body can’t keep its temperature at a normal, consistent level. Dehydration happens when your body lacks the proper amount of fluids and electrolytes to keep working properly.

Common dehydration symptoms include:

  • Thirst
  • Less frequent urination than normal
  • Darker urine color
  • Dry skin
  • Tiredness
  • Dizziness and/or fainting
  • Headache

In infants and young children, signs may include:
  • Dry mouth and tongue
  • Crying without tears
  • No wet diapers for several hours
  • Fever
  • Unusual sleepiness or drowsiness

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What is heat exhaustion?

Heat exhaustion is a serious condition. Heat exhaustion happens when the body loses a great deal of water and salt (which is usually caused by profuse sweating). Cases of heat exhaustion are made much worse whenever there is a high level of humidity or physical activity involved. Notably, heat exhaustion can cause any pre-existing conditions to worsen or become more apparent. It is important to know the signs of heat exhaustion, as it could take place within a very short period of time.

Signs and symptoms of heat exhaustion include:

  • Muscle cramping
  • Fatigue
  • Headache
  • Nausea and/or vomiting
  • Dizziness and/or fainting

Someone with heat exhaustion may have cool or even slightly wet skin, which indicates the body is still working toward cooling itself; however, the individual will likely have a fast and faint pulse and exhibit quick and shallow breathing.

 

What is heat stroke?

Heat exhaustion left untreated may result in heat stroke. This is a life-threatening condition that can result in damage to the brain or other important organs. In some cases, heat stroke may cause multiple organ systems to fail and can ultimately cause death. During heat stroke, the body’s core temperature rises above 106° F within a time frame of 10 to 15 minutes. This rapid increase in body heat coupled with a failing sweating mechanism leaves the body without the ability to cool itself.

Heat stroke symptoms include:

  • Confusion
  • Altered mental state
  • Slurred speech
  • Unconsciousness
  • Hot, dry skin or extreme sweating
  • Seizure
  • Extremely high blood pressure

When to seek emergency care for heat illness

Call 911 or seek emergency medical care right away if someone:

  • Has symptoms of heat stroke
  • Is confused or unconscious
  • Has a very high body temperature
  • Stops sweating in extreme heat

If symptoms of dehydration or heat exhaustion do not improve with rest, cooling and fluids, seek medical care as soon as possible.

For milder symptoms, or if you’re not sure which kind of care you need, start with your primary care physician (PCP). Because your PCP knows your complete health history and how you respond to medication, he or she can develop the best course of treatment. If a higher level of care is needed, they can guide you to the right care facility.

 

How to prevent heat‑related illness

  • Drink water regularly, even if you do not feel thirsty
  • Take breaks in shaded or air‑conditioned areas
  • Avoid strenuous activity during the hottest parts of the day
  • Wear lightweight, light‑colored clothing
  • Pay close attention to children, older adults and people with chronic conditions

A little preparation goes a long way — stay cool, stay hydrated and take care of yourself and those around you this summer.

 

This article was originally published Jun 21, 2022 and has since been updated.

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