Heat exhaustion is a heat-related illness that develops when your body loses too much water and salt through sweating and can no longer cool itself effectively. Core body temperature may rise but typically stays below 104°F (40°C). It’s the stage before heatstroke, and recognizing it early makes the difference between a quick recovery and a medical emergency.
What Happens in Your Body
When you’re exposed to high heat or exercising hard in warm conditions, your body cools itself primarily by sweating and by sending more blood to the skin’s surface. Both of these strategies cost your body fluid and shift blood flow away from your core organs. Heat exhaustion sets in when the fluids lost through sweating aren’t replaced fast enough. Blood volume drops, blood pressure falls, and your cardiovascular system struggles to keep up with the demand to cool the skin while also supplying your muscles and brain.
The result is a cascade of symptoms driven by dehydration and electrolyte loss. Your body is still trying to cool itself (you’re still sweating, unlike in heatstroke), but it’s losing the battle.
Symptoms to Recognize
Heat exhaustion comes on gradually, which makes it easy to dismiss early signs as just feeling “off.” The hallmark symptoms include:
- Heavy sweating
- Cool, moist skin, sometimes with goose bumps despite the heat
- Dizziness or faintness
- Nausea
- Headache
- Fatigue and weakness
- Weak, rapid pulse
- Muscle cramps
- Low blood pressure when standing up
- Decreased urine output
Many of these symptoms are posture-dependent, meaning they worsen when you stand. That’s because blood is already pooling near the skin’s surface for cooling, so standing up makes it even harder for your heart to deliver enough blood to your brain.
How It Differs From Heatstroke
The critical distinction is what’s happening in the brain. With heat exhaustion, you may feel foggy or irritable, but you’re coherent and aware. Heatstroke crosses into neurological territory: confusion, slurred speech, seizures, loss of consciousness. Body temperature in heatstroke can spike to 106°F or higher within 10 to 15 minutes.
Another key difference is sweating. In heat exhaustion, you’re still sweating heavily. In many heatstroke cases, sweating stops entirely because the body’s cooling system has failed. If someone who was showing signs of heat exhaustion suddenly becomes confused or stops sweating, that’s a medical emergency.
Who Is Most at Risk
Anyone can develop heat exhaustion, but certain groups face significantly higher risk. Older adults and young children are especially vulnerable because their bodies regulate temperature less efficiently. Children also have a higher surface-area-to-body-weight ratio, meaning they absorb heat faster. Older adults often have a blunted thirst response, so they may not drink enough even when losing fluid rapidly.
People with heart disease, diabetes, kidney disease, or obesity also face elevated risk. Excess body weight acts as insulation, trapping heat. Cardiovascular conditions limit the heart’s ability to ramp up blood flow to the skin for cooling.
Medications That Increase Risk
Several common medications interfere with your body’s ability to handle heat, and many people taking them don’t realize it. The mechanisms vary:
- Diuretics (water pills): Deplete fluid and electrolytes, reduce thirst sensation
- Beta-blockers: Limit blood vessel dilation and reduce the heart’s ability to increase output for cooling
- Antidepressants (SSRIs, tricyclics): Can alter sweating, either increasing or decreasing it
- Antihistamines (like diphenhydramine): Block sweating through anticholinergic effects
- ADHD stimulant medications: Interfere with the brain’s temperature regulation
- Anti-inflammatory painkillers (NSAIDs): Can reduce kidney blood flow when you’re already dehydrated
Combinations of these medications compound the risk. If you take a blood pressure medication paired with a diuretic, for example, the effect on heat tolerance is more than additive. This is worth keeping in mind during heat waves or before spending extended time outdoors.
Environmental Thresholds
Risk doesn’t start at extreme temperatures. Research using wet-bulb globe temperature (WBGT), a measurement that accounts for heat, humidity, wind, and sun exposure, found that heat illness risk increases significantly above 80°F (27°C) WBGT. Categories used by occupational health organizations break it down: 80 to 84°F WBGT is a warning zone, 84 to 89°F is a severe warning, and anything above 89°F is considered dangerous.
Humidity matters as much as temperature. A 90°F day with 30% humidity is manageable for most people. The same 90°F day with 80% humidity can be genuinely dangerous because sweat doesn’t evaporate efficiently, crippling your body’s primary cooling mechanism.
What to Do When It Happens
If you or someone around you shows signs of heat exhaustion, act immediately. The goal is to cool the body and replace lost fluids before the condition progresses.
- Move to shade or an air-conditioned space
- Lie down with legs elevated slightly to help blood return to the core
- Remove any tight or heavy clothing
- Sip cool water or a sports drink (diluted to half or quarter strength works well)
- Spray or sponge skin with cool water while fanning to promote evaporation
- Apply cool, wet cloths to the neck, forehead, and armpits
The emphasis on sipping rather than gulping matters. Drinking too much too fast when you’re already nauseated can cause vomiting, which worsens dehydration. Small, steady amounts of fluid over 15 to 30 minutes are more effective. Sports drinks help because they replace sodium and potassium lost through sweat, but they don’t need to be full strength. Diluting them reduces the sugar concentration, which helps your stomach absorb the fluid faster.
Most people start to feel better within 30 to 60 minutes of aggressive cooling and rehydration. If symptoms don’t improve within that window, or if confusion, vomiting, or loss of consciousness develops, that suggests progression toward heatstroke and requires emergency medical care.
Recovery and Returning to Activity
Feeling better after an episode doesn’t mean your body has fully recovered. Heat exhaustion leaves you more susceptible to heat illness for days afterward. Your fluid balance, electrolyte levels, and cardiovascular system all need time to normalize.
For athletes and people with physically demanding jobs, guidelines from the National Athletic Trainers’ Association recommend a rest period of 7 to 21 days before returning to activity after a significant heat illness episode. Even after that rest period, the return should be gradual: starting with light activity, progressing to moderate exercise, and only resuming full intensity after several days of tolerance at each stage. In documented cases of severe exertional heat illness in athletes, full clearance for unrestricted activity has taken up to 60 days.
For non-athletes who experienced heat exhaustion during yard work, a hike, or a hot day outdoors, the recovery is typically faster. But it’s still wise to avoid heavy exertion in the heat for at least 48 to 72 hours and to pay close attention to hydration during that window. Your body’s heat tolerance resets over days, not hours, and pushing back into the same conditions too soon is the most common path to a repeat episode.