Exercising in hot weather is safe for most people, but it requires real adjustments to your routine. Heat forces your body to cool itself and fuel your muscles at the same time, and when conditions get extreme enough, your cardiovascular system simply can’t do both. In 2023, U.S. emergency departments recorded nearly 120,000 heat-related visits, with men visiting at roughly 2.7 times the rate of women. The risks are manageable if you understand what heat does to your body and where your personal limits sit.
What Heat Does to Your Body During Exercise
When you exercise in the heat, your body faces a tug-of-war for blood flow. Your working muscles need blood to deliver oxygen, and your skin needs blood to radiate heat away from your core. During shorter bouts of exercise, your body handles this competition reasonably well: your heart pumps harder, your blood vessels redirect flow from your gut and organs to your skin, and sweat begins evaporating to cool you down. Active muscles generally win the competition for blood flow in the short term, meaning your performance holds up initially.
The trouble starts when exercise is prolonged and you lose fluid through sweat without replacing it. As dehydration sets in, your heart’s output can drop by nearly 4 liters per minute. That’s a massive reduction. Your stroke volume (the amount of blood pumped per heartbeat) falls, your heart rate climbs to compensate, and blood flow to both your muscles and your skin decreases. The result is that you overheat faster and your muscles fatigue sooner. Staying hydrated largely prevents this cascade. Studies show that drinking enough fluid during exercise in the heat fully prevents the decline in stroke volume and the spike in heart rate that dehydration causes.
Humidity Matters More Than Temperature Alone
Your body’s primary cooling tool is sweat evaporation, and humidity determines how well that tool works. At around 33% relative humidity, roughly half of the sweat you produce actually evaporates and cools you. At 70% humidity, only about 28% of your sweat evaporates. At nearly 90% humidity, that number plummets to around 16%. The rest pools on your skin and drips off your body, doing nothing to lower your temperature.
This is why a 85°F day in New Orleans feels far more dangerous for exercise than a 95°F day in Phoenix. In dry heat, your sweat works efficiently. In humid heat, your body keeps producing sweat at the same rate but gets a fraction of the cooling benefit, and you lose fluid faster for less return. If you live in a humid climate, you need to be more conservative with exercise intensity and duration than the air temperature alone might suggest.
Temperature Thresholds for Outdoor Activity
Weather services and athletic organizations use a measurement called the Wet Bulb Globe Temperature (WBGT), which combines air temperature, humidity, wind, and sun exposure into a single number. It’s a better gauge of danger than temperature alone. Many weather apps now report it, and portable WBGT monitors are available for under $100.
Guidelines from high school athletic associations offer a practical framework anyone can use:
- Below 80 WBGT: Normal activity, with water breaks every 30 minutes.
- 80 to 85 WBGT: Standard exercise for fit individuals. New or unconditioned exercisers should reduce intensity. Rest and water breaks every 25 minutes.
- 85 to 88 WBGT: Reduce intensity, take rest and water breaks every 20 minutes, and watch closely for symptoms of heat illness.
- 88 to 90 WBGT: Strip down to minimal clothing, take breaks every 15 minutes, and keep exercise light.
- Above 90 WBGT: Suspend outdoor exercise entirely. Move indoors to air conditioning or wait for cooler conditions.
Building Heat Tolerance Takes 7 to 14 Days
Your body can adapt to heat, but it needs a gradual introduction. The CDC recommends increasing your exposure over 7 to 14 days. If you’re new to exercising in the heat, start with about 20% of your normal duration or intensity on the first day and add roughly 20% each subsequent day. If you’ve exercised in heat before but took time off, you can be a bit more aggressive: start at 50% on day one, move to 60% on day two, 80% on day three, and full effort by day four.
During acclimatization, your body learns to start sweating earlier, produce more dilute sweat (preserving electrolytes), increase blood plasma volume, and lower your resting core temperature. These adaptations are significant, but they fade within a few weeks of avoiding heat exposure. A vacation in a cool climate or a stretch of mild weather means you’ll need to re-acclimatize when the heat returns.
How Much to Drink
Sweat rates during exercise range from about half a liter to 4 liters per hour, depending on your size, fitness level, and the conditions. A general target is roughly 200 milliliters (about 7 ounces) every 15 to 20 minutes. That’s a few good gulps from a water bottle at regular intervals.
Plain water is fine for most sessions under an hour. For longer or more intense efforts, you need sodium too. Sweat sodium losses vary enormously between people, ranging from 0.2 to 7.3 grams per hour. If you notice white salt stains on your clothing after exercise, you’re a heavier salt sweater and should add electrolytes to your drinks or eat salty snacks during longer workouts. Sports drinks, electrolyte tablets, or even a pinch of salt in water all work.
Thirst is a reasonable guide for casual exercise, but it lags behind actual fluid needs during intense or prolonged efforts. Weighing yourself before and after a workout gives you a precise picture: each pound lost represents about 16 ounces of fluid you should have consumed.
Heat Exhaustion vs. Heat Stroke
Heat exhaustion is your body waving a yellow flag. You’ll feel heavy fatigue, nausea, headache, dizziness, or heavy sweating. Your skin may feel cool and clammy. This is your cue to stop exercising, get to shade or air conditioning, and drink fluids. Most people recover within 30 minutes to an hour with rest and cooling.
Heat stroke is a medical emergency. Your core body temperature can spike to 106°F or higher within 10 to 15 minutes. You may stop sweating entirely, become confused, slur your speech, or lose consciousness. The most effective immediate treatment is full-body cold water immersion, which cools the body faster than any other method. If someone shows signs of heat stroke during exercise, getting them into cold water quickly, even a bathtub, kiddie pool, or horse trough, while waiting for emergency help can be the difference between full recovery and lasting organ damage.
Who Faces Higher Risk
Children absorb more heat per pound of body weight than adults when the air temperature exceeds skin temperature, because they have a higher surface area relative to their mass. Their sweat glands are also about 27% smaller than adult glands, producing less sweat per gland. Children rely more on increasing blood flow to the skin than on sweating, which works well in mild heat but becomes a disadvantage when conditions get truly hot. They need more frequent breaks and closer monitoring than adults exercising in the same conditions.
Older adults face compounding risks: reduced thirst sensation, lower cardiovascular reserve, and a higher likelihood of taking medications that interfere with heat regulation. Common medications that increase heat vulnerability include diuretics and blood pressure drugs (which can cause fluid loss, reduce blood vessel dilation, and blunt thirst), antihistamines like diphenhydramine (which reduce sweating), beta-blockers (which limit the heart’s ability to increase output and impair sweating), and many psychiatric medications including antidepressants and antipsychotics (which can interfere with the brain’s temperature control center or alter sweating). If you take any of these, you’re starting at a disadvantage in the heat and should adjust your expectations accordingly.
Practical Ways to Exercise Safely in Heat
Timing is your most powerful tool. Early morning, typically before 10 a.m., offers the coolest temperatures and lowest sun exposure. Evening workouts can work too, though pavement and buildings radiate stored heat for hours after peak temperatures. If you run or cycle, choose shaded routes over exposed ones.
Clothing makes a real difference. Light-colored, loose-fitting fabrics allow air circulation and reflect sunlight. Dark, tight clothing traps heat against your skin. Wetting a bandana or hat before heading out provides evaporative cooling that supplements your own sweating.
Pre-cooling your body before exercise, by drinking an ice slurry, taking a cold shower, or applying cold towels, lowers your starting core temperature and gives you a larger buffer before overheating. Cold water immersion before exercise and combined cooling methods (cold drinks plus external cooling) appear to be the most effective pre-cooling strategies for performance in the heat.
Most importantly, adjust your intensity. Your normal pace will feel harder in the heat because your heart is working overtime to cool you. Slowing down by 10 to 20 percent on hot days isn’t a sign of weakness. It’s your cardiovascular system telling you it’s already working near capacity just to keep your temperature stable. Listen to that signal.