A few specific, evidence-based habits prevent the overwhelming majority of cases I see:
Hydrate on a schedule, not on thirst. By the time you feel thirsty you are already behind. Drink roughly 16 to 20 ounces of fluid in the two hours before you start, then about 6 to 12 ounces every 15 to 20 minutes during the workout, adjusted up if you are a heavy or salty sweater.
Train in the cooler windows. Move runs and outdoor lifting to before 9 a.m. or after 7 p.m. Midday, when the sun is highest, is when I see the worst cases come through the door.
Respect the heat index, not just the temperature. Humidity is the hidden danger. At 90 percent humidity your sweat cannot evaporate, which is your body’s main cooling system, so a 90 degree day can stress you like a 105 degree day. Check the heat index before you head out and scale back when it climbs.
Dress for cooling. Light colored, loose, moisture-wicking clothing and a hat or visor. Skip the heavy cotton and the sweat-it-out gear.
Pre-cool and use shade. Start cool rather than already warm, take breaks in shade, and pour cool water over your head and neck if you start heating up.
Build in rest intervals. Even a 60 to 90 second pause in the shade every 15 minutes lets your core temperature settle.
When is water alone enough, and when should you add electrolytes?
For most people, plain water is fine for any easy to moderate workout under about 60 minutes. Your body has plenty of stored electrolytes for that.
Add electrolytes, especially sodium, when any of these apply: the session runs longer than 60 to 90 minutes, you are sweating heavily, it is hot and humid, or you are a visibly salty sweater (white crust on your skin or clothes after you dry). In those situations you are losing sodium fast enough that water alone does not replace it.
One caution that surprises people: you can overdo plain water. Drinking very large amounts of water without any sodium during long efforts can dilute your blood sodium and cause hyponatremia, which looks a lot like heat illness (headache, nausea, confusion) and can be dangerous. The fix is balance. On long, hot sessions, replace fluid AND salt, do not just chug water.
Early warning signs of heat exhaustion you should never ignore, and what to do
These are the signs I tell everyone to treat as a hard stop:
- Heavy sweating with cool, clammy, pale skin
- Lightheadedness, dizziness, or feeling faint
- Nausea or a headache that came on during the workout
- Muscle cramps
- A racing, weak pulse and a feeling that your legs are suddenly heavy or weak
- Unusual fatigue or irritability that does not match your effort
The single most important rule: if you notice these, stop immediately. Do not try to finish the set, the mile, or the game.
Then cool down aggressively. Get into shade or air conditioning, sit or lie down, and elevate your legs. Take off extra clothing, hats, and equipment. Sip cool water with electrolytes. Cool the skin fast with cool water, a wet towel, or ice packs to the neck, armpits, and groin, where large blood vessels run close to the surface.
Most people turn the corner within 30 to 60 minutes. Get emergency help (call 911) if symptoms do not improve in that window, if the person starts vomiting and cannot keep fluids down, or if you see ANY change in mental status: confusion, slurred speech, stumbling, or loss of consciousness. Those signal a slide toward heat stroke, which is a true emergency.
Common mistakes people make exercising in extreme heat
The patterns I see over and over:
Pushing through the early signs. Athletes are conditioned to ignore discomfort. In the heat, that instinct is exactly backward. The dizziness IS the signal.
Working out at the worst time of day. Midday “I will just power through it” sessions fill ER beds in the summer.
Skipping acclimatization. Going from an air conditioned spring straight into an all-out summer workout, or traveling from a cool climate to a hot one and not easing in.
Under-hydrating before they even start. Showing up already a little dehydrated from coffee, a busy morning, or last night’s drinks.
Overhydrating with plain water on long efforts. The opposite mistake, which can cause hyponatremia, as I mentioned.
Ignoring humidity. People look at the temperature and feel fine about it, not realizing the humidity has shut down their cooling system.
Overdressing or wearing sweat suits and heavy gear to “burn more.” That traps heat and is genuinely dangerous in summer conditions.
Not accounting for medications and supplements. Diuretics, some blood pressure medicines, stimulants, certain antihistamines, and even high caffeine pre-workouts can impair temperature regulation. If you take these, you need to be extra cautious.
What happens physiologically, heat exhaustion vs. heat stroke, and who is at higher risk
How your body normally cools itself: when you exercise, your muscles generate a lot of heat. Your body sheds it two main ways: it sends blood to the skin to radiate heat out, and it sweats so that evaporation pulls heat off the skin. Both of those compete with your muscles for blood flow and for fluid.
What heat exhaustion is: when you lose too much fluid and salt through sweat, and your cardiovascular system cannot keep up with the demand to cool the skin AND fuel the muscles, the system starts to fail. Blood pressure drops, you feel weak, dizzy, and nauseated, and your core temperature is usually elevated but typically still under about 104 degrees Fahrenheit. Critically, in heat exhaustion your brain is still working normally. You are miserable but mentally clear.
How heat stroke is different: heat stroke is the emergency. The dividing line is two things together: a core temperature above roughly 104 degrees Fahrenheit AND a change in mental status, meaning confusion, agitation, slurred speech, seizures, or loss of consciousness. At that point the body’s cooling has failed completely and the high temperature begins to damage the brain, kidneys, muscles, and other organs. Exertional heat stroke, the kind athletes get, can come on fast and is fatal if it is not cooled quickly. The mental status change is the red flag I want every coach, parent, and gym buddy to know. Confusion plus hot, exhausted athlete means call 911 and start cooling now.
Who is more predisposed: people who are not yet acclimatized to the heat, who are dehydrated, or who are unfit relative to the effort. Older athletes, because the ability to sweat and regulate temperature declines with age. People carrying more body weight. Anyone on medications that impair cooling (diuretics, some blood pressure drugs, stimulants, certain antihistamines). Athletes wearing heavy equipment, such as football players in pads. People with a prior heat illness, who are at higher risk of another. And athletes with sickle cell trait, who are at particular risk during all-out exertion in the heat. Anyone in those groups should ramp up slowly, hydrate deliberately, and stop at the first warning sign.