When exercising in the heat, you should hydrate before and during your workout, wear lightweight clothing, ease into intensity over one to two weeks, and watch for early warning signs of heat illness. Heat forces your body to cool itself and fuel your muscles at the same time, which puts serious strain on your cardiovascular system. The good news: with the right preparation, you can train safely even in high temperatures.
Drink Early and Often
A good starting point is roughly 200 to 300 milliliters of fluid (about 7 to 10 ounces) every 15 minutes during exercise. That pace replaces about one liter per hour of sweat, which works for moderate sweaters. But individual sweat rates range from one liter per hour to as much as three liters per hour, so you may need more. If you’re a heavy sweater, know that your stomach can only absorb about 1.2 liters per hour, so complete replacement during exercise isn’t always possible. The goal is to minimize the deficit, not eliminate it entirely.
Weigh yourself before and after a workout to get a sense of your personal sweat rate. Each pound lost is roughly 16 ounces of fluid. After your session, aim to drink 125 to 150% of whatever volume you lost. That extra margin accounts for the fluid you’ll lose through urination during recovery.
Replace Sodium, Not Just Water
Your sweat contains a significant amount of sodium. People who are acclimatized to heat lose around 1 gram of sodium per liter of sweat, while those who aren’t acclimatized lose closer to 1.5 grams per liter. Over a long, hot workout, that adds up fast. Drinking plain water without replacing sodium can dilute your blood’s electrolyte balance, which in extreme cases leads to a dangerous condition called hyponatremia.
A sports drink with at least a modest sodium content helps, but many commercial options contain relatively little. For longer sessions in the heat (90 minutes or more), adding electrolyte tablets or packets to your water, or salting your pre-workout meal, helps keep levels in a healthy range. If you notice white streaks on your clothes or hat after a workout, you’re a salty sweater and should pay extra attention to replacement.
Give Your Body Time to Adjust
Heat acclimatization is one of the most effective things you can do, and it takes 7 to 14 days of gradually increasing exposure. During that window, your body makes measurable changes: you start sweating sooner and in greater volume, your sweat contains less sodium, your resting heart rate during exercise drops, and your core temperature stays lower at the same effort level. Skin blood flow also increases, which helps move heat from your core to the surface more efficiently.
If you normally train in air conditioning and then suddenly do a long outdoor workout on a 95-degree day, you’re skipping this adaptation period entirely. Start with shorter, lower-intensity sessions and build up. Even cutting your usual volume in half for the first few days makes a real difference.
Choose the Right Clothing
Loose-fitting, light-colored clothing allows air to circulate across your skin, which is essential for sweat evaporation. Cotton absorbs sweat but holds onto it, leaving you damp and heavy. Synthetic fabrics designed for performance tend to wick moisture away from the skin and dry faster. Bamboo-based and Tencel fibers offer good breathability with strong moisture absorption, making them practical options for hot conditions.
The key factor is how well a fabric lets moisture vapor pass through it. Tightly woven materials trap humid air against your skin, which slows evaporative cooling. If you’re choosing between two shirts, pick the one that feels more open and breathable, even if it offers less sun coverage.
Protect Your Skin to Protect Your Cooling
Sunburn doesn’t just hurt. It actively impairs your body’s ability to cool itself. UV radiation damages the sweat glands and the ducts that carry sweat to your skin’s surface. Research published in the American Journal of Physiology found that sunburned skin produced significantly less sweat than unburned skin, with reductions measured on both the forearm and back within 24 hours of UV exposure. The likely mechanism is damage to the cells lining the sweat ducts, which physically blocks sweat from reaching the surface.
This means sunscreen isn’t just about preventing skin cancer when you exercise in the heat. It’s a thermoregulation tool. Apply it generously before heading out, and reapply if you’re sweating heavily.
Try Pre-Cooling Before Intense Sessions
If you have a particularly hard workout or competition in the heat, cooling your core temperature before you start gives you a larger thermal buffer. Drinking an ice slurry (crushed ice blended into a drink) 15 to 30 minutes before exercise consistently lowers core temperature in studies of trained athletes. Cold towels draped on the neck and shoulders, ice vests, and brief cold water exposure also work.
Pre-cooling won’t prevent heat illness on its own, but it effectively delays the point at which your body reaches a critical temperature. Think of it as buying yourself extra minutes of safe, high-quality effort.
Know the Warning Signs
Heat illness exists on a spectrum, and recognizing where you are on it can be lifesaving.
Heat exhaustion is the earlier, less dangerous stage. It shows up as muscle cramps, headache, dizziness, weakness, fatigue, nausea, and rapid breathing. Your skin may look pale, and your body temperature rises to between 101°F and 104°F. At this point, stop exercising, move to shade or air conditioning, and drink cool fluids. Most people recover fully with rest and cooling.
Heat stroke is a medical emergency. The hallmark is a core temperature above 104°F combined with confusion, slurred speech, aggression, hallucinations, or loss of consciousness. The skin often appears dry and red because the body has lost its ability to sweat. Seizures can occur. If someone shows these signs, call emergency services immediately. The single most effective field treatment is cold water immersion, which cools the body at roughly 0.13 to 0.35°C per minute and has produced 100% survival rates in documented cases when applied quickly.
Monitor Conditions, Not Just Temperature
Air temperature alone doesn’t tell you how dangerous conditions are. Humidity matters just as much, because high humidity slows sweat evaporation, which is your body’s primary cooling mechanism. The Wet Bulb Globe Temperature (WBGT) combines temperature, humidity, wind, and solar radiation into a single number that reflects actual heat stress.
The American College of Sports Medicine recommends modifying activity at a WBGT of about 82°F (27.9°C) and making further reductions at 86°F (30.1°C). At 90°F (32.3°C) WBGT, even acclimatized athletes in light clothing should cancel outdoor practice. Many weather apps now report WBGT or heat index. Check before you head out, and adjust your plans accordingly. On the most oppressive days, shifting your workout to early morning, moving indoors, or simply taking the day off is the smartest decision you can make.