How to Exercise Safely in the Heat (and When to Stop Entirely)

Summer invites people outside, whether for morning runs, weekend bike rides, or afternoon gardening. But the same season that makes outdoor activity appealing can make it dangerous, and the line between a tough workout and a medical emergency is a shorter distance than most people realize.

Knowing when to push through, when to scale back, and when to stay inside is not about avoiding exercise. It is about making choices that keep you active all season long.

The Heat Index: What the Thermometer Does Not Tell You

Air temperature alone does not tell the full story of how hard your body has to work outside. The heat index, sometimes called the apparent temperature or feels-like temperature, accounts for both air temperature and humidity to estimate how hot conditions actually feel to the human body.

This matters because sweating is your body's primary cooling system. Sweat evaporates from skin to carry heat away. High humidity slows that evaporation significantly, reducing the cooling effect even as your body keeps producing sweat. The result is that 90°F at 75 percent humidity can feel like 105°F in terms of physiological stress.

The National Weather Service uses the heat index to issue heat advisories and excessive heat warnings. When those alerts are active in your area, outdoor exertion should be treated as a medical risk management decision, not just a fitness one.

Timing Your Workout Around the Heat

The simplest adjustment most people can make is when they exercise. Solar radiation and surface heat accumulate throughout the day, with peak temperatures typically occurring between noon and 3 p.m. The hours before 9 a.m. and after 6 p.m. offer meaningfully cooler conditions and significantly lower UV exposure.

Early morning has the added advantage of giving you a full night's rest before exerting yourself, which is relevant during heat waves when overnight temperatures may stay high and sleep quality suffers. If you exercise in the evening, watch for residual heat radiating off pavement and buildings, which can keep ambient temperature elevated even after the sun sets.

During an active heat dome, even the early morning and evening windows may carry risk. In those conditions, the safest option is moving activity indoors entirely.

Hydration: More Than Just Drinking Water

Most people know to drink water during summer exercise. Fewer know they should start hydrated before going outside. By the time you feel thirsty, you are already in mild dehydration, and mild dehydration measurably impairs both physical performance and the body's ability to regulate temperature.

A practical approach: drink 16 to 20 ounces of water in the hour before outdoor exercise. During activity, aim for six to eight ounces every 15 to 20 minutes, adjusting based on sweat rate and intensity. After exercise, each pound of body weight lost during activity represents roughly 16 ounces of fluid that needs to be replaced.

For workouts lasting longer than 60 minutes in high heat, an electrolyte drink can help replace the sodium lost through sweat, which water alone does not restore.

What to Wear and How to Slow Down

Clothing choice affects how much heat your body retains. Lightweight, loose-fitting, light-colored fabrics allow air circulation and reflect solar radiation rather than absorbing it. Moisture-wicking synthetic materials move sweat away from skin more effectively than cotton, which saturates and stays wet.

Intensity matters as much as clothing. Heat forces your cardiovascular system to divide its effort between muscle oxygenation and skin cooling. The result is that the same pace that feels easy at 65°F becomes genuinely taxing at 90°F. Reducing your effort level in high heat conditions is not weakness. It is physiology.

People new to heat or arriving from cooler climates should allow 10 to 14 days to acclimate. During that window, plasma volume expands, sweat rate increases, and sweating starts earlier in exertion. These adaptations are real and meaningful, but they do not happen overnight.

Recognize the Warning Signs

Heat exhaustion and heat stroke share some early symptoms but require very different responses. Dizziness, nausea, heavy sweating, and muscle cramps are signs of heat exhaustion. Stop activity immediately, move to shade or air conditioning, apply cool water to skin, and rehydrate.

Heat stroke is a medical emergency. Signs include confusion, loss of coordination, and a body temperature above 103°F. Call 911 and cool the person aggressively with ice packs or cool water while waiting for help. Do not wait to see if the symptoms resolve on their own.

Some medications increase heat sensitivity, including diuretics, antihistamines, and beta blockers. People taking GLP-1 medications like Ozempic or Wegovy should discuss heat and dehydration risks with their provider before ramping up summer outdoor activity.

Weather changes fast, so help your community stay prepared. Share this story with friends, family, or your group chat, and keep your forecast in the now with Weather Forecast Now.