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.