When your body has to work extremely hard to cool you down, you can develop heat exhaustion. During heat exhaustion, the body’s core temperature is usually less than 104 degrees Fahrenheit, but blood pressure is low and the heart is not pumping blood as efficiently as it should.
At this stage, the body is still doing what it’s supposed to. “You’ll be very fatigued and sweating a lot and thirsty—so those natural defenses against heat and dehydration are still working,” says Peter Shearer, MD, associate director of the Mount Sinai Hospital emergency department in New York City.
Heat exhaustion does not necessarily lead to heatstroke—but it could, says exercise physiologist Michael Bergeron, PhD, president and CEO of Youth Sports of the Americas. Bergeron describes exertional heatstroke as “a clear medical emergency affecting multiple body systems,” which usually occurs when the body’s core temperature rises above 104 degrees.
Heatstroke causes the central nervous system to malfunction. It can also damage the brain, heart, liver, kidneys, spleen, and muscular tissue. “Your body loses the ability to thermoregulate, so at that point it’s much more difficult to reverse itself,” says Dr. Shearer.