Is the most severe form of heat illness and is a life-threatening emergency.?

The answer is: heatstroke

Heatstroke is the most severe form of heat illness and is a life-threatening emergency. It occurs when the body's temperature rises to 104 degrees Fahrenheit or higher and the body is unable to cool itself down. Heatstroke can cause damage to the brain, heart, kidneys, and other organs. If heatstroke is not treated immediately, it can lead to death.