The two main types of heat-related illnesses are heat exhaustion and heat stroke. Heat stroke is more severe than heat exhaustion. Heat exhaustion is defined as an elevated core body temperature between 98.6 and 104 degrees Fahrenheit resulting from exposure to a hot environment. Heat exhaustion is often associated with dehydration. The signs and symptoms of heat exhaustion are increased thirst, profuse sweating, dizziness, headache, tachycardia, hypovolemia, vomiting, and weakness, which usually resolve quickly with the treatment of cooling down and rehydration. Signs and symptoms of heat exhaustion are milder than heat stroke. Patients with heat exhaustion will not typically experience neurocognitive impairment, as is commonly seen with heat stroke. 

Heat stroke is defined as an elevated core body temperature above 104 degrees Fahrenheit with a central nervous system complication that occurs following exposure to a hot environment. Heat stroke is also often associated with dehydration. The signs and symptoms of heat stroke are similar to heat exhaustion but more severe. They often include cognitive impairment and possibly additional neurologic signs and symptoms such as delirium, seizure, and coma. There is a lack of sweating in heat stroke, unlike heat exhaustion, despite elevated core body temperature. Heat stroke is associated with organ damage. The elderly and infants are most vulnerable to heat stroke. 

Heat stroke that occurs due to increased physical activity and exertion in hot environments is associated with exertional rhabdomyolysis and exertional hyponatremia. Rhabdomyolysis is caused by the breakdown of skeletal muscle and accumulation of the protein myoglobin and electrolytes in the bloodstream, which can cause kidney damage, cardiac arrhythmias, muscle pain, nausea and vomiting, and or seizures. Rhabdomyolysis can be fatal. As with heat exhaustion and heat stroke, rhabdomyolysis is associated with dehydration. The first treatment for heat-related illnesses is removing the patient from the hot environment, rest, rehydration, and some form of cooling (active or passive depending on the severity of illness). 

Reference:

Danzl DF. Heat-Related Illnesses. In: Jameson J, Fauci AS, Kasper DL, Hauser SL, Longo DL, Loscalzo J. eds. Harrison's Principles of Internal Medicine, 20e. McGraw Hill; 2018. Accessed February 02, 2022.