“Heat stroke is a severe heat-related illness that involves a significant elevation in body temperatures, usually as a result of prolonged exposure to high temperatures or physical exertion in high temperatures,” Dr. Bangash said, noting “heat stroke is seen most commonly when there’s a sudden increase in environmental temperatures.”

“The early summer heat wave is when it’s the highest risk of heat stroke, as people have not acclimated to the temperature changes,” he said. “Geographically, it can happen in all hot and humid regions of the country—along the East, West, Midwest.”

“In a majority of the cases of heat stroke, body temperatures are usually over 104º F or 40º C,” Dr. Bangash said. In terms of the outside environment, when the heat index is more than 90º F, this poses “the highest risk of developing a heat stroke.”

The heat index, a combination of air temperature and relative humidity, is what the temperature feels like to the human body.