The Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report says men accounted for three-quarters of the cases and people age 15-19 accounted for one-third of those sick from the heat and physical activity.
The two most common activities leading to heat-related hospital emergency department visits were football and exercise, the report says.
"Anyone can be susceptible to heat illness, and thus it's necessary to take appropriate steps to prevent it," the report says. "These include taking frequent rest breaks, consuming plenty of fluids and limiting activity during very hot or humid days."
Other prevention tips include giving your body several days to acclimate to hot weather and wearing lightweight, light-colored, loose-fitting clothing.
"All persons who are physically active, including those who participate in unstructured sports and recreational activities, need to take appropriate actions to protect themselves from heat illness," the report says. "When heat illness occurs, prompt recognition and response can save lives."