Deadly heat stroke can be prevented. But experts say that a lot of people still haven’t gotten the message. And every summer thousands of people are admitted to the ER for heat-related illness after exercising or playing sports in the hot summer sun.

That’s because hot temperatures combined with humidity raise your internal temperature, increase your heart rate, and make it hard for your sweat to evaporate. And if you don’t drink enough cool water, you can easily get dehydrated and overheated. Which can lead to organ failure, brain damage, and death. In fact, heat stroke and dehydration are the leading reasons why heat waves kill more North Americans than all other severe weather events combined. And that includes tornadoes and hurricanes.

Hot weather doesn’t discriminate. But a new study by the Centers for Disease Control found that the most cases of heat exhaustion and dehydration occur among boys between 10 and 19. Experts say it’s because boys spend more time playing sports and practicing for the upcoming school year. In fact, the most common activities that lead to heat-related ER visits are football and exercise drills – like walking, jogging, and like jumping jacks.

That’s why experts say that on hot days, coaches should keep outdoor practice time to a minimum, and encourage kids to drink more fluids.

And, know the early warning signs of heat illness: Which can include excessive thirst, nausea, vomiting, headaches, and dizziness.