Some marathons are warning runners when conditions increase the risk of heatstroke. iStockphoto hide caption

Portugal's Cristiano Ronaldo takes a water break during the 2014 World Cup soccer match between Portugal and the U.S. in Manaus, Brazil, on June 22. Siphiwe Sibeko/Reuters/Landov hide caption

Ray Copeland, a football coach at Bishop McGuinness High School, puts his players through a workout in Oklahoma City in 2007. As is often the case in much of the U.S., the first day of high school football practice that year began in a heat wave. AP hide caption