New Delhi, Aug. 14 -- Paul Grinvalds has completed 114 marathons in all 50 states, but he still wilts when he races in temperatures beyond 60 degrees Fahrenheit (15.5 Celsius).

It's the same for most runners, as the body can't cool down during exertion, and heat makes that even harder.

"One thing about marathoners is typically we're kind of stubborn," said Grinvalds, 68, from West Des Moines, Iowa, who finds even temperatures in the 50s F (10 C) a bit warm. "So if we have a goal and it's hot, a lot of times we're still going to try to do that goal, which is just stupid because you're almost guaranteed to have a very painful experience."

Such an experience can also be dangerous, a reality that marathon organizers are having to confront ...