Originally Posted by John C. Ratliff
If you were to see it anywhere, you'd see someone falling over from heat exhaustion on the Tour de France. So far, in the last two seasons, I haven't seen that reported. This has more to do with keeping ahead with your fluids then whether or not the person was wearing a helmet.
John
Might help to be superbly trained athletes whose vital signs are being constantly monitored and tended to by support vehicles too. IMO the TDF is the last place I would try to extrapolate information from to represent regular cyclists.