*** So, personal story here: my son is a flight medic/Search and Rescue, and there was a half marathon here in our area this year. Most of the runners in the half marathon probably didn't take heed, but there was a mass casualty incidence where many people got heat illnesses (multiple ambulances called). One of the runners collapsed far up on the trail, and had to be medevac'd out by helicopter. That runner died. So... heat during a marathon... not a small matter.