High humidity is being blamed. I just don’t remember reports like this at other times. (Colorado has the Pikes Peak marathon where hypothermia is a problem. Run to the top of Pikes Peak and back down. Gnarly)
I been to many marathons and ultras. The weather today was very unusual for this time of year in the northeast.
Chicago 2013, one runner died, several more hospital, many more collapsed. They called it off long before the scheduled ending time.