The only thing I can come up with is the heat dialates the vascular system ? I would like to see the supporting medical information. Where I live its 90-110 all summer. I guess I will spend more time out side/. There is my 2 cents worth
I had a heart surgeon tell me few years ago that they see more heart attacks in the winter. Reason? People are more sedentary and the cold constricts blood vessels raising your risk.
That's one of the beneficial effects of cardio. It dilates your blood vessels as the need to pump blood through your body increases.