Eh, i dunno. mid to upper 90's and above, really anything at or above body temperature is potentially dangerous due to less cooling from sweat. Yea, on dry heat days sweat helps, but on days when its 98 and 80% plus humidity..... you gotta be careful.
I have recently changed my cool down procedure when arriving home. I put my box fan on "3" the highest setting, and let the air cool me off. The cooler & dryer the ambient air, the better... my ideal is 30% and below and 70 degrees. I do this while drinking water. Doing this has cut my cool-down time from up to an hour to less than 15 minutes on the hottest most active days. It should be mentioned again that i do not ride to and from an office, but ride around to places all day, so this may not be a solution for A to B commuting.
- Andy