You are expected to make a judgment call. You can easily gauge and declare a hotter zone has a higher DC, using the usual chart of target numbers, but also realizing that if characters are required to make several rolls over time, that even low DCs will rack up failures.
Now if you want to throw a little more consideration into it, you can start looking up stuff on exertional heat stroke. This report on fatal heat stroke in sports has one case of fatal heat stroke in the 70 degree range with high humidity though most it lists are in the 80s to the 100s range. So, at least as far as the D&D5E rules are concerned, they're being pretty generous in making it a DC5 roll.
An interesting thing to look into would be sauna related injuries or deaths, for example this article points out in a sauna competition, the winner stayed in for 16 minutes over 230 degrees. Mind you, in a sauna you're naked or near naked, and not doing anything - far from running and fighting and the usual adventurer stuff.
A few years back a few people died in a new age sweatlodge retreat where the temperatures were at 120 degrees, though the lack of ventilation, number of people, and the fact they had been in it for 2 hours straight also contributed significantly.
D&D is not particularly great at specific simulation, and the reality is that our current science points to massive variances based on age, fitness, humidity and many factors we're not exactly sure on to totally model heat exhaustion/death. That is, you'll find a lot that once the internal temperature goes above a certain amount, we have significant problems, but how fast we reach that point compared to the outside temperature is extremely variable.
So, even with science, it's going to be an arbitrary judgement.