Much depends upon acclimatization, whether it's temps (high or low) or altitude.
Read an article last summer related to all the heat-related deaths in Europe, as they had record-breaking temps in many areas. As researchers discovered, heat is relative. For example, in Phoenix, where the average summer temps are much higher than what Europe was experiencing, few if any heat-related deaths would have been expected. Of course, some form of cooling (evaporative or A/C) is common in Phoenix whereas it's not as widespread in Europe, so that's a factor. Again, much depends upon giving our bodies time to adapt.
