The normal human body temperature is 98.6-degree Fahrenheit, which corresponds to 37-degree Celsius. Anything above is called fever, which can lead to hyperthermia in a heat wave condition. It could be fatal.
It is commonly held that the maximum temperature at which humans can survive is 108.14-degree Fahrenheit or 42.3-degree Celsius. A higher temperature may denature proteins and cause irreparable damage to brain. Simply put, the human body can turn into a scrambled egg.