Note that "wet-bulb temperature" (WBT) is not a specific value for temperature, it is a way of measuring temperature. You can cave a WBT of 0C, or a WBT of 35C. WBT is the temperature shown by a thermometer covered in a wet cloth over which air is passing.
This is relevant because it indicates the temperature that human skin can reach with any amount of sweat and wind. It is normally much lower than air temperature because of evaporative cooling effects; but, as humidity increases, WBT and air temperature get closer and closer; at 100% humidity, there is no more evaporation, so WBT becomes equal to air temperature.
The problems for human activity start at a WBT of 32C (90F), and a WBT of 35C (95F) means that even healthy humans with access to unlimited water will die of heat stroke in the shade after a few hours.
