When one exercises hard, much of the blood flow goes to the skin to promote heat loss. Obviously, venting heat at high temperatures, like 100 degrees will be more challenging. If the humidity is low, even 100 degrees will not be that bad. But if the humidity is high, it can potentially dangerous.
As your body tries to vent heat, it perspires to promote evaporative cooling. Of course, this sweating comes at the cost of loss of water and electrolytes from the body.
The first danger is heat exhaustion followed by heat stroke. Both of these are caused by loss of water. Heat stroke is potentially fatal. It is important to take in enough fluids at high temperatures while exercising.
The second danger is hyponatriumia. In this case, the person is drinking enough water but as they perspire, they lose progressively more sodium by sweating. If your sodium concentration in the blood gets low enough, it can be fatal. Even drinking electrolyte replacement drinks can be inadequate. These drinks do not contain adequate sodium. If you put enough sodium in the drink to replace the losses, you get a nasty tasting drink which people will not buy. The companies are in business to sell the product and the product does a pretty good job on the other electrolytes. You can take an amount of table salt with you and eat some as necessary.
Heat exhaustion is far more common than hyponatriumia. I suppose that if the humidity is really high and the temperature is at or above 100 degrees one could get into trouble from having an sort of artificial fever. If the body gets above 105 degrees for very long, brain damage can occur. I have never heard of this happening, but there is always a first time.