When outside temperature + relative humidity = <130, most horses can keep their body cool, with the exception of very fat or very muscular horses. When outside temperature + relative humidity = >150 or above, it’s hard for a horse to keep cool because their ability to sweat is compromised — especially if the humidity contributes over half of the 150.
Source: AAEP, Facebook, July 9, 2021 at 12:31 pm. Link. Scroll down the July 9 posts. Exercise is a major source of body heat, and horses that cannot sweat will usually overheat very rapidly, even in cooler weather with a small amount of exercise.
Also see: Heat stroke, AAEP. Link.