Which environmental factor is typically not a primary cause of heat stress in horses?

Wind is typically not a primary cause of heat stress in horses because it can actually be beneficial in regulating body temperature. Wind helps dissipate heat from a horse's body, facilitating cooling. Conversely, high humidity can hinder the evaporation of sweat, making it difficult for horses to cool down effectively. Overexertion leads to increased body temperature and energy expenditure, raising the risk of heat stress. Sun exposure directly contributes to increased temperatures and can exacerbate heat stress, especially when coupled with high humidity and inadequate shelter. Therefore, while wind is often a refreshing factor in hot conditions, the other options are more closely associated with heat stress in horses.