Which environmental factor is NOT included in the definition of heat stress?

The concept of heat stress is primarily concerned with the effects of environmental conditions on the body's ability to regulate temperature. Factors like humidity, air temperature, and airflow are directly related to how heat is managed by the human body. High humidity can prevent sweat from evaporating, which is a key cooling mechanism. Air temperature determines the degree of heat exposure, while airflow helps facilitate cooling by increasing evaporation rates.

Altitude, on the other hand, does not directly influence heat stress in the same way. While altitude can affect oxygen levels and have various physiological impacts, it is not a primary factor in the thermal regulation process or how the body copes with heat. Thus, it does not fit in the typical definition of heat stress as it pertains to the body's thermal environment.