Individuals wearing encapsulating garments require auxiliary cooling systems to sustain physical and cognitive performance when exposed to high temperatures or workloads. Heat transfer in such cooling systems is typically based on either air or liquid as the heat exchange medium. Designing air-cooled systems requires knowledge of the quantity of heat to be extracted and cooling system design criteria, inlet air temperature and humidity and ventilation rates.This paper addresses this issue by viewing the human as a simple time averaged heat source whose temperature must be maintained within a specified range. Integrating heat production over time permits heat extraction to be separated from physiological thermoregulation.Framing physical workload and ambient conditions in terms of military relevant scenarios for rear cabin helicopter aircrew (25 yr old male working at 45% VO2max), families of curves were identified that define air conditioning system design criteria for given conditions.