The Kestrel 4400 heat stress tracker offers accurate and localized measurement of heat stress conditions which is a key component of a heat illness prevention plan. Human heat stress results from a combination of many environmental factors – air temperature and humidity along with radiant heat from the sun and surfaces, balanced by the cooling effect of breezes or air flow. Anyone who has stood under welcome shade adjacent to a sizzling parking lot understands that these conditions must be evaluated exactly where peoples' activities are taking place. The most common composite measurement used to determine appropriate exposure to heat stress conditions is Wet Bulb Globe Temperature or “WBGT." The American College of Sports Medicine, the American Conference "of Governmental Industrial Hygienists, and the US Military all have published threshold limit values (TLV's) or reference guidelines for conditions, activity levels and hydration based on WBGT, clothing levels and acclimatization of participants.