Portable handheld device Kestrel 4400 Heat Stress Tracker fits in a pocket or small belt carrying case and provides accurate waterless wet bulb globe temperature (WBGT) measurements within minutes of exposure to the air. The most common composite measurement used to determine appropriate exposure to heat stress conditions is WBGT. The vane mount accessory allows the user to mount the tracker on a tripod for longer-term monitoring and logging of heat stress conditions, and optional Bluetooth® wireless data transfer enables transfer of the logged conditions to a computer or smart phone for later documentation. The Kestrel 4400 also displays thermal work limit (TWL), another recognized composite heat stress prediction tool. For both WBGT and TWL, the Kestrel 4400 provides on-screen alarms when conditions enter the caution and danger zones, providing clear and immediate guidance that heat illness prevention steps must be taken. It also displays natural wet bulb temperature, black globe temperature, and mean radiant temperature.