Comparison of frequency of measurements in different American College of Sports Medicine heat-safety categories based on wet bulb globe temperature (WBGT). The WBGT guidelines for activity modification were WBGT ≤27.8°C (normal activities), 27.9°C to 30.0°C (limit practice time), 30.1°C to 32.2°C (very limited practice time), and ≥32.3°C (cancel). a The reference unit was custom designed and built by Kestrel engineers (Nielsen-Kellerman Co, Boothwyn, PA) to meet the specifications for a wet bulb globe monitor as determined by the International Organization for Standardization. b General WBGT8778 Heat Index Checker (General Tools & Instruments LLC, Secaucus, NJ). c REED Heat Index WBGT Meter (model SD-2010; REED Instruments, Wilmington, NC). d Extech HT30 Heat Stress Wet Bulb Globe Temperature Meter (FLIR Commercial Systems Inc, Nashua, NH). e QUESTemp° 34 (3M Detection Solutions, St Paul, MN). f Kestrel 4400 Heat Stress Tracker (Neilsen-Kellerman Co, Boothwyn, PA).g WBGT-103 Heat Stroke Checker (Kyoto Electronics Manufacturing Co, Ltd, Kyoto, Japan).