The Zelus System
The WBGT index integrates three temperature measurements using the following formula:
WBGT = 0.7 × Tnwb + 0.2 × Tg + 0.1 × Tdb
WBGT Adoption
The Wet-Bulb Globe Temperature (WBGT) index represents one of the most successful heat stress assessment tools in history, evolving from a military necessity in the 1950s to a global standard used by international organizations, federal agencies, and sports authorities worldwide. This timeline reveals the essential milestones from military origins to comprehensive international adoption.
Military Origins
Captain David Minard (USN) and Constantin P. Yaglou officially published the WBGT methodology in “Control of heat casualties at military training centers” (AMA Archives of Industrial Health), establishing the scientific foundation for heat stress assessment.
Federal Agency Adoption
First federal agency to formally adopt WBGT in “Criteria for a Recommended Standard: Occupational Exposure to Hot Environments” (Publication No. 72-10269), establishing WBGT as the preferred method for measuring environmental heat stress in workplaces.
International Standards
1989 – Published ISO 7243:1989 “Hot environments — Estimation of the heat stress on working man, based on the WBGT-index,” creating the first international standard for WBGT application in occupational settings.
2017 – Published ISO 7243:2017 “Ergonomics of the thermal environment — Assessment of heat stress using the WBGT index,” replacing the 1989 version with improved methodology for spatial and temporal variations.
Incorporated WBGT guidelines into the Code of Practice on Ambient Factors in the Workplace (Chapter 8), establishing international workplace heat stress standards.
Sports Organizations
Developed the first comprehensive WBGT guidelines for sports activities (Medicine & Science in Sports & Exercise, March 2007), establishing WBGT as the gold standard for assessing environmental heat stress during physical activity.
Released preseason heat-acclimatization guidelines for secondary school athletics based on WBGT, forming the foundation for state-level policy adoption across the United States.
Developed comprehensive recommendations for sport events in the heat, incorporating WBGT measurements for environmental monitoring and athlete protection during Olympic Games.
Implemented WBGT-based heat policy requiring mandatory cooling breaks when WBGT exceeds 32°C (89.6°F), with breaks at 30th and 75th minutes during international football matches.
Recent Mandatory Implementations
Multiple states including New Jersey (2020), Louisiana (2020), Texas (2023), and California (2024) mandated WBGT monitoring for high school athletics, requiring on-site measurement, emergency action plans, and specific work/rest ratios based on WBGT readings.
Starting with Captain David Minard’s groundbreaking work in 1957, WBGT has achieved unprecedented adoption across diverse sectors. Today, 38 U.S. states use WBGT-based heat safety policies for high school athletics, international sports organizations rely on it for major competitions, and occupational health agencies worldwide have integrated it into workplace safety standards.
Scientific grade on-site WBGT measurement devices (thousands of dollars), while accurate if used properly, can be cumbersome to set up properly and require special training, limiting their widespread use. In addition, less expensive hand held units that approximate the WBGT (i.e. Kestrel 5400, Extech HT30) can be labor intensive, suffer from large errors due to improper deployment, instrument calibration drift and bias, especially in the hot temperature regimes that are so important to measure accurately. The need for accessible and reliable real-time WBGT monitoring tools is evident, especially in settings with limited resources.