How to compute the WBGT Wet Bulb Globe Temperature: environmental heat stress index developed by Constance Yaglou and David Minard (USA 1957) for US Marines Corps Parris Island South Carolina after numerous heat strokes among recruits during summer training. Outdoor formula with sun: WBGT = 0.7*T_w + 0.2*T_g + 0.1*T_d, where T_w wet bulb temperature (°C natural ventilation), T_g globe temperature Vernon globotherometer (°C 150 mm black sphere radiation absorbing), T_d dry bulb air temperature (°C dry bulb thermometer). Indoor or cloudy formula: WBGT = 0.7*T_w + 0.3*T_g. Critical values: WBGT32°C extreme critical heat stroke. International standard ISO 7243:2017 "Ergonomics of the thermal environment - Assessment of heat stress using the WBGT index", USA NIOSH OSHA ACGIH TLV Threshold Limit Values, Australia AS 1885.1, UK HSG 25.

Essential tool for: construction laborers (OSHA Washington DC USA, NIOSH Cincinnati OH, AGC Associated General Contractors Arlington VA), steelmaking industry (US Steel Pittsburgh PA, Nucor Corporation Charlotte NC, Cleveland Cliffs Cleveland OH), foundries glass plastic factories, military forces (US Army Department of Defense Arlington VA), firefighters (NFPA Quincy MA, IAFF Washington DC), sports professional NFL MLB NBA NHL athletic events, marathons NYC Boston Chicago LA, motor racing NASCAR IndyCar, tennis US Open Flushing Meadows NYC, Olympics IOC Lausanne Tokyo 2020 Paris 2024 LA 2028 Brisbane 2032 thermal monitoring.

WBGT formulas and parameters

Outdoor WBGT with sun: WBGT = 0.7*T_w + 0.2*T_g + 0.1*T_d. Weight coefficients: 70% wet bulb (humidity evapotranspiration), 20% globe temp (solar radiation), 10% dry bulb (air temperature). Outdoor formula valid for direct sun exposure exteriors, standard meteorological conditions. Indoor WBGT = 0.7*T_w + 0.3*T_g (no Td because indoor T_g includes thermal radiation indoor sources).

Wet bulb T_w: measured thermometer with wet wick covered by water, ambient air natural ventilation (NOT forced fan), evaporative cooling. T_w<=T_d always, equal at 100% RH. Stull 2011 approximation formula: T_w = T_d * atan(0.151977 * sqrt(RH+8.313659)) + atan(T_d+RH) - atan(RH-1.676331) + 0.00391838 * RH^1.5 * atan(0.023101*RH) - 4.686035 (T_d °C, RH %). Real measurement Assmann ventilated psychrometer 0.1°C accuracy.

Globe temperature T_g Vernon globotherometer: black copper sphere diameter 150 mm matte painted (high emissivity epsilon=0.95), thermometer at center. Measures combination convection + thermal radiation (sun, walls equipment radiated heat). T_g>T_d outdoor sun (5-15°C above), T_g~T_d indoor uniform. USA industry: Kestrel Nielsen Kellerman Boothwyn PA, Reed Instruments Wilmington NC Heat Stress meters R6200, 3M Quest Technologies Oconomowoc WI (TSI 2018) QuesTemp 32-34-36 series, Extech Instruments Boston MA HD550 humidity meters.

Comparative indices: WBGT more rigorous vs Heat Index US NWS (T_d + RH only), Humidex Canada (T_d + dew point), Apparent Temperature Australia, Thermal Discomfort UTCI ISO 11079. WBGT includes solar radiation (important outdoor work), Heat Index daily weather forecasts. Combine metabolic rate workload heavy/moderate/light for personalized thresholds.

Workload categories ACGIH/ISO 7243: Light 500 W (construction heavy lifting, mining): threshold 22-23°C. Acclimatized workers tolerate +1-2°C above unacclimatized thresholds.

History and context

Constance Yaglou (USA Harvard University 1882-1955) and David Minard (USA Naval Medical Research 1916-1996) Marines Corps Parris Island South Carolina developed 1957 WBGT index in response to numerous heat stroke deaths among USMC recruits during South Carolina hot humid summer training (1956 1957 several deaths). Published "Control of Heat Casualties at Military Training Centers" Archives of Industrial Hygiene Occupational Medicine 16:302-316 establishing Threshold Limit Value WBGT 30°C suspend rigorous training. Charles Vernon (Great Britain 1875-1953) Medical Research Council UK 1932 invented globotherometer 150 mm copper. Carl Gunnar Aschoff (Germany 1894-1963) German Federal Health Office body thermoregulation contributions. James Currie (Great Britain 1756-1805) Liverpool 1797 first cold immersion therapy experimentation. Richard Stull (USA Air Force 2011) UC Berkeley simplified wet bulb approximation formula widely used industrial monitoring devices. NIOSH National Institute Occupational Safety Health Cincinnati OH USA founding 1970 OSHA Occupational Safety Health Administration Washington DC heat stress workplace. ACGIH American Conference Governmental Industrial Hygienists Cincinnati OH TLV Threshold Limit Values updated annually 2025. AIHA American Industrial Hygiene Association Falls Church VA. NIOSH 2016 publication 2016-106 "Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments" comprehensive update.

Industry global WBGT meters: Kestrel 5400 Heat Stress Tracker Nielsen Kellerman Boothwyn PA USA global leader (acquired Nielsen-Kellerman 1979) digital WBGT meters used Olympic events military sports, Reed Instruments Wilmington NC Heat Stress meters R6200, 3M Quest Technologies Oconomowoc WI (TSI 2018 acquisition) QuesTemp 32-34-36 series, TSI Inc Shoreview MN USA precision thermal monitors, Casella CEL UK heat stress monitors. Industry PPE Personal Protective Equipment heat: Gore-Tex W.L. Gore Newark DE USA breathable membrane, Nike Beaverton OR USA athletic Pro Cool gear, Under Armour Baltimore MD HeatGear sport, Adidas Herzogenaurach Germany ClimaCool HEAT.RDY, Decathlon Villeneuve d'Ascq France Quechua Forclaz outdoor cooling. USA construction industry: AGC Associated General Contractors Arlington VA, ABC Associated Builders Contractors, Bechtel Reston VA major contractor, Fluor Irving TX EPC, Skanska Stockholm Sweden-USA Manhattan NYC. USA steelmaking: US Steel Pittsburgh PA founded 1901 J.P. Morgan, Nucor Corporation Charlotte NC largest USA steel producer 2024, Cleveland Cliffs Cleveland OH integrated mining steel, ArcelorMittal Luxembourg-USA, Steel Dynamics Fort Wayne IN.

Sports: FIFA Zurich Switzerland mandate cooling breaks WBGT>32, FIFA World Cup Qatar 2022 controversies outdoor stadium temperatures cooling air-conditioning stadium, USA Major League Baseball MLB rules extreme heat games suspended, NFL National Football League regulation games postponement WBGT>34, NCAA college sports heat policy. Marathon: NYC Marathon November typical safe T, Boston Marathon April typical safe T, Chicago Marathon October, LA Marathon March, Las Vegas Marathon December. UCI cycling Lausanne Switzerland Tour de France stage cancellation heat, US Tour de France LA 2024+. ATP US Open Flushing Meadows NYC tennis extreme heat policy WBGT>32 men 5-set rule. International Olympic Committee IOC Lausanne organizing committee Tokyo 2020 (deferred 2021) Paris 2024 LA 2028 Brisbane 2032 cooling stations 1000+ urban centers spectators. American universities heat stress research: Harvard School of Public Health Boston MA founded 1922, Yale School of Medicine New Haven CT, Stanford CA School of Medicine, University of California San Francisco UCSF, Johns Hopkins Bloomberg School Public Health Baltimore MD, University of Pittsburgh Public Health, Emory University Atlanta GA. NOAA Boulder CO marine forecast models heat advisories, NWS National Weather Service heat advisories alerts.

WBGT thresholds and typical values table

• WBGT<25°C: low risk all workloads, standard hydration
• WBGT 25-28°C: moderate risk, hydration 400-500 ml/h heavy workload
• WBGT 28-30°C: high risk, work-rest 50/50 ratio NIOSH ACGIH
• WBGT 30-32°C: very high risk, suspend heavy workload
• WBGT>32°C: extreme critical risk, stop all outdoor work
• Light workload <200 W: WBGT threshold 30°C ACGIH
• Moderate 200-350 W: WBGT threshold 27.5-28°C
• Heavy 350-500 W: WBGT threshold 25-26°C
• Very Heavy >500 W: WBGT threshold 22-23°C
• Acclimatized worker: threshold +1-2°C above unacclimatized
• Outdoor sun formula: WBGT=0.7*Tw+0.2*Tg+0.1*Td
• Indoor formula: WBGT=0.7*Tw+0.3*Tg
• Globe temp Vernon 150 mm: black copper sphere emissivity 0.95
• Wet bulb thermometer: water wick natural ventilation NO forced fan
• Heat Index US NWS: combines T_d + RH no solar radiation
• Humidex Canada: T_d + dew point variant
• UTCI ISO 11079: Universal Thermal Climate Index includes wind
• FIFA cooling break: mandatory WBGT>32°C professional soccer
• USA NYC Marathon Nov: typically WBGT<20 cool autumn
• USA Boston Marathon April: typically WBGT<22 spring
• Las Vegas Marathon Dec: typically WBGT<25 winter desert
• Australian Open Melbourne: extreme heat policy WBGT>32 suspension
• Tokyo 2020 Olympics: WBGT alert system spectators athletes
• WBGT USA summer July: 28-32°C south, 25-28°C north typical
• WBGT winter January: <15°C cold stress separate not WBGT

Detailed numerical examples

Example 1 - Phoenix AZ summer hot 42°C 30% RH. Phoenix Arizona summer July: T_d=42°C, RH=30%, T_g=50°C (intense desert sun). Tw approximated Stull formula: Tw=24.4°C (low because dry desert). WBGT outdoor=0.7*24.4+0.2*50+0.1*42=17.08+10+4.2=31.28°C. Risk VERY HIGH 30-32°C. Heavy workload threshold 25°C: STOP heavy work mandatory. NIOSH 2016 recommendation: hydration 1+ L/h, work-rest 25/75 ratio. Application: USA Sun Belt cities Phoenix Las Vegas Tucson Albuquerque construction crew, OSHA Heat Illness Prevention Program federal initiative.

Example 2 - Death Valley extreme heat 52°C 10% RH. Death Valley California summer July (record 56.7°C July 1913): T_d=52°C extreme, RH=10% bone-dry, T_g=65°C (extreme sun). Tw=21.2°C (very low due to extreme dryness). WBGT outdoor=0.7*21.2+0.2*65+0.1*52=14.84+13+5.2=33.04°C. Risk EXTREME >32°C critical. Even very heavy workload threshold 22°C: ABSOLUTELY STOP. NPS National Park Service Furnace Creek closure visitor warnings. Application: USA Death Valley NP California Nevada border, military training Fort Bliss Texas.

Example 3 - US Steel Gary IN steel mill indoor 35°C. US Steel Gary Works Indiana steel mill casting bay: T_d=35°C ambient, T_w=28°C (RH 55%), T_g=45°C (radiation Siemens electric arc furnace 100 t load). WBGT indoor=0.7*28+0.3*45=19.6+13.5=33.1°C. EXTREME RISK indoor. Heavy workload threshold 25°C: AUTOMATIC SUSPENSION cyclical 25/75 work-rest. NIOSH 2016 publication 2016-106 hydration 1+ L/h electrolytes, heat-resistant Nomex PPE clothing, medical surveillance. Application: US Steel Pittsburgh PA, Nucor Charlotte NC, Cleveland Cliffs Cleveland OH integrated mining steel.

Example 4 - Spring NYC safe day 22°C 50% RH. NYC April spring T_d=22°C, RH=50%, T_g=24°C (mild sun). Tw=15.3°C. WBGT outdoor=0.7*15.3+0.2*24+0.1*22=10.7+4.8+2.2=17.7°C. LOW RISK <25°C. All workloads safe. Application: NYC Marathon November typically WBGT<18 ideal racing, Boston Marathon April typically WBGT<20, Chicago Marathon October typically WBGT<22 cool autumn. Recreational sport cycling running outdoor cycling NYC Central Park, professional NFL preseason training acclimatization.

Example 5 - Construction site summer heavy work. Construction site Phoenix AZ summer August: T_d=38°C, RH=20% (dry desert), T_g=52°C (intense sun). Tw=20.6°C (low dry). WBGT outdoor=0.7*20.6+0.2*52+0.1*38=14.42+10.4+3.8=28.62°C. HIGH RISK 28-30°C. Heavy workload threshold 25°C: STRESS CRITICAL, work-rest 50/50 mandatory. ACGIH TLV 25°C heavy continuous: 25%/75% rest required. AGC Associated General Contractors recommend

• hydration 750+ ml/h water+electrolytes 0.5 g/L sodium
• light-color cotton breathable clothing
• early/late hours 6-10 AM + 4-8 PM avoid 11 AM-3 PM worst hours
• light Nomex PPE avoid heavy synthetic. OSHA 1910.269 Federal regulation employer responsibility worker safety

Limits and considerations

1. WBGT does not consider wind velocity: high wind cool down 2-5°C effective body temperature, reduced risk underestimated.
2. Direct sun radiation T_g measured ASSUMES black globe 150 mm Vernon: real workers may be partially shaded.
3. Acclimatization 7-14 day adaptation period: unacclimatized workers thresholds 1-2°C lower.
4. Individual factors: age, fitness, hydration status, alcohol caffeine, medications (anticholinergic diuretics) modify tolerance ±3°C.
5. Clothing impact: occlusive impermeable PPE (Tyvek hazmat suits) increase effective WBGT 5-10°C above ambient measurement.
6. Workload metabolic rate variable: light 200 W vs heavy 500 W personalize thresholds significant.
7. WBGT static moment: rapid weather change frontal passage adjustment +5°C 1 hour, calibration frequency every 30 min.
8. Pregnancy children elderly: thresholds 2-3°C lower vs healthy adult worker, separate guidelines.
9. Indoor workplaces: HVAC ventilation control reduces WBGT but introduces stratification, multi-point measurement needed.
10. Sport regulation: FIFA cooling breaks mandatory WBGT>32, ATP tennis extreme heat policy WBGT>32, IOC Olympics specific event guidelines.

Common errors

1. Confusing wet bulb T_w with dew point: T_w always <=T_d, dew point separate phenomenon.
2. Neglecting globe temp T_g: indoor formula 0.7*Tw+0.3*Tg requires T_g, not substitutable T_d.
3. Assuming WBGT valid for intense work without lowering threshold: heavy workload threshold 25°C not 30°C.
4. Computing Tw from Stull approximation incorrectly: precision ±2°C, for critical measurements use sling psychrometer.
5. Confusing outdoor formula with indoor: outdoor 0.7Tw+0.2Tg+0.1Td, indoor 0.7Tw+0.3Tg.
6. Neglecting acclimatization: unacclimatized workers thresholds 2°C lower than acclimatized.
7. Assuming WBGT same all body parts: localized heat exposure (face hands) not represented.
8. Computing instant WBGT without time average: 60-min weighted moving average per FAA regulation.
9. Neglecting PPE clothing: hazmat suits add 5-10°C effective WBGT.
10. Confusing WBGT with Heat Index US NWS: HI no solar radiation, WBGT includes T_g.

Numbered applications

1. USA construction OSHA: AGC Associated General Contractors Arlington VA, ABC Builders Contractors, Bechtel Reston VA, Fluor Irving TX EPC.
2. USA steelmaking: US Steel Pittsburgh PA, Nucor Charlotte NC largest, Cleveland Cliffs Cleveland OH integrated, Steel Dynamics Fort Wayne IN.
3. USA professional sports: NFL National Football League Atlanta heat games, MLB Major League Baseball, NCAA college, NBA NHL leagues.
4. USA marathons: NYC Marathon November, Boston Marathon April, Chicago Marathon October, LA Marathon March, Las Vegas Marathon December.
5. USA military forces: US Army Department of Defense Arlington VA, USMC Marines Corps, US Air Force, Navy training centers.
6. USA firefighters: NFPA National Fire Protection Association Quincy MA, IAFF International Association Fire Fighters Washington DC.
7. WBGT instrumentation: Kestrel 5400 Boothwyn PA leader, 3M Quest Oconomowoc WI, Reed Instruments Wilmington NC, Extech Boston MA.
8. FIFA Olympics cooling: World Cup Qatar 2022 controversies, Tokyo 2020 Paris 2024 LA 2028 Brisbane 2032 cooling stations.
9. USA universities research: Harvard School of Public Health Boston MA, Yale Stanford Johns Hopkins, UCSF, Pittsburgh, Emory Atlanta GA.
10. Educational teaching: NIOSH Cincinnati OH 2016 publication 2016-106, OSHA Heat Illness Prevention Program federal, ACGIH annual TLV updates.

Support

For technical questions or reports: info@calcoi.com. Educational tool for preliminary occupational health. For professional engineering ISO 7243:2017 'Ergonomics of the thermal environment - Assessment of heat stress using the WBGT index', NIOSH USA 'Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments' 2016 publication 2016-106, ACGIH 'TLVs and BEIs' annual 2025, OSHA Heat Illness Prevention Program federal initiative, Yaglou CP Minard D 1957 'Control of Heat Casualties at Military Training Centers' Arch Ind Hyg Occup Med 16:302.

Updated July 2026