What WBGT Measures
The military and OSHA heat-stress index, explained.
WBGT combines humidity, radiant heat, and air temperature into a single number that predicts thermoregulatory failure.
Heat does not kill through raw temperature. It kills when the body cannot shed metabolic heat fast enough, and the core temperature creeps up. Wet bulb temperature captures evaporative capacity — the lower the wet bulb, the better sweat evaporates. Globe temperature captures radiant heat. WBGT combines both plus air temperature into a single number. It is the standard OSHA, ACGIH, and military use because it actually predicts heat illness.
Quick answer
WBGT combines humidity, radiant heat, and air temperature into a single number that predicts thermoregulatory failure.
Key points
- ▸ Outdoor formula (sun): WBGT = 0.7·Tw + 0.2·Tg + 0.1·Td. Indoor: WBGT = 0.7·Tw + 0.3·Tg (no dry-bulb term).
- ▸ Wet bulb temperature (Tw): air temperature a wet thermometer reads with full evaporation. Accounts for humidity.
- ▸ Globe temperature (Tg): black sphere thermometer reading. Captures direct sun and radiant heat from hot surfaces.
- ▸ ACGIH thresholds: nominal below 27°C, moderate 27-30, high 30-32, extreme 32-35, fatal risk above wet bulb 35°C.
- ▸ Fundamental limit: at wet bulb 35°C, sweat cannot evaporate fast enough to shed metabolic heat — human thermoregulation fails.
Examples
- 75°F wet bulb · 85°F globe · 88°F dry · sunWBGT = 26.1°C (79°F). Nominal band; normal work pace OK.
- 80°F wet bulb · 95°F globe · 100°F dry · sunWBGT = 30.6°C (87°F). High heat-stress band — reduced work/rest cycles, aggressive hydration.
- Wet bulb 86°F · no shadeWBGT crosses 35°C fatal-risk territory. Outdoor work must stop regardless of hydration, acclimatization, or fitness.
When to use which tool
Related
- Thermal Failure · WBGTWet-bulb globe temperature — the heat-stress index used by OSHA, ACGIH, and military for work/rest cycle decisions.
- Interpreting WBGTACGIH work/rest schedules and hydration targets by heat band.
- Heat Stress Edge CasesClothing, age, medication, and humidity extremes all shift the real safe limit.
- Electrolyte Loss in SweatSodium, potassium, chloride, and magnesium — how much each litre of sweat costs you.
Frequently asked questions
› Why not just use heat index? Troubleshooting
Heat index combines temperature and humidity only. It ignores radiant load (direct sun, hot pavement) and convective factors. WBGT captures those, which is why OSHA and ACGIH mandate WBGT not heat index for high-heat workplaces.
› Can I estimate WBGT without a globe thermometer? Trust & accuracy
Approximations exist using temperature, humidity, and wind, but they are rough. A proper WBGT device has three thermometers (wet, globe, dry). Consumer heat stress monitors with built-in WBGT are available for $100-300.
› How should I use this guide with a Kefiw tool? How-to
Use the guide as the plan and the linked Kefiw tool as the check. Read the steps first, try the move manually, then use the tool to compare outputs, catch edge cases, and decide whether the result actually fits your task.
› What mistake do tool guides help avoid? Troubleshooting
Tool guides help avoid using a utility mechanically without understanding what you are trying to accomplish. Most word, writing, and text utilities are fast, but speed can hide context mistakes. Know whether you are solving a puzzle, cleaning copy, drafting a line, or checking a rule.
› Can a tool guide help me learn the skill? How-to
A tool guide can help you learn if you pause before accepting the output and ask why it worked. Compare your first guess with the tool result, look for the rule or pattern, and repeat that review. Passive copying solves one task; active review builds the skill.