WBGT · The Heat Stress Standard

Wet-bulb globe temperature is OSHA, ACGIH, and military doctrine. Wet-bulb 35°C is the absolute human limit.

WBGT = 0.7·Tw + 0.2·Tg + 0.1·Td (outdoor) blends humidity, radiant, and air temp; ACGIH bands drive work/rest cycles; wet-bulb 35°C kills regardless of hydration or fitness.

Heat kills through evaporative failure, not raw temperature. At wet-bulb 35°C, sweat cannot shed heat faster than metabolism generates it — fatal within hours regardless of shade, hydration, or fitness. WBGT (wet-bulb globe temperature) is the index that captures this: 70% wet-bulb (humidity), 20% globe (radiant load), 10% dry-bulb (air temp) for outdoor conditions. OSHA, ACGIH, and the military all run work/rest cycles off WBGT bands.

Part of: Environmental Stressors

Quick answer

WBGT = 0.7·Tw + 0.2·Tg + 0.1·Td (outdoor) blends humidity, radiant, and air temp; ACGIH bands drive work/rest cycles; wet-bulb 35°C kills regardless of hydration or fitness.

What you are trying to do

Wet-bulb globe temperature is OSHA, ACGIH, and military doctrine. Wet-bulb 35°C is the absolute human limit.

Best next step

Thermal Failure · WBGT

Limit to remember

Treat this as a practical aid for the task, not a replacement for professional judgment.

Key points

▸ WBGT outdoor formula: 0.7·Tw + 0.2·Tg + 0.1·Td. Indoor: 0.7·Tw + 0.3·Tg (no direct sun).
▸ Wet-bulb (Tw): temperature of a thermometer with a wet wick — reflects cooling capacity of evaporation. Dominates the index (70% weight).
▸ Globe (Tg): black-globe thermometer — captures radiant load from sun, hot surfaces, machinery.
▸ Dry-bulb (Td): normal air temperature — least weighted because humidity matters more than raw temp for heat stress.
▸ ACGIH bands: <27°C nominal, 27-30°C moderate (reduce work), 30-32°C high (alternate work/rest), 32-35°C extreme (rest cycles dominate), >35°C fatal-risk (work stops).
▸ Wet-bulb 35°C: hard physiological limit. No amount of hydration or acclimatization compensates — skin cannot shed heat faster than metabolism creates it.

How to

Measure or look up wet-bulb, globe, and dry-bulb temperatures (°F or °C).
Toggle direct-sun if outdoors.
Enter the three temps into the calculator.
Read WBGT and ACGIH status band.
Apply the band's work/rest ratio — moderate = 75/25, high = 50/50, extreme = 25/75, fatal-risk = stop work.
Acclimatized workers tolerate one band higher than unacclimatized; first 5-7 days of heat exposure require the stricter ratio.

Examples

Tw 75°F, Tg 85°F, Td 88°F, direct sun

WBGT = 0.7×75 + 0.2×85 + 0.1×88 = 52.5 + 17 + 8.8 = 78.3°F = 25.7°C. Nominal band — normal work cycle.
Tw 82°F, Tg 100°F, Td 95°F, direct sun (humid summer afternoon)

WBGT = 0.7×82 + 0.2×100 + 0.1×95 = 57.4 + 20 + 9.5 = 86.9°F = 30.5°C. High band — alternate work/rest 50/50.
Wet-bulb 35°C (South Asian heat dome, rare but real)

WBGT ≥ 35°C regardless of globe/dry readings. Fatal-risk band. No outdoor work, no outdoor exercise, no unconditioned sleep. Hours of exposure kill healthy adults.

When to use which tool

Thermal Failure · WBGT

Before outdoor work or training in any summer-like conditions; re-check every 2 hours as temps shift.

Wet-bulb globe temperature — the heat-stress index used by OSHA, ACGIH, and military for work/rest cycle decisions.

▸ Operational Thresholds

CYAN · STABLE — WBGT <27°C — nominal; normal work/training cycle.
GOLD · GUARDED — WBGT 27-32°C — moderate to high; reduce intensity, alternate work/rest, hydrate and electrolyte-load.
MAGENTA · CRITICAL — WBGT >32°C — extreme or fatal-risk; work stops, exercise dangerous; wet-bulb 35°C kills regardless of precautions.

▸ Pivot

Heat risk high. Balance electrolytes first — sodium loss in sweat accelerates in the moderate+ bands.

Ion Balance · Electrolyte Loss →

Frequently asked questions

› Why WBGT instead of heat index? Troubleshooting

Heat index (NOAA) combines air temp and humidity only — it misses radiant load. WBGT adds the globe term for sun and hot surfaces, which can push effective heat stress 5-10°C above heat index alone. WBGT is the ISO 7243 and ACGIH TLV standard for exactly this reason.

› Can acclimatization push past wet-bulb 35°C? Trust & accuracy

No. Wet-bulb 35°C is a physics limit: skin temp must be below air temp for evaporative cooling to work. Above 35°C wet-bulb, your body gains heat from the environment faster than it can shed it. Acclimatization helps up to about wet-bulb 32°C; above that, no human tolerates prolonged exposure.

› What about indoor WBGT with AC?

AC drops both dry-bulb and wet-bulb (dehumidification). A conditioned office typically sits at WBGT 20-23°C — nominal band regardless of outdoor conditions. Problem rooms are warehouses, server rooms with high radiant load, or kitchens.

› 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.