What Is WBGT?

WBGT (Wet Bulb Globe Temperature) is the gold-standard index for assessing heat stress risk. Unlike air temperature alone, it combines humidity, radiant heat, and air movement — the three factors that determine how your body actually copes with heat.

Two days might share the same 35°C air temperature, but one with high humidity creates dramatically higher heat stress. WBGT captures this difference accurately.

How WBGT Is Calculated

Outdoor Formula (Natural Ventilation)

WBGT (outdoor) = 0.7 × NWB + 0.2 × GT + 0.1 × DB

NWB (Natural Wet Bulb Temperature): Temperature of a thermometer wrapped in wet gauze — reflects the cooling effect of evaporation. The dominant factor (70% weight) because humidity most strongly affects heat stress.
GT (Globe Temperature): Temperature inside a black copper sphere — reflects solar and radiant heat load.
DB (Dry Bulb Temperature): Ordinary air temperature.

Indoor Formula

WBGT (indoor) = 0.7 × NWB + 0.3 × DB

No globe temperature component since indoor radiant heat load is negligible.

Heatstroke Risk CheckerEstimate Wet Bulb Globe Temperature (WBGT) and heatstroke risk from temperature and humidity. Unit ConverterEasily convert length, weight, temperature, and more on the fly.

WBGT Exercise Guidelines (Japan Sports Association)

WBGT	Risk Level	Exercise Guidance
≥31°C	Stop exercise	Cancel all exercise activities. Extreme heatstroke risk.
28–31°C	Severe caution	Stop exercise for unfit or unacclimatized individuals.
25–28°C	Caution	Avoid intense/prolonged exercise. Take frequent breaks. Hydrate actively.
21–25°C	Attention	Heat stress risk increases. Active water and electrolyte replacement needed.
<21°C	Nearly safe	Low heatstroke risk under normal conditions.

Special Considerations for Vulnerable Groups

WBGT thresholds apply to healthy adults. Children, the elderly, and those with medical conditions face risk at lower WBGT levels.

Children: Underdeveloped thermoregulation; larger surface-area-to-body-weight ratio absorbs heat faster
Elderly: Reduced heat sensation, decreased sweat production
Certain medications: Diuretics, anticholinergics, and beta-blockers increase heat stress risk

Japan's Heat Warning Alert System

Japan's Ministry of the Environment and Japan Meteorological Agency issue Heatstroke Warning Alerts by prefecture.

Alert Level	Trigger	Recommended Action
Heatstroke Warning Alert	Prefectural WBGT forecast ≥33	Avoid unnecessary outdoor activity; use air conditioning
Heatstroke Special Warning Alert (from 2024)	Nearly all points in prefecture forecast WBGT ≥35	Strongly discourage all outdoor activity

Workplace Heat Management

Japan's Ministry of Health, Labour and Welfare recommends WBGT-based heat management in workplaces.

Approximate WBGT limits by work intensity:

Work Level	Approximate WBGT Limit
Resting (minimal movement)	Up to ~33°C
Light work (standing, machine operation)	Up to ~28°C
Moderate work (walking with load)	Up to ~26°C
Heavy work (shoveling, heavy lifting)	Up to ~25°C

People not yet acclimatized to heat are at highest risk — particularly at the start of the hot season (post-rainy season in Japan).

Practical Heat Precautions by WBGT Level

WBGT 25–28°C (Attention):

Hydrate every 15–20 minutes (100–200ml per session)
Seek shade outdoors

WBGT 28–31°C (Caution):

Avoid intense or prolonged exercise
Take frequent active rest breaks
Stay in shade or air conditioning when possible

WBGT ≥31°C (Severe Caution):

Stop intense exercise entirely
Prioritize cool indoor spaces
Heightened vigilance for children, elderly, and unacclimatized individuals

WBGT ≥35°C (Danger):

Cancel all outdoor athletic events
Avoid going outside
Stay in air-conditioned spaces

Hydration Guidelines During Exercise

During activity: 150–250ml every 15–20 minutes
Sessions over 1 hour: Water plus sports drink (electrolyte replacement)
During heavy sweating: 0.1–0.2% saline solution to replace lost sodium

Heatstroke Risk CheckerEstimate Wet Bulb Globe Temperature (WBGT) and heatstroke risk from temperature and humidity.

Summary

WBGT	Risk Level	Primary Action
≥31°C	Dangerous / Stop	Cancel all outdoor exercise
28–31°C	Severe Caution	Stop intense exercise; frequent rest
25–28°C	Caution	Regular breaks; active hydration
21–25°C	Attention	Active water and electrolyte intake
<21°C	Nearly Safe	Standard precautions

WBGT is the most accurate publicly available measure of real-world heat stress risk. Use it — not just air temperature — to make safe decisions about summer activities.