What Is WBGT?
WBGT, or wet-bulb globe temperature, is an indicator used to assess heat stress risks in extremely hot environments.
Also known as the wet-bulb black globe temperature or heat stress index, WBGT considers humidity, temperature, and radiant heat, which significantly impact the body’s heat balance. It can be calculated using dry-bulb temperature (Td), wet-bulb temperature (Tw), and black-bulb temperature (Tg). In the presence of solar radiation, WBGT is expressed as WBGT = 0.7Tw + 0.2Tg + 0.1Td. In indoor or shaded conditions, it is WBGT = 0.7Tw + 0.3Tg.
The dry-bulb temperature is the air temperature measured by a thermometer. The wet-bulb temperature indicates air and vapor characteristics. The black-bulb temperature, measured using a virtual blackbody sphere (also known as globe temperature Tg), reflects radiant heat exposure. All measurements for WBGT, Tg, Tw, and Td are in degrees Celsius (°C). Airflow influences black-bulb and wet-bulb temperatures, thus being incorporated into WBGT calculations.
Uses of WBGT
First proposed in the United States in 1954 to prevent heatstroke, WBGT was adopted at U.S. military training facilities for its effectiveness. Now, it is an internationally recognized guideline for sports and occupational environments, standardized by ISO.
WBGT is a standard for sports activities and is also used in workplaces and daily life to ensure comfort and avoid overexertion in hot and humid conditions. Regular reassessment of work environments and practices like salt and fluid replenishment, prioritizing sleep, and ensuring adequate ventilation are crucial. WBGT serves as a vital indicator for maintaining safe conditions.
Principles of WBGT in Preventing Heat Stroke
- WBGT below 21: Low heatstroke risk, but caution needed in events like marathons.
- WBGT 21-25: Increased risk of heatstroke; stay hydrated and watch for symptoms.
- WBGT 25-28: Regular breaks recommended, especially during intense activities.
- WBGT 28-31: High heatstroke risk; limit intense activities and take frequent breaks.
- WBGT above 31: Exercise should be discontinued, particularly for children.
Guidelines for daily life include:
- WBGT below 25: Low risk but caution during intense activities.
- WBGT 25-28: Incorporate breaks during vigorous activities.
- WBGT 28-31: Avoid outdoor exposure in hot weather.
- WBGT above 31: High risk for older adults; stay indoors and avoid heat exposure.
Types of WBGT
WBGT calculations are based on three measurements: dry-bulb, wet-bulb, and black-bulb temperatures.
1. Dry-Bulb Temperature
Direct air temperature measurement using a standard thermometer.
2. Wet-Bulb Temperature
Measured with a thermometer wrapped in wet gauze, indicating equilibrium temperature with cooling from water evaporation.
3. Black-Bulb Temperature
Measured using a thermometer in a black-painted, thin copper ball, reflecting perceived temperature in shaded conditions.
How to Choose WBGT
WBGT values can be determined using online heat index information or by using a WBGT measuring device for more accurate readings. Handheld types, prevention guideline-based types, and large-display types are available, though accuracy can be affected without a black bulb for measuring solar and radiant heat.