THERMAL ENVIRONMENT · REFERENCE
Heat Stress, Wet Bulb and WBGT
Understand easily confused concepts and why occupational heat assessment depends on field measurements and work conditions.
What is heat stress?
Occupational heat stress combines environmental heat load with metabolic heat produced during work. Air temperature, humidity, air movement and radiation influence heat exchange between a person and their surroundings; workload, clothing, individual circumstances and acclimatization also affect risk.
Heat stress and the body's physiological response are different quantities. A single temperature reading cannot determine whether an individual is safe or has a heat-related illness. For symptoms and first aid, consult NIOSH heat-related illness information.
Wet-bulb temperatures are not all the same
Psychrometric wet bulb
Moist-air calculations often estimate thermodynamic wet-bulb temperature from dry-bulb temperature, humidity and pressure. NIOSH also describes psychrometric wet bulb measured by a wetted sensor under forced ventilation; a calculation model and the measurement conditions of a field sensor are not interchangeable.
Natural wet bulb
Occupational measurement uses a wetted sensor exposed to the naturally prevailing air movement. Sensor setup and measurement method affect the result; follow the applicable measurement standard.
Wet-bulb globe temperature
WBGT is an environmental heat-stress index based on natural wet bulb and globe temperature, and in applicable conditions dry-bulb temperature. It is not a single wet-bulb reading or a body temperature.
Our moist-air properties calculator reports a model-estimated wet-bulb temperature. It is not a field measurement of natural wet bulb and cannot be used directly as WBGT or as an occupational exposure assessment.
Understanding occupational WBGT
NIOSH describes WBGT measurements using natural wet-bulb and black-globe temperatures; outdoors in sunshine, air dry-bulb temperature is also measured. The globe sensor responds to radiation and the air environment, while natural wet bulb is affected by humidity and local air movement. Measurement and calculation differ between indoor and outdoor sunshine conditions.
The heat index is also different from WBGT. NIOSH explains that heat index mainly uses air temperature and relative humidity, whereas occupational heat stress is also affected by radiation, air movement and metabolic heat from work.
Limits
This page explains concepts only. This site does not measure or calculate occupational WBGT, perform occupational safety assessment, classify individual health risk, diagnose illness or determine permissible work duration.
Occupational assessment requires appropriate field instruments and measurement methods, with workload, exposure duration, acclimatization, clothing and other factors considered by qualified personnel under applicable local requirements. In particular, special protective clothing can make an ordinary environmental index incomplete as an assessment of personal heat load.
Seek medical help promptly for possible heat-related symptoms; contact local emergency services immediately for a suspected serious condition. Consult the NIOSH resources below for recognition and response.
Official NIOSH resources
- Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments (NIOSH 2016-106): Chapter 5 discusses measurement and Chapter 9 discusses WBGT and its limitations.
- OSHA–NIOSH Heat Safety Tool and FAQs: describes heat index and WBGT distinctions.
- Heat-related illnesses: symptoms and first aid.
This page is independently written from official resources. Linked material may change; consult the applicable version and local requirements for occupational assessment.