What Is WBGT
Wet Bulb Globe Temperature (WBGT) is a composite measurement that accounts for air temperature, humidity, wind speed, and solar radiation to assess heat stress risk. Unlike standard thermometer readings, WBGT reflects how the human body experiences heat in real conditions, making it the standard tool for determining safe work exposure limits in hot environments.
OSHA does not have a federal heat standard, but it references WBGT thresholds established by the National Institute for Occupational Safety and Health (NIOSH). NIOSH recommends limiting work in conditions exceeding 30.6°C (87°F) WBGT for acclimatized workers without protective equipment. For non-acclimatized workers, the limit drops to 27.5°C (81.5°F). These thresholds assume continuous outdoor work; light indoor work tolerates higher WBGT values, while heavy exertion lowers them further.
WBGT measurement requires specific equipment: a natural wet bulb thermometer, a black globe thermometer, and a dry bulb thermometer. The calculation weighs each component differently depending on whether work occurs indoors or outdoors in direct sun. Outdoors in sunlight, the formula is: WBGT = 0.7(wet bulb) + 0.2(globe) + 0.1(dry bulb). Indoors or outdoors in shade: WBGT = 0.7(wet bulb) + 0.3(globe).
Workplace Safety Requirements
Safety managers must monitor WBGT levels during heat-prone seasons and work activities. OSHA citations under the General Duty Clause have increased for employers who failed to implement heat controls. Practical requirements include:
- Establish work-rest schedules based on WBGT thresholds. At 28°C (82°F) WBGT, limit work to 75 minutes per hour with 15-minute breaks in shade.
- Provide unrestricted access to drinking water. Workers should consume 200 to 300 milliliters every 15 to 20 minutes, even if not thirsty.
- Designate a heat monitor or supervisor trained to recognize early signs of heat illness prevention protocols.
- Acclimatize new workers over 10 to 14 days, gradually increasing exposure time.
- Implement controls like shade structures, cooling vests, or schedule adjustments to reduce heat stress exposure.
- Document WBGT readings daily during heat seasons as part of safety audits.
Home and Residential Context
Homeowners and property managers use WBGT to assess risk during outdoor maintenance, yard work, and emergency preparedness. Heat-related deaths during home repairs and landscaping exceed workplace incidents in the general population. Monitoring conditions through local heat index data (which approximates WBGT for outdoor shade) helps determine when outdoor work should pause or shift to early morning hours.
During emergency preparedness planning, especially for wildfire zones or areas prone to extreme heat, establish protocols for when outdoor response activities must cease. Heat illness during evacuation or post-disaster cleanup claims lives each year.
Common Questions
- How often should WBGT be measured? For workplace safety, measure WBGT hourly during hot shifts, or more frequently if conditions are changing rapidly. Use the peak reading to set rest-work schedules for that period.
- Can I use a regular thermometer instead of WBGT equipment? No. A standard thermometer shows air temperature only and ignores humidity and radiation, both critical factors. At 35°C (95°F) dry temperature with high humidity, actual heat stress can be equivalent to much higher temperatures. Proper WBGT measurement requires the three-component setup.
- What should I do if WBGT exceeds safe limits but work must continue? Reduce work intensity, increase break frequency and duration, enforce mandatory hydration, monitor workers for symptoms of heat illness, and use engineering controls like shade or cooling systems. Consider rescheduling non-urgent tasks to cooler hours.