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Wet Bulb Globe Temperature (WBGT) in Heat Safety
Components of WBGT
WBGT equipment uses three critical environmental factors to assess heat stress risk, capturing the thermal environment affecting human comfort and health.
• Natural wet bulb temperature. Measures the cooling effect of evaporation on the body using a thermometer covered in a water-soaked cloth. This reflects the temperature and humidity as well as the body’s potential for evaporative cooling. High humidity decreases evaporation, leading to higher perceived heat stress.
• Globe temperature. Measured with a globe thermometer, this gauges the amount of radiant heat present, significantly affecting heat stress levels. The black globe absorbs radiant heat, mimicking the heat absorption by the human body.
• Air temperature. The ambient air temperature is measured using a standard thermometer. While straightforward, it doesn’t fully represent heat stress risk without considering humidity, wind speed or radiant heat.
Calculating the Impact
These components are measured using the following specific instruments to ensure accuracy:
• The natural wet bulb temperature is measured with a sling psychrometer.
• The globe temperature requires a black sphere globe thermometer.
• Air temperature is measured with a standard thermometer.
These measurements are weighted and combined according to a formula that accounts for their relative contributions to heat stress. For example, in direct sunlight, the globe temperature carries more weight due to the significant impact of solar radiation.
WBGT in Workplace Safety
WBGT helps organizations mitigate heat stress risks by adjusting work-rest cycles, particularly in high-temperature environments. For instance, in construction or agricultural sectors, WBGT guidelines determine the maximum duration workers should be exposed to heat. In foundries, WBGT guidelines are critical due to high ambient temperatures and radiant heat from machinery.
Developing Safety Protocols
By integrating WBGT readings into safety protocols, companies can proactively manage health risks in hot conditions, such as adjusting the duration and intensity of work based on WBGT levels and implementing hydration breaks. Policies might dictate that when WBGT reaches a certain threshold, outdoor work is limited to 45 minutes followed by a 15-minute rest in a shaded or air-conditioned area. Safety protocols can include guidelines for acclimatization, gradually increasing heat exposure to help workers adapt.
This article originally appeared in the July/August 2024 issue of Occupational Health & Safety.