Safety Tidbit 4.35 - Wet-Bulb Globe Temperature

Safety Tidbit 4.35 – Wet-Bulb Globe Temperature

Reference:        ACGIH -Threshold Limit Values (TLV) 2018

                        National Weather Service (https://www.weather.gov/tsa/wbgt)

Last Summer I and two of close friends rode our motorcycles from San Francisco back to Pennsylvania. One our stops along the way was Death Valley. But we couldn’t just visit “Death Valley” we had to visit in the middle of Summer and the middle of the afternoon. The temperature was 118 degrees F. Everyone told us it wouldn’t feel as bad as the heat back East because it was a “dry heat.” Let me tell you, when riding a motorcycle, 118 degrees is hot wet or dry! Since the forecast is for extreme heat over the next few days. I thought I would spend a little time discussing measuring heat stress. See Safety Tidbit 2.41 for more on the Heat Index.

The Wet-Bulb Globe Temperature (WBGT) is a measure of the heat stress in direct sunlight, accounting for: temperature, humidity, wind speed, sun angle and cloud cover (solar radiation). This differs from the heat index, which takes into consideration temperature and humidity and is calculated for shady areas. If you work or exercise in direct sunlight, this is a good element to monitor. Military agencies, OSHA, and many nations use the WBGT as a guide to manage workload in direct sunlight.

The WBGT can be calculated using the formula

            WBGT = 0.7GB + 0.2WB + 0.1DB

Where GB is globe temperature, WB is the web-bulb temperature and DB is the dry bulb temperature. The globe temperature is the heat loading from radiant sources. Outside in the summer the largest source of radiant heat being the sun. This temperature is found by using a copper globe painted in black matte paint with a thermometer inserted so that the bulb is in the center of the globe. As you can see this portion is 70% of the over WBGT. The wet bulb temperature describes humidity and is 20% of the calculation. Lastly, at only ten percent of the calculation is the common household dry bulb temperature.

Once the WBGT is calculated precautionary actions can be taken when working outdoors. A WBGT of 80 degrees is usually the starting point where fifteen minutes breaks out of every hour of work outdoors are necessary. The most severe is when the WBGT exceeds 90 degrees when the ratio is reversed, and 45 minutes of rest are necessary for fifteen minutes of work outdoors.

  Take breaks in the shade. Strenuous outdoor activities should be reduced, especially in direct sunlight where there is little ventilation.

  Drink plenty of water or other non-alcohol fluids. Your body needs water to keep cool. Drink plenty of fluids even if you don’t feel thirsty.

  Don’t get too much sun. Sunburn makes the job of heat dissipation that much more difficult

Hope this was helpful and thank you for reading my Safety Tidbits! Comments and questions are always welcome. ~ Bryan

P.S. If you have a new safety or health question, please let me know.