How to set work-rest cycles from WBGT
Air temperature alone does not tell you how dangerous a hot job is. The same 92 °F can be harmless in dry shade and dangerous in humid sun, because sweat cannot evaporate and radiant heat keeps loading the body. Wet bulb globe temperature combines humidity, radiant heat and air temperature into one index for exactly this reason.
Outdoors with a solar load, WBGT = 0.7 × natural wet bulb + 0.2 × globe + 0.1 × dry bulb. Indoors or with no solar load it becomes 0.7 × natural wet bulb + 0.3 × globe. Heavier work generates more metabolic heat, so the screening criterion drops as intensity rises.
| Work-rest cycle | Light | Moderate | Heavy |
|---|---|---|---|
| Continuous work | 85.1 °F | 81.5 °F | 78.8 °F |
| 75% work, 25% rest | 86.9 °F | 83.3 °F | 81.5 °F |
| 50% work, 50% rest | 88.7 °F | 85.1 °F | 83.3 °F |
| 25% work, 75% rest | 90.5 °F | 87.8 °F | 86.0 °F |
These are the ACGIH TLV screening criteria for workers acclimatized to heat, converted from the published Celsius values. Unacclimatized workers, the first days of a heat wave, older workers and anyone on medication that affects heat tolerance all need a more conservative schedule. Breaks have to be taken in shade or air conditioning with water available, and a heat illness prevention plan should be reviewed by an occupational health professional.
Frequently asked questions
WBGT folds humidity and radiant heat into one index. Outdoors with sun it is 0.7 times natural wet bulb plus 0.2 times globe plus 0.1 times dry bulb; indoors it is 0.7 wet bulb plus 0.3 globe.
Light covers seated or standing hand work, moderate covers walking while handling material, and heavy covers shoveling, pick work and similar sustained effort.
In a shaded, ventilated or air conditioned area with water available. A break taken in the sun does little to bring core temperature back down.