The WBGT index is the heat stress index of preference in the Occupational Health and Safety Act,No. 85 of 1993. The purpose of this investigation was to determine if equivalent WBGT values with low and high relative humidity levels would result in similar physiological heat loads. Un-acclimatised subjects were exposed to 24, 28, 30 and 32 WBGT with 30% (LH) and 70% (HH) relative humidity levels respectively using a metabolic rate of 450 watts. Subjects were exposed for five hours using a work:rest cycle of 45 min work:15 min rest. Final core temperature at 30 and 32
WBGT was significantly higher in HH. Final heart rate was at all the experimental conditions significantly higher in HH. Sweat rate increased significantly only at 32 WBGT (HH). Tolerance time decreased significantly at 30 and 32 WBGT with HH. The results indicate that WBGT index values
above 30 with HH levels underestimate thermal load and that un-acclimatised employees may be at risk to develop heat illnesses if work schedules are not properly managed.