This study aimed at determining the effect of heat exposure on physical workload and maximum acceptable work duration in a hot and dry climate. Wet Bulb Globe Temperature, as an environmental index, and heart rate and core body temperature, as physiological indices, were used to assess environmental heat stress. For this purpose, the study was conducted on 86 acclimatized workers of six different sites of an oil industry. The comparison between heart rate at rest and work hours and core body temperature at rest and work hours revealed the existence of a significant different between heart rate and Core Body Temperature (P value < 0.001). The results also indicated a significant difference between WBGT(TWA) and the limit value (P value < 0.001). However, the results uncovered that the MAWD should be reduced in some units and, consequently, the work-rest regimens were modified based on this reduction. In addition, the results of this study demonstrated a statistically significant increase in and heart rate after work in excessive heat stress conditions. Heat exposure is perceived as a significant problem by the workers who worked in such workplaces. The results of this study can help managers and workers better plan and control thermal stresses.