Introduction: In many industrial and mining environments, heat stress not only leads to a lower productivity and poor morale, but also it increases the cost of accidents. Heat stress indices have been developed to assess heat stress at workplace. The objective of this study was to evaluate heat stress and to investigate the correlation between ESI index and some physiological parameters of workers in an open pit mine.

Methods: This cross-sectional study was conducted on 120 healthy miners. Tympanic and skin temperatures were measured using a FT70 model thermometer manufactured by Beurer Co, Germany. Heart rate and blood pressure were measured using Emsig BO26 model (Taiwan) digital instrument. All environmental and physiological parameters were simultaneously measured and recorded during rest and work. The ESI index was calculated using the formula. Finally, the correlations were determined using Pearson's correlation test.

Results: The average ESI index calculated for individuals was 29.23^oC. Drilling and transportation units had the highest and lowest ESI values of 30.45 ^oC and 28.03 ^oC, respectively. The statistically significant correlation was found between ESI heat stress Index and physiological parameters (p<0.01). The Pearson's correlation coefficient of ESI with tympanic temperature, skin temperature, heart rate, systolic and diastolic blood pressure was 0.614, 0.507, 0.482, 0.355 and 0.247, respectively. The highest correlation of 0.614 was related to the tympanic temperature.

Conclusion: Heat stress at the studied open pit mine was high. Drilling workers were faced to higher thermal stress and ESI had an acceptable correlation with physiological parameters. Further studies on ESI at different working environments are recommended.