Background: Working under hot climatic conditions, especially in summer, has a high potential for inducing heat strain in workers. The wet-bulb globe temperature (WBGT) index has an inherent limitation to estimate the heat strain due to not taking into account the non-environmental factors. The aim of this study was to evaluate the combined application of WBGT index and a physiological strain index (PSIHR) based on heart rate in the estimation of the heat strain under hot climatic conditions.

Materials and Methods: This cross-sectional study was conducted on 122 men in the center and south of Iran (51 and 71 workers from Isfahan steel company and Assaluyeh national petrochemical company, respectively) between July and September 2010. The WBGT index, heart rate and the auditory canal temperature were measured at rest and during the work. Data were analyzed using descriptive statistics and logistic regression.

Results: Results of the logistic regression analysis revealed that the WBGT index is a poor predictor for heat strain with a sensitivity and specificity of 53% and 65%, respectively. However, the combined application of the WBGT and PSIHR indexes was a better predictor for heat strain and its sensitivity and specificity were 75% and 69%, respectively.

Conclusion: A combined application of the WBGT and PSIHR indexes can be a valid estimator of heat strain for hot climatic conditions in the center and south of Iran.