Workers of Iron and steel plants are exposed to extreme environmental heat that causes discomfort and limits their performance. This study investigates the influence of heat load on workers' health and activity in Kardemir Steel Factory in Karabuk-Turkey using several heat stress indices. Combined field measurements and questionnaires were carried out over a period from June to August 2016. A total number of 100 workers regularly working in the steel plant from five different workplaces were selected. The wet bulb globe temperature (WBGT), the physiological strain (PSI), and the heat stress (HSI) indices were calculated. Workers' productivity level was evaluated by analyzing the relationships between work capacities and different WBGT levels against work intensities' curves and by using the predicted mean vote (PMV)-productivity model. The highest values of WBGT were recorded in August, notably within the blast furnace area and continuous casting unit with mean values of 31.32 +/- 0.8 degrees C and 31.34 +/- 0.74 degrees C respectively, while the maximum HSI was calculated at the rolling mills unit with a value of 137.83% +/- 18.45. About 86% of participants complained of thermal discomfort during summer as a result of heat waves, dirt and gas emissions. Strong correlations were found between PSI and WBGT indices with core body temperature (r = 0.725 and r = 0.721 respectively) as well as the rate of heartbeat (r = 0.648 and r = 0.517). These are considered as the most applicable indices for evaluating heat load impact on workers' health and performance. (C) 2018 Karabuk University. Publishing services by Elsevier B.V.