<p>An increasing number of workers are facing various physical factors, such as heat stress due to the inevitable climate change. This study aimed to develop and validate an Outdoor Environmental Heat Index(OETI) without using measurement instruments, relying only on meteorological and geographical data and using structural equation modeling. This cross-sectional study was conducted on 201 men who worked outdoors in both hot and dry, as well as hot and humid environments. Alongside collecting personal information from the participants, tympanic temperature and heart rate were measured both at rest and 90&#xa0;min after the start of work. Concurrently, the climatic conditions of the workplace were assessed using instruments, along with geographic information and data from meteorological databases. Additionally, the metabolic rate and insulation value of the individuals’ clothing were estimated, and information related to other observed parameters was also recorded. The results revealed that the main factors, with a total coefficient of 0.8 had a significant direct effect on tympanic temperature. According to the results, the indirect effect coefficients of the most important variables were as follows: globe temperature (0.59), dry bulb temperature (0.55), relative humidity (0.53), work place (0.59) and heat source (0.55). The overall index score was divided into four categories based on established optimal cut-off points of 18.5, 20.02, and 21.46. Furthermore, the results showed that the OETI developed using device data and with meteorological data was able to justify 69% and 59% of the changes in tympanic temperature. OETI was developed based on a combination of observational, geographic, and meteorological data, making it a practical tool for assessing heat stress without need to measurement ins As a result, this index justifies 69% of the tympanic temperature (R² =0.69).</p>