Background and Objectives: Nowadays, indicators related to human physiology have special roles in human bioclimatic and environmental studies. One of these indices is Physiological Equivalent Temperature (PET). In the present study, trends of physiological equivalent temperature index over the past half century, which is the foundation of plans, especially in the field of health and environment, were evaluated in four climatic regions of Iran (Mashhad, Rasht, Esfahan, Tabriz and Bandar Abbas).
Material and methods: In this study, the data required to calculate the heat stress in the daily scale for 50 years during 1961-2010 were obtained from Iranian Meteorological Organization. Then, PET index at the daily scale was calculated and values greater than 35°C were identified for warm periods of the year. In the next step, the seasonal trends were analyzed by Mann-Kendall test.
Results: Changes in the frequency of days with heat stress in stations of Mashhad, Isfahan, and Tabriz have been rising during spring and summer. In other words, the greatest amount of positive change has occurred over the long-term average in these cities. The highest frequency of hot days was observed in Rasht and Bandar Abbas stations. The main reason for this subject has to do with how adjacent they were to the sea and the high rate of evaporation in such areas.
Conclusion: The 1980s-1990s have been allocated as the most days of hot stress. Upward trend of thermal stress occurring in the studied stations has increased the attention to the problem of heat stress and disease (thermal attack, fainting, muscle cramps) from the field of planning and management of the crisis.
