Iran is located in the arid region of the world and having high temperatures is one of its climate characteristics. As the country's capital, the metropolis of Tehran is located in the geographical heart of the country and has an arid climate. Studies show that in recent years, the intensity of temperature increase in Iran and the metropolis of Tehran has increased, and besides that, temperature waves with more frequency are occurring. These events have a direct effect on thermal stress, which directly affects the health of Iranians and the metropolis of Tehran. This paper examines the current and future characteristics of temperatures affecting thermal stress in the Tehran metropolis. This study has explained the statistical characteristics of heat stress by using daily temperature data and statistical methods including frequency and trend calculation. The results show that during recent years, the temperature of the country, including the metropolis of Tehran, has increased by at least 2 degrees Celsius, which is also significant in the maximum temperatures. According to this increase, the frequency of days with heat stress affecting the health of the metropolis of Tehran is also increasing, and at least we have seen an increase of one month in the days with heat stress in this metropolis, which has had its effect on the human mortality rate. The study of the future conditions under the influence of different climate models and scenarios in the metropolis of Tehran indicates that the thermal stress conditions in Tehran are increasing and we should expect even worse conditions. To deal with the impact of these conditions in the country and the metropolis of Tehran, adaptation solutions must be considered to reduce the severity of the damage to health.