Assessing the risk of mortality caused by heat stress in Iran's metropolises

Thermal stress is one of the risk factors for mortality. The research aims to identify the potential of the Universal Thermal Climate Index (UTCI) as a heat-related health risk indicator in Iran. Using 32 years of daily meteorological data, UTCI values in different percentiles were computed to identify excess mortality periods in five cities of Tehran, Ahvaz, Mashhad, Shiraz, and Isfahan in the summer season. Frequency patterns of summer UTCI index classes were identified. Threshold percentiles of minimum and maximum index values were extracted for each station. The results of the research showed that the summer values of the index vary between -1.9 and 55.4 degrees Celsius. In total, 18.08% of the night temperature (minimum temperature of thermal climate index) and 97.64% of the daily temperature (maximum temperature of thermal climate index) of the studied days were associated with heat stress. During the studied period, 10% of summer days are in the conditions related to the maximum UTCI index values in three classes of strong, very strong, and severe heat stress. Correlation with mortality data from 5 stations showed that the relationship between UTCI and the number of deaths depends on the bioclimate of the station and that the number of deaths increases in very strong and severe heat stress conditions, that is when UTCI Above 38 and 46 degrees Celsius. The fitting of the categories of heat stress on the number of deaths with the trend of the LOWESS function showed that the distribution of deaths according to the UTCI index had a diverse nature and was seen as V-, U-, and J-shapes. The increase in mortality depends on the levels of heat stress, and at 35 degrees Celsius and above, i.e. entering the moderate heat stress category of the UTCI index, an increase in mortality is evident.