One of the general concerns of the current world is climate change and global warming. As temperatures rise over the next century due to climate change, increasing heat stress becomes more important, especially for outdoor workers such as farmers, industrial workers, construction workers, and so on. Due to the consideration of safety principles, workers should use personal protective equipment such as shoes, helmets, face masks and ear muffs. Doing strenuous physical activity with protective equipment in hot weather conditions can increase the risk of heat stress . The output of results of models showed the minimum and maximum air temperature in different months of the year in the three studied climatic zones an increasing trend. The temperature will increase more during the summer season. In a way, this rate of temperature increase in July and August is significant. Also, the trend of temperature increase until 2099 will be about 5 degrees Celsius. The study of Haghtalab et al., (2013) showed similar results for the city of Tehran . Vaghefi et al., (2019) stated that Iran will have warmer climates during the period 2025–2049 than during the period 1980–2004, which is more severe in the southern part of the country . This findings support the results of the present study. Also, several studies have surveyed the effect of climate change on disease rates as well as mortality. By using the AR5 scenario, Petkova et al., (2013) stated that heat-related deaths in New York, Philadelphia, and Boston would increase in 2020, 2050, and 2080, indicating more difficult conditions for heat adaptation and most people are vulnerable to heat increase . Nastos and Matzarakis (2012) studied the effect of air temperature and heat stress indices, with the application of the Universal Thermal Climate Index (UTCI) and the Physiologically Equivalent Temperature (PET) on mortality over a 10-year period in Athens. The results showed a significant relationship between air temperature, UTCI, PET and mortality rate . Considering that physiological capacities will be more limited in heat conditions, if the Earth's temperature rises to at least 7 degrees Celsius, over populated areas of the world will be uninhabitable due to climate change. In this study, the WBGT index was used to assess heat stress in agricultural workers. This index is one of the most widely used tools for estimating heat stress worldwide and the WBGT index is well correlated with physiological parameters .The study findings showed that the WBGT index has an increasing trend until 2099, which is much higher in Khuzestan province and Ahvaz city. The results also showed that the mean monthly WBGTs occur in March in Khuzestan (caution, 24-28 ° C). So more farm workers can be affected by this risk factor in following years. In a study based on time series modeling, the trend of past and future heat stress in outdoor workplaces in Tehran showed an increasing trend in the average annual WBGT (1961–2009). It also estimated a 1.6 ° C increase in WBGT index until 2050 . In another study using the WBGT index, the effect of climate change during the period of 1973 to 2003 was examined for 15 regions of the world on increasing the heat stress threshold. The results showed an upward trend in all zones except the northeastern United States and northeastern Australia . Also, using the HadCM3 model, regional changes in mean summer air temperature, RH% and WBGT were identified under the A1B scenario in 2020-2050. The results showed that WBGT index is incremental for all zones, but changes in air temperature and RH% depend on climatic conditions in the study areas (Willett and Sherwood, 2012). Similar finding is obtained in this study. Maeda et al., (2006) revealed that deaths in August and July were higher than in other months of the year . Furthermore, the results of the study showed that the WBGT index increased to 5.5 ° C in July and August, thus outdoor workers can be the most vulnerable. Studies have shown that global warming and rising temperatures will be a major challenge in the coming years. Developed countries have a significant role to play in increasing fossil fuel consumption. And developing countries are more vulnerable to the effects of the global warming. Earth temperatures are estimated to rise between 2.5 and 7 degrees Celsius during the current century. Therefore, attention to this phenomenon should be considered . Due to climate change, outdoor workers can be the most vulnerable affected population. Extreme heat combined with various job needs such as clothing, workload, metabolism, long hours of exposed to heat, and personal protective equipment may lead to severe heat stress. And other complications associated with hot weather such as skin rash, muscle cramps, heat exhaustion, and death may occur in hot months. Also, ignoring heat stress in the workplace can lead to a decrease in physical capacity and mental performance and has a negative effect on the work ability, in addition to a wide range of heat-related complications and diseases [6, 9, 25].
In the present study, temperature changes in the coming years were modeled using WBGT index and general circulation models. The results indicate an increase in temperature and an increase in heat stress load for people working outdoors, including agricultural workers. Therefore, exposure to heat stress in Khuzestan is increasing in most months of the year, especially in the hot months due to global warming. Rising temperature is known to be a health concern in the coming decades and it can exert adverse effects on the health of people, especially vulnerable people such as the elderly, children and patients. Heat stress can also affect the level of health, safety and productivity of outdoor workers, including farmers. It is suggested that working on farms should be limited to early morning and late evening hours in order to reduce heat stress risk as much as possible.