Influence of the temperature and relative humidity on human heat acclimatization during training in extremely hot environments

•The effects of heat acclimatization on workers were presented.•The heat acclimatization training day was obtained.•Cox regression method was adopted to rank influencing factors of heat acclimatization.•The formation stage of heat acclimatization was quantified.

Heat-related disorders, decreases in productivity and security risks are common phenomena when people live in extremely hot environments. To a certain extent, humans can adapt to extremely hot environments, but few studies have examined the causes of and factors influencing the aforementioned phenomena, especially in hot environments. In this paper, a climate chamber was built to simulate an extremely hot environment. Sixteen healthy male university students were asked to simulate manual labour by running. The physiological indexes (rectal temperature, heart rate and sweat production) were measured. The effects of human heat acclimatization in extremely hot environments were determined via paired sample t-tests. The time required to acclimate to the heat (in days) was determined based on the standard of thermal adaptation improvement. A Cox regression method was adopted to quantify the development of heat acclimatization and to rank the effects of several variables on heat acclimatization to determine the relationship between thermal parameters and heat acclimatization. The effects of heat acclimatization are significant, and heat acclimatization training can improve the adaptability of humans to extremely hot environments. The Cox regression method can be used to analyse heat acclimatization effectively. Both the dry bulb temperature and the relative humidity significantly affect heat acclimatization, with the effect of the former being more significant.