The strain in different functional systems of human body maintaining thermal homeostasis that develops during work under heating conditions results in impaired working capacity and efficiency and may be harmful to health. One of the most efficacious measures is the reduction of exposure to the adverse conditions and its rational alternation with rest in the comfortable environment. Based on the mathematical and statistical analysis of the results of multiple-factor experiments, we derived a multiple regression equation describing the quantitative dependence of the integral index of human body thermal regime on the totality of factors responsible for thermal strain. The equation permits to determine the heat content in the human body formed by exothermal and endothermal strain, to estimate the contribution of each individual factor, and to predict the risk of overheating in order to take measures for reducing the thermal strain. Recommendations are proposed on the duration of thermal exposure during a workshift depending on the overheating risk level and on the optimal relationship between the duration of staying in the heating microclimate and the duration of the rest (work) in the comfortable environment.