Objective: in this article the calculation and analysis is performed, of thermal risk index and the concept of the Synthesis-Reduced Temperature (SRT), by calculating heat stress (WBGT) in the atmosphere of an underground coalmine. Methods: Field data included measures of climate variables such as Dry Temperature (DT) and the Relative Humidity (RH), and the time of exposure of workers to such conditions; these measures were taken in the Coalmine San Jose. Result: Became apparent that although the average (SRT) of the mine environment be classified as moderately warm acceptable, there fronts where the heat index experienced workers is high and risky. Conclusions: ISSN 0122-820X It is concluded that the relative humidity is the most influential variable in warm environments of underground mines in the region; similarly, it is concluded that it is not enough determine the WBGT, it is necessary to establish the thermal sensation experienced by workers, from the calculation of the heat index.