In the past, it was crucial to ventilate the old mines, in order to make the miners' work possible. In most mountainous areas, the galleries where equipped with ventilation shafts. However, in many short galleries, such shafts were not bored and air removal was achieved thanks to natural ventilation. It seems that beyond a critical distance, natural ventilation is no longer efficient and the turnover of breathable air is not achieved. The work presented here deals with the physical analysis of air removal in a horizontal mine gallery, using three quantitative approaches. In the first approach, air velocity and temperature measurements were used to establish an empirical expression of the air velocity within the gallery with respect to the temperature difference between the inner and the outer air. This law quantifies the breathable air change rate and gives the distance (of about 50 meters) above which ventilation does not work anymore. The second approach is based on a theoretical analysis which yields the following expression of the air velocity, where T(out) and T(end) . are respectively the temperatures outside and at the end of the gallery, g is the gravitational acceleration, H is the gallery neight and x is the distance from the gallery entry. In the last approach, the flow field is solved with a commercial CFD (Computational Fluid Dynamics) code based on the finite volume method. Although convergence was not actually reached due to the strong dynamic uncoupling effect between the inner and the outer parts of the gallery, the qualitative results computed with this code are in agreement with the previous ones.