In the context of an underground repository, one of the main concerns lies within the management of the smoke produced by a fire. The propagation front of smoke is the first indicator received by the detectors and this front can be represented by a gravity current. In this study, air/helium experiments on the non-Boussinesq gravity currents generated from an instantaneous buoyancy source in horizontal and tilted tunnels have been carried out. A large range of density ratios γ = ρc/ρ0 (0.29 ≤ γ ≤ 0.88), between the density of the release (ρc) and the ambient (ρ0) is investigated. First, similarly to the open ambient case, we note that the front of the gravity current propagates in three phases: the slumping, inertial and viscous phase. In each phase, a power-law exists between the front location of the gravity current (x) and the time (t). In the early inertial deceleration phase, x ∝ t2/3 is valid for both Boussinesq and non-Boussinesq gravity currents. Moreover, in the late deceleration phase, the viscous effects become more important and a power-law x ∝ t1/2 has been identified. Then, experiments on tilted tunnels with angles varying from 1% to 17 % have been performed to consider the influence of the slope. It is found that the transition between inertial and viscous propagation decreases when the slope increases. One possible explanation is that the mixing layer increases in the higher angle percentages which causes the buoyancy contained within the head of the density currents to decrease more rapidly. A high angle of slope can be seen potentially as a disadvantage for the propagation of the smoke.