In the nuclear decommissioning and dismantling operations, a dynamic confinement is applied to all openings in order to prevent the transfer of pollutants outside depressurized enclosures and to insure the safety of workers. To guaranty an efficient dynamic confinement, ISO 16647 and ISO 17873 standards recommend to maintain a constant value for the inward flow velocity near the opening depending on the level of radioactive pollution hazard. The main purpose of this work is to identify the possible conditions under which flow inversions near the opening may lead to gaseous pollutant leakage and then failure of the dynamic confinement. We aim at quantifying the amount of this pollutant backflow. The leakage from an experimental ventilated enclosure with a small opening on its frontal wall has been investigated. Laser flow visualizations and Particle Image Velocimetry (PIV) measurements showed that the presence of an additional turbulent jet flow in competition with the inward confinement flow is among the main causes leading to the leakage through the opening. The gas tracing technique has provided experimental data to quantify the pollutant backflow and allowed to compare the different scenarios. We conclude that a new criterion based on local aeraulic conditions near the opening is relevant to guaranty an efficient confinement.