The safety of nuclear site dismantling operations is often based on potential gas and aerosol releases from confinement facilities through nominal or accidental openings. This study is a follow-up analysis in which we experimentally characterized the efficiency of dynamic confinement, addressing only gas release from a rectangular opening caused by internal and external perturbations. Our present study focuses on an internal aeraulic disturbance (countercurrent jet) involving gas and 5 µm diameter particles. We experimentally and numerically characterize the local backflow near the opening under established conditions. In addition, we added an external envelope to collect all the releases from the enclosure. The experiments provided quantitative information on the backflow threshold. We found that the dimensionless criterion based on local aeraulic conditions is also relevant to maintain the efficient confinement of the particulate pollutant. The global coefficients expressed in terms of mass release offer a robust method to quantify global leakage. We have shown that the aeraulic behavior of gas and 5 µm aerosol is very similar under our aeraulic and geometric configurations. Finally, the reliability of computational fluid dynamics based on hybrid SST-DES turbulence modeling coupled to aerosol dispersion model is qualitatively and quantitatively validated to predict backflow.