A new technique, named Progressive Flux has been developped by the Cemagref for the shielding of food products against airborne contamination when they are laying on a working table or a conveyor, the protected area remaining open to human interventions. Numerical simulation was used to investigate the design of the clean air handling system. Experimental works were necessary both to validate the simulations and to control the consequences of turbulence and local unsteadiness on the size of the totally protected zone. A particular attention was paid to the problem of obstacles, like human arms, penetrating into the clean air flow from outside the protected area. The problem was approximated by studying the interaction between the wake of a cylinder and a plane mixing layer. The cylinder can be taken as a generic shape for the arms, while the interface between the clean and the polluted air can be considered correctly represented by a mixing layer with high speed on the the clean side. An experimental study was carried out in a wind tunnel, involving both hot wire anemometry and Particle Image Velocimetry. From the analysis of mean flow topology and turbulence characteristics, we could draw conclusions on the systematic generation of a secondary flow leading to a local injection of polluted air into the clean side of the mixing layer. Actions to prevent that pollution in practical cases could be deduced from the analysis of the three dimensional flow. New studies, both experimental and numerical, are carried out to go further in the analysis of such interactions between air flows and obstacle wakes, aiming of being able to control the consequences of such interactions on pollution transfers in a wide range of industrial situations.