The perturbations of localised ultra-clean air delivery systems due to continual access of operators to handle products have been investigated. It was exhibited that the keystone of localised air delivery systems was a turbulent mixing layer modeling the interface between the polluted and the clean air. Furthermore the obstacle penetrating into the clean air flow was represented by a generic shape, the circular cylinder. Hence the problem of perturbation was approached by studying the interaction between the wake of a circular cylinder and a turbulent plane mixing layer. From the analysis of experimental results a local injection of contaminant has been highlighted. Actions to prevent that pollution were deduced and studied on the Progressive Flux, a localised delivery system developped by the Cemagref. The inherent drawback of uni-directional flow systems was demonstrated and explained.