The purpose of this presentation is to describe the theoretical relations for the dispersal of airborne contaminants and to illustrate the validity of these equations occurring during factual situations, where a number of observations on air movements in open unidirectional air flow units supplied with HEPA-filters are described. In factual situations the aerodynamic system which governs the dispersion of contaminants in reality is always very complicated that risk situations must be mapped and assessed empirically. The presence of a person can give risk to wakes that may be stable or unstable. The unstable situations are in most cases caused by the influence of arms and hands. As part of the microbiological assessment of aseptic processes carried out in clean zones, it is important to investigate that such vortices do not occur in the clean working areas. As the level of airborne contaminants in the operational environment may have an effect on the level of product contamination, the microbiological assessment of aseptic processes is important. A system is described for microbiological assessment in unidirectional air flow units by using visual illustrative methods and particle challenge tests (measured by particle counter) for the dispersion and/or induction of particles.