Models derived from a computational fluid dynamics (CFD) package (CF4X) were used to predict the fate of micra-organisms lost in aerosols as an incidental feature of the normal operation of bioprocess equipment. The model predics the tracks of particles from their assumed source to their point of capture in an Aerojet cyclone impinger. In a set of controlled experiments micro-organisms were sprayed into a small cabinet (volume = 0.36 m(3)). The efficiency which CFD predicted for their capture in the cyclone was compared with the actual efficiency measured with a quantitative polymerase chain reaction (QPCR) which was specific for the released organism. The capture efficiency was constant at about 40% over six orders of microbial concentration in the aerosol. This is consistent with the CFD predictions provided the coefficient of restitution of 50% of the particles is about 0.2. This coefficient determines the momentum with which the particles will rebound from the surface and continue their flight. Experiments in a bioprocess pilot plant, where QPCR was used to measure the quantity of process organisms and CFD was used to predict the capture efficiency of the released aerosols, confirm that the release from well-maintained equipment is very low. Only when the primary containment is broken as part of the normal operation of the process is there a significant release. Even this is small, amounting to about 10 mu l of fermentation broth, or 0.2 mu l of the concentrated process fluid. These quantitative methods for measuring levels of containment should aid the design of equipment and processes in which a proper balance is struck between the releases which are incidental to the normal operation of a process and the much larger losses which would follow an accidental failure in the equipment. In the long term this balance defines the safety of the containment.