The quality of air within factory buildings is controlled by many food manufacturers. Ventilation system hygiene must be regularly cleaning to prevent the build up of dust, product or condensate that may provide a focus for microbial growth. In the present study we predict the particle deposition velocity in ventilation ducts of three different food factories. At these locations, during several days, aerosol particle number and mass size distribution were measured using optical particle counter and cascade impactor, respectively. The concentration of particulate matter of aerodynamic diameters in the size range 0.3-20 mu m varied from 0.2 to 1.7 mu g m(-3). The measured mass concentration and the predicted particle deposition velocity were used to calculate the deposited particle mass flux (DMF) in the ventilation ducts of each site. The results indicate that the DMF at the floor is about 2-20 times larger than that at vertical walls. Thus, the deposited particle mass on the floor is sufficient to check whether a ventilation duct should be cleaned. According to the chosen cleaning initiation criteria for ducts, ventilation systems would take approximately 1-9 years to meet the cleaning time. So, cleaning procedures and maintenance intervals are strongly influenced by several parameters, such as flow conditions, particle concentration, characteristics of the ventilation duct and the filter efficiency. The modeling approach combined with mass concentration measurements enables the analysis and the quantification of the influence of these parameters.