Thesis title: Efficacia della ventilazione degli ambienti confinati per la mitigazione del rischio di diffusione di patogeni aerotrasmessi.
The CoViD-19 pandemic has highlighted the relevance of air conditioning systems in the airborne transmission of pathogens. In response to the emergency, numerous organizations (including WHO, ISS, ASHRAE, REHVA) have issued guidelines for the management of the plants. However, these recommendations have often been formulated on precautionary principles, not always supported by solid scientific evidence, leading to sometimes conflicting indications, especially regarding the flow of external air, recirculation and filter management.
This thesis collects and analyzes the available scientific evidence on the interactions between ventilation systems and airborne pathogens. The aim is to provide a basis for planning structural interventions and developing more efficient management strategies, improving preparedness to react to future epidemics.
The analysis delves into the mechanisms of transmission, distinguishing between droplets (droplets) and airborne (aerosols). It shows that aerosols (<10 μm) can remain suspended in the air for long periods, travel long distances and be significantly more infectious than larger particles. The traditional particle size classification is critically analyzed, demonstrating how air motions (generated by ventilation or human movement) can keep even larger particles (up to 20 μm) in suspension, unlike what is assumed in still air conditions.
Key factors such as human movement, which increases air mixing and the resuspension of deposited particles (mainly from the floor), and the impact of environmental conditions on pathogen transport are also examined. Finally, the paper compares different ventilation systems, noting that systems with uneven distribution tend to be more effective in removing contaminants.
Subsequently, a quantitative evaluation of the effectiveness of the ventilation air flow rate is developed, with the aim of determining the flow rate actually useful for limiting the risk of transmission. Despite the fact that it was not possible to obtain conclusive results. It has been found that flow rates of more than four changes per hour do not bring appreciable risk reductions.
In addition, a simplified model was developed for assessing the role of air recirculation through ventilation systems in pathogen transmission. It was found that in the case of environments with high mixing, recirculation does not significantly alter the distribution of pathogens. On the other hand, in environments with low mixing, recirculation can lead to appreciable increases in the probability of the presence of pathogens in portions of the environment distant from the source.
On the basis of the technical literature examined and the further insights presented, it was possible to formulate general recommendations for the management of the plants in the event of airborne epidemics and for a design of the same that allows a more effective use in these circumstances.