**1. Introduction**

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514 Current Air Quality Issues

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The study of OT ventilating and IAQ conditions is particularly important. Campaigns of experimental measurements in OTs are usually expensive, invasive and, in many countries, require complex procedures for legal authorization. CFD simulation has opened up low cost methods to provide useful indications on proper indoor microclimate conditions and IAQ. [1, 2,3]. There is a great dealing the literature on airflow patterns, air velocity and temperature distribution in unidirectional (vertical downward) airflows, as well as other applied ventilat‐ ing layouts, such as air curtains use to achieve a chosen air quality level inside OTs using CFD transient simulations [4,5,6,7,8]. Other authors have studied the airflow and temperature distribution modifications due to different surgical lamp shapes and locations [9]. In particular [10], the effectiveness of the ventilation system and airborne bacteria removal due to the HVAC plant have been analysed by means of CFD simulation supported by experimental data obtained in a test room. A comprehensive experimental and numerical analysis has been proposed by Kameel and Khalil [11] concerning both the airflow regimes and heat transfer inside an OT considering the correct operational use conditions. Few analyses have been made concerning the effect of moving objects on OT airflow: the "computationally expensive" moving mesh approach has been applied in order to manage the fluid-solid interface during transient simulations [12,13], but similar investigations were proposed for a hospital isolation room [14,15] and an industrial cleanroom [16]. An impressive study concerning the influence of movements on contaminant transport has been proposed by Brohus et al. [17] using the indirect approach, based on distributed momentum and turbulent kinetic energy sources. Recently, the present authors proposed an alternative indirect numerical approach in order to simulate solid objects moving on a fixed mesh in forced [18] and natural convection flows [19].

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Definition and quantification of the contaminant sources under OT effective use conditions is a complex task. Existing databases refer to constant particle mass emission of a person as a function of his/her activity. Several studies analyze different sources and human activities causing high levels of indoor particle concentration, diffusion and re-suspension processes. Some authors [20] present studies on the influence of periodic bending movement of the surgeon on the air flow field and bacteria carrying particle distribution using a combined approach based on an Eulerian RANS model, modified drift-flux and moving mesh. In the study by Brohus et al. [17], bacteria attributed to the skin flake emission of staff and patient are simulated as a gaseous contaminant. The most widely used experimental methods allow particle size calculation for discrete intervals, so as to check whether microclimatic and IAQ conditions are in compliance with standards and specific rules. Some authors have developed a useful method for determining individual emission rates from ambient air contaminant concentrations caused by multiple indoor sources [21]. Other studies provide a sampling method integrated with statistical analysis. [22]. In a recent paper bacteria sources are inves‐ tigated [23] in a university classroom both occupied and vacant as concentration differences due to the two conditions. Besides the influence of the airflow pattern, it has been demonstrated that particle dynamic behaviour is strongly dependent on particle size and size-related forces [24]. Chih-Shan Li et al. [25] carried out a series of field tests by active and passive sampling of air and surface measuring bacterial and fungal concentrations without referring to particle size and diameter. Another paper [26] based on particle counting combined with statistical analysis, evaluates aerobic bacterial sedimentation and connected index of microbial air contamination. There, results refer to different particle diameters but not emission sources. Some authors [1] have carried out transient simulations using the Renormalization Group (RNG) k-ε turbulence model, assuming a released rate of bacteria-carrying airborne particu‐ lates from surgical staff of 100 BCP/min per person and for patients of 400 BCP/min per wound. In general, CFD simulation concerning aerosol particle transport and diffusion processes can be solved by three main different numerical approaches: the first method consists in solving the particle concentration field by using diffusion-transport equations based on "passive scalar transport" in which the vector of particle transport is the motion field of the fluid but gravi‐ tational effects and frictional forces on particle motion are disregarded; the second is the Eulerian-Eulerian method and the third is the Eulerian-Lagrangian one. These last two methods solve the airflow field based on the Eulerian approach, but the particle phases are treated differently: in the first, particle concentration is directly solved using gravitational effect in the transport term, in the second one an ordinary differential equation is solved for any particle path. A modified drift-flux model based on the second approach has been applied to modelling particle transport [27,28,20]. At present, there are not many studies in the literature concerning numerical modelling in which the contaminant concentration for the IAQ analysis has been performed starting from sources of particle emission with a distribution dependent on particle diameter [29,30]. However, Italian and international standards require, for environment classification, a particle count for diameter dimension. Actually, there is a lack of data concerning specific emission sources in terms of particles issued in time unit by people. A recent study provides the quantification of size-resolved particle concentrations in indoor air of a classroom under occupied and vacant conditions [29]. Another important numerical study provides information on airflow, particle deposition and movement in two different spaces equipped with displacement and mixing ventilation modes [30]. In our present research airflow and climate in an existing orthopaedics OT under both the "at rest" and real use conditions were investigated by experimental measurements and numerical simulations. The "operational" conditions should be understood with faked surgery, split into two conditions respectively "correct operational use" and "incorrect operational use". Two main purposes were pursued in our investigation: the first one consists in exploiting the experi‐ mental acquisitions in order to check whether the environmental parameters respect standard requirements. Numerical results are then correlated with the microclimatic parameters suggested by the standards to verify the model suitability for assessing the OT indoor climate according to suggested standard limits, starting from the design variables of the room and HVAC system. The second purpose focuses on the airflow and microclimatic parameter variations induced by the operative use conditions. Our study develops an investigation and applied research to obtain a distribution of emitted particles as a function of their diameter, assuming that the amount of particle mass emitted by a person in a time unit does not change with their size. Starting from this assumption, the distribution of emitted particles will consist of a larger number of a smaller size, and vice versa. Our study investigates the thermo-fluid dynamics combined with the analysis of the concentration of diluted gas phase and particle aerosol dispersion in the air. Some IAQ indexes, widely used in the literature, were calculated as global and local values referring to the total volume of the room but also to the critical zones defined by ANSI/ASHRAE [31]. The present investigation is organized, referring to air flow and climate, ventilation efficiency and IAQ in the investigated real OT, as follows: the second section presents the experimental measures; the third one concerns the numerical tools and model validation; results are reported in the fourth section; then, conclusions, basic findings and possible developments due to the application of our proposed approach to similar cases, follow.
