## 1 Introduction
In addition, it is necessary to maintain temperature and humidity within determined values that avoid the proliferation of bacteria, viruses and fungi, while providing the most comfortable working conditions possible for the surgical personnel [27; 28; 29]. However, hygothermal and ventilation requirements in ORs may significantly vary depending on the standard considered for designing the HVAC equipment, as different countries define their own technical guidelines [30; 31; 32]. A detailed comparative of different standards can be found in [21]. The high-energy intensity associated with surgical facilities provides an opportunity to develop and implement energy saving measures in ORs, particularly those aimed at minimizing their HVAC loads. However, reducing the energy consumption in ORs is a complex and challenging task, as it inevitably involves preserving or enhancing the safety and comfort conditions for patients and staff. In fact, energy efficiency is mostly overlooked by current standards, while the number of investigations addressing the analysis and optimization of the energy performance in ORs is rather limited. The installation of heat exchangers to recover energy from the exhaust airflow has been proposed to reduce the air conditioning energy demand in surgical facilities [33; 34]. However, the use of heat recovery units is not always possible due the risk of cross-contamination of the fresh airstream. [PERSON] et al. [35] have estimated that the energy consumption of Turkish ORs could be reduced up to 74% by introducing heat recovery systems and lowering the number of air changes per hour (ACH) during night periods by half. Decreasing the airflow intake during off-use hours is a simple measure that could effectively minimize the energy use for fans and air conditioning, but it might also involve difficulties in keeping adequate overpressure levels in the surgical areas or producing an ineffective removal of smoke from cleaning activities. Hence, energy saving proposals based on reducing the ventilation rates beyond the limits recommended by standards, either during occupied or unoccupied periods, should include a thorough assessment of the contaminant concentration in the OR environment, as airflow patterns have a major influence in their elimination and dilution [36]. In this regard, [PERSON] et al. [37] have evaluated the performance of different ventilation systems for ORs, concluding that a temperature-controlled airflow can provide high levels of air cleanliness, comfortable working conditions and reduced energy consumption. Likewise, [PERSON] et al. suggest that energy savings up to 51% are achievable in a Spanish hospital by reducing the ventilation rates by half while complying with airborne particle standards [38]. In turn, the use of air recirculation is supported by most of the current standards in the design of HVAC systems for ORs [39]. Thus, it has been recently estimated that the energy demand of a conventional Spanish OR may be lowered by 55% if the fresh air intake is reduced to 50% of the total ACH [40], which is the highest recirculation rate contemplated in the Spanish standard [41]. Similar results were obtained by [PERSON] et al. [21] for an existing OR in Spain, although they have additionally found that energy savings could increase up to 79% if the recirculation rate was 80%, as recommended by the American standard [42]. [PERSON] et al. [21] have also revealed that approximately 80% of the total thermal energy demand of a surgical suite corresponds to periods of inactivity, thus confirming that large amounts of energy are currently spent on maintaining comfort and aseptic conditions in ORs when no surgical works are being performed. However, it is known that, assuming high filtering efficiency and an adequate number of ACH with effective air distribution, the concentration of infectious particles is determined by their ratio of generation, the principal source of such particles being the staff and the surgical activities [43]. In this vein, [PERSON] et al.