## 2 Materials and Methods  
### Description of the Surgical Suite and the Experimental Procedure  
#### 2.1.1 The Surgical Suite and Its HVAC System  
The surgical suite studied in this article is located in the Lucus Augusti University Hospital (HULA) in Lugo, in the northwest of Spain. The electrical energy consumption of the hospital in 2018 was 22.08 GWh and its thermal energy consumption in 2018 was around 30.8 GWh (Liquefied Natural Gas - LNG). As the HULA has an area of 185,000 m\({}^{2}\), this implies a consumption of 285 kWh/m\({}^{2}\)/year (119 kWh/m\({}^{2}\)/year electrical energy and 166 kWh/m\({}^{2}\)/year thermal energy). This means an energy bill of around 3.1 M\(\cels\). This data highlights the importance of an efficient use of energy and the consequent economic savings that this would entail. However, energy savings must be implemented guaranteeing air indoor quality (AIQ) conditions. The hospital incorporates six surgical suites and four dedicated ORs. For its part, the studied surgical suite consists of three identical ORs, a sterile entrance corridor, an unsterile exit corridor, two scrub rooms, a storage room, a pre-anesthesia room, an anteroom and a hall. Detailed information of the hospital, the surgical suite and the characteristics of the ORs, as well as construction characteristics of the surgical suite envelope and its interior partition walls can be found in [21; 38].  
The internal loads of the surgical suite are occupancy, lighting and equipment. The occupancy includes patients and medical staff present during operations, so it depends on the operations schedule. Lighting comes from fluorescents and LED lamps placed in ORs, ancillary rooms and corridors. Moreover, the medical and surgical equipment comprises batteries, monitors, electrosurgical units, laparoscopy tower, etc. This equipment does not always work simultaneously. Table 1 summarizes the lighting and equipment power in the different spaces of the surgical suite. Further information can be found in [21; 38].  
The surgical suite is generally used to conduct ordinary surgeries in the mornings of working days; it is not used for emergency operations. The schedule of the working days is from 8 m to 3 pm from Monday to Friday. Therefore, the surgical suite is not occupied in evenings and weekends  
Regarding the HVAC system, the surgical suite comprises six Air Handling Units (AHUs) (Figure 1), one for each OR, one for the ancillary rooms and one for each corridor. It is allocated in the upper floor. The total net area of the surgical suite is 425.37 m\({}^{2}\). The Spanish standard UNE 100713:2005 Air conditioning in hospitals [41] establishes a minimum airflow of 2400 m\({}^{3}\)/h with a minimum of outdoor air of 1200 m\({}^{3}\)/h and with a minimum of 20 ACH for air mixing diffusion systems. Therefore, taking into account the dimensions of the surgical suite and the Spanish standard recommendations [41], the ORs operate with 2400 m\({}^{3}\)/h or 21.5 ACH and the remaining spaces with 2680 m\({}^{3}\)/h or 6 ACH. At present, the HVAC system maintains a constant ventilation 24/7 with 100% outdoor airflow intake without reduction in periods of inactivity. The supply air outlets use High Efficiency Particulate Air (HEPA) filters. Temperature and RH conditions are maintained throughout the surgical suite according to Spanish standards [41], with temperature between 22 and 26 \({}^{\circ}\)C and RH between 45% and 55%, and positive pressurization of ORs to ensure adequate conditions of comfort and asepsis.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Lighting**} & \multicolumn{2}{c}{**Electrical Equipment**} \\ \hline
**Space** & **Power (W)** & **Space** & **Power (W)** \\ \hline ORs & 3300 & ORs & 31,500 1  \\ Ancillary rooms & 1500 & Ancillary rooms & - \\ Corridors & 1000 & Corridors & - \\ Total & 5800 & Total & 31,500 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Lighting and electrical equipment power in the surgical suite.