## 2 Materials and Methods  
### System Description  
UV-SAN is an UltraViolet SANitizing System designed for the sterilization of Ambulances fleets and for real-time remote monitoring of sterilization level. The system exploits the germicidal power of ultraviolet light emitted in the UVC to sterilize surfaces inside the passenger compartment of Ambulances that come into contact with the human body (e.g., hands) and which are a persistent way for the transmission of viruses and bacteria. The total germicidal effect on the vehicles, below also called Sterilization Level, is proportional to the UVC dose absorbed by surfaces, where the dose is defined as the total time-integrated amount of irradiance received. The system is equipped with four commercial low pressure mercury vapor lamps (LSE Lighting) installed on the top of the patient's compartment in order to ensure a uniform irradiation of the compartment. The system is designed to be used several times a day, for example during transfers when, for safety reasons, there are no patients on board, or immediately after the patient offloading, or before a new transfer. To ensure a complete geo-traceability of the sterilization operations, UV-SAN is equipped with a Satellite Navigation System (SAT-NAV) that provides position and duration of each treatment. This information is real-time transmitted, by means of a Satellite Communication System (SAT-COM-IRIDIUM Edge(r) with Iridium Short Burst Data Service(r)) to a Central Control System (CCS) that tracks all the sterilization operations, when and where they are performed, allowing knowing, at any time, the sterilization level of each ambulance. The UV-SAN system is composed of two main components like shown in Figure 1.  
The Sanitation Equipment (SE) is the main component of the UV-SAN system. It is installed in the rear compartment of the ambulance (Figure 2) where patients are transported and where the risk of contamination of air, surfaces and instruments is the greatest. The SE is equipped with a touchscreen that allows the healthcare operator to start the sanitation switching on the UVC lamps mounted on the ceiling of the compartment. The position of the lamps is chosen in such a way to irradiate, as uniform as possible, the surfaces of the compartment as demonstrated by direct measure of the irradiance (i.e., the flux of radiant energy per unit area measured in W/m\({}^{2}\)) performed during the pilot stage. A presence sensor ensures that the sanitation does not start if a healthcare operator, or a patient, is on board and automatically stops the irradiation whenever someone enters during the sanitation. This safety mechanism has been implemented to ensure the highest level of safety of the system with the scope to reduce at maximum the risk of accidental exposure to UV radiation. As noted, the UVC radiation stimulates the production of ozone (O\({}_{3}\)) starting from molecular oxygen of the atmosphere [38]. O\({}_{3}\) is highly corrosive to equipment and is lethal to humans with prolonged exposure at concentrations above 4 ppm. The U.S. Food and Drug Administration (FDA)'s maximum allowed ozone concentration in the air for residential areas is 0.05 ppm ozone by volume. The SE is equipped with an O\({}_{3}\) and UVC sensors (respectively Spec-Sensor DSG-03 and D-SICONORM by Z-E-D Electronics) that measure the gas concentration and the UV irradiance level avoiding they exceed the safety level defined by the international normative. The CCS is the component that records all the operations carried out by each SE in order to maintain full traceability of the operations performed and the sterilization level of each ambulance. The CCS can also update configuration parameters in order to guarantee the same performances to all the equipment installed. Data collected can  
Figure 1: UV-SAN System Architecture. Figure 2: UV-SAN Sanitation Equipment installed in the ambulance patient compartment.