## 4 Discussion
This background concentration of CO\({}_{2}\) is consistent with the average Italian data measured outdoors in 2019 and equal to 410 ppm [38], considering that there were still people present in the museum even during closing hours (staff, restorers, maintenance workers, etc.). The graph also shows that the highest concentration of CO\({}_{2}\) was reached between 10:30 and 16:00, indicating that these times were also those with the greatest turnout of visitors. Regarding the VOCs concentration (Figure 8), comparison of the values from sampling carried out in Room 10 when the museum was closed with days of high visitor turnout confirmed the hypothesis of human influence on the release of volatile organic compounds. The two VOCs values relating to the first and second sampling in Room 10 on the day of closure to the public provide significant information on the effectiveness and importance of the air exchange provided by the ventilation system: two hours between the passage of the first group of visitors (11:00) and the following group (13:00) were sufficient to reduce the VOCs concentration by 44%. The values compared with the recommended limits reported in Table 3 showed comfortable (Room 35, Room 41, and Room 71) or acceptable levels for the VOCs concentrations. Furthermore, many VOCs classes (halogen-aliphatic, halogen-aromatic, halogen-unsaturated, nitrogen-aromatic, oxygen-acid, ether, oxygen-acid, phenol, sulfur-mercapan, sulfur-sulphide) were not detected. For the VOCs classes that were detected, to evaluate the contribution and investigate the origin, the cut-off value of the compound was set at 1%. This value represents the concentration of the pollutant in the compound to be considered significantly present. The most present classes in the monitored rooms were aldehydes and alcohols, which were highlighted by the sum of the concentration values of the individual VOCs classes. As previously mentioned, these pollutants are present in furnishings, and especially in cleaning products (perfumes, deodorants, additives to cleaning products) and disinfectants, but their concentration can also be related to the presence of visitors. In particular, the values of aldehydes were higher at the second floor entrance, where all visitors pass through before being distributed to the different rooms, and where there was no mechanical ventilation. High and persistent concentrations of aldehydes can be associated with respiratory problems in sensitive subjects. Table 8 summarizes the substances with concentrations greater than or equal to the cut-off values for each sampling point. As the CO\({}_{2}\) and VOCs trends were similar, strategies to improve the IAQ can be effective for the control of both CO\({}_{2}\) and VOCs. The analysis of the biological pollutants (Figure 9) confirmed the data that emerged from the chemical investigation: the sampling point most vulnerable to the concentration of biological pollutants was the entrance on the second floor. Based on the reference values shown in Table 4, the following observations can be made: the concentration of TBL in all the rooms was at a mean pollution level (less than 500 CFU/m\({}^{3}\)), while the concentration  
Figure 10: Trend of CO\({}_{2}\) values (ppm) for the period from 3 to 6 June 2019 detected by the continuous monitoring system in Room 160 owned by the museum. of mold in all the rooms was at a low pollution level (less than 100 CFU/m3). Nevertheless, these biological pollutants must be constantly monitored as indicators of indoor air quality, together with the other inorganic and organic pollutants. The observed environmental molds were of the Aspergillus and Penicillium genera. The first species is ubiquitous and, therefore, is often monitored in different environments. Flavus and Fumigatus species (classified in [31] as allergenic molds) were recorded, while Aspergillus niger was not identified. Penicillium genera can generate respiratory tract disorders in predisposed or immunosuppressed subjects.