## 4 Discussion
### Efficacy of ULV Sprayer on Total Bacteria on Indoor Surfaces
Disinfectant microparticles sprayed from ULV interact with small particles such as aerosols, droplet nuclei, and air. The microparticles also combine with infectious agents or dust particles (chemical irritants or toxins) in the air to kill pathogens or make them heavy and sink [15]. In this study, the sprayed small particles (PM\({}_{10}\), PM\({}_{2.5}\), and PM\({}_{1.0}\)) stayed in the air for 90 min and then sank (Figure 4). In addition, the number of particles in the air and disinfection efficiency increased 30, 60, and 90 min after the ULV spray (Figure 5). A study conducted by [PERSON] et al. (2005) also reported that as time passed from 30 min to 2 h after spraying, fogged disinfects increased _Salmonella_ and _Staphylococcus_ disinfection efficiencies [15]. Therefore, we concluded that sprayed (or fogged) disinfectant aerosol adhered to the surface for approximately 30 min to 2 h while floating in the air, increasing the disinfection efficiency. Optimal Usage Conditions of the ULV Sprayer Considering Factors Affecting Total Bacteria Inactivation  
The disinfectant concentration, spray amount, and room size, factors that affected the inactivation of microorganisms, had an interaction effect on bacteria disinfection efficiency (Table 3). With regard to room size, the optimum conditions when using the recommended dilution factor of 100 were as follows: a spray amount of 3.08-6.40 L in 160 m\({}^{3}\), 3.78-7.22 L in 230 m\({}^{3}\), and 5.68-8 L in 300 m\({}^{3}\) (Figures 7-9). These conditions predicted the bacteria inactivation efficiency to be above 1.10 log\({}_{10}\) (92%) at all surfaces. Among various factors, disinfect concentration, spray amount, and space size are the main factors that affect disinfection efficiency, either individually or interactively. In general, there is an appropriate concentration for maximum disinfection efficiency. For example, ethanol is known to have maximum efficiency at 70% [38]. In studies using a disinfectant such as Virkon(tm) Micro, which was used in this experiment, the disinfection effect significantly increased as the concentration of disinfectant increased to 2 and 4% based on the recommended dilution factor of 1% (100 times). In contrast, the disinfection effect significantly decreased as the concentration of disinfectant decreased from 1 to 0.1 and 0.05% [25]. In addition, the increase in the amount of spraying increased disinfectant efficiency by increasing the contact time [20], which is consistent with the results of this study showing a positive correlation among the disinfectant concentration, spray amount, and disinfection efficiency (Table 1). This study also identified the interactive effect of the factors on disinfection efficiency, unlike previous studies, which observed only the disinfection efficiency of individual factors. The result showed that the disinfectant concentration and spray amount had a significant interaction effect on the total disinfection efficiency under the assumption that space size cannot be changed during quarantine (Table 3). Meanwhile, when observing the total bacteria disinfection efficiency at locations 1 to 6, the significance of the \(p\)-values of linear coefficients and the interaction coefficients were varied. It seemed to be because each area was affected by the direct/indirect spray zones and the distance away from the ULV sprayer. In particular, the effects were not observed well at 3, which showed the highest disinfection efficiency, because 3 was a direct spray zone, and the direct spray zone had the most significant impact on disinfection efficiency other than the three factors. However, the results in which the \(p\)-values of the coefficients of disinfection concentration, spray amount, and the interaction term were statistically significant at most locations were notable. To the best of our knowledge, this is the first study that confirmed the disinfection efficiency of the ULV sprayer as a method for inactivating bacteria present on indoor surfaces, and found the optimum conditions using BBD.