## 4 Discussion  
### Efficacy of ULV Sprayer on Total Bacteria on Indoor Surfaces  
In this study, the disinfection of bacteria on indoor surfaces using the ULV sprayer was shown to be effective. The disinfection efficiency of total bacteria using the ULV sprayer was 0.56-2.46 log\({}_{10}\) reductions at all surfaces (Figure 5). Few studies have demonstrated the efficacy of microbial inactivation using foggers (or sprayers), including the ULV sprayer, and some controversy continued regarding their effectiveness. [PERSON] and [PERSON] (2005) found that spraying was ineffective in removing microbial pollutants outside direct spray zones ([PERSON], [PERSON] et al. (2006). Meanwhile, [PERSON] et al. (2006) tested the efficacy of methicillin-resistant _Staphylococcus aureus_ (MRSA) and _Acinetobacter baumannii_ when decontaminating environmental surfaces using a Sterilox fogger. As a result, initial counts of approximately 10\({}^{9}\) CFU/mL for both microorganisms were reduced approximately 10\({}^{4}\)-fold for MRSA and 10\({}^{5.8}\)-fold for _A. baumantii_ when using single fogging ([PERSON] et al. (2020). A recent study by [PERSON] et al. (2020) also found a disinfection efficiency of 0.32 log\({}_{10}\) reductions (52.14%) when they disinfected the methicillin-resistant _Staphylococcus pseudintermedius_ (MRSP) using hydrogen peroxide and silver fogging systems ([PERSON] et al., 2020). Our study showed a high total bacteria inactivation efficiency of 0.56-2.46 log\({}_{10}\) reductions (72.68-99.65%), compared to other studies. The advantage of using a ULV sprayer over the existing method is that it allows disinfectant in small aerosol particles to reach the unreachable spaces by workers ([PERSON] et al., 2012; [PERSON] et al., 2013). Our results show high efficiency values of 0.93 log\({}_{10}\) and 1.42 log\({}_{10}\), respectively, even on the ceiling and underneath the desk, which are hard-to-reach areas, after 90 min of disinfection (Figure 5). Previous studies have also reported that aerosol particles dispersed from the fogger stuck to various locations and increased disinfection efficiency. [PERSON] et al. (2005) determined the disinfection efficacy of fogging Virkon S on the survival of _Staphylococcus aureus_ and _Salmonella enterica_ on different hospitals' surfaces. As a result, the disinfection efficiency of _Staphylococcus_ and _Salmonella_ direct spray zones was high, at 3.40 and 4.92 logs direct spray zones, respectively, whereas that of outside direct spray zones showed a low reduction of 0.95 and 0.02 logs ([PERSON] et al., 2012). A study by [PERSON] et al. (2012) found that the removal efficiency of dry fogging on adenovirus, vesicular stomatitis virus, _S. aureus_, _E. coli_, and _B. atrophaeus_ spores in 29 different areas showed a 0.2-1.6 logs reduction ([PERSON], 2017). Although the removal efficiency varied from place to place, the sprayed  
Figure 9: Overlay plot of the disinfect concentration and spray amount on the disinfection efficiency at each point when the room size is 300 m\({}^{3}\). disinfectant aerosols reached all sites and showed a disinfection effect. This was true for the current study as well. The disinfection efficiency for removal of total bacteria was >0.70 log\({}_{10}\) (80%) for not only 3 the wall in the direction of the spray but also 5 the ceiling, and 6 the bottom of the desk (Table 1). Similar results were observed even when the space size increased. Meanwhile, 8 the ceiling showed the lowest disinfection efficiency (0.93 log\({}_{10}\)) compared to other walls because the ceiling was far, approximately 1.8 m above the ULV installation position and 4-6 m forward from the ULV sprayer. The forward dispensing particles are sufficiently convective within a certain height. Still, they are considered to have a limited ability to reach the ceiling. This is consistent with our assumption that a non-directly dispensed direction from the wall would show relatively less efficiency. It is nevertheless worth noting that it showed a high efficiency of more than 0.7 log10.