###### Abstract  
Physical and chemical cleaning for the removal of indoor microorganisms, which can cause allergic reactions and respiratory diseases, is labor-intensive and time-consuming. An ultra-low volume (ULV) sprayer, a newly introduced device to inactivate pathogenic microorganisms, allows the disinfect particles to reach hard-to-reach spaces indoors and is more cost-effective than the existing methods. However, few studies have been conducted to verify the efficiency of the ULV sprayer. Here, we verified the disinfection efficiency of the ULV sprayer for inactivating total bacteria present on indoor surfaces, considering the factors affecting bacteria inactivation, and presented the optimal ULV sprayer usage conditions to achieve the highest disinfection efficiency depending on room size. The total bacteria removal efficiency was high (range: 0.56-2.46 log\({}_{10}\) reductions), including hard-to-reach spaces. A response surface model was developed to identify the individual and interactive effects of the disinfect concentration, spray amount, and room size on total bacteria disinfection efficiency. These three variables had interactive effects on the total bacteria disinfection efficiency. The experimental data were fitted to a second-order polynomial model, with high coefficients of determination (\(R^{2}\)) for all models (\(R^{2}\) > 0.82). The optimum conditions were 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\({}^{3}\) surface area when using dilution rates of 100 times. These conditions predicted a bacterial disinfection efficiency of >1.10 log\({}_{10}\) reductions (92%) on all surfaces. Our results clearly indicate that the ULV sprayer effectively inactivates total bacteria present on indoor surfaces. b 20 April 2021 20 April 2021 20 April 2021 20 April 2021 20 April 2021 2021 20 April 2021 20 indoors [7]. Several disinfection methods have been developed to control this spread effectively [8; 9; 10]. Thus far, physical removal methods or chemical agents are used to control indoor microorganisms, including bacteria [10; 11]. A typical physical removal method is to wipe the surfaces of the objects with a cloth or sponge soaked in chemical disinfectant. Cleaning (or the physical removal method) is considered the most effective method as it physically removes the source of infection. However, this method has disadvantages, e.g., the spreading of pollutants and organic matter (e.g., blood, secretions, and excretions), if the user does not use the appropriate disinfectants, such as hydrogen peroxide or alcohol. Additionally, physical cleaning requires one to completely empty the indoor space that is to be disinfected and remove all equipment and furniture so that the workers can thoroughly scrub all the surfaces. Therefore, this method is time- and labor-intensive. Spraying disinfectants that can kill or inactivate microorganisms is a widely used cleaning method as an alternative to physical cleaning [12; 13]. The ultra-low volume (ULV) sprayer (Figure 1), which is mainly used to kill pests in agricultural fields, is nowadays being used to disinfect quarantine areas in order to eliminate influenza viruses and other infectious microorganisms from such spaces [13; 14; 15]. The advantage of a ULV sprayer is that it produces aerosols of a small particle size of 0.1 to 50 um in the air, making it possible to reach spaces that are inaccessible to workers and reducing the spread of airborne infectious agents and dust particles in the air, by causing them to combine with the dispersed aerosol droplets and sinking them to the floor [15]. Most of all, this method is cost-effective because it can spray a small amount of disinfectant for a long time. A small disinfectant particle for single-use requires a much smaller amount than the disinfectant used for physical or chemical disinfection methods.