Humidity is one of the crucial factors in reducing microorganisms by ClO2 gas. Previous studies reported that inactivation of bacterial contaminants by ClO2 gas was negatively impacted by a low relative humidity (Lowe et al., 2013). This study also revealed that disinfection of eggshells by ClO2 gas was less effective under the dry condition (30±5%) than under the wet condition (RH 80±5%). Regardless of the bacterial species and organic matter present, the reduction in bacteria by ClO2 gas was greater under high RH conditions. A significant difference was found at a high concentration of ClO2 gas between dry (RH 30±5%) and wet conditions (RH 80±5%).