Uses of gaseous hypochlorous acid for controlling microorganisms in indoor spaces

Hypochlorous acid (HOCl) is an active species in the chlorination process. Hypochlorite salts that release hypochlorite ion (OCl^-) have been used for more than 200 years as disinfecting, cleaning, deodorizing, and decolorizing agents in various technological fields. In the food industry, sodium hypochlorite is the most widely used among chlorine compounds. The antimicrobial activity of a dilute hypochlorite solution is attributed largely to HOCl because of its cell membrane permeability. OCl^- exhibits an excellent cleaning action for organic soils on solid surfaces. HOCl has been used as an aqueous solution, and its objects to be treated are things. In hypochlorite solution, HOCl is volatile and easily volatilized by stirring, bubbling, atomizing, or forced-air vaporization. On the other hand, OCl^- is non-volatile and stays in the solution. Recently, the scope of objects to be treated with hypochlorite solution has been expanded to indoor spaces, and the use of gaseous hypochlorous acid (HOCl_（g) ) has been studied intensively. This review describes the mechanisms of actions of hypochlorous acid as liquid-based and gaseous disinfectants and provides the evidence for the safety and effectiveness of HOCl_（g) for controlling microorganisms in indoor spaces.