Volatilization and disinfection efficacy of gaseous hypochlorous acid from an air washer-type humidifier in a large space.

Abstract

An air washer-type humidifier has two useful functions: humidification, and air purification, and it applies to large indoor spaces. In this study, the efficacy of an air washer-type humidifier fed with 24 L of weakly acidic electrolyzed water（WAEW) at pH 5.0 and 30 mg/L in disinfecting attached bacteria and airborne microorganisms was studied in a 480 ^m3 indoor space. The humidifier was operated at a shower volume of 9.0 L/min of WAEW and at an air flow rate of 29 ^m3/min. Volatilization of gaseous hypochlorous acid（HOCl_(g)) proceeded according to first-order kinetics during the 60 min of operation. Fresh WAEW was supplied to the humidifier every 60 min, and the HOCl_(g) concentration in the indoor space was maintained within the range of 25-52 ppb for at least 180 min of operation. The number of viable bacterial cells on wet agar plates placed on the floor at a distance of 5-20 m away from the humidifier decreased by 2.0-3.0 log after 30 min of operation, and no viable cells were detected after 60 min of operation. A logarithmic reduction of more than 2.7 was achieved within 15 min against bacteria-attached plates placed at a 1.5 m-height position where the outlet airflow from the humidifier was directly exposed. This indicates that the disinfection efficacy of HOCl_(g) volatilized from the humidifier depends on the rate of outlet airflow reaching the bacteria-attached plates. The number of viable airborne microorganisms decreased by approximately 54% after 180 min of operation. This study demonstrated that an air-washer-type humidifier can spread HOCl_(g) evenly throughout a large indoor space and is effective in disinfecting attached bacteria and airborne microorganisms.