Inhalation of gaseous hypochlorous acid and its effect on human respiratory epithelial cells in laboratory model systems.

During the disinfection of indoor spaces using gaseous hypochlorous acid (HOCl_(g)), inhalation is the most common route of exposure for humans. In this study, an artificial human respiratory tract model was exposed to 12-140 ppb HOCl_(g) at an aspiration flow rate of 800 mL/s for 15 h in a 1 m³ chamber. The respiratory tract model was equipped with 5th order bronchi and all gas-contact parts were made of silicone rubber with no other chlorine-consuming substances. The concentration of HOCl_(g) reaching the lung pseudo-space was approximately 47.4% of the HOCl_(g) concentrations in the chamber and was calculated to be very close to zero when the chamber concentration was less than 20.5 ppb. The disappearance of HOCl_(g) during inhalation is likely due to the adsorption of HOCl_(g) on the gas-contact silicone rubber surfaces. The cytotoxicity of HOCl_(g) on respiratory epithelial cells was also examined using human air-liquid-interface airway tissue models. Human nasal epithelium and bronchiolar epithelium were exposed to 100 ppb and 500 ppb HOCl_(g) for 8 h and 5 d, respectively. No significant effects of HOCl_(g) on cell viability and ciliary activity were observed in any cell type, indicating that low concentrations of HOCl_(g）, less than 500 ppb, had no cytotoxic effect.