Emission of iron and aluminum oxide particles from ultrasonic humidifiers and potential for inhalation.

Ultrasonic humidifier use is a potential source of human exposure to inhalable particulates. This research evaluated the behavior of insoluble iron oxide and aluminum oxide particles in water used to fill room-sized ultrasonic humidifiers. Solutions of 10 mg/L Fe, as iron oxide particles, or 5 mg/L Al, as aluminum oxide suspension, were added into tap water used to fill ultrasonic humidifiers. The humidifiers were operated for 14 h; samples were obtained over time and monitored for soluble and particulate Fe and Al, as well as particle sizes in the humidifier reservoir and emitted in aerosols. Denser, settleable particles of approximately 1.5 µm diameter of iron or aluminum oxides accumulated at the bottom of the humidifier reservoir. Smaller, suspended metal oxide particles of 0.22-0.57 µm diameter were emitted as aerosols from the humidifier. Soluble anions and cations in tap water were also present in the aerosols emitted from humidifiers. The results indicate that a typical 1.6 MHz ultrasonic humidifier can emit 0.22-0.57 µm particles and dissolved minerals from fill water into breathable air.