Ultrasonic humidifier use is a potential source of human exposure to inhalable particulates. This paper focused on the behavior of insoluble iron oxides particles, and aluminum oxide particles in ultrasonic humidifiers. 10 mg/L Fe oxide particles and 5 mg/L Al oxide suspension solutions were added into tap water, as fill water for ultrasonic humidifiers operated for 14 hours. Denser, heavier particles of approximate 1.5 um diameter of iron or aluminum oxides accumulated in the humidifier reservoir. Smaller, suspended metal oxide particles of 0.22-0.57 um diameter were emitted as aerosols from humidifiers. Soluble anions and cations in tap water were present in the aerosols emitted from humidifiers. The results indicate that if suspended particles and dissolved minerals are present in source water, they will be transported in aerosolized waters. / M. S.