Size and mineral composition of airborne particles generated by an ultrasonic humidifier

This study describes the size distribution and concentration of particles expelled by a portable, 3-L ultrasonic humidifier. The ultrasonic humidifier was filled with waters of varying mineral content and hardness. Aerosol size distributions were measured during 8hours of humidifier operation in a typical bedroom. Humidifiers produced approximately 1.22 x 10(10) -2.50 x 10(10) airborne particles per milliliter of water consumed, resulting in airborne particle concentrations of 3.01-5.91 x 10(4) #/cm(3), with modes ranging between 109 and 322 nm in diameter. The emission rate of particles varied by water type from 1.02 x 10(9) to 2.27 x 10(9) #/s. Lower mineral waters produced fewer, smaller particles when compared to higher mineral waters. Chemical analyses of particles collected with a cascade impactor indicated that the minerals in emitted particles had the same relative mineral concentrations as the fill water. Our results demonstrate that ultrasonic humidifiers should be considered a source of inhalation exposure to minerals dissolved in water, and that the magnitude of exposure to inhalable particles will vary with water quality.