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 8 hours of humidifier operation in a typical bedroom. Humidifiers produced approximately 1.22 × 10¹⁰ -2.50 × 10¹⁰ airborne particles per milliliter of water consumed, resulting in airborne particle concentrations of 3.01-5.91 × 10⁴  #/cm³ , with modes ranging between 109 and 322 nm in diameter. The emission rate of particles varied by water type from 1.02 × 10⁹ to 2.27 × 10⁹  #/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.