An overlooked route of inhalation exposure to tap water constituents for children and adults: Aerosolized aqueous minerals from ultrasonic humidifiers

Fine particulates and aerosols emitted by commonly used, room-sized ultrasonic humidifiers may pose adverse health effects to children and adults. The literature documents adverse effects for children exposed to minerals emitted from humidifiers. This study performs novel and comprehensive characterization of bivariate particle size and element concentrations of emitted airborne aerosols and particles from ultrasonic humidifiers filled with tap water, including size distribution from 0.014 to 10 μm by scanning mobility particle sizer and AeroTrak; corresponding metal and elemental concentrations as a function of particle size by inductively coupled plasma mass spectrometer; and calculations of deposition fraction in human lungs for age-specific groups using the multi-path particle dosimetry model (MPPD). Deposition fraction is the ratio of mass deposited to total mass inhaled. When filled with tap water, water evaporated from emitted aerosols to form submicron particles that became essentially “dried tap water” with median size 146 nm and mean concentration of 211 μg-total elements/m(3)-air including 35 μg-calcium/m(3)-air in a room of 33.5 m(3) and air exchange rate at ∼0.8 hr(−1). Approximately 90% of emitted particles deposited in human lungs were <1 μm as shown by MPPD model. The smaller particles contained little water and higher concentration of minerals, while larger particles of >1 μm consisted of lower elemental concentrations and more water due to low evaporation. Deposition fraction in pulmonary region was ∼2-fold higher, and deposited particulate mass was 3.5-fold higher for children than adults, indicating greater inhalation exposure to children compared to adults. Modeled data of total particles mass per body weight (BW) that will deposit in adult and child lungs after 8-h humidifier exposure were respectively 2.8 μg/kg-BW and 9.8 μg/kg-BW, where calcium contributes 0.4 μg/kg-BW and 1.6 μg/kg-BW. This comprehensive study of bivariate inorganic chemical composition as a function of particle size expanded, quantified, and modeled exposure for children and adults to aerosolized calcium and other inorganic constituents in water.