This research investigated air–water quality interactions during single room humidification to increase indoor relative humidity. Ultrasonic humidifiers were filled with five different water quality types ranging from low to high total dissolved solids (TDS), hardness, and iron content. Aerosols emitted were evaluated for particle size distribution and dissolved metal and nonmetal constituents with inductively coupled plasma–mass spectroscopy. Emitted aerosols were in the inhalable range; the 90th percentile aerosol concentrations occur between 0.020 and 0.040 μm. Findings indicated that humidifier aerosols contained 85–90% of constituents present in the source water for low TDS, hardness, and iron tap waters. Deviations from this pattern were found in high hardness and high iron well waters due to precipitation of some constituents within the humidifier reservoir during operation. Deviation were seen only in the constituents that were precipitating as determined by comparison of filtered and unfiltered reservoir water. There was no evidence to suggest that failure to remove remaining humidifier reservoir water after a humidifier cycle and before refilling the humidifier reservoir for the subsequent cycle would result in increased exposures after five consecutive cycles regardless of increased concentrations in the humidifier reservoir during a single cycle. Results indicated that inhalable aerosols represent exposure to dissolved constituents present in source water.
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