An indoor air quality study was conducted in Boise, ID, residences to evaluate the range of aerosol concentrations that result from using tap water in portable home humidifiers and to characterize the physical and chemical properties of the humidifier aerosol. Aerosols having particles smaller than 10 micrometers (PM10) concentrations exceeded 650 microgram/cu m and 7000 microgram/cu m when an ultrasonic humidifier, charged with tap water containing 303 mg/L of dissolved impurities, was operated under whole-house and single-room conditions, respectively. Elements measured in high concentrations were uniformly present as soluble salts in both the fine and coarse particles with small differences observed between the ultrasonic and impeller humidifier-generated aerosols. In a separate study, nearly 50% of 39 samples of U.S. commercial bottled waters collected had dissolved mineral concentrations exceeding 10 mg/L while 4 samples exceeded 100 mg/L. The results of the limited-scale study suggest that personal exposures to ultrasonic and impeller humidifier-generated aerosols can be minimized by using water of low impurities.