Various health effects are associated with or directly caused by respirable airborne particles and microbial agents. To reduce the human exposure to these indoor pollutants, numerous techniques have been developed over the years. In this study, we have investigated the effect of unipolar air ionization on airborne dust particles and microorganisms in indoor environments. The concentration and particle size distribution were measured in real time using optical and aerodynamic particle counters with a special focus on the bacterial particle size range of 0.5 to 2 µm. The tests were conducted in three indoor chambers of different volumes (ranging from 26 L to 24.3 m3 ) at different ion emission rates (producing air ions at ~104  to ~105  ions/cm3  as measured at ~1 m from the

source). The concentration decay occurring due to ionic emission was compared to the natural decay for four types of challenge aerosols. Resulting from the interaction with unipolar air ions, airborne particles exhibited considerable electric charges of the same polarity as the emitted ions. Due to electrostatic repelling forces, the particles migrated toward the indoor surfaces and rapidly deposited on these surfaces. Two small, battery operated ionic emitters tested in this study showed significant air cleaning efficiency for respirable (sub- and

super-micrometer) particles. This effect was more pronounced in smaller air volumes. The efficiency of ion emission in reducing the viability of airborne microorganisms in indoor air was also evaluated in a specially designed set-up. Two species of Gram-negative bacteria (Pseudomonas fluorescens and Escherichia coli) and one species of Gram-positive bacteria (Staphylococcus epidermidis) were tested. It was found that a significant percentage of airborne viable bacteria could be inactivated by the ion emission: up to 92% of E. coli was inactivated during a one-minute exposure in dry air. It was concluded that the ion-driven decrease in the aerosol concentration combined with the bactericidal effect can significantly reduce human exposure to indoor air pollutants, such as particles and microorganisms.