Student Research: Brian D. Smith

The Effect of Surface Charge, Negative and Bipolar Ionization on the Deposition of Airborne Bacteria
Faculty Advisor: 


Studies from a hospital setting have suggested that static charges on some surfaces may attract bacteria in excess of normal sedimentation or diffusion, perhaps contributing to the occurrence of nosocomial infections. Bacteria can accumulate significant static charge during aerosolization. In a bipolar atmosphere, this charge will eventually stabilize to the Boltzmann distribution at a rate dependent on the air ion density. This suggests that enhancing the air ion density may result in reduction of bacterial deposition. This study investigated the interaction between surface electrostatic potential and the deposition of airborne bacteria in an indoor environment using nutrient agar settle plates, charged with electric potentials of 0, +/- 2.5kV and +/- 5kV, exposed to the air for an average of 24.4 hours. Results showed that bacterial deposition on the plates increased proportionally with increased potential to over twice the normal sedimentation rate at 5kV. A slightly larger deposition rate was observed on the positively charged surface at 5kV compared to the negative surface, consistent with studies showing that charged bioaerosols end toward an overall net negative charge. Experiments were repeated under similar conditions in the presence of either a negative ion generator or bipolar ionizer. Releasing negative and bipolar ions into the air during testing resulted in bacterial deposition onto the charged surface nearly equal to that of normal sedimentation, consistent with the assumption that ions introduced into the air reduce the potential on oppositely-charged airborne bacteria towards the Boltzmann equilibrium before deposition. However, there was an indication during unipolar ionization that diffusion charging had occurred, resulting in an even greater deposition onto the oppositely charged surface than without ionization. Results support the hypothesis that electrostatic attraction can significantly contribute to the deposition of airborne bacteria onto surfaces under select conditions. Conversely, implementing a bipolar ionization strategy can be effective in reducing both electrostatic interactions and diffusion charging. This suggests that the healthcare industry could benefit from further research into the development of air ionization-type engineering strategies designed to reduce the effect of electrostatic attraction, and thus to help control the deposition of potentially harmful airborne bacteria, similar to that used already in the semiconductor industry to control the settling of particulate matter in clean room environments.