Classroom scale measurements of the efficacy of ventilation, filtration, and electronic air cleaners for the removal of aerosol particles

Exposure to aerosol particles in the indoor environment is of particular importance due to the confinement of particles in poorly ventilated spaces. Reduction in the number concentration of aerosol particles can be achieved by increasing supply air flow rates and installation of in-room particle filtration devices. Electronic air cleaning technology has been proposed as a low-cost mitigation strategy, although the efficacy and unintended consequences on indoor air quality are not well characterized. We determined the efficacy of ventilation, in-room filtration, and air ionizers for the reduction of aerosol number concentrations in an unoccupied classroom. We focus on particles larger than 500 nm to assess the efficacy of this technology for reducing the number concentration of virus containing particles. We show that increases in supply air flow and in-room HEPA filtration result in reductions in particle number concentrations at clean air delivery rates that match those measured at the supply air register or specified by the filtration unit. We find no evidence for a reduction in particle number concentrations, or enhanced deposition, for the air ionizers tested. For two of the air ionizer units tested rapid new particle formation and growth (10 nm h−1) was observed. We directly measured the speciation of air ions from both tube-based and needlepoint bipolar ionizer (BPI) devices. We do not observe highly reactive ions such as O2− in the immediate vicinity of the BPI device. In contrast, we observe stable ions, such as NO3−, that have limited disinfecting power when exhausted to the room.