Impact of needle-point bipolar ionization system in the reduction of bioaerosols in collective transport

Abstract

The transmission rate of SARS-CoV-2 is higher in collective transport than in other public environments. Transport companies require preventive strategies to mitigate airborne risk of contagion which not imply responsible use at the individual level. Air purification systems, such as UV-C or needle-tip bipolar ionization, are attractive alternatives. However, only a few studies addressing the validation of this technology against bioaerosols in actual operation conditions have been published so far. In this work, the efficiency of a bipolar ionization unit in the Zaragoza Tram has been evaluated. Against environmental bioaerosols, ionization (~25.7 · 109 ions/m3, on average) reduced the concentration of colony-forming units (CFU) by ~46 % and ~69 % after 30 and 90 min. No clear benefits were obtained against microorganisms on inner tram surfaces (seats, grab bars, walls, and windows). ‘Pre-pandemic’ filtration equipment located in the HVAC based on a Coarse 45 %-type filter removed ~73 and ~ 92 % of aerial CFU by itself after 30 and 90 min. Microscopic visualization of the CFUs revealed that they were mostly >1 μm, much larger than the SARS-CoV-2 virion (~100 nm) and SARS-CoV-2-loaded bioaerosols (from 0.25 μm). Then, we studied the filter behavior under normalized laboratory methods. The filters efficiency against submicron particles was limited (between 5 and 12 % against 0.1 to 0.3 μm NaCl particles). Another ionization strategy was to generate aerosol agglomerates to enhance filtration performance, but the combined action of ionization and filtration did not improve substantially. The effect of these technologies was also characterized using the clean air delivery rate (CADR). Relative to untreated air (CADR = 0.299 m3/min), ionization and filtration reduce ambient CFUs (CADR = 5.153 and 9.261 m3/min, respectively; and CADR = 13.208 m3/min, combined) which implies that it has a substantial impact on indoor bioaerosols.