A comprehensive review on mitigating the risk of airborne particles using add-on systems

Inadequate indoor ventilation can lead to microbial contamination in enclosed spaces, necessitating improvements in indoor air quality (IAQ). To address this, 'add-on systems' can be employed-additional, independently installable devices within indoor environments, often separate from the HVAC system. This study comprehensively assesses various add-on systems in diverse applications to identify their potential and primary limitations in enhancing IAQ. The analysis suggests that for efficient contaminants reduction in the occupants' breathing zone (BZ), conventional personalized ventilation (PV) and ductless PV (DPV) should maintain supply flow rates at 7-14 L/s and 10-20 L/s, respectively. Additionally, DPV is recommended for spaces with non-uniform air distribution, especially those delivering cooler air at lower room regions. For HEPA portable air cleaners (HEPA-PAC), optimal placement involves positioning them at 1 m above the floor, away from contaminant sources. Conversely, ION-PAC and ultraviolet germicidal irradiation PAC (UVGI-PAC) exhibit inferior particle removal efficacy compared to HEPA-PAC. Thus, it is advisable to incorporate multiple-layer filters with them. The rotating upper-room UVGI (UR-UVGI) demonstrates superior disinfection efficiency due to its larger disinfection area compared to fixed UR-UVGI. Additionally, combining UR-UVGI with ceiling fans is suggested for high disinfection efficiency. Concerning physical barriers, an optimal height of 0.6-0.7 m effectively reduces particle concentrations in occupants' BZ. In conclusion, various add-on devices exhibit substantial efficacy in reducing contaminants. Future research should focus on comparing all add-ons within the same room environment and under identical conditions to establish a comprehensive summary of each add-on's effectiveness.