Airborne transmission of COVID-19 and mitigation using box fan air cleaners in a poorly ventilated classroom

Abstract

Many indoor places, including aged classrooms and offices, prisons, homeless shelters, etc., are poorly ventilated but resource-limited to afford expensive ventilation upgrades or commercial air purification systems, raising concerns on the safety of opening activities in these places in the era of the COVID-19 pandemic. To address this challenge, using computational fluid dynamics, we conducted a systematic investigation of airborne transmission in a classroom equipped with a single horizontal unit ventilator (HUV) and evaluate the performance of a low-cost box fan air cleaner for risk mitigation. Our study shows that placing box fan air cleaners in the classroom results in a substantial reduction of airborne transmission risk across the entire space. The air cleaner can achieve optimal performance when placed near the asymptomatic patient. However, without knowing the location of the patient, the performance of the cleaner is optimal near the HUV with the air flowing downwards. In addition, we find that it is more efficient in reducing aerosol concentration and spread in the classroom by adding air cleaners in comparison with raising the flow rate of HUV alone. The number and placement of air cleaners need to be adjusted to maintain their efficacy for larger classrooms and to account for the thermal gradient associated with a human thermal plume and hot ventilation air during cold seasons. Overall, our study shows that box fan air cleaners can serve as an effective low-cost alternative for mitigating airborne transmission risks in poorly ventilated spaces.

A Sanitation Argument for Clean Indoor Air: Meeting a Requisite for Safe Public Spaces

“…They are part of design recommendations for setting up temporary, negative pressure isolation units ( 81 , 82 ) and also a part of the WHO Roadmap for ventilation in face of the COVID-19 pandemic ( 57 ). Multiple studies during the past months have focused on PACs, due to the ongoing pandemic, and have used approaches CFD modeling ( 83 ), experiments in actual spaces ( 84 – 88 ), and study involving actual COVID-19 patients ( 89 ) to validate that PACs are an effective mitigation measure. The studies using PACs, to date, have mostly focused on particulate matter pollution ( 80 ).…”

Rapid evidence review to inform safe return to campus in the context of coronavirus disease 2019 (COVID-19)

“…A combination of interventions (masks along with natural ventilation and HEPA filtration) were the most effective, producing a 30-fold decrease in cumulative aerosol dose 38 . Aerosol scientists have begun to develop and test home-made, low-cost box fans fitted with HEPA filters as a quick and effective solution for improving mechanical ventilation in poorly-ventilated spaces 46 .…”

Effect of recirculation zones on the ventilation of a public washroom

“… 6,7 The inlet and outlet ports set up an air circulation pattern in a given room, which determines the pattern of aerosol spread from a potential source. 8 Of particular interest are the recirculation zones formed at the corners of a room, and around obstacles. 3,9 Such regions of recirculating flow are characterized by low air flow velocity and high residence time.…”

“…The deposition of aerosol on surfaces is also found to be correlated with the location and size of recirculation zones. 8 …”

“…It is found that the efficacy of these mitigation measures depends critically on the air flow pattern in the room and the location of the source of infection. 8–10 …”