The Impact of Large Mobile Air Purifiers on Aerosol Concentration in Classrooms and the Reduction of Airborne Transmission of SARS-CoV-2

Abstract

In the wake of the COVID-19 pandemic, an increased risk of infection by virus-containing aerosols indoors is assumed. Especially in schools, the duration of stay is long and the number of people in the rooms is large, increasing the risk of infection. This problem particularly affects schools without pre-installed ventilation systems that are equipped with filters and/or operate with fresh air. Here, the aerosol concentration is reduced by natural ventilation. In this context, we are investigating the effect of large mobile air purifiers (AP) with HEPA filters on particle concentration and their suitability for classroom use in a primary school in Germany. The three tested APs differ significantly in their air outlet characteristics. Measurements of the number of particles, the particle size distribution, and the CO2 concentration were carried out in the classroom with students (April/May 2021) and with an aerosol generator without students. In this regard, the use of APs leads to a substantial reduction of aerosol particles in the considered particle size range of 0.178–17.78 µm. At the same time, the three APs are found to have differences in their particle decay rate, noise level, and flow velocity. In addition to the measurements, the effect of various influencing parameters on the potential inhaled particle dose was investigated using a calculation model. The parameters considered include the duration of stay, particle concentration in exhaled air, respiratory flow rate, virus lifetime, ventilation interval, ventilation efficiency, AP volumetric flow, as well as room size. Based on the resulting effect diagrams, significant recommendations can be derived for reducing the risk of infection from virus-laden aerosols. Finally, the measurements were compared to computational fluid dynamics (CFD) modeling, as such tools can aid the optimal placement and configuration of APs and can be used to study the effect of the spread of aerosols from a source in the classroom.

A review of facilities management interventions to mitigate respiratory infections in existing buildings

“…(2) Social outcomes: We only identified three articles that explored the social effects of limited-coverage FM strategies. Two studies gathered feedback from occupants and stakeholders on the noise level generated by portable air purifiers installed in classrooms; however, the findings were inconsistent [ 82 , 83 ]. Moreover, Zhang et al analysed the effects of varying occupant schedules on occupant productivity with a somehow unwarranted assumption that extended work hours boost worker productivity [ 76 ].…”

“…First, when filters are not correctly sealed, particles may escape and become circulated by the fan, as illustrated by the experiment in a dental clinic [ 51 ]. Additionally, filters gradually get saturated and congested, lowering airflow through the filter and hence the number of particles filtered in the room over time, and thus require changing filters regularly [ 82 , 83 ]. The changing frequency of filters differs across different equipment types.…”

Can 10x cheaper, lower-efficiency particulate air filters and box fans complement High-Efficiency Particulate Air (HEPA) purifiers to help control the COVID-19 pandemic?

“…73 To achieve these ACH targets, cost and noise generation concerns with portable air filtration or ventilation upgrades need to be addressed. 59 60…”

“…Approaching 15 ACH as in hospitals and passenger aircraft [9] would require further HEPA units putting it further out of reach. If portable HEPA air purifiers of sufficiently low noise generation with sufficiently high clean air delivery rate (CADR) are unaffordable for many communities, how else can they achieve target ACH while addressing [10] [11] both cost and noise generation concerns?…”

Computational Study of Thermal Comfort and Reduction of CO2 Levels inside a Classroom

“…Some of these studies represent strictly applied research, such as [ 10 ], in which the effect of CO 2 sensors was studied to improve the performance of forced convection ventilation systems. On the other hand, studies such as [ 11 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ] were conducted in real spaces, such as industrial buildings for welding processes, hospitals, computer laboratories, offices, mosques, kitchens, buildings, dorm rooms, or schools.…”