Tracing of Sars-CoV-2 aerosols with tracer gases in an occupied classroom with mobile air cleaners

The placement of mobile air cleaners (MACs) in classrooms was widely discussed between parents, teachers, and authorities in Germany during the peak of Corona infections in 2020 and 2021. Measurements of mobile air cleaner efficiencies in larger laboratory rooms indicated that there are substantial efficiency differences between test re-sults in a real room compared to results measured in a standardized 28m³ well-mixed clean test room according to a standard. The test method described here overcomes the multiple problems and uncertainties of aerosol particle decay tests. However, this method can only be applied to mobile air cleaners having a particle filter and an active carbon filter.  
Sars-CoV-2 viruses are always attached to aerosol particles. 99% of exhaled aerosol particles are smaller than 2 microns and therefore completely airborne. This is the reason why a tracer gas can be used to mimic the flow pattern of aerosol particles where potential viruses could be attached to. Tracer gases in general are not affected by particle filters, they go just through. While aerosol particles accumulate in the HEPA filter of the MAC Per-fluorocarbon tracer (PFT) are at the same time adsorbed by a downstream or upstream active carbon filter. The proportionality between particles removed by the particle filter and PFT removed by the active carbon filter is used to study the aerosol particle removal efficiency and spreading in a classroom. The Perfluorocarbon tracer PMCH was used to measure the effective clean air delivery rate in the occupied zone of a classroom. Constant injection of diluted PDCB was used to mimic the spreading of aerosol particles by an infected pupil sitting at a desk. 
The paper describes an efficiency test for mobile air cleaners in an occupied classroom with 22 pupils and a teacher present. Tracer gas sampling was done by the pupils themselves at every desk in the breathing zone, at the inlet of each air cleaner and at the outlet of each air cleaner using 60cc syringes. The syringes were later analysed by an Autotrac 101 tracer gas monitor, a gas chromatograph with electron capture detector. With this strategy a complete picture of aerosol resp. tracer gas migration and decay could be established.  
SF6 as a third tracer gas was used to account and correct for removal of particles by infiltration. Local PMCH decay data provided local removal efficiencies at all desk locations. By evaluating PDCB migration data, local exposure values could be calculated for every desk and pupil in the classroom.  
By comparing the PMCH concentrations in the occupied zone with the PMCH inlet concentration of the mobile air cleaners it was possible to quantify short circuiting around the MACs, i.e., the flow of cleaned air at the MAC outlet which circulates directly to the MAC inlet without affecting the occupied zone.