Visualisation of the airflow pattern of exhaled droplets in a classroom

Abstract

The airborne transmission of SARS-CoV-2 in educational buildings has raised concerns during the current COVID-19 pandemic. In this study, a portable fog generator system was designed and assembled to visualise the airflow pattern of exhaled droplets in a classroom. The system consists of five components: medium, fog generator, buffer, pump, and manikin head. The medium was made of a combination of glycol and demineralised water, which produced a fog composed of droplets mimicking to some extent human breath. The fog was produced with the fog generator and passed through a pipe into the buffer for build-up. After accumulation, the fog is pumped through another pipe and is exhaled out of the mouth of the manikin. The experiments were conducted in a simulated classroom. The lights of the room were dimmed and six lasers were used to make the fog more visible. Four different ventilation regimes were examined: no ventilation, natural ventilation (open windows and door), mixing ventilation (600 m3/h), and a combination of natural + mixing ventilation. The experiments were recorded with a camera and analysed to determine the horizontal distance of the path taken by the fog and to measure the time it remained visible after exhalation from the mouth. During the experiments, it could be observed with the naked eye that the glycol droplets travel much further and linger in the air for longer than what is captured in the recordings. Furthermore, not all the droplets were visible with the camera, especially the smaller ones of a few micrometres in diameter. The recordings showed that at the natural ventilation regime the droplets travelled the furthest distance (1.8 metres). This wind coming through the door pushed the drops further into the room. The combination of natural + mixing ventilation regime had the highest indoor air velocity, causing the droplets to reach the shortest distance (0.5 metres). In conclusion, the type of ventilation, classroom layout, and exhalation location play an important role in determining the airflow pattern, as they affect how long and how far the glycol droplets travel. Applying any type of ventilation is critical in transmission control and reduces the possibility of aerosol accumulation in the classroom as smaller droplets can linger in the air for many hours until they settle on a surface.