984967 - Indoor Air Purifiers in the Effort to Reduce the Spread of Coronavirus Disease 2019 (COVID-19): The Advantage of Circumferential Outflow Diffusers

Indoor air filtration is one of the two primary recommended interventions aimed at reducing the spread of COVID-19 inside buildings, along with ventilation. It had been suggested that a directional outflow from an indoor air purifier could entrain unpurified surrounding air and increase the distance potentially infectious aerosol particles could spread in indoor spaces. It has been argued that such an air current indoors could lead to a higher risk of transmission of SARS-CoV-2 between people who are at a certain distance apart compared to the scenario with the absence of indoor air currents. Because only the case of a directional outflow has been examined to date, we investigated an indoor air purifier that does not have a unidirectional outflow and has a circumferential outflow diffuser instead. Circumferential outflow diffusers constitute a second widespread type of outflow ports used in indoor air purifier design, alongside directional outflow ports. We measured the airflow velocity at two different angles to the circumferential outflow diffuser's surface and two motor-fan speeds. We visualized in scattered light the deflection of a vertical mist spray cone from a sneeze-simulating nebulizer parallel to the side of the air purifier. We found a significant difference in airflow velocities for different angles to the circumferential outflow diffuser: 0.01 m/s – 0.02 m/s for 0° vs. 0.01 m/s – 0.65 m/s for 45° at 1 m distance. We observed no significant deflection of the sneeze-simulating spray cone at the minimum blower speed and a 5 cm deflection at the maximum speed under the experimental conditions. We found that circumferential outflow diffusers may constitute a lower risk of COVID-19 spread indoors compared to directional outflow vents as they do not create fast air currents indoors. Outflow velocities from circumferential diffusers are lower than for sneeze ejecta, and we observed a minimal deflection of sneeze-mimicking mist by circumferential outflow under the experimental conditions.