Adequacy of air change rate as the sole indicator of an air distribution system's effectiveness to mitigate airborne infectious disease transmission caused by a cough release in the room with overhead mixing ventilation: A case study

Abstract

In indoor environments where airborne infectious disease transmission is of concern, air change rate is conventionally used as the sole indicator of air delivery system performance. This indicator, based on the total volume dilution reasoning, suggests that increase of the supply flow rate will reduce risk of airborne infectious disease transmission. Results obtained from recent studies on cough release conducted in the field environmental chamber (FEC) at the National University of Singapore indicate that increase of supply flow rate may cause increase in the airborne infection risk transmission for several positions of the cough source and the susceptible person in relation to the supply and return air grills. Particle image velocimetry (PIV) was used for airflow field investigation, while a Grimm 1.108 aerosol counter was used to measure droplet concentration in the FEC. Results from this study imply that a local airflow pattern is an important factor influencing dispersion of cough droplets and consequential exposure. It is demonstrated that increase in supply flow rate can lead to an increase in exposure under certain circumstances. This further implies that air change rate should not be used as the sole indicator of the air delivery system's ability to reduce exposure to airborne infectious droplets.