Adequacy of air exchange rate as a sole indicator of air delivery system's protective effectiveness against airborne infections disease transmission

Abstract

In the indoor environments where airborne infectious disease transmission is of concern, air exchange rate (ACH) 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 of the airborne infection risk transmission for several positions of the cough source and the susceptible person in relation to the supply and return air grilles. Particle Image Velocimetry (PIV) was used for airflow field investigation, while Grimm 1.108 aerosol counter was used to measure droplet concentration in the FEC. These results show that airborne droplet transmission effectiveness cannot be characterized solely based on ACH. Air flow patterns needs to be incorporated into characterization because the type of the supply air terminals, supply flow rate and momentum, positions of the supply and return air terminals generated air flow patterns that can enhance or decrease transmission risk. These results have implications on codes and recommendations for indoor environments with high risk of airborne infection transmission.