A Method for Monitoring the Effective Air Change Rate for Respiratory Aerosols Using Real-Time Tracers

Ventilation is one of the most critical components in a layered approach toward reducing the spread of airborne infectious diseases in indoor spaces. However, building ventilation systems act together with natural ventilation, local filtration systems and other aerosol removal processes to remove infectious aerosols from an occupied space. Airflow-based determinations of ACH do not account for the full range of aerosol removal processes; however understanding the effective aerosol removal rate is critical to providing airborne infection control. In this study, we investigated the relationship between the calculated air change rate of a space (i.e. volumetric airflow based) and the effective air change rate for aerosol particle removal within the breathing zone based on direct measurements of the rate of change in tracer particle concentrations at representative occupant locations in a room. Further, we examined positional effects under well mixed and non-well mixed conditions. Our results demonstrate that tracer particles combined with real-time sensors can be used to make rapid, accurate measurements of the effective air change rate (eACH) for respiratory aerosols within the breathing zone of non-well mixed rooms. We used two experimental test beds for these analyses. First, numerical simulation (computational fluid dynamic simulation, CFD) was conducted to visualize airflow and particle removal paths within a realistic large room. Here, simulated sensors were placed in concentric zones around a nebulizer providing test-particle releases. This CFD model allowed a direct comparison of the differences between eACH and airflow ACH values under varying levels of mixing and airflow, in a fully controlled system. We then recapitulated this system in physical space to validate the CFD results under real-world conditions that include all mechanisms of particle removal that contribute to true aerosol clearance rates, including deposition and leakage. Here, we measured eACH using the decay of DNA tracer aerosols nebulized and monitored in real-time. We find that a standard sampling time of 15 minutes from the end of nebulization is sufficient to produce an accurate eACH value under non-well mixed conditions. The availability of a rapid direct test for eACH will enable empirical optimization of a wide range of ventilation and filtration mechanisms to reach and maintain target aerosol clearance rates that deliver reliable airborne infection control in typical indoor environments.