Investigation of a DNA Tagged Aerosol Tracer Method for In Situ Evaluation of Germicidal UV Air Cleaner Effectiveness
Transmission of respiratory pathogens occurs primarily in indoor settings, interventions to reduce the risk of their transmission include increases in outdoor air introduction, filtration, and ultraviolet germicidal irradiation (UVGI). However, validating these interventions is challenging, particularly in actual applications. This study introduces an aerosol tracer system utilizing DNA as tracer molecule, aimed at quantitative characterization of the performance of indoor air cleaning systems.
Two DNA tracers, designed one to be relatively UV-resistant and another relatively UV-sensitive, were employed to assess air quality changes related to filtration and ventilation and the contributions of UVGI fixtures in various built environments. We conducted controlled UV exposure experiments of DNA-tagged tracers on foil coupons, aerosolization studies in a test chamber, and in a commercial building conference room. The DNA tracer results provided insights at the point of sampling into the effects of complex airflow dynamics.
Additionally, a way to scale the DNA tracer results to MS2 bacteriophage is proposed. Four distinct UV devices challenged with MS2 in a chamber test produced equivalent clean airflow rates of 13–147 CFM. Scaled equivalent clean airflow rates in a commercial building setting using the UV-sensitive tracer varied from 47 percent less to 101 percent more than the chamber results, possibly due to differences in airflow patterns, equipment configuration, and other factors.
Our findings provide quantitative understanding of the interaction between UV-sensitive aerosols and the built environment, with implications for environmental monitoring, measuring UVGI fixture impact in field settings, and addressing current technologies limitations for assessing UVGI disinfection efficacy.
Introduction
Airborne exposure to pathogens, including viruses like coronaviruses (responsible for COVID-19 and some common cold strains), influenza, measles, respiratory syncytial virus (RSV), Human metapneumovirus (HMPV), and bacteria such as M. tuberculosis, is a significant factor in the transmission of the infectious diseases they cause [1]. Mitigating the airborne spread of these pathogens is crucial for public health. Interventions such as increased air exchange, filtration, and ultraviolet germicidal irradiation (UVGI) aim to reduce the concentration of viable pathogens in the air, hence, reducing the risk of infection by reducing inhalational exposure [2]. Among these interventions, UVGI systems hold great promise for effective pathogen control, offering both significant efficacy and cost-effectiveness in terms of capital and operating expenses [3].
The effectiveness of an intervention may vary across different settings. Designing and optimizing such systems necessitates a comprehensive approach that combines numerical simulations and empirical testing. For example, an air cleaning system for a hospital might greatly benefit from design and optimization, whereas for non-critical environments such as a residential setting, such modeling and optimization would be impractical and cost prohibitive [4]. Numerical simulations, such as Computational Fluid Dynamics (CFD) and fluence rate field models (e.g., ray tracing), provide insights into the potential distribution and impact of UVGI within enclosed spaces. While CFD was not employed in the current study, it is worth noting that CFD is commonly used to simulate indoor air quality (IAQ) dynamics in the built environment, although its broad application in commercial building settings can be challenging [5,6].
Empirical testing, on the other hand, involves experiments using non-pathogenic challenge agents to quantify inactivation or physical separation under various operating conditions. Conventionally, such tests have been conducted in specialized environments like IAQ chambers. However, a critical challenge arises in translating test results from controlled environments to commercial building application settings. Currently, there is no method that combines mimicking the particle size distribution of a human upper respiratory aerosol emission that can be used for onsite testing of UVGI system efficacy, highlighting the need to develop robust new test methods and associated analyses.
An important limitation on effective application of the current methods for reducing human exposure to airborne pathogens in public spaces is the lack of a safe, single indicator that can represent the effectiveness of air purification devices, while taking into account the multiple mechanisms that affect the pathogen decay rate. Given the inability to assess installed effectiveness, it is often difficult to justify the cost of air filtration and cleaning devices, such as filters or UVGI fixtures.
