Estimating and Mitigating Indoor Airborne Pathogens to Support Healthy Buildings

The global pandemic has caused myriad damages to the lives of millions of people worldwide. Several studies confirm that indoor spaces are the main hotspots of COVID-19 outbreaks resulting in multiple confirmed instances of human-to-human transmission. Therefore, quantifying the impact of indoor environments and human activities on the transmission of infectious disease is key to stopping the spread of COVID-19 and prepare for future outbreaks. This dissertation is a multidisciplinary collaboration between designers, engineering, biologists, and public health experts to answer a question: “what is the airborne viral exposure risk indoors and how can building design and operations help to effectively reduce the risk of disease transmission indoors during the COVID-19 pandemic?” We aimed to answer these questions through following the projects: Chapter.II. A quantitative aerosol risk estimation platform. Chapter.III. Environmental mitigation of aerosol viral load. Chapter.IV. Respiratory exposure at alternate distances. In Chapter.II, we describe a quantitate aerosol risk estimation platform that is more mechanistic in nature than traditional risk estimates for airborne infectious disease. It enables the inclusion of aerosol size distributions and emissions from infected individuals with several predefined assumptions. In Chapter.III we provide the first real-world evidence that building related interventions described in Chapter.II significantly impact the dispersion and abundance of SARS-CoV-2 virus in the presence of individuals who were diagnosed with COVID-19. We also provide novel insights about the relationships of human and environmental viral loads (aerosols and surfaces) in near and far fields. In Chapter.IV, we describe a novel gas-tracing technique to quantify the degree of exposure to bioaerosols at alternate distances. Here we provide quantitative data to better explain the application of the well-mixed room assumption as well as insights about the distance from emitter variable that underly aerosol risk exposure estimates. In this dissertation, we conclude that buildings have a substantial impact on the risk of COVID-19 transmission. We offer an estimation platform for better understanding the risk of infection transmission indoors and provide proof that environmental mitigation strategies substantially reduce the viral load in a controlled study with infected participants. This dissertation includes both previously published/unpublished and co-authored material

A Novel VOC Breath Tracer Method to Evaluate Indoor Respiratory Exposures in the Near- and far-fields; implications for the spread of respiratory viruses

Background Several studies suggest that far-field transmission (\u3e6 ft) explains a significant number of COVID-19 superspreading outbreaks. Objective Therefore, quantifying the ratio of near- and far-field exposure to emissions from a source is key to better understanding human-to-human airborne infectious disease transmission and associated risks. Methods In this study, we used an environmentally-controlled chamber to measure volatile organic compounds (VOCs) released from a healthy participant who consumed breath mints, which contained unique tracer compounds. Tracer measurements were made at 0.76 m (2.5 ft), 1.52 m (5 ft), 2.28 m (7.5 ft) from the participant, as well as in the exhaust plenum of the chamber. Results We observed that 0.76 m (2.5 ft) trials had ~36–44% higher concentrations than other distances during the first 20 minutes of experiments, highlighting the importance of the near-field exposure relative to the far-field before virus-laden respiratory aerosol plumes are continuously mixed into the far-field. However, for the conditions studied, the concentrations of human-sourced tracers after 20 minutes and approaching the end of the 60-minute trials at 0.76 m, 1.52 m, and 2.28 m were only ~18%, ~11%, and ~7.5% higher than volume-averaged concentrations, respectively. Significance This study suggests that for rooms with similar airflow parameters disease transmission risk is dominated by near-field exposures for shorter event durations (e.g., initial 20–25-minutes of event) whereas far-field exposures are critical throughout the entire event and are increasingly more important for longer event durations

A Quantitative Risk Estimation Platform for Indoor Aerosol Transmission of COVID-19.

Aerosol transmission has played a significant role in the transmission of COVID-19 disease worldwide. We developed a COVID-19 aerosol transmission risk estimation model to better understand how key parameters associated with indoor spaces and infector emissions affect inhaled deposited dose of aerosol particles that convey the SARS-CoV-2 virus. The model calculates the concentration of size-resolved, virus-laden aerosol particles in well-mixed indoor air challenged by emissions from an index case(s). The model uses a mechanistic approach, accounting for particle emission dynamics, particle deposition to indoor surfaces, ventilation rate, and single-zone filtration. The novelty of this model relates to the concept of "inhaled & deposited dose" in the respiratory system of receptors linked to a dose-response curve for human coronavirus HCoV-229E. We estimated the volume of inhaled & deposited dose of particles in the 0.5-4 μm range expressed in picoliters (pL) in a well-documented COVID-19 outbreak in restaurant X in Guangzhou China. We anchored the attack rate with the dose-response curve of HCoV-229E which provides a preliminary estimate of the average SARS-CoV-2 dose per person, expressed in plaque forming units (PFUs). For a reasonable emission scenario, we estimate approximately three PFU per pL deposited, yielding roughly 10 PFUs deposited in the respiratory system of those infected in restaurant X. To explore the model's utility, we tested it with four COVID-19 outbreaks. The risk estimates from the model fit reasonably well with the reported number of confirmed cases given available metadata from the outbreaks and uncertainties associated with model assumptions

A novel VOC breath tracer method to evaluate indoor respiratory exposures in the near- and far-fields; implications for the spread of respiratory viruses

BackgroundSeveral studies suggest that far-field transmission (>6 ft) explains a significant number of COVID-19 superspreading outbreaks.ObjectiveTherefore, quantifying the ratio of near- and far-field exposure to emissions from a source is key to better understanding human-to-human airborne infectious disease transmission and associated risks.MethodsIn this study, we used an environmentally-controlled chamber to measure volatile organic compounds (VOCs) released from a healthy participant who consumed breath mints, which contained unique tracer compounds. Tracer measurements were made at 0.76 m (2.5 ft), 1.52 m (5 ft), 2.28 m (7.5 ft) from the participant, as well as in the exhaust plenum of the chamber.ResultsWe observed that 0.76 m (2.5 ft) trials had ~36-44% higher concentrations than other distances during the first 20 minutes of experiments, highlighting the importance of the near-field exposure relative to the far-field before virus-laden respiratory aerosol plumes are continuously mixed into the far-field. However, for the conditions studied, the concentrations of human-sourced tracers after 20 minutes and approaching the end of the 60-minute trials at 0.76 m, 1.52 m, and 2.28 m were only ~18%, ~11%, and ~7.5% higher than volume-averaged concentrations, respectively.SignificanceThis study suggests that for rooms with similar airflow parameters disease transmission risk is dominated by near-field exposures for shorter event durations (e.g., initial 20-25-minutes of event) whereas far-field exposures are critical throughout the entire event and are increasingly more important for longer event durations.Impact statementWe offer a novel methodology for studying the fate and transport of airborne bioaerosols in indoor spaces using VOCs as unique proxies for bioaerosols. We provide evidence that real-time measurement of VOCs can be applied in settings with human subjects to estimate the concentration of bioaerosol at different distances from the emitter. We also improve upon the conventional assumption that a well-mixed room exhibits instantaneous and perfect mixing by addressing spatial distances and mixing over time. We quantitatively assessed the exposure levels to breath tracers at alternate distances and provided more insights into the changes on "near-field to far-field" ratios over time. This method can be used in future to estimate the benefits of alternate environmental conditions and occupant behaviors