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 (>6 ft) explains a significant number of COVID-19 superspreading outbreaks. Objective: Quantify the ratio of near- and far-field exposure to emissions from a source to better understand human-to-human airborne infectious disease transmission and risk. Methods: In an environmentally controlled chamber, volatile organic compounds (VOCs) uniquely associated with a healthy participant consuming breath mints were measured using PTR-TOF-MS at 0.76 m, 1.52 m, 2.28 m from the participant, and in the exhaust plenum (volume-averaged concentration). Results: Concentrations at 0.76 m were ~36–44% higher than other distances during the first 20 minutes, underscoring the importance of near-field exposure before mixing distributes emissions to the far-field. After ~20 minutes and toward the end of 60-minute trials, the human-sourced tracer concentrations 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: For rooms with similar airflow parameters, transmission risk is dominated by near-field exposures for shorter event durations (initial 20–25 minutes), whereas far-field exposures are critical throughout and increasingly important for longer durations. Impact: We demonstrate a novel VOC breath-tracer approach as a proxy for bioaerosols, enabling real-time, distance-resolved exposure assessment and improving upon idealized well-mixed assumptions by addressing spatial and temporal mixing.