The COVID-19 pandemic highlighted the critical need to understand airborne disease transmission, particularly the role of far-field transmission (>6 ft). Quantifying the ratio of near- to far-field exposure is crucial for developing effective infection control strategies. Previous research suggested that far-field transmission contributes significantly to superspreading events. Existing models often rely on the assumption of a well-mixed indoor environment, which may not accurately reflect reality due to factors such as thermal stratification and low mixing flow rates. These factors can lead to non-uniform distribution of bioaerosols, making the probability of infection dependent on distance from the source. Carbon dioxide (CO2) has been used as a tracer, but its limitation lies in its inability to uniquely identify the source of emissions in a multi-occupant environment. This study aims to overcome these limitations by using a novel breath tracer method involving VOCs released from breath mints consumed by a participant. The unique VOCs serve as a proxy for bioaerosol emissions, allowing for the direct study of transport, mixing, and exposure implications of exhaled breath constituents from a specific individual. This approach provides a more accurate assessment of exposure risk by isolating emissions from a single source and measuring the concentration of the proxy tracer compounds over time. The goals of this study are to evaluate the effectiveness of breath mint-derived VOCs as tracers and to characterize the impact of distance on the distribution of exhaled bioaerosols.

Epidemiological studies and engineering risk assessments have increasingly demonstrated the importance of far-field exposure in COVID-19 transmission. Several studies have shown a correlation between far-field exposure and superspreading events. The conventional assumption of a well-mixed indoor space in many indoor air quality and infectious disease transmission models has been challenged by evidence showing non-uniform distribution of bioaerosols due to various factors such as thermal stratification and low ventilation rates. Previous studies have attempted to address spatial variations using CO2 as a tracer, but its limitations were highlighted in relation to identifying a specific emission source in spaces with multiple people. Existing literature highlights the need for improved methods that consider both spatial and temporal factors in evaluating exposure risks.

This study employed a custom-built environmentally controlled chamber (27 m³) with a ventilation rate of approximately 3 air changes per hour (ACH). A single healthy male participant consumed breath mints at a rate of one every 10 minutes during 60-minute trials. VOCs were measured using proton transfer reaction–time of flight–mass spectrometry (PTR-TOF-MS) at 1-second resolution. Sampling locations were 0.76 m (2.5 ft), 1.52 m (5 ft), and 2.28 m (7.5 ft) from the participant's mouth, along with the chamber's exhaust plenum. Duplicate trials were conducted at each location. To identify unique tracer compounds, additional experiments were performed with breath mints placed in a 250 mL glass container and with the participant exhaling into the container while consuming a mint. Menthone, menthol, and monoterpenes were identified as unique breath tracer compounds, showing substantially higher concentrations when breath mints were present. The emission rate of these tracer compounds was estimated to be approximately 130 µg/h, significantly higher than endogenous emissions. Data analysis included subtracting baseline VOC concentrations from trial data and normalizing the tracer compound concentrations at each distance by the volume-averaged concentration (VAC). Statistical analysis employed paired t-tests, Taylor expansion for uncertainty calculation, and Cohen's d effect size.

The study found that during the first 20 minutes of each trial, near-field (0.76 m) concentrations of breath tracer compounds were approximately 36–44% higher than the VAC. This highlights the dominant role of near-field exposure in the initial phase of an emission event. However, as the trials progressed toward the end of the 60-minute period, far-field exposure became increasingly important. The concentration differences between the sampling locations and the VAC decreased over time. During the final 5 minutes of the experiment, the magnifiers (concentration at a distance normalized by VAC) were approximately 18%, 11%, and 7.5% above VAC at 0.76 m, 1.52 m, and 2.28 m, respectively. These findings suggest that while near-field exposure is initially more critical, the significance of far-field exposure increases over longer durations. The results are comparable to previous studies examining CO2 and particle concentrations in the near and far fields, providing further support for the breath tracer approach.

The findings demonstrate the utility of the novel breath tracer methodology in studying the spatiotemporal dynamics of respiratory emissions indoors. The results show a clear difference in near-field and far-field exposures over time, challenging the well-mixed assumption commonly used in indoor air quality models. The observed patterns highlight the importance of considering both near- and far-field exposures in risk assessment and mitigation strategies for airborne infectious diseases. The findings suggest that near-field mitigation strategies might be prioritized for shorter-duration events, while far-field strategies are critical for longer events. The quantitative assessment of exposure levels at different distances provides valuable insights for future studies investigating the impact of environmental conditions and human behaviors on disease transmission risk.

This study successfully demonstrated a novel breath tracer methodology for evaluating near- and far-field respiratory exposures indoors. The results highlight the importance of both near-field and far-field exposures, particularly their time-dependent relationship. This method offers significant advantages over previous approaches and can be utilized for future studies investigating the impact of different ventilation strategies, occupant behaviors, and environmental conditions on respiratory exposure. Future research could focus on expanding the study to include various ventilation rates, chamber volumes, and multiple participants to enhance the generalizability of the findings and improve the accuracy of the near-field to far-field multipliers.

This pilot study has several limitations. The study included only duplicate trials with a single participant and a constant ventilation rate in a specific chamber. The generalizability of the results might be limited by the controlled nature of the experimental setup. Future studies should involve more participants, diverse ventilation strategies and air exchange rates, as well as the exploration of different emission sources and spatial configurations to validate the approach further and generate more robust magnifiers for various scenarios. The use of a single healthy participant may also limit the applicability of the findings to other populations.