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

SARS-CoV-2 is transmitted via bioaerosols generated by respiratory activities such as breathing, talking, singing, and coughing. Epidemiological and modeling studies indicate a substantial role for far-field (>2 m) exposure in COVID-19 transmission, emphasizing the need to quantify exposure as a function of distance from the source. The conventional well-mixed assumption neglects spatial gradients and the time required for mixing, which can be influenced by stratification, ventilation rates, and room flows. CO2, while useful for ventilation assessment, is not a unique tracer of emissions from a specific infectious individual in multi-occupant spaces. The study aims to tag emissions from an individual with unique breath mints-derived VOCs to directly track transport, mixing, and exposure at near- and far-field distances, thereby improving characterization of exposure risk beyond idealized models.

Prior work shows well-mixed assumptions may not hold due to thermal stratification and low mixing, creating spatial gradients in contaminants, including 1–10 µm particles whose gradients are controlled by ventilation rates. Studies in clinical settings found higher virus RNA copies, CO2, and particle counts in the near-field compared with far-field around COVID-19 patients, and elevated CO2 at 0.5 m during HFNC therapy. CO2 has been used historically as a tracer and proxy for rebreathed air and ventilation rate but cannot uniquely identify emissions from specific individuals. Real-time VOC measurements have been used to study indoor pollutant dynamics. Chewing gum and mint products emit identifiable VOCs (menthone, menthol, monoterpenes), and PTR-TOF-MS allows high time-resolution detection of such compounds with minimal fragmentation, enabling breath tagging for exposure studies. The study situates itself within this context to provide empirical measures of near- vs far-field exposure under controlled conditions.

Design and instrumentation: Experiments were conducted in a custom climate chamber (27 m³) with filtered supply via ceiling plenum and exhaust via floor plenum, both through activated carbon filters, at ~3 air changes per hour (ACH) during trials and >20 ACH flush for ≥20 minutes between trials. Air change rate was monitored with in-duct thermal anemometers and multifunction ventilation meters. VOCs were measured with PTR-TOF-MS (mass range 17–490 amu, 1 s resolution). Target tracers were menthone (m/z 155.150, [M+H]+), menthol (reported as dehydrated ion, m/z 139.137), and monoterpenes (reported as sum of m/z 137.144 and fragment m/z 81.070), selected due to strong association with peppermint breath mints and minimal background in natural breath. Participant: One healthy male (25–30 years, height 1.89 m, seated height 1.2 m) followed pre-trial restrictions (no cologne, detergent residues; consistent diet) and consumed breath mints at a controlled rate. Experimental protocol: Each trial began with a 20+ minute high-ventilation flush (20 ACH) to minimize residual VOCs, verified by menthone background. The participant then entered, sat, and breathed normally for 5 minutes (baseline, no mints). Subsequently, one breath mint was consumed every 10 minutes at 0, 10, 20, 30, 40, and 50 minutes (without chewing), with typical dissolution over ~10 minutes, while remaining silent and minimizing movement to stabilize emissions. Sampling locations and trials: Single sampling line at 1.2 m height (breathing zone) was positioned at distances: 0.76 m (2.5 ft; Trial A), 1.52 m (5 ft; Trial B), 2.28 m (7.5 ft; Trial C), with duplicate 60-minute trials each (1 Hz, 3600 samples/trial). An additional line sampled the floor plenum exhaust (Trial D) to approximate the volume-averaged concentration (VAC), also in duplicate. Trials were randomized over 3 days; duplicate trials at each location were averaged into a single time series per location. Additional verification experiments: Trial E placed one mint in the headspace of a 250 mL glass container with flow-through (~100 cc/min) and monitored VOCs for 20 minutes. Trial F had the participant consume one mint while exhaling once into the same container, then monitored for 20 minutes. Background (3 minutes) preceded both. Statistical analysis and data processing: Paired t-tests compared first vs last minute of baseline (no-mint) for key compounds (menthone, menthol, monoterpenes, isoprene, acetone). Because isoprene and acetone changed during baseline (likely from natural breath and other sources), the unique breath tracer was defined as the sum of menthone, menthol, and monoterpenes. For consistency, the average baseline concentration (minutes −5 to 0) for each compound was subtracted from the 60-minute trial signals. Distance-specific tracer concentrations were normalized by the VAC at each time point to yield magnifiers (ratios >1 indicate above VAC). Uncertainty was assessed using Taylor expansion (propagate package in R). Effect sizes were evaluated with Cohen’s d. Emission rates of breath tracer compounds were estimated from accumulation in the 250 mL chamber (E and F) at ~4.8 air changes per minute for the initial ~30 seconds.

- Menthone, menthol, and monoterpenes did not change during baseline (p > 0.05), whereas isoprene and acetone did, confirming the former as unique mint-associated breath tracers and excluding the latter from tracer definition. - Trials E and F confirmed substantial increases of menthone, menthol, and monoterpenes in mint headspace and during exhalation while consuming a mint. Estimated combined emission rate of breath tracer compounds was ~130 µg/h (monoterpenes ~90 µg/h; menthone ~37 µg/h; menthol ~5 µg/h), an order of magnitude greater than endogenous emissions reported elsewhere. - Early period (first 5–10 minutes): Distance measurements at 0.76 m and 1.52 m exceeded VAC; 2.28 m approached VAC by ~10 minutes, exceeding by ~20 minutes, indicating plume effects prior to full mixing. - First 20 minutes: 0.76 m exhibited ~36–44% higher concentrations than other distances and VAC, highlighting elevated near-field exposure before mixing distributes emissions to the far-field. - Steady-state/late period (final 5 minutes of 60-minute trials): Distance-specific magnifiers above VAC were ~18% (±25%; Cohen’s d large) at 0.76 m, ~11% (±21%; large) at 1.52 m, and ~7.5% (±18%; large) at 2.28 m. - Over 60 minutes, mean magnifiers were 1.21 (0.76 m), 1.07 (1.52 m), and 1.05 (2.28 m). Early 0–5 minute magnifier at 0.76 m was 2.74; at 1.52 m, 1.72; at 2.28 m, 0.73 (below VAC), consistent with spatial and temporal mixing dynamics. - Despite large effect sizes relative to VAC, expanded uncertainties suggest more replicates are needed to refine near-/far-field multipliers. - Comparison to literature: Near-/far-field differences for CO2 and 1–2.5 µm particles (8–12%) in a related COVID-19 patient study align with the present near-field (~0.76 m) vs far-field (~2.28 m) magnifiers during steady-state (~10% higher near-field).

The VOC breath-tracer approach enabled real-time, distance-resolved assessment of emissions from a single tagged source, overcoming limitations of CO2-based rebreathed fraction that aggregates emissions from all occupants. Findings demonstrate that near-field exposure dominates early in an event due to incomplete mixing and concentrated exhalation plumes. As mixing proceeds (and with ~3 ACH), far-field exposure approaches the volume-averaged concentration and remains critical throughout, increasingly driving cumulative exposure for longer durations. Magnifiers show that while near-field remains somewhat elevated even at steady-state, far-field differences narrow. Comparisons with clinical studies using CO2 and particles show similar near-/far-field ratios at steady-state, supporting the validity of VOC breath tracers as proxies for bioaerosols. These results suggest that mitigation strategies should prioritize near-field controls (e.g., masking, distancing, source control) during initial periods and maintain far-field controls (e.g., ventilation, filtration) throughout. The methodology offers empirical inputs to models that currently rely on ideal mixing, enabling superposition of near-field doses onto far-field estimates for improved risk assessment.

This study introduces and validates a novel method using VOCs from breath mints (menthone, menthol, monoterpenes) as unique tracers of an individual’s exhaled emissions to quantify near- and far-field exposures in real time. In a controlled chamber at ~3 ACH, near-field concentrations were substantially elevated during the first 20 minutes (~36–44% higher at 0.76 m), while at steady-state, distances of 0.76 m, 1.52 m, and 2.28 m remained ~18%, ~11%, and ~7.5% above volume-averaged concentrations, respectively. The approach supports refined exposure assessments that account for spatial and temporal mixing, informing mitigation strategies and risk models beyond idealized well-mixed assumptions. Future work should expand to multiple participants, varied ventilation strategies and rates, diverse room geometries, additional distances and orientations, and increased replicates to develop robust near-/far-field multipliers and heterogeneous indoor air models.

Pilot study with one participant and duplicate trials only; fixed ventilation (~3 ACH) and a specific chamber volume and overhead ventilation pattern limit generalizability. Movement and speech were minimized, not reflecting real occupant behavior. Uncertainties in PTR-TOF-MS measurements and limited replicates lead to wide expanded uncertainties for magnifiers. Potential residual VOCs from chamber surfaces and filters required baseline subtraction. The mint-tracer emissions differ from pathogen-laden bioaerosols in physical properties, and the proxy assumes similar transport and mixing behavior.