Continuous aerosol monitoring and comparison of aerosol exposure based on smoke dispersion distance and concentrations during oxygenation therapy

The aerosols generated by breathing primarily exhibit sizes below 1 μm, with the majority falling between 0.5 to 1 μm^25,26. It is important to note that the SARS-CoV-2 virus can remain viable in aerosols for a duration of approximately 1.1 to 1.2 h⁵. In our simulation study, we employed tracer gas particles with a diameter of 0.5–0.7 μm (and a median aerodynamic diameter, MMAD, of 0.62 ± 0.16 μm)—a measurement that is nearly comparable to, albeit slightly smaller than, previous research findings^25,26. Despite this size difference, the behavior and movement of these particles within the flow field demonstrate remarkably similar comparative characteristics.

Numerous studies have documented aerosol dispersions from various oxygen-delivery devices^27,28,29. Hung et al.²⁹ reported maximum aerosol dispersion at different minute ventilation settings recorded within 3 min: 51.49 ± 19.47 cm for the reference group (no oxygen device), 64.31 ± 14.39 cm for HFNC 30 L/min, 67.09 ± 12.74 cm for HFNC 70 L/min, 85.55 ± 7.28 cm for NCO 15 L/min, and 63.08 ± 15.33 cm for NRM 15 L/min at 20 L/min. Hui et al.²⁷ also reported aerosol-dispersion distances for NCO at 1, 3, and 5 L/min of < 42 cm, while those for NRM at 6, 8, 10, and 12 L/min were < 10 cm. In our study, spanning a 10-min observation period, we found an aerosol dispersion of NCO at 4 L/min to be 117.17 ± 14.26 cm in length and 80.71 ± 29.48 cm in width. Interestingly, despite the lower flow rate used in Hui's study, both Hung's and our studies exhibited larger aerosol dispersion lengths and widths compared to the 3-min and 10-min periods, implying a time-dependent increase in aerosol dispersion for the reference group (no oxygenation therapy) and NCO. The aerosol dispersion of NRM, although increased with time, remained the lowest among the non-invasive oxygen therapy methods. During COVID-19 outbreak, HFNC was considered the most effective approach for hypoxia as it decreased the intubation rate in patients with respiratory failure^18,19. However, the risk of aerosol transmission while using this method remains unknown^{19,21, 22}. Hui et al.²⁸ reported aerosol-dispersion values of 17.2 ± 3.3, 13.0 ± 1.1, and 6.5 ± 1.5 cm in normal lungs for HFNC at 60, 30, and 10 L/min, respectively. In our study, the length and width of aerosol dispersion in HFNC were the longest. Following a 10-min observation, the aerosol dispersion of HFNC 50 L/min was 1.35 ± 0.10 m in length and 1.01 ± 0.20 m in width. In contrast, in Hung’s study of HFNC with flow rates of 70 L/min, the maximum dispersion was 1.00 ± 0.01 m in length and 0.57 ± 0.02 m in width. This finding suggests that Hui’s study may have observed a shorter period than both the 3-min period in Hung's study and the 10-min period in our study²⁹. Overall, the aerosol dispersion distance of HFNC remained the largest among the non-invasive oxygenation devices studied^27,28,29.

HFNC involves an air/oxygen blender, active humidifier, single heated tube, and nasal cannula. It delivers adequately heated and humidified medical gas at a flow of 0–70 L/min, affording several physiological advantages than other standard oxygen therapies, including reduced anatomical dead space, upper airway positive end-expiratory pressure, constant FIO₂, and good humidification. Moreover, it decreases the breathing frequency and work, thereby reducing intubation requirement in patients with COVID-19^18,19. In severe hypoxia, escalating respiratory support requirement should be considered alongside oxygenation, which increases oxygen reserves to prevent hypoxemia during apnea before intubation. Driver et al.¹⁶ reported that airway oxygenation using flush-rate NRM provided an airflow of 50–54 L/min, which was noninferior to that of BVM devices. Herein, flush-rate NRM yielded the highest aerosol concentration compared with HFNC and the remaining oxygenation devices (Fig. 3A,B). Notably, this contrasted with the aerosol-dispersion distance, as a greater distance did not reflect a higher concentration. Unsealed interface around the airway and oxygen devices may explain these findings as aerosol leak more from the flush-rate NRM than in HFNC. The visible condensed aerosols can be more easily captured via the high-sensitivity camera, however, the invisible accumulated aerosol concentration may reflect the aerosol exposure more authentically.

In patients with mild-to-moderate hypoxia, oxygen can be delivered using NCO or simple masks with an oxygen flow of approximately 4–6 L O₂/min followed by titration of the flow rates by monitoring pulse oximetry, aiming for an oxyhemoglobin saturation on pulse oximetry (SpO₂ > 88%)³⁰. In all the study groups, the aerosol concentrations were higher in the foot area vs. the trunk or head areas (Table 2). Gravity affected the aerosols exhaled from the mouth of an upright mannequin at a 30-degree angle, which, according to projectile motion, exhibited a parabolic trajectory. With decreasing kinetic energy, the aerosols remained suspended at the foot area, thus accumulating and showing increased concentration (Fig. 3). Hung et al.²⁹ reported that aerosol concentration during NRM at 15 L/min was higher in the head area vs. the foot and trunk areas in a 3-min observation period; owing to the holes on the upper part of the mask, the particle concentration in the head area rapidly increased. Conversely, this study was conducted over 10 min, which could explain this difference. We assumed that the aerosol was exhaled from the mannequin’s mouth toward the foot area; therefore, the aerosol concentrations were significantly higher in the foot area vs. other sites regardless of the oxygenation device, especially in the HFNC and flush-rate NRM groups (Table 2). Additional studies are necessary to evaluate the effects of aerosols according to the patient’s inclination angle.

NRM at 15 L/min and NCO at 4 L/min yielded the lowest aerosol concentration in the three areas (Table 2). Increasing the NCO flow to 15 L/min also resulted in an increase in aerosol concentrations. Moreover, the oxygenation devices that use masks (NRM at 15 L/min, NCO at 15 L/min combined with NRM at 15 L/min, and simple mask at 6 L/min) yielded lower particle concentration levels than other devices (Table 2). Compared with NCO and HFNC, the oxygen mask covers the mouth and nose, and the air leaking from the gap between the edge of the mask and the face flows easily toward the ground in round-shaped mask settings. As the mask in NRM seals more tightly than a simple mask, the former causes less aerosol leaks, resulting in lower aerosol concentrations. The aerosol concentrations in NCO combined with NRM at 15 L/min were lower than those of NCO alone at 15 L/min in the three areas (Table 2). Therefore, except at a flow rate ≥ 15 L/min, nasal cannula devices with poorer sealing place HCWs at a higher risk of exposure to contaminated aerosols compared to oxygen mask devices.

Limitations

This simulation study might not perfectly replicate real clinical conditions. The chosen fixed ventilation rate of 20 L/min was intended to simulate a desaturated patient with breathlessness, and PAO particles were released as traceable aerosols. However, it may not precisely mirror lung physiology due to various physiological factors, such as respiratory irregularities, depth, droplet distribution, tidal volume, and virion deactivation rates, which are influenced by factors like droplet radius, temperature, and humidity. Nonetheless, the standardized experimental setup allowed us to delineate aerosol dispersion and exposure among the seven common oxygenation devices for the study's purpose. Additionally, it's important to note that this study did not account for scenarios where the mannequin wears a face mask, which might not accurately reflect real-life conditions where patients wear masks.