Influence of Aerosol Mask Design on Fugitive Aerosol Concentrations During Nebulization

Abstract

BACKGROUND:

Secondhand exposure to fugitive aerosols may cause airway diseases in health providers. We hypothesized that redesigning aerosol masks to be closed-featured would reduce the fugitive aerosol concentrations during nebulization. This study aimed to evaluate the influence of a mask designed for a jet nebulizer on the concentration of fugitive aerosols and delivered doses.

METHODS:

An adult intubation manikin was attached to a lung simulator to mimic normal and distressed adult breathing patterns. The jet nebulizer delivered salbutamol as an aerosol tracer. The nebulizer was attached to 3 aerosol face masks: an aerosol mask, a modified non–rebreathing mask (NRM, with no vent holes), and an AerosoLess mask. An aerosol particle sizer measured aerosol concentrations at parallel distances of 0.8 m and 2.2 m and a frontal distance of 1.8 m from the manikin. The drug dose delivered distal to the manikin's airway was collected, eluted, and analyzed using a spectrophotometer at a 276 nm wavelength.

RESULTS:

With a normal breathing pattern, the trends of aerosol concentrations were higher with an NRM followed by an aerosol mask and AerosoLess mask (P < .001) at 0.8 m; however, the concentrations were higher with an aerosol mask followed by NRM and AerosoLess mask at 1.8 m (P < .001) and 2.2 m (P < .001). With a distressed breathing pattern, the aerosol concentrations were higher with an aerosol mask followed by an NRM and AerosoLess mask at 0.8 m, 1.8 m (P < .001), and 2.2 m (P = .005). The delivered drug dose was significantly higher with AerosoLess mask with a normal breathing pattern and with an aerosol mask with a distressed breathing pattern.

CONCLUSIONS:

Mask design influences fugitive aerosol concentrations in the environment, and a filtered mask reduces the concentration of aerosols at 3 different distances and with 2 breathing patterns.

Introduction

Nebulizers generate aerosols containing fine particles of drugs that are used to treat lung diseases. Aerosol particles are inhaled into the airways, reducing systemic side effects and maximizing drug absorption and utilization.¹ Compressed gas is often used in hospitals to deliver bronchodilators to patients with asthma or COPD during exacerbations. The easy-to-use and inexpensive features of jet nebulizers make them irreplaceable for clinical applications in developing countries.^2–4

A constant-output jet nebulizer can expel fugitive aerosols into the environment. Bioaerosols exhaled from patients receiving aerosolized medication delivery from a nebulizer for the treatment of respiratory-related infection may be transmitted by air or droplets from within a range of 0.1 to > 100 µm.⁵ Fugitive aerosols generated by a jet nebulizer during aerosol therapy may be inadvertently released from aerosolized drugs.⁶ Fugitive aerosols can be inhaled and can irritate the eyes, mouth, or nose of individuals in close proximity. If the patient is treated for an airborne infectious disease, the evaporative fugitive aerosols generated from respiratory droplets, forming smaller droplet nuclei suspended in the air, carrying pathogens, are transported farther away by the air flow of the jet nebulizer.⁷ Studies have shown that fugitive aerosols may cause side effects. For example, improper face mask use leads to fugitive aerosols of ipratropium bromide being deposited into the eyes, resulting in dilated pupils with poor response to light.⁸ Caregivers are frequently secondhand exposed to aerosolized medications, which may be associated with an increased risk of developing chronic pulmonary diseases.^9–11

The aerosols produced from a jet nebulizer strongly correlate with the transmission distance propelled by the gas.¹² A previous study has shown that aerosols spread at decreasing concentrations with increasing distance from the head of a spontaneously breathing adult patient.¹³ The amount of fugitive aerosols that leaked as aerosolized droplets was less in a normal lung simulation than in the severe lung injury model, where a lower concentration of particles at a longer distance was implicated.¹² The delivery of nebulized drugs through the mouth due to different breathing patterns, which refers to the combination of tidal volume, breathing frequency, inspiratory time, and inspiratory flow, has been identified as the primary factor that alters the delivered dose. The total lung deposition or inhaled dose fraction can be increased 2–3 times by changing the tidal volume and breathing frequency.^14–15

Redesigned aerosol masks with a tighter fit are likely to reduce this risk. A previous study showed that a tightly fitting aerosol mask increased the inhaled mass and reduced the drug deposition on the face, eyes, and mask in a simulated pediatric model.¹⁶ For example, the open feature of an aerosol mask can be redesigned without vent holes or by replacing vent holes with filters. Some masks have a one-way valve to direct exhaled gas to a bacterial filter, whereas others are redesigned with soft cushions for a better fit to the face. However, the effectiveness of these masks in reducing fugitive aerosols remains poor. We hypothesized that masks designed without holes would reduce fugitive aerosol concentrations during nebulization. This study aimed to compare the concentrations of fugitive aerosols measured at different distances during jet nebulization using different designs of face masks.

QUICK LOOK.

Current knowledge

A previous study showed that the fugitive aerosol concentration generated by a jet nebulizer was higher with face masks than with mouthpieces. However, no data on fugitive aerosol concentrations resulting from standard versus newly designed aerosol face masks concerning breathing patterns and distances are available.

What this paper contributes to our knowledge

A filtered and tightly fit face mask reduced the concentration of fugitive aerosols at different distances around the manikin, with both normal and stressed breathing patterns. The highest aerosol concentrations were detected with an open-featured mask at a close distance. When treating patients with distressed breathing patterns, health care providers should wear an N-95 mask, avoid standing parallel to the head of the patient and keep a safe distance of at least 1.8 m to mitigate risk of fugitive aerosol exposure.

Methods

An adult intubation manikin (Laerdal Medical, Stavanger, Norway) was attached to a lung simulation system (Michigan Instruments, Grand Rapids, Michigan) with compliance of 0.05 L/cm H₂O and resistance of 5 cm H₂O/L/s. The simulator was set to mimic a healthy adult breathing pattern with a breathing frequency of 15 breaths/min, tidal volume of 500 mL, inspiratory-expiratory ratio (I:E) of 1:3, and distressed/fast adult breathing pattern with a breathing frequency of 30 breaths/min, tidal volume of 500 mL, and I:E of 1:1. Figure 1 shows the apparatus used in the experimental setup.

Fig. 1.

Illustration of the experimental setup. The left panel presents three aerosol masks used in the experiment. The collecting filter was only placed during inhaled dose determination.

Ventolin nebules (GSK, London, England) containing salbutamol sulfate were nebulized as an aerosol tracer. A pneumatic jet nebulizer (Besmed, New Taipei City, Taiwan) was filled with a unit dose of salbutamol (5.0 mg/2.5 mL) and was powered by 50 psig of compressed air at a flow of 8 L/min. The nebulizer was stopped when the aerosol was produced inconsistently (onset of sputtering) for 30 s. The performance of the nebulizer with a unit-dose salbutamol was first tested with an Andersen Cascade Impactor (Thermo Fisher Scientific, Waltham, Massachusetts), resulting in a median mass aerodynamic diameter of 4.3 ± 0.2 μm and a fine-particle fraction < 5 microns of 73.5 ± 6.2% with a residual volume of 1.10 ± 0.05 mL. The nebulization time was 6.3 ± 0.3 min.

Three adult aerosol face masks were compared: a standard aerosol mask (GaleMed, Taipei, Taiwan), an AerosoLess SafetyNeb mask (AerosoLess Medical, Margate, Florida), and a modified non–rebreathing mask (NRM) (Hsiner, Taichung, Taiwan). The aerosol mask is a commonly used interface for a nebulizer with a 2-cm hole on each side. The AerosoLess mask is a new product that utilizes water-resistant viral filters over the vent holes on both sides, water-resistant pads on the upper and lower edges to minimize fugitive aerosol leakage, and a faceplate for tight sealing to a user's face. The modified NRM was redesigned for the COVID pandemic, with a no vent-hole mask attached to a Y adapter. The Y adapter was connected to a 500 mL reservoir at one end and a high-efficiency particulate air filter at the other end, with 2 one-way valves placed on each side of the Y adapter to guide gas flow during inspiration and expiration. Both aerosol and AerosoLess masks were placed on the face of the manikin in sitting position at 45 degrees. The manikin with the NRM was tested in the supine position to avoid compromising the orientation of the T-connecting tube and jet nebulizer in a perpendicular position.

For drug deposition measurement, the dual-lung simulator was connected to a collecting filter (VADI Medical Technology, Taoyuan, Taiwan) distal to the bronchial tree of the manikin to quantify the mass of the salbutamol delivered. At the end of each nebulization, the collecting filter, representing the inhaled drug dose, and the test mask were removed for drug assay. Drugs deposited on masks were eluted with 5 mL of distilled water, whereas collecting filters were eluted with 10 mL of distilled water with gentle agitation for 2 min. The sample concentration was quantified using ultraviolet spectrophotometry (BioMate 3S, Thermo Fisher Scientific) at a wavelength of 276 nm. A known salbutamol sulfate concentration to ultraviolet absorbance correlation was determined to develop a linear regression for the calculation (r² = 0.99). Each condition was repeated 5 times.

An aerodynamic particle sizer (APS) (Grimm 11-D, Grimm Aerosol Technik, Hamburg, Germany) was used to detect fugitive aerosol emission concentrations. The APS continuously measured the mass concentrations and size distributions (0.225–34.000 µm) of the aerosols over 30 min. Particulate matter (PM) data at 10 µm (PM₁₀), 2.5 µm (PM_2.5), and 1 µm (PM₁) were retrieved for particles likely to be deposited in the upper and central airways and alveoli. The size of the laboratory at Chang Gung University was 24.5 m (depth), 5.8 m (width), and 2.8 m (height), with 3 internal doors, and 2 fresh-air diffusers as air inlets mounted on the ceiling. The ambient temperature during testing was 20.3 ± 1.6°C with a relative humidity of 55.3 ± 4.8%. To minimize the variation in aerosol concentrations, the experiment was carried out with the air conditioning turned off after working hours when no one entered the room.

The collecting filter representing the inhaled drug dose was removed during the aerosol concentration determination. The APS was placed at distances of 0.8 m and 2.2 m, parallel to the head of the manikin. The 0.8 m distance was representative of a health care provider standing near a patient during nebulization therapy. A side distance of 2.2 m was selected as the approximate distance between the beds without separate compartments in a general hospital ward, mimicking a patient lying in a bed next to the patient receiving aerosol therapy. The particle concentration at a frontal distance of 1.8 m from the manikin was measured, representing the fugitive aerosols from the vertical distance of the mask at the end of the patient's bed. The APS recorded data at 6 s intervals for a total of 30 min for each measurement. The first 5 min established a baseline for ambient aerosol counts prior to the initiation of nebulization. In the subsequent 6 min, aerosol counts were measured during nebulization; and in the remaining 19 min, fugitive emissions were monitored during subsequent aerosol decay. After each nebulization, the washout period for fugitive emissions was 30 min. After the washout period, aerosol concentrations were set as a new baseline. Figure 2 shows the timeline of the measurements. The protocol involved testing with different masks, and each mask consisted of triplicate measurements at each distance. Each scenario with different breathing patterns and distances was repeated 3 times.

Fig. 2.

Timeline measurements of the experimental design.

Data were analyzed using commercial statistical software (SPSS 26.0, IBM, Armonk, New York). Considering the experiments were performed in quintuplicate for drug depositions and triplicate for aerosol concentrations, data were reported with median and interquartile range (IQR). Comparisons of the drug mass for each component and particle concentrations at different distances were analyzed by Kruskal-Wallis tests for within groups and Mann-Whitney tests for between groups. Statistical significance was set at P < .05 (2 tailed).

Results

Table 1 presents the aerosol concentrations under all conditions during the 6-min nebulization. Nebulization using the AerosoLess mask yielded the lowest fugitive aerosol emission concentrations at different distances. The lowest aerosol concentrations (PM_2.5) were detected at 2.2 m for a normal breathing pattern and at 1.8 m for a distressed breathing pattern. With a normal breathing pattern at 0.8 m, the aerosol concentrations were higher with an NRM followed by an aerosol mask and AerosoLess (25 [IQR 18–28], 12 [IQR 5–18], and 4 [IQR 1–8] µg/m³, respectively, P =.002). The concentrations (PM_2.5) were higher with an aerosol mask (27 [IQR 11–27] µg/m³) followed by an NRM (27 [IQR 11–27] µg/m³) and AerosoLess (5 [IQR 1–7] µg/m³) at 1.8 m (P < .001). With a distressed breathing pattern, the aerosol concentrations were higher with an aerosol mask followed by an NRM and AerosoLess (34 [IQR 16–84], 27 [IQR 10–35], and 14 [IQR 9–24] µg/m³, respectively) at 0.8 m (P = .002) and 2.2 m (25 [IQR 14–34], 25 [IQR 19–28] and 7 [IQR 5–9] µg/m³, respectively, P < .001).

Table 1.

Median Aerosol Concentrations Measured by Aerodynamic Particle Sizer in 6 min of Nebulization

Figure 3 illustrates the trends of the PM_2.5 concentrations among the 3 masks and distances with 2 breathing patterns. The trends of aerosol concentrations over a 30-min measurement period were similar for PM₁₀, PM_2.5, and PM₁; the concentrations of PM₁₀ and PM₁ are presented in Supplements 1 and 2, respectively (See related supplementary materials at http://www.rcjournal.com). Aerosol concentrations under each scenario showed significant differences among the 3 masks (all P < .01). For PM_2.5 concentrations, the highest fugitive concentration was found with an AM measured at 0.8 m (120 µg/m³) under a stressed breathing pattern, approximately 4-fold > those of the other 2 face masks (30 µg/m³, P < .001).

Fig. 3.

Comparisons of PM_2.5 concentrations among 3 masks and 3 distances (0.8 m [A and D], 1.8 m [B and E], 2.2 m [C and F]) during the 30-min measurement with normal (A, B, C) and stressed breathing patterns (D, E, F). P values were analyzed by Kruskal-Wallis test. PM = particulate matter.

Figure 4 shows that aerosol deposition is influenced by the design of the face mask. With the frequency of normal breathing pattern set at 15 breaths/min, the inhaled drug dose was significantly higher with the AerosoLess mask (4.2% [IQR 4.0–4.4]) followed by the aerosol mask (3.7% [IQR 3.5–3.8]) and NRM (2.7% [IQR 2.4–3.0], P < .001). With a distressed breathing pattern, the inhaled drug dose showed a significant difference between the standard aerosol mask and NRM (P = .02).

Fig. 4.

Comparisons of drug deposition (%) with 3 masks and 2 patterns. Box plots display as median and interquartile range, and whiskers represent the minimum and maximum values. NRM = non–rebreathing mask.

The amount of drug deposited on the face mask was greater with an NRM, regardless of the breathing pattern. The drug dose deposited on the face masks was significantly higher with an NRM (4.1% [IQR 4.0–4.3]) followed by the AerosoLess (2.7% [IQR 2.5–2.9]) and aerosol masks (1.1% [IQR 0.9–1.3], P < .001) for a breathing frequency of 15 breaths/min; and 5.4% [IQR 2.3–5.7) for NRM, 2.8% [IQR 2.6–2.9] for AerosoLess, and 1.4% [IQR 1.3–1.6] for the standard aerosol mask (P < .001) with a distressed breathing pattern.

Discussion

This study explored nebulized particle deposition in simulated situations to detect delivered drug dose and emission concentrations using different aerosol face masks under 2 breathing patterns. The results of our study showed that minimum fugitive aerosol concentrations were achieved with a tightly fitted and filtered face mask regardless of the distance from the manikin and breathing patterns.

Secondhand exposure to medical aerosols has become a concern for health care providers because of the increased incidence of asthma among them.^9–11 We detected 4-fold greater aerosol concentrations at 0.8 m and a 10-fold lower concentration at 1.8 m with a fast breathing pattern. The results may imply that the health care provider has a greater chance of exposure to aerosols while standing parallel to the patient's head and a much lower chance of exposure while standing at the end of the bed during aerosol therapy for a patient with fast breathing. For normal breathing, the health care provider standing at the side of the patient's head (2.2 m) has a lower chance of exposure to aerosols than standing at a closer distance or at the end of the bed.

With a rapid breathing frequency, aerosol concentrations at a closer distance from the manikin using an AM were affected more than those on the vertical side (1.8 m). We speculated that increasing the breathing frequency led to a more rapid expulsion of aerosols escaping from the open-featured mask at a distance parallel to the head, especially at 0.8 m. A previous study showed that turbulent air flow promotes the sedimentation deposition of aerosols and pushes fugitive aerosols out of the mask.¹⁷ In our study, the turbulence to the aerosols also promoted sedimentation deposition on the face mask, resulting in a slightly higher dose on the mask. The highest concentration was accumulated approximately 3 min after the initiation of nebulization, followed by rapidly declining concentrations before the end of the nebulization process. The results imply that a person standing at a close distance may have a greater chance of exposure to high aerosol concentrations. Health care providers should have adequate respiratory protection to mitigate the risk of transmission of nosocomial infections within 0.8 m of patients with distressed breathing pattern and illnesses of unknown etiology.¹²

A nebulizer with an AerosoLess mask, designed with viral filters and a faceplate, expelled the lowest aerosol concentrations in the environment over a 30-min detection period at different distances. The faceplate of the AerosoLess mask was designed to hold the mask in shape, and 2 water-resistant pads were placed over the nose bridge and chin to create a better seal and less aerosol escape. The faceplate over the AerosoLess mask reinforced fixation for a tight fit to the face and reduced fugitive aerosols even during nebulization, resulting in steady fugitive aerosol concentrations with different breathing patterns measured at 3 distances.

The redesigned NRM with one-way valves to direct the gas flow of inspiration and expiration and a face mask without vent holes for use during the COVID pandemic was purposely designed to minimize aerosols expelled to the environment. Surprisingly, our results showed that the highest aerosol concentration at a closer distance (0.8 m) was during normal breathing with the NRM. The NRM was placed over the face of the manikin in the supine position, which allowed the aerosol to flow directly toward the nose and mouth. It was difficult to properly fit the large configuration with a Y-piece and a T-adapter for a nebulizer on the NRM, leaving gaps below the chin and around the nose bridge. Without vent holes on the mask, aerosols escaped through the gaps. Thus, more fugitive aerosols were detected at a close distance parallel to the head of the manikin. Compared to the features of the NRM and AerosoLess masks, we speculated that a face mask that fits the manikin's face tightly was more important than closing the vent holes to reduce fugitive aerosols.

The results showed that a fast breathing frequency with a high minute ventilation increased the drug deposition in both the inhaled dose and face mask. Ari et al³ reported that the inhaled dose in percent from a jet nebulizer connected to a standard AM was 3.6–4.4%, as opposed to 4.4–6.0% with a valved face mask. The inhaled drug mass via a smaller-diameter side-hole pediatric face mask was > that associated with the standard pediatric aerosol face mask.¹⁸ We speculated that blocking the vent holes on the sides increased the turbulence during expiration, resulting in greater drug deposition on the face mask and lower inhaled dose. However, Berlinski et al¹⁹ compared a jet nebulizer connected to face masks with different dead spaces, blocking the side holes in various proportions to form different degrees of resistance. Their results showed that increasing the resistance due to various side hole sizes had no impact on the inhaled dose.

A T-adapter was optionally applied for the NRM, connecting a nebulizer to a face mask due to the manikin's position and keeping the nebulizer in a perpendicular position. When a nebulizer is connected to a 90-degree angled T-piece, it acts as a baffle, with some aerosol lost due to inertial impaction, whereas a nebulizer entering the mask at 90 degrees has less impactive losses. Our results showed a lower inhaled dose and a greater face mask dose with the NRM connected to the nebulizer through a T-adapter.

The tightness of a standard aerosol mask on the face impacts both the fugitive aerosol concentrations and inhaled dose. Tight-fitting face masks could improve drug delivery and impact facial and eye deposition of aerosol.²⁰ Although the NRM mask was redesigned with a no-side-hole mask, one-way valves, and an expiratory filter, the enlarged configuration creates difficulty in fitting the face of the manikin tightly, resulting in a greater leak of nebulized aerosols, increasing fugitive aerosol, and reducing the inhaled dose.

Our results illustrated that fugitive aerosol concentrations were affected by the interface used and breathing pattern. Hui et al¹² found that a small-volume jet nebulizer with an aerosol mask and a gas flow of 6 L/min could disperse aerosols as far as 0.45 m. Higher fugitive aerosol concentrations were observed when the manikin breathed with a fast breathing pattern using an open-featured aerosol mask at 0.8 m. Thus, we recommend avoiding an aerosol mask for nebulization therapy, especially for patients with fast breathing frequency. Using a no-vent face mask with a tight fit can minimize fugitive aerosol emissions. The filtered mask also minimizes fugitive aerosols; however, purchase price difference and the total cost associated with aerosol mask replacement during the course of therapy may need to be considered when selecting an aerosol interface. The prices for masks were approximately United States $0.20 for a standard aerosol masks, United States $8.00 for a non-vented aerosol mask, and United States $15.00 for a filtered aerosol mask. The efficiency of a static filter decreases when the filter is wet, requiring replacement after each treatment.

Owing to the highly contagious nature of SARS-CoV-2, a single patient room is preferred during the COVID-19 pandemic; however, this may not be possible for health care in populated countries. The potential for bioaerosol exposure and the transmission of infectious pathogens to health care providers and other bystanders who are in close proximity to patients receiving aerosol therapy are of great concern. The use of personal protective equipment is strongly recommended during aerosol therapy.^21–23 We observed that the return of the particle concentration level to the baseline required 2–3 h after nebulization. Fugitive aerosol concentration would have minimal impact on health care providers who are equipped with personal protective equipment, but fugitive aerosol could negatively impact other patients and visitors who remain unprotected in the room not only during but up to 3 h following nebulized aerosol therapy. If an aerosol mask is used, the use of a filtered face mask or a no-vent mask with a tight fit to the face reduces fugitive aerosol emissions and possible transmission of bioaerosols to others who may be in close proximity during nebulization therapy. Our study supports a previous study that maintained that a distance of at least 1.8 m from the patient should be considered when the nebulization treatment is administered.

This study had several limitations owing to the parameters used in a bench study setting. Laboratory simulations of patient breathing patterns may not fully represent a patient's real and varied breathing patterns. For example, the lung mechanics of patients with distress patterns may be altered rather than an increase in breathing frequency. Patients may speak or cough during nebulization; thus, aerosol concentrations may be altered at different distances. The results are limited to the conditions studied in our adult model. Our results may not be representative of the range of air ventilation conditions, as testing was performed in a university laboratory room rather than in a negative-pressure hospital room. Many studies support the efficiency of a vibrating mesh nebulizer owing to its minimum residual volume and unidirectional water flows, leading to a lower chance of contamination by secretions.^24,25 A high emitted dose from a vibrating mesh nebulizer may yield different environmental aerosol concentrations. The performance of jet nebulizers in the market varies owing to different particle size and residual volumes, from 40–60%. The residual volume of the nebulizer used in this study was approximately 50–55%. In this study, one local brand of jet nebulizer was used with a 2.5 mL unit-dose salbutamol. Performance of the nebulizer was predetermined within the recommended rage. In addition to nebulizer performance, the fill volume of the nebulizer changes the delivered dose, as the total delivered dose is altered.^26,27 Salbutamol was chosen for testing because it is the most commonly prescribed aerosolized agent used in hospital settings, and it has the most defined method for chemical assay. However, chemical properties influence the performance of a jet nebulizer, resulting in a different fugitive aerosol behavior.

The fugitive aerosol concentrations may vary with the performance of different nebulizers; however, the measurements of aerosol concentrations between masks are relevant. The APS measures aerosol concentration over a wide range of particle sizes in real time. The increased aerosol concentrations in the time series represent fugitive emissions from nebulization and exhaled aerosols into the environment over a period of 2–3 h. Aerosol concentrations during the long washout period may have been altered by the room temperature and humidity; therefore, a new baseline was set for each experiment. In future studies, laser imaging or a fast-motion camera could be employed to detect the motion of nebulized aerosol particles for a comprehensive process of fugitive aerosol escape to the environment. Furthermore, the breathing simulation was conducted under dry conditions rather than with humid expiratory gas and airway. Aerosols may more likely age with humidity and behave differently compared to a dry simulated expiratory flow. Thus, the fugitive aerosol concentrations and inhaled drug dose may be altered, and our study provides a relative comparison.

Conclusions

In this simulated adult model, the design of aerosol interfaces affected the aerosol concentrations in the environment during nebulization. A filtered aerosol mask tightly fitted to the face reduced the concentration of fugitive aerosols at 3 different distances and with 2 breathing patterns. With the other masks, the aerosol concentrations were impacted at different distances and directions, depending on the fit of the mask to the manikin, as well as the direction and accumulation of escaped aerosols to the environment. The study suggested that, when treating patients with distressed breathing patterns, health care providers should avoid standing parallel to the patient's head and keep a safe distance of at least 1.8 m for a normal breathing pattern and 2.2 m for a distressed breathing pattern to mitigate the potential for fugitive aerosol exposure and transmission. Proper personal protective equipment should also be utilized.