key: cord-1028700-iu3kp9kb authors: Daniel, Dan; Lin, Marcus; Luhung, Irvan; Lui, Tony; Sadovoy, Anton; Koh, Xueqi; Sng, Anqi; Tran, Tuan; Schuster, Stephan C.; Jun Loh, Xian; Thet, Oo Schwe; Tan, Chee Keat title: Effective design of barrier enclosure to contain aerosol emissions from COVID‐19 patients date: 2021-04-20 journal: Indoor Air DOI: 10.1111/ina.12828 sha: c9a7c1fc1a555ff74d31e29a14430c312da572f6 doc_id: 1028700 cord_uid: iu3kp9kb Facing shortages of personal protective equipment, some clinicians have advocated the use of barrier enclosures (typically mounted over the head, with and without suction) to contain aerosol emissions from coronavirus disease 2019 (COVID‐19) patients. There is, however, little evidence for its usefulness. To test the effectiveness of such a device, we built a manikin that can expire micron‐sized aerosols at flow rates close to physiological conditions. We then placed the manikin inside the enclosure and used a laser sheet to visualize the aerosol leaking out. We show that with sufficient suction, it is possible to effectively contain aerosol from the manikin, reducing aerosol exposure outside the enclosure by 99%. In contrast, a passive barrier without suction only reduces aerosol exposure by 60%. The prolonged nature of the coronavirus disease 2019 (COVID- 19) pandemic has resulted in global shortages of personal protective equipment (PPE), especially N95 respirators. As a result, some clinicians have resorted to using barrier enclosures typically mounted on the hospital bed over the patient's head to contain any aerosol emissions, especially during aerosol generating procedures such as intubation. [1] [2] [3] Different barrier designs with varying levels of complexities have been proposed, ranging from a simple carton box, 4 plastic drapes and boxes 1,5,6 to a custom-built acrylic box with active suction and filtration system. 7 Despite all these innovations, there is little validation on the effectiveness of the various designs. To assess the performance of barrier enclosures, some groups have resorted to simulating a cough/sneeze (for example by using an exploding balloon 1 or a water spray 7 ) and comparing the splatter pattern of droplets with and without the enclosure. Such an approach can confirm the effectiveness of the enclosures at blocking larger respiratory droplets (tens of microns to millimeters in diameters), but not for small micron-sized aerosol droplets which are airborne and can potentially travel over much longer distances. 8 To detect aerosol spread, some groups have successfully used particle counters to measure the concentration of micron and submicron droplets leaking out of the barrier enclosure 5,9 and concluded that active suction is critical to effectively contain aerosol droplets. 10 However, the amount of suction required to achieve containment remains unexplored. This is crucial because COVID-19 patients are often subjected to treatment modalities involving high gas flow rates, for example, high flow nasal cannula (HFNC) therapy to provide supplemental oxygen, [11] [12] [13] which can quickly disperse bioaerosol over large distances, increasing the infection risk to healthcare workers. In this paper, we would like to establish the design criteria-in particular, the minimum suction required-for a barrier enclosure to effectively contain aerosol emissions especially with high (and medically relevant) gas flow rates. We used two complementary techniques (one qualitative and the other quantitative) to assess the performance of the barrier enclosure. The first is to use laser sheets to visualize the aerosol flow from a custom-built manikin expiring micron-sized water-glycerin droplets at flow rates close to physiological conditions. The second is to collect the aerosol leaking out using air samplers and subsequently to quantify the collected amount using spectrofluorometry. Given the growing evidence that COVID-19 is airborne and can spread through aerosol, 14-16 a well-designed barrier enclosure can be useful as an additional layer of protection for healthcare workers. In this study, we looked at the effectiveness of aerosol containment for diferent Q O2 = 0-60 L min −1 (with oxygen concentration set at 75%) and Q suction = 0-120 L min −1 . The HFNC was placed over the nose of a custom-built manikin which can emits aerosol at flow rates close to physiological conditions as described in the next section. In a typical experiment, the Ambu bag was compressed for 3 seconds and re-inflated for another 3 seconds to mimic continuous exhalation of aerosol for 20 minutes ( Figure 2B and Video S1). A metronome was used to help the operator keep in time. The manikin does not simulate the inhalation cycle, but this should not affect the results greatly, since aerosol is generated mostly during the exhalation cycle. Aerosol typically refers to droplets <5 μm in diameter and thought to be responsible for airborne transmission of diseases, since they can stay suspended in air for a long period of time. 17, 18 Aerosol generated during respiratory activities (such as talking and coughing) and in healthcare settings (such as intubation) is thought to vary between 0.1 and 10 μm. [18] [19] [20] The droplets generated by the fog machine span this range with a mean diameter of about 1 μm, as measured using a particulate matter sensor Sensirion SPS30 ( Figure 2C ). The aerosol flow can be visualized by shining 2D laser sheets. The We first dissolved a small amount of fluorescein sodium salt (Sigma Aldrich) at a concentration of 0.5 g L −1 in the water-glycerin solution. The aerosol (glycerol-water droplets with fluorescein) leaking out of the barrier enclosure was collected using SASS 3100 Air Samplers fitted with standard filter cartridges from Research International running at a flow rate of 60 L min −1 for 20 minutes. The hospital room we were using had a total air change rate per hour of 10, that is, it took about 6 minutes for the air in the room to be well-mixed, shorter than the experimental time. SASS 3100 air sampler has been used previously to collect bioaerosols from the air. 22 The filter paper was then placed in a tube Calibration curve for different concentration standards. Dashed line is the best fit curve with a slope of 1, that is, intensity is linearly proportional to concentration (Duetta, HORIBA scientific) by comparing its fluorescence intensity at 514 nm with those from calibration standards of known concentrations (See Figure 3 for the calibration curve). The minimum concentration that can be measured using spectrofluorometer is 0.02 μg L −1 or 0.04 ng in 2 mL solution. For aerosol to be effectively contained, we need to generate a negative pressure inside the enclosure. This is achieved when the flow rate of the suction Q suction exceeds the sum of the oxygen flow rate Q O2 and the expiration rate of the patient/manikin Q air , that is, For example, at the maximum oxygen flow rate Q O2 = 60 L min −1 , we are able to contain aerosol emissions by turning on the two suction wall units with a combined Q suction = 120 L min −1 ( Figure 4A and Video S2). To visualize the aerosol flow, we shone two laser sheets: a blue laser sheet at the sagittal z-y plane and a green laser sheet at the transverse x-z plane in front of the barrier. As expected, there is no aerosol leakage and the green laser sheet is not visible since there is minimal aerosol outside the barrier to scatter the light. We also observed fresh air, which was free from aerosol and therefore appeared dark, continually being drawn through gaps at the bottom of the plastic drape. In contrast, with just one (Q suction = 60 L min −1 ) or no suction (Q suction = 0 L min −1 ), the aerosol cannot be contained and spread quickly throughout the entire room (with a floor area of about 30 m 2 ) within minutes. The aerosol leaking out scatters light strongly and renders the green laser sheet visible ( Figure 4B and Video S3). 23 The enclosure chamber is also now completely filled with aerosol, and there is no fresh air being drawn in. To test the effectiveness of the aerosol containment under different conditions, we varied the suction rate Q suction from 0 (no suction) to 60 and 120 L min −1 (1 and 2 wall suction units, respectively), while at the same time subjecting the manikin to different oxygen flow rates Q O2 = 0, 30 and 60 L min −1 ( Figure 4C ). Experimentally, we found that Equation 1 correctly predicts the criterion for effective aerosol confinement (The transition from effective to ineffective containment as predicted by Equation 1 is indicated by the gray line in Figure 4C ). In our experiment, the suction ports are located on the two sides of the enclosure. The exact positionings of the suction port, together with the detailed geometry of the enclosure, need to be optimized to avoid deadspots, where there are little circulation and potential (1) Q suction > Q O2 + Q air F I G U R E 4 A, Aerosol is effectively contained when Q suction > Q O2 + Q air . B, Otherwise, aerosol leakage from the enclosure can be readily observed. C, Phase diagram for effective (blue filled dots) and ineffective aerosol containment (unfilled red dots) F I G U R E 5 A, To collect the aerosol leaking out, we placed two air samplers outside the barrier enclosure. B, Aerosol droplets (with added fluorescein) were trapped by the filter on the air sampler, which can then be detected using spectrofluorometer. C, The amount of fluorescein collected by air samplers 1 and 2 for enclosure barrier with and without suction can then be compared to the control, that is, no barrier. Error bars are the standard deviation for triplicates accumulation of aerosol. This can be done using detailed computational fluid dynamics study and could be part of a future study. To assess the level of protection afforded by the barrier enclosure with and without suction, we added a small amount of fluorescein to the water-glycerin solution. The level of aerosol exposure can then be quantified by measuring the amount of fluorescein collected by filters of two air samplers placed 50 and 110 cm away from the barrier (samplers 1 and 2 in Figure 5A , respectively). The amount of fluorescein trapped by the filter ( Figure 5B ) can be deduced by spectrofluorometry. See Section 1D, for experimental details. We found that for HFNC oxygen therapy with no barrier at Q O2 = 60 L min −1 , the amount of fluorescein collected after 20 minutes by samplers 1 and 2 are 15 ± 5 ng and 11 ± 6 ng, respectively ( Figure 5C ). With a passive barrier and no suction (corresponding to Figure 4B and case b in Figure 4C ), the amount of fluorescein collected by samplers 1 and 2 was reduced by about 60% to 5 ± 2 and 4 ± 1 ng, respectively. At maximum suction of Q suction = 120 L min −1 (corresponding to Figure 4A and case a in Figure 4C ), the amount of fluorescein reaching the two samplers was reduced by 99% to 0.15 ± 0.08 and 0.19 ± 0.01 ng, respectively. The amount of fluorescein collected by the two air samplers is similar to each other, because the aerosol droplets are uniformly distributed within the experimental time of 20 minutes. Finally, we would like to point out that although a passive barrier with no suction provides some level of protection, large amount of aerosol can accumulate inside the barrier over time, which will be released into the room if the barrier were to be dismantled, for example during a medical emergency. In short, we have established the criterion for effective containment of aerosol for barrier enclosure, namely that the suction rate must exceed the oxygen flow rate and the expiration rate of the human breath. We show explicitly that for high (and medically relevant) oxygen flowrate of 60 L min −1 , it is possible to significantly reduce aerosol exposure outside the enclosure by 99% with sufficient suction. Given that the barrier enclosure can be made from readily available material such as acrylic and that suction points are commonly found in hospital rooms, we believe such a device can potentially be scaled up and provide additional protection for healthcare workers. The authors would like to acknowledge funding from A*STAR, Singapore, "A*CRUSE project on airflow and aerosol particles studies for public agencies" project number SC25/20-8R1640, as well as from the National Medical Research Council, Singapore, project number MOH-000411. The peer review history for this article is available at https://publo ns.com/publo n/10.1111/ina.12828. The data that support the findings of this study are available from the corresponding author upon reasonable request. Barrier enclosure during endotracheal intubation More on barrier enclosure during endotracheal intubation The intubox: enhancing frontline healthcare worker safety during Coronavirus disease 2019 (COVID-19) A carton-made protective shield for suspicious/ confirmed COVID-19 intubation and extubation during surgery Measurement of airborne particle exposure during simulated tracheal intubation using various proposed aerosol containment devices during the COVID-19 pandemic Modified wake forest type protective shield for an asymptomatic, COVID-19 nonconfirmed patient for intubation undergoing urgent surgery Protective device to reduce aerosol dispersion in dental clinics during the COVID-19 pandemic Violent expiratory events: on coughing and sneezing Effectiveness of a negativepressure patient isolation hood shown using particle count Protective Barrier Enclosures Without Negative Pressure Used During the COVID-19 Pandemic May Increase Risk to Patients and Health Care Providers -Letter to Health Care Providers High flow nasal cannula compared with conventional oxygen therapy for acute hypoxemic respiratory failure: a systematic review and meta-analysis High-flow nasal cannula for COVID-19 patients: low risk of bio-aerosol dispersion The utility of high-flow nasal oxygen for severe COVID-19 pneumonia in a resourceconstrained setting: A multi-centre prospective observational study It is time to address airborne transmission of COVID-19 Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1 Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission Natural ventilation for infection control in health-care settings Size distribution and sites of origin of droplets expelled from the human respiratory tract during expiratory activities Airborne transmission of covid-19 Aerosol generating procedures and risk of transmission of acute respiratory infections to healthcare workers: a systematic review Visualizing droplet dispersal for face shields and masks with exhalation valves Microbial communities in the tropical air ecosystem follow a precise diel cycle Generalized lorenz-mie theories