key: cord-0780699-dysf5gl4 authors: Lommel, Michael; Froese, Vera; Sieber, Moritz; Jentzsch, Marvin; Bierewirtz, Tim; Hasirci, Ümit; Rese, Tim; Seefeldt, Josef; Schimek, Sebastian; Kertzscher, Ulrich; Paschereit, Christian Oliver title: Novel measurement system for respiratory aerosols and droplets in indoor environments date: 2021-06-07 journal: Indoor Air DOI: 10.1111/ina.12860 sha: 07b01f094a998cb4b93f1b8d4c2b55fe3cdb75e4 doc_id: 780699 cord_uid: dysf5gl4 The SARS‐CoV‐2 pandemic has created a great demand for a better understanding of the spread of viruses in indoor environments. A novel measurement system consisting of one portable aerosol‐emitting mannequin (emitter) and a number of portable aerosol‐absorbing mannequins (recipients) was developed that can measure the spread of aerosols and droplets that potentially contain infectious viruses. The emission of the virus from a human is simulated by using tracer particles solved in water. The recipients inhale the aerosols and droplets and quantify the level of solved tracer particles in their artificial lungs simultaneously over time. The mobile system can be arranged in a large variety of spreading scenarios in indoor environments and allows for quantification of the infection probability due to airborne virus spreading. This study shows the accuracy of the new measurement system and its ability to compare aerosol reduction measures such as regular ventilation or the use of a room air purifier. ground within a shorter time due to gravity. The middle-sized (20- 100 µm) and smaller droplets (5-20 µm) , as well as aerosols (<5 µm), remain airborne over a longer period of time and evaporate quickly, depending on environmental conditions like humidity or temperature and their initial diameter. 2, 3, [5] [6] [7] The evaporation time of aerosols with an initial diameter of 1 µm is 1-2 ms; for small droplets of 10 µm, it is 250-550 ms; and for large droplets of 100 µm, it is 5-30 s. 3, 7, 9 After evaporation, the average size distribution of the solid droplet nuclei of sputum is 0.0786-26.2 µm 4 and can linger in still air for 20-60 min. 6 Because of the higher mass of large-and middle-sized droplets, the resulting viral load is significantly higher than in aerosols and small droplets. Additionally, there is an increased chance of survival for viruses of this magnitude. [9] [10] [11] Thus, after evaporation, the originally middle-sized droplets become small, high-risk droplet nuclei, carrying a great number of active viruses. 4, 10 In their review, Mao et al. 4 combine this finding with studies on the deposition probability as a function of aerosol size in the upper respiratory tract and the alveolar region. [12] [13] [14] [15] [16] [17] They conclude that aerosols and droplets with a size of 2-10 µm carry the highest risk of infection. Therefore, it is important to investigate aerosol and droplet dispersion at these orders of magnitude, which are mostly responsible for infectious transmissions. Especially in scenarios with poor ventilation, the ambient air can become enriched with viruses up to a critical concentration. All persons in the environment are simultaneously exposed to a high concentration of viruses and the risk of so-called superspreading events emerges. Currently, there are no experimental studies that quantitatively determine viral spreading in everyday environments. Most studies use particle size analyzers to evaluate the dispersion and spread of aerosols and droplets in rooms. Noti et al. 5 Simulation Chamber using a breathing and a coughing simulator. to determine the influence of air humidity on the risk of infection. The amount of infectious viruses was determined by real-time qPCR analysis and a viral plaque assay (VPA). In another study, Lindsley et al. 18 examined the efficiency of a face shield as protective equipment in the same scenario. The measuring method is very well suited for the direct measurement of virus spread via aerosols. Alsved et al. 19 determined the size of the aerosols and droplets during singing and talking by using this method. However, measurements with a particle size analyzer are limited to a determined range of droplet and aerosol sizes. Moreover, the direct measurement of the virus spread or aerosol and droplet measurements with a particle size analyzer can only be done with much effort in clean room environments. Thus, these measurements are not applicable in everyday environments. Another important factor is the experimental substitute for sputum, which is used to track the aerosol spread. Most studies use water as a single component droplet model to analyze the evaporation and dispersion. 2, [19] [20] [21] [22] However, as stated above, the composition of sputum is much more complex than that of pure water and the evaporation differs strongly. 4, 5, 8 Another substitute that has been used in experimental studies is NaCl-water solution. 3, 7, 9 Although the evaporation time of NaCl-water droplets is more similar to water than to sputum, the resulting droplets and aerosols also shrink to a solid nuclei that stays airborne. 9, 23 To consider and investigate a larger range of parameters that influence the spread of aerosols and droplets, several studies simulate the dispersion of droplets in closed rooms numerically. 2, 6, 7, 9, 24 The simulations allow for the analysis of the aerosol dispersion at any time and position in a room. Important parameters like the velocity during exhalation, humidity, temperature, ventilation pattern or droplet nuclei size (solid phase in the aerosol or droplet), and ventilation rate can be included. 2 Nevertheless, a limitation of these simulations is that the airflow in a specific room is influenced by many factors like ventilation slits or door and window columns that cannot be covered by simplified boundary conditions in the simulation. 9 In most studies, the background flow is solely driven by coughing or sneezing or by defined ventilation. Additionally, turbulent fluctuations are not considered at all or only in a limited way. Therefore, it is necessary to validate the simulation results with experimental measurements. This study presents an experimental measurement method, which is suitable to measure the spread of aerosols in everyday situations and reproduce the spread of viruses between humans. For this purpose, an emitter was developed that simulates the human droplet and aerosol emission. It releases a NaCl-water solution. The NaCl serves as a tracer embedded in small-and middle-sized droplets or aerosols that evaporate, while the NaCl nuclei remain in the air and follow the room airflow. The size of the NaCl nuclei is in the range of the size of the sputum nuclei after evaporation. Additionally, re- an analytical model was set up which calculates the time-dependent concentration of droplets and aerosol depending on the distance to the emitter. The model was also used to determine the effect of the emission mode on the measurement results. In addition, a field study was conducted to demonstrate the suitability of the new system to measure the effect of aerosol reduction measures of ventilation and indoor air purification in indoor environments. The setup of the aerosol measurement system consists of one portable emitter and a number of portable recipients that can be positioned in different spatial arrangements with respect to one another. The system is designed to measure the transmission of tracer particles in aerosols and droplets from the emitter to the recipients and to evaluate and compare the infection risk of different configurations with and without protective measures. The emitter developed for this study enables the repeatable emission of aerosols and droplets. A siphon fed two-fluid nozzle (XA-SR 050; BETE Fog Nozzle, Inc.) is used to disperse the tracer solution into an aerosol and droplet spray. The emitted fluid is a solution of distilled water with a certain amount of NaCl (sodium chloride ≥ 99.5 %, p.a., ACS, ISO, Carl Roth). During dispersion, the NaCl is released and acts as a tracer particle dissolved in aerosols and droplets or crystallized as a solid nucleus after evaporation. The tracer solution is kept in a water reservoir with a constant liquid level that is aligned with the outlet of the spray nozzle to avoid hydrostatic pressure differences. This prevents a back flow or fluid leaking from the system between the emissions, which would affect the measurement accuracy. Furthermore, the tracer solution is placed on a microscale (FC-2000, GRAM Group) to measure the total amount of the emitted fluid. Through the two channels, compressed air and tracer solution are fed into the nozzle and are mixed at the outlet ( Figure 1A) . A bypass flow (sheath flow), which is injected through four radial inlets, is integrated and positioned circularly around the nozzle to adjust the exhaled volume of air as well as the momentum of the emitted aerosol. The volume flow through the nozzle and bypass system is controlled independently by pressure regulators (DR021-01-3, Landefeld GmbH, Germany) and solenoid valves (SLP15E4, E.MC Machinery Co., Ltd.). The schematic diagram of the nozzle is shown in Figure 1B . As described in the following chapters, the droplet size distribution and outlet velocity are a function of the mass flow and applied pressure to the nozzle and bypass. An exemplary emission is shown in Figure 2 . The emitter can be operated in a pulsed or continuous emission mode. The number and frequency of the emissions and the duration of the pauses in between are set by an in-house built pulse generator. The head of the emitter is 3D-printed from polylactide (PLA). The basic head geometry is obtained from free3d. com. 25 It is extended by an elliptical mouth opening with an average surface of a woman's mouth during coughing (3.37 cm 2 ). 26 The nozzle is placed at the back of the head so that the emitted aerosol exits through the mouth (see Figure 1A ). Except for the mouth opening, the human appearance of the emitter is only for representative purposes and future studies on face masks. In order to characterize the dependence of emitted aerosols and droplets on the supply pressure and air mass flow, a phase Doppler anemometry (PDA) system (SprayExplorer, Dantec Dynamics GmbH) is used. 27 The measurement point of the focused laser beams is The PDA measurements are conducted with continuous air supply to the nozzle. The supplied air mass flow is controlled using a pressure regulator valve with manometer (DR021-01-3, Landefeld GmbH, Germany) and is monitored using a Coriolis mass flow meter (Promass A, Endress+Hauser AG). The aerosol-absorbing recipients ( To determine the sensitivity, a 0.9% NaCl solution (sodium chloride solution 0.9%, CELLPURE ® , Carl Roth) was pipetted into the measurement system of four recipients. The detectable amount of NaCl was determined by the increase in conductivity. For the determination of the ability of the water lung to dissolve the tracer from the aspirated air, two water lungs were connected in series. The filter efficiency was determined from the ratio of the increase in conductivity in the two lungs. (see following paragraph). Moreover, the data obtained from each recipient were compared to the others to determine the accuracy of the measurement method. The verification measurement followed the measurement procedure stated above. A lead-in time of 300 s was chosen to sufficiently measure the background concentration. The aerosol was released with low momentum (see Table 1 ) and within an emission period of 500 s with 100 ejection cycles to ensure a good signal to noise ratio. The lead-out time was predetermined to 1300 s. Therefore, the en- Between the measurements, the room air was fully ventilated until the conductivity did not further increase and the background concentration of particles was detected anew for each measurement. To further evaluate the new system and to verify its responsiveness to the aerosol reduction measures venting and room air purification, a field study was conducted. The measurements were performed in a conference room with a volume of 100 m 3 and two windows (3 m 2 opening area) on one side of the room (see Figure 4 ). During the measurement procedures, the room was fully closed with sealed windows and doors to minimize the air exchange rate. The approach was the same as described in the measurement procedure and verification measurements. Settings of the emitter, recipients, positioning, lead-in time (300 s), emission period (500 s with 100 ejection cycles), and lead-out time (1300 s) were performed accordingly. Three different measures were conducted, and each measurement was repeated three times: • A measurement without any aerosol-reducing mechanisms. • A measurement by using a room air purifier (Philips AC2882/10) with HEPA filter. The device was positioned centrally near one wall, that has no windows or doors, at a distance of 3 m from the emitter. It was started at the beginning of the lead-in time with an air exchange rate of 333 m 3 h −1 . • A measurement with ventilation. The room was ventilated by opening both windows two times for 120 s after 480 s and 800 s after the beginning of the measurement. The conditions. Therefore, the values of the diffusion rate, which are ad- increase, while the average particle size distribution shifts to smaller particle diameters. A summary of the investigated parameters and the results of the PDA measurements is given in Table 1 . An average diameter of 2.52-4.37 µm was measured. The De Brouckere mean diameter (D (4, 3) ), the mean of the particle size distri- After evaporation, the calculated remaining average tracer particle diameter was 0.83 µm and D (4,3) was 2.7 µm. The encoder resolution of the conductivity meter was 0.01 µS. Thus, the smallest increase detectable by the device corresponds to The time series at selected conditions is displayed in Figure 8 indi- Similarly, a complete decay of the concentration in the investigated time frame is only observed for large dissipation rates. To further describe the scaling of the absorbed concentration with an increasing distance, the PF (2) is computed for the entire simulation time (t 0 = 0 s and t 1 = 1800 s, see Figure 8 ). The scaling of the PF is found to be exponential with the distance as indicated in Figure 9A . To relate this exponential trend to the simulation parameters, the radial dependency is approximated by an exponential function in the range from r = 1 m to r = 5 m. The PF scaling exponent α is found to be almost perfectly proportional to √ ∕ as displayed in Figure 9B . An excellent agreement could be found between the verification measurement and the analytical model. Two exemplary graphs are shown in Figure 10 . The calibrated model parameters for the four F I G U R E 6 Average increase in conductivity for four verification measurements within the same setup: mean values of four recipients, negligible airflow, distance 1.5 m, lead-in time: 300 s, aerosol emission until 800 s, leadout time 1300 s, and overall sampling duration 2100 s. The dashed lines for each measurement represent the basic increase during the lead-in, which are subtracted from the measurement results in the postprocessing F I G U R E 7 Verification measurements: the overall average increase in conductivity after a sampling duration of 2100 s and the standard deviation of the four recipients is shown. Four measurements with four recipients were performed. The distance between the recipients and the emitter was 1.5 m. As tracer solution a 5% NaCl solution was used. A total of 100 exhalations with a duration of 2.5 s and a pause of 2.5 s were performed, during which a total of 6.36 g ± 0.38 g tracer solution was emitted. Thus, the exhalation to pause ratio was 1: Thus, both show a standard deviation of <10%. Two different aerosol reduction measures were compared to a reference measurement without any measures in the same room (see Figure 11 ). All three measurements were repeated three times with four recipients, respectively. The reference measurement showed very similar results and curve progression to the verification measurement described above (see The overall average increase in conductivity after a sampling duration of 2100 s is shown. The dashed vertical lines mark the start 300 s and end 800 s of aerosol emission of each measurement. Each curve is based on three measurements with four recipients, respectively. The yellow curve represents the measurement with no aerosol-reducing measures. The measurement shown by the red curve has been conducted with a room air purifier, filtering the air with a HEPA filter from the start to the end of the measurement. During the ventilation measurement (blue curve), the room was ventilated two times (after 480 s and 800 s) for 120 s (blue highlighted sections). An almost perfect ability to follow the airflow is assumed for all particles smaller than 20 µm after evaporation. 30 In the measurements, a higher mass flow is emitted than during speaking. This increases the humidity in the immediate vicinity of the emitter. It is assumed that this does not influence the results of the measurements significantly due to the distance of 1.5 m between emitter and recipients. In the measurements, the emitted air has not been heated, so the trajectory deviates from body heated exhaled air. In addition, the airflow caused by the plume was not mimicked, so the exposure of the recipient could be underestimated. The error depends on the principle of room air distribution. The errors can be prevented by further improvements of the measurement system. The electrostatic charge of the materials has not been checked. It is therefore possible that charged NaCl particles have deposited on the walls. The aerosol uptake may therefore have been underestimated. In As shown in this field study, of increase of the conductivity in the lead-in time, which than will be subtracted from the overall increase. In Figure 4 , in the verification measurement, the increase due to the background concentration is represented by the dashed lines, whereas in the system response measurement (Figure 11 ), it has already been subtracted from the entire curves respectively. The measurement system is portable and can be placed on seats or chairs in a room, and the positioning can be changed easily within minutes. In order to obtain the most expressive and comprehensive mea- Currently, the spread of viruses in everyday situations is mainly evaluated by flow simulations. However, the flow simulations make strong simplifications and cannot take the many disturbing influences into account, such as local temperature fluctuations, convection through window gaps and door slits, and evaporation of droplets. Therefore, it is important to validate the results of the simulations. This system is very well suited for this purpose, as it simultaneously delivers time-resolved measurement data at several positions in the environment. For this study, the human appearance of the recipients and the emitter is for representative purposes and for an easier positioning only. The physiological human heat emission can be simulated in future studies by heating pads. Furthermore, in a current redesign we plan for heating the liquid phase of the aerosol before atomization in order to prevent a temperature drop due to the evaporation. However, the realistic representation of human plume in experimental investigations is beyond the focus of the current paper. The recipients and physiological human breathing differ from each other, since the inhalation of the recipients is continuous. This is sufficient for the examination of the spreading of aerosols and droplets, as long as a transport of aerosols and droplets mainly driven by large flow structures can be assumed. The local pulsatile flow dynamics due to in-and exhaling processes would not play a significant role in this context. However, for the future potential evaluation of masks, it would be more appropriate to include an inhalation and exhalation rhythm. In this context, the flow dynamics in the direct vicinity of the individuals become more important. The system will be further developed for these applications, and the human appearance will be necessary for a realistic fit of the masks. Medical face masks are currently tested according to the DIN EN standard on requirements and test procedures for medical face masks. Herein, the tests are performed with a particle size of 0.65-7 µm (DIN EN 14683:2019-1). With the emitter, it is possible to generate the corresponding droplet and aerosol distribution and evaluate the masks accordingly. The system could be suitable to evaluate schools or offices as well as event locations, cinemas or public transport. An individual protection strategy could be developed, and the efficiency of protective measures could be quantified. In addition to the measurements of this study, the measurement system has already been tested in a large concert hall, a train, and a dentist's examination room with promising results. Initial measurements outdoors are also in progress. The results of these additional measurements in indoor and outdoor environments will be presented in future studies. As long as a representation of airborne salt crystals is valid in terms of size, the model can also be used to recreate and analyze the propagation processes of other bioaerosols like bacteria, fungi, or pollen and aerosols consisting of inorganic harmful substances. Thus, the measurement system has proven to be reliable due to its high sensitivity and low measurement fluctuation. This has also been demonstrated in the system response measurement. The two different measures ventilation and room air purifying could be represented and evaluated very well by the measurements, where both processes were able to reduce aerosol uptake by about onethird after 2100 s. Despite the expected high fluctuations during ventilation, the standard deviation was only 2.8%. In conclusion, this method is suitable to assess the respiratory hazards of everyday situations in private and public indoor environments, enabling the evaluation of ventilation strategies, air purification technologies, and further protective measures. The valuable and inspiring support of Joshua Gray is gratefully acknowledged. The peer review history for this article is available at https://publo ns.com/publo n/10.1111/ina.12860. 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