key: cord-1054411-9cu57280 authors: Mahjoub Mohammed Merghani, Khansa; Sagot, Benoit; Gehin, Evelyne; Da, Guillaume; Motzkus, Charles title: A review on the applied techniques of exhaled airflow and droplets characterization date: 2020-11-18 journal: Indoor Air DOI: 10.1111/ina.12770 sha: e93e42885236063786aa23c1b3046c485533510b doc_id: 1054411 cord_uid: 9cu57280 In the last two decades, multidisciplinary research teams worked on developing a comprehensive understanding of the transmission mechanisms of airborne diseases. This article reviews the experimental studies on the characterization of the exhaled airflow and the droplets, comparing the measured parameters, the advantages, and the limitations of each technique. To characterize the airflow field, the global flow field techniques ‐High‐speed photography, schlieren photography and PIV‐ are applied to visualize the shape and propagation of the exhaled airflow and its interaction with the ambient air, while the pointwise measurements provide quantitative measurements of the velocity, flow rate, humidity and temperature at a single point in the flow field. For the exhaled droplets, intrusive techniques are used to characterize the size distribution and concentration of the droplets' dry residues while non‐intrusive techniques can measure the droplet size and velocity at different locations in the flow field. The evolution of droplets' size and velocity away from the source has not yet been thoroughly experimentally investigated. Besides, there is a lack of information about the temperature and humidity fields composed by the interaction of the exhaled airflow and the ambient air. and experimental studies that consider the ventilation systems at the room scale and investigating its 2 influence on the droplet dispersion. This review focuses on the first category and analyzes the 3 experimental techniques applied for the characterization of exhaled airflows and droplets. The first 4 part of this review is dedicated to the measurement systems involved in the exploration of the 5 gaseous exhaled flow, while the second part concerns the techniques applied to respiratory droplets. 6 Characterization of the exhaled airflow 7 This section reviews the published papers concerning the applied measurement techniques used to 8 characterize exhalation flows while breathing, sneezing, coughing, and speaking. Here, we have 9 identified the most relevant articles and separated them, technical-wise, into two main categories as 10 follows: global flow-field measurements and pointwise measurements. Table 1 shows the 11 corresponding articles and the main parameters measured. It is important to note that such parameters 12 strongly depend on the adopted technique, and ultimately results are difficult to compare. In the 13 following subsections, we provide an analysis of these applied techniques and discuss their pros and 14 cons. Global flow-field measurements 16 The global flow-field measurements techniques provide information regarding the flow properties The high-speed photography is used for flow visualization and provides information regarding the 3 flow shape and its propagation such as the flow direction, spread angle and propagation velocity. To 4 visualize the airflow, a high-speed camera is used to capture the motion of tracer particles like 5 cigarette smoke or theatrical fog. The used tracer particles are fine enough to closely follow the 6 airflow. 7 This technique was applied by Gupta et al. to identify the boundary conditions of the airflow exhaled 8 during coughing 14 breathing and speaking 15 . In this study, cigarette smoke was used as a tracer 9 assuming that the 0.2 µm smoke particles closely follow the airflow 14 . In order to visualize the 10 airflow, five smoking subjects were photographed using a black background and a light source 11 beneath the subject's face. This technique provides images with visible smoke particle concentration 12 gradient which allows a rough estimation of both airflow direction and spread angle. As shown in 13 Figure 1 , the flow direction is defined as the angle between the flow center and the horizontal. Such 14 angles can be determined by analyzing the side view photography. Table 2 compares the angle values 15 obtained by Gupta 14, 9 with other studies from literature 16, 17, 18, 19 . 16 Likewise, front view photography of the subjects was used to determine the mouth and nose opening 17 areas. The latter information is important to provide boundary condition for flow modeling. Gupta et 18 al. photographed the subject's nose and mouth during coughing 14 speaking 9 and breathing 9 . The 19 values of these areas are shown in Table 1 . Though a noticeable difference in nose and mouth 20 opening area has been found between subjects. 21 Bourouiba et al. 20 used the high-speed imaging technique to analyze sneezing and coughing. The Figure 1 , the first part of the flow near the mouth (prior to the change of flow direction) was referred 2 as the jet phase by Bourouiba et al., while, the subsequent phase was referred as the puff phase. The 3 authors defined the entrainment coefficient  as the slope of the linear relation between the jet radius 4 and the travel distance from the mouth. With this definition, the relation between  and the spread 5 angle   is  =tan(    As for coughing, Bourouiba et al. reported an entrainment coefficient of 6 0.24 in the jet phase which corresponds to a spread angle of 27°. This value matches well with those 7 obtained by Tang et al. 12 and Gupta et al. 14 as shown in Table 2 . Thanks to the high-speed 8 photography technique, the authors identified the value of the puff phase entrainment coefficient, 9 which is almost half of the jet phase one, corresponding to a spread angle of 15°. Likewise, a similar 10 decomposition of the flow into a jet phase and a puff phase was also made for the sneezing activity. The entrainment coefficients values were 0.13 and 0.055 for the jet and puff phases respectively, 12 which corresponds to spread angles of 14.8° and 6.3°. 13 The high-speed photography technique was not only applied on human subjects. Liu and 14 Novoselac 21 built a cough box or cough generator with a volume of 15.6 L. Air was injected inside 15 this box with a square wave temporal profile, and with a time step of 1s. The airflow was visualized 16 by using a fog machine that generates particles with diameters lower than 2.5 µm in size. The 17 analysis of the flow shape revealed that the jet propagates more than 0.2 m in 0.1s. The observation of 18 the flow boundaries allowed the determination of the spread rate, which is defined as the ratio 19 between the radial expansion of the jet and the horizontal propagation. The captured images showed 20 the creation of a circular vortex, also known as the leading tip, in the front of the jet during the first 21 0.15 s. The schlieren photography 23 The schlieren photography technique is based on the variation of the air refractive index with the 24 density. In the case of human exhalation flows, a density gradient is induced by the temperature 25 difference between the ambient air and the exhaled air. Although the exhaled air temperature varies 26 with the climatic conditions, it is usually higher than the ambient air. In laboratory conditions with 27 50% relative humidity and 20°C, the air is exhaled from the nose at a temperature value close to 34°C 28 22 . The schlieren technique can be used to study the flow shape and analyze its propagation in the Accepted Article 1 surrounding environment. Tang et al. implemented this technique to observe the breathing 11 , 2 sneezing 11 and coughing 23 of 20 healthy volunteers (10 females and 10 males). They used a high-3 speed camera to capture images of exhalation that were later automatically processed. The 4 automation of the image processing allowed the authors to analyze a large number of images and to 5 extract data about the visible projected area and the propagation distance at a high frequency. Tang et 6 al. 23 reported that the average horizontal propagation distance of coughing was in the range of 0.16 m 7 to 0.64 m however, when they reprocessed the images in another study 11 considering the propagation 8 on the main flow direction they found that the maximum distance is 0.7 m and the maximum derived 9 velocity is 5 m.s -1 . Pepper sneeze stimulus was used to help six volunteers to sneeze and the sneeze 10 puff traveled 0.6m with a propagation velocity of 4.5 m.s -111 . By analyzing the schlieren imaging for 11 nasal and mouth breathing, they found that the air travels 0.6m with a velocity of 1.4 m.s -1 in the case 12 of nasal breathing, while it propagates more slowly at a velocity of 1.3 m.s -1 over longer distance -13 0.8m-in the case of mouth breathing 11 . When analyzing these data, it is important to keep in mind 14 that these distances are characterized by the temperature/density gradient and not the local airflow 15 velocity. Further results of the breathing flow shape and direction obtained by 11 are presented in 16 Table 2. Another study by Xu et al. 16 used schlieren photography technique to visualize the breathing airflow 18 in standing and lying positions of 18 healthy subjects at 23°C room temperature. They noticed that the 19 flow shape, direction and propagation seem to vary from one person to another. However, they were 20 able to determine average values of the jet direction and spread angle for mouth only and nose only 21 breathing which are reported in Table 2 . Images recorded by the high-speed camera demonstrated that The Particle Image Velocimetry (PIV) technique has been widely used in the field of indoor air 3 because it provides essential detailed quantitative information on the flow fields 24 . In this PIV 4 technique, a pulsed laser light sheet illuminates the tracer particles in a measurement plane, and a 5 synchronized CCD camera acquires two single exposed images taken shortly after one another. By 6 comparing these two pictures, the traveled distance of individual particles can be determined, and it 7 corresponds to the displacement field of tracer particles. The velocity field is then calculated by 8 dividing the displacement field by the time step of a few microseconds order. The PIV has been used 9 to characterize the airflow field formed by coughing 25,19,26 , 27 and speaking 19, 27 . In these studies, the 10 subjects were asked to cough into a chamber which was filled with submicronic particles of titanium VanSciveret al. 25 asked 10 males and 19 females to cough into a 120 cm × 76 cm × 67 cm chamber to 20 analyze the airflow field and the maximum coughing velocity. They recorded a wide range for this 21 maximum velocity; from 1.15 m.s -1 to 28.8 m.s -1 while the average maximum velocity was 10.2 m.s -1 . The authors found that the maximum velocity of the airflow usually happens at a distance of 3.9 mm Considering these values of the velocity and the size of the box the authors suspected that the results Table 2 . 8 As discussed above, health hazards arise when using the PIV technique on human subjects, it 9 requires to apply safety procedures, in particular those to make sure that the tracer particles are that although the flow shape is almost the same, the propagation distance for the (test M) was higher 15 than (test F) and the diffusion angle was wider, 20.5° for (test M) and 22.7° for (test F). 16 Pointwise measurements 17 Here, pointwise measurements refers to quantitative measurements performed by using probes at a al. 14 measured the airflow rate of 25 healthy subjects for coughing, breathing and speaking using a 4 spirometer. They found that the cough airflow rate can be described using a combination of gamma 5 probability distribution functions while the normal breathing airflow rate follows a sine function over 6 time and this function depends on the subject height and weight 9 . The value obtained for the duration 7 of a single cough was in the range of 0.26-0.78 s, which is in accordance with the value obtained by 8 means of high-speed photography of 0.3 s by Bourouiba et al. 20 . The airflow rate during speaking 9 might be the most difficult to measure as it depends on the pronounced sounds. To characterize the 10 airflow rate for speaking, Gupta et al. 9 used a spirometer to measure the exhaled airflow during three 11 exercises: counting from 1 to ten, pronouncing six letters and reading a passage. Pronouncing T letter 12 resulted in 2 L.s -1 flowrate and it was the highest expelled flowrate. Consequently, pronouncing 13 number two and ten produced more airflow. The peak flow rate measured during passage reading was 14 1.6 L.s -1 . In addition to the airflow measurements, Xu et al. 16 Where U m is the centerline velocity and isthe distance from the mouth/nose. and are constants 25 which depend on gender and mode of breathing (mouth or nose). As discussed above, manikins can be used to overcome technical constrains but there are differences The authors compared those results with the results of the previous studies 11,23 obtained by applying 14 the schlieren photography technique and they suggested that using the schlieren technique might 15 overestimate the propagation distance of the flow. This overestimation might be attributed to 16 temperature diffusion rather than the travel of the gas. particles exhaled from 37 seasonal-influenza infected patients. The study showed that fine particles -8 smaller than 5 µm-contain 8.8 fold more viral copies than coarse particles -bigger than 5µm-. 9 Similarly, Lindsey et al. 39 found that 42% of influenza RNA is detected in particles smaller than 1 10 µm, while 23% is in particles of 1 to 4 µm, and 35% in coarse particles bigger than 4µm. Fabian et 11 al. 40 conducted another observational study on 12 laboratory-confirmed influenza patients. In this 12 study, they observed that during tidal breathing, 87% of the particles sampled from patients' After their release, the droplets go through a very rapid evaporation process and transform to dry 20 residues which are mainly composed of NaCl and organic component 41 . Table 4 is an important point when 9 considering any aerosol size measurement technique. As each measurement technique is based on a 10 specific physical property, the equivalent diameter is "the diameter of a sphere having the same value 11 of a specific physical property" 43 . For example, the Aerodynamic Particle Sizer (APS) measures the 12 particle aerodynamic equivalent diameter which is defined as the diameter of a sphere having a 13 density of 1000 kg.m -3 and the same gravitational settling velocity as the measured particle. The 14 optical diameter corresponds to the diameter of the spherical particle scattering the same light amount 15 as the analyzed particle. Baron et al. 43 define the mobility equivalent diameter as "the diameter of a 16 sphere with the same mobility as the particle in question". Meanwhile, the geometric diameter is the 17 diameter of the spherical particle which is usually obtained through direct measurement of the particle using 18 a microscope or through a geometric analysis of the particle's image.Thus, the equivalent diameter should breathes to gather the exhaled air. Then, the captured air passes through a conventional 5µm cut-off 20 diameter slit sampler to collect large droplets. A water vapor condensation process on the fine 21 particles, less than 5µm, was then used to grow the size of these particles so that they could be 14 Other techniques 15 In addition to the techniques mentioned above, there are a few less frequently used techniques. 16 Loudon and Roberts 46 tried to directly observe with a microscope the dry residues collected on a filter 17 at the outlet of the box described in section 0 to collect the dry residues suspended in the air, inside 18 the box. Authors observed that dry residues under 1 µm were not detectable although the lower 19 detection limit of the microscope was 0.5 µm. to measure the fine dry residues of the droplets generated by coughing. The particle size range of the 5 SMPS is 0.02 µm to 0.6 µm. In this study Yang et al. SMPS has used in addition to APS to cover a 6 wider size range, but almost no particles were observed in the size range of the SMPS. Most of these techniques are used to determine the size distribution of the exhaled droplets. Using a 10 nonintrusive technique eliminates the bias due to physical phenomena such as evaporation and 11 condensation during the sampling process. This makes these nonintrusive techniques particularly 12 suitable for measuring the initial droplets size distribution and for studying the evaporation process. High-speed photography 16 High-speed photography technique was previously discussed in section 0 for the characterization of Accepted Article 1 The particle concentration was 170 part.L -1 , and the average size was 1.7 µmat 1.5 cm from the 2 subject's mouth. Shao et al. observed irregular particle shapes in addition to well-rounded droplets. 3 By defining a roundness threshold, the authors estimated that 33% of the particles emitted at a 4 distance of 1.5 cm are not droplets. which highlights the necessity for more quantitative measurement on the exhaled droplets motion and 10 evaporation. In our review, we classified the exhaled airflow measurements techniques into two categories: global 13 flow-field measurements, and pointwise measurements. Three global flow-field measurement 14 techniques were identified, namely the high-speed photography, schlieren photography and Particle 15 Image Velocimetry (PIV). These techniques provide information on the whole flow-field, and helps 16 understanding the interactions between the exhaled flow, the thermal plume and the room airflow. The high-speed photography and the schlieren photography were applied to study the flow shape and Accepted Article 1 The second part of the article summarizes the techniques applied for the characterization of the 2 exhaled droplets and dry residues. These techniques can be used to determine three important 3 parameters: droplet or dry residues sizes distribution, droplet velocity distribution, and traveled 4 distance. Apart for three articles, all the reviewed articles in this section measuredthe size distribution 5 of the droplets and/or the dry residues. Solid impaction, droplet deposition analysis, optical particle 6 counter, aerodynamic particle sizer, electrical low-pressure impactor scanning Mobility Particle Sizer, 7 Liquid impingers and filtration are the techniques used to measure the size distribution of the dry 8 residues. 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