key: cord-0734306-11bh9v5i
authors: Wang, Qiaoqiao; Gu, Jianwei; An, Taicheng
title: The emission and dynamics of droplets from human expiratory activities and COVID-19 transmission in public transport system: A review
date: 2022-05-24
journal: Build Environ
DOI: 10.1016/j.buildenv.2022.109224
sha: fde67d689943e15f86b6d6ef12ce894b10758ffe
doc_id: 734306
cord_uid: 11bh9v5i

The public transport system, containing a large number of passengers in enclosed and confined spaces, provides suitable conditions for the spread of respiratory diseases. Understanding how viruses are transmitted in public transport environment is of vital importance to public health. However, this is a highly multidisciplinary matter and the related physical processes including the emissions of respiratory droplets, the droplet dynamics and transport pathways, and subsequently, the infection risk in public transport, are poorly understood. To better grasp the complex processes involved, a synthesis of current knowledge is required. Therefore, we conducted a review on the behaviors of respiratory droplets in public transport system, covering a wide scope from the emission profiles of expiratory droplets, the droplet dynamics and transport, to the transmission of COVID-19 in public transport. The literature was searched using related keywords in Web of Science and PubMed and screened for suitability. The droplet size is a key parameter in determining the deposition and evaporation, which together with the exhaled air velocity largely determines the horizontal travel distance. The potential transmission route and transmission rate in public transport as well as the factors influencing the virus-laden droplet behaviors and virus viability (such as ventilation system, wearing personal protective equipment, air temperature and relative humidity) are also discussed. The review also suggests that future studies should address the uncertainties in droplet emission profiles associated with the measurement techniques, and preferably build a database based on a unified testing protocol. Further investigations based on field measurements and modeling studies into the influence of different ventilation systems on the transmission rate in public transport are also needed, which would provide scientific basis for controlling the transmission of diseases.

Introduction 1 The Coronavirus disease 2019 (COVID- 19) was first detected in 2019, and was soon declared as a 2 global pandemic by the World Health Organization (WHO) on March 11, 2020. By April 30, 2022, 3 510 million confirmed cases and 6.2 million deaths were reported by WHO [1] . The unexpected 4 rapid spread of COVID-19 worldwide was largely associated with the convenient and advanced 12 The public transport system is a special indoor environment with many passengers in an enclosed 13 and confined space, favoring the propagation of diseases between passengers. There is ample 14 evidence showing that the public transport environment contributes to the transmission of influenza 15 and coronaviruses [6] . The transmission of SARS-CoV-2 in a variety of public transport systems 16 has also been recently reported, including aircraft cabins [7, 8] , buses [9, 10], trains [11, 12] and 17 cruise ships [13] . The ventilation system in public transport needs to provide conditioned air 18 (heated or cooled) and to ensure passenger thermal comfort and an acceptable cabin air quality. 19 Due to high population density, it requires high energy consumption. The ventilation systems and 20 air distribution in common public transport systems such as buses, trains, airplanes are also 21 different from the general indoor environment, and the main exposure and transmission routes of 22 respiratory transmissible diseases are not fully understood yet. 23 Historically, public health communities have discovered two major transmission routes for 24 respiratory-borne diseases, namely, short-distance droplet transmission and contact with 25 contaminated surfaces (or fomites) [14, 15] . At the early COVID-19 transmission stage, 26 government and health agencies recommended social distancing to avoid the short-distance droplet 27 transmission. Regarding the fomite route, hand washing and surface disinfection were 28 recommended to reduce the possibility of picking up viruses via touching the contaminated 29 surfaces. These measures are essential but may not be sufficient, as small droplets can evaporate 30 quickly and remain in the air for a prolonged time. Aerosol scientists argued that viruses may also 31 J o u r n a l P r e -p r o o f

In this review paper, literature was collected by searching in Web of Science and PubMed using 23 keywords followed by manual screening for suitable topics and scopes. The keywords used in Web 24 of Science included "emission", "aerosol", droplet", "respiratory", "coughing", "sneezing", 25 "speaking", "evaporation", "dynamic", "SARS-CoV-2", "COVID-19", "bus", "subway", "metro", 26 "underground", "vehicle", "train", "rail", "airplane", "flight", "public transport/transportation", 27 "face mask", "filtration", and "virus survival". For COVID-19 transmission in public transport, 28 literature search in PubMed was also conducted using keywords of "SARS-CoV-2", "COVID-19", 29 J o u r n a l P r e -p r o o f "bus", "subway", "metro", "underground", "vehicle", "train", "rail", "airplane", and "flight". Some 1 of the papers were collected by cross-referencing other reviewed papers. Three main methods have been applied to study the droplet number and size from expiratory 10 activities: optical microscopy, on-line particle size spectrometer and on-site detection techniques.

Optical microscopy, applied in early studies, uses collection media such as glass slides to collect 12 deposited droplets, and then the stains of droplets are counted and measured with a microscope [20,

13 25]. Dyes are commonly applied in the mouth to leave a colorful stain, while no dye is needed if 14 water-sensitive paper is used. Although the microscope is capable of measuring stains as small as 15 0.25 -0.50 μm, the method is only effective in measuring droplets > 10 µm, because small droplets 16 do not deposit effectively on the collection media but instead remain airborne for an extended time.

Only a few studies endeavored to collect small droplets (< 10 µm) by air sampling followed by 18 microscope analysis (e.g. Duguid [20] , Loudon and Roberts [25] ). 19 The on-line particle size spectrometer uses an aerosol instrument with a tubing system to measure 20 particles in a wide size range, focusing on small ones in general [26] [27] [28] . For instance, the in a size range of several nanometers to sub-micrometer. It is worth noting that droplets may dry 25 out in the tubing system, leading to their size reduction, while droplets near the upper size detection 26 limit (e.g. 10 -20 μm) may be lost due to gravitational settling and inertia in the tubing system.

More recently, some studies utilized on-site detection techniques such as Interferometric Mie 28 Imaging and laser particle size analyzer to determine droplet sizes (e.g. Chao Table 1 and Table 2 summarize the characteristics of the droplets emitted from respiratory activities   4 including breathing, speaking, singing, coughing, and sneezing from previous studies. For 5 breathing, results are only available based on the on-line particle size spectrometer, limited to 6 particles with diameters smaller than 10 -20 µm. Only the study by Asadi et al. [26] reports En of 7 0 -2 s -1 (0 -11 L -1 ), which is at the lower end of the range of En for speaking or singing. Most 8 studies report droplet concentrations in the exhaled breath in the range from 14 to 1.7×10 4 L -1 . The 9 wide range spanning three orders of magnitude can be largely attributed to individual differences, 10 as some individuals are "super emitters" [31] . The variation can also be partly explained by the 11 change in breathing maneuver, with higher concentrations for breathing to residual volume or with 12 airway closure [32, 33] . The droplet sizes from breathing were mainly below 1.0 µm, with peaks 13 at 0.07 µm, 0.2 -0.5 µm and 0.75 -1.0 µm in different studies. It is also possible that peaks at 14 larger size could be omitted due to the detection limit associated with the on-line particle size 15 spectrometer. 16 For speaking or singing, emissions were measured when counting 1 to 100 in English, which was 17 defined as one event in most studies. Reported droplet number emission rates (En) are in a wide 18 range of 100 -6720 event -1 [20, 21, 26, 29, 34] . The size distribution of emitted droplets also varies 19 largely, with the main peaks located at ~ 1 µm, 6 µm, 12 µm, 63 µm, and 100 µm in different 20 studies. Three studies have identified a bimodal size distribution, showing both a major peak at < 21 10 µm and a sub-peak at > 50 µm [28, 29, 34] . The study by Asadi et al. [26] suggested that En is 22 linearly correlated with the vocalization loudness but the corresponding number size distribution 23 is independent of loudness. Intensive activities such as coughing and sneezing tend to have higher En as well as larger variation. 26 However, considering the duration, total emissions from breathing and speaking can be of more 27 importance. In addition, most respiratory activities can produce droplets both smaller and larger 28 than 10 µm in size, which will have different aerodynamic behaviors and subsequent exposure 29 routes in the public transport. It is also worth noting that the major modes are all below 10 µm for Some studies suggest that v0 values from coughing and speaking are higher for males than females, 1.64 m s -1 ) than males (1.08 -1.56 m s -1 ). However, the evidence is too limited to conclude any 10 gender difference. After expiration, the air velocity follows an exponential decay over the distance 11 from the mouth [43] . [1] 23 where ρp is the particle density, dp is the particle diameter, g is the gravitational acceleration, Cc is 24 the slip correction factor which is important for small particles, and η is the viscosity of air.

The particle VTS in the transition region ( 

where ρg is the air density, and CD is the drag coefficient in Newton's resistance law. In the 2 transition region, CD has a complicated relationship with Re. However, the VTS can be calculated 3 empirically based on the value of CDRe 2 as shown in Eq. 3.

The evaporation of a pure water droplet is controlled by two competing processes: (a) ambient 5 water vapor arrives and sticks onto the droplet surface, and (b) water molecules evaporate and leave 6 the droplet surface. The complete evaporation time of a water droplet (tE) in still air is a function 7 of the particle diameter, particle density, temperatures of the droplet and the ambient air, as well as 8 vapor pressures on the droplet surface and in the ambient air [45] . It can be calculated by Eq. 4:

where R is the gas constant, Dv is the diffusion coefficient of water vapor, M is the molecular weight 11 of water, p∞ and pd are the water vapor pressures in the ambient air and on the droplet surface, 12 respectively, T∞ and Td are the temperatures of the ambient air and the droplet, respectively.

However, the exhaled droplets contain some solutes. For instance, the major components of mucus 14 include Na + , Cl -, K + , lactate and glycoprotein [46] , which would form a droplet nuclei with a 15 diameter of about half of the original droplet size after evaporation [47] . The evaporation time for 16 a droplet to form a nucleus about half of its original size (half evaporation time, t1/2E) can be 17 calculated by Eq. 5:

Based on Eq. 1 -3, the VTS is about 2.1 m s -1 for a 500-µm droplet, 0.25 m s -1 for a 100-µm droplet, 6.4 s for the 100-µm droplet, 5.2×10 2 s for the 10-µm droplet, to 5.3×10 4 s for the 1-µm droplet.

This clearly indicates that the residence time of a droplet in still air is largely controlled by the 24 droplet size. It is also worth noting that it takes ≥ 5.2×10 2 s for droplets in the size range of < 10 25 J o u r n a l P r e -p r o o f µm to fall onto the ground, which may be sufficiently long for them to be inhaled and lead to 1 significant exposure. 100-µm droplet and 3.3×10 2 s for a 500-µm droplet. In other words, droplets < 10 µm evaporate 12 almost instantaneously, while larger droplets (> 100 µm) take a significantly longer time to 13 evaporate. In addition, high ambient RH slows down the droplet evaporation, while low RH 14 accelerates the evaporation. The t1/2E for the same droplet increases by one order of magnitude 15 when RH increases from 0% to 90%. The calculations indicate that, in the still ambient air under 16 typical indoor temperature and RH, large respiratory droplets (> 100 µm) settle and fall onto the 17 ground quickly with a limited airborne time (commonly less than a few seconds), while small 18 droplets (roughly < 10 µm) evaporate completely and form the droplet nuclei within one second. 19 The decrease in size due to the evaporation prolongs the droplet residence time in the air. Several sophisticated models have been applied to simulate the transport and related dynamics of 2 droplets emitted from respiratory activities, as summarized in Table 3 . Among them, physical 3 models, mathematical models and Lagrangian models consider several forces acting on the droplets 4 in still ambient air, such as the gravitational force, the drag force, the buoyancy force, etc., and take 5 into account the droplet evaporation in most cases [48] [49] [50] [51] . The exhaled air is commonly treated as 6 a turbulent jet with varying initial velocities (ranging from 1 to 50 m s -1 ) entering the still ambient 

When the air is exhaled in a horizontal direction in still air condition, the droplet dynamics can be 12 simplified into evaporation, vertical falling, and horizontal travel, which are heavily influenced by 13 the droplet size, the initial air velocity and RH. As illustrated in Fig. 1 , the evaporation rate is 14 controlled by droplet size, with small droplets evaporating rapidly to form dried nuclei and large 15 droplets evaporating slowly. Note that there is no absolute division for small and large droplets as 16 it is also influenced by environmental factors (temperature, RH, etc.).

Vertically, large droplets fall onto the ground quickly because of gravity [21, 50]. Small droplets 18 have a low gravitational falling velocity, and the reduction in size due to rapid evaporation helps to travel a short distance horizontally. The distinct behaviors of droplets in the exhaled jet can be 18 explained by the relaxation time (the time required for a droplet to adjust its velocity to new forces), 19 which is proportional to the square of the diameter [45], e.g., 3.1×10 -4 s, 3.1×10 -2 s and 6.7 s for 1 droplets of 10 μm, 100 μm and 1500 μm, respectively. Therefore, small droplets quickly follow the 2 exhaled air stream while very large droplets tend to maintain their own velocity. Moreover, while 3 high RH promotes the gravitational falling, it also leads to a longer horizontal travel distance in 4 still air [50].

As demonstrated above, large, medium and small droplets have different transport and dynamic 6 patterns. Assuming a typical scenario with two persons facing each other at a short distance, 7 questions arise as to whether this scenario causes the exposure to expiratory virus-laden droplets? 8 If yes, which droplet size and initial velocity (corresponding to different expiratory activities) (1) Droplets > 400 μm travel > 2 m.

(2) Droplets of 75 -400 μm travel the shortest distance (< 2 m) and fall onto the ground rapidly.

(3) Droplets < 75 μm follow the air stream and be widely dispersed.

(4) Regarding exposure, the airborne route is more important than the large droplet route. Modeling studies on dynamics of respiratory droplets in public transport are presented in Table 4 . (1) The through flow and back door exhaust (case 3) has the highest droplets removal ability but also the longest dispersion distance.

(2) The no through flow and lower exhaust (case 2) shows the minimum impact to other passengers.

High speed train CFD 4 types of air suppy diffusers on the sidewall or ceiling; outlets on the bottom of sidewalls (1) Gas and particle show different dispersion patterns with the same diffisuer.

(2) Diffuser type 1 is best in restricting gas from dispersing to other passengers. (3) Diffuser type 2 and 3 lead to smaller average particle volume fraction in breathing zone. Overall, current literature points to high transmission rates on coach or tour buses, which can be 1 attributed to the insufficient ventilation [18] and prolonged time of close contact among passengers 2 (ranging from a few hours to a few days). Based on very limited evidence, it appears that the use 3 of transit bus or school bus is associated with much lower transmission risk, which might be 4 explained by the wearing of face mask, shorter riding duration and frequent opening of bus doors.

On the other hand, epidemiological study on city transit bus may be more challenging as it is 6 difficult to collect the riding information and trace the passengers. High transmission rates occurred at close distance, e.g., 3.8% for those sitting within two rows 1 from the index case compared to an overall transmission rate of 0.2% [78] , and 9.2% for those 2 sitting adjacent to the index case compared to an overall transmission rate of 0.33% -0.60% [12] . 3 Swadi et al. [7] found that all the infected cases were seated within two rows and two columns in the air [102] . In other words, low RH favors the airborne transmission. It is recommended to 6 keep the indoor RH in the range of 40% -60%, which lowers the virus survival rate and infectivity, 7 reduces the risk of airborne transmission, and is also comfortable to humans [103]. 137.5 µm deposit around 5 s and 3 s, respectively [38] . The horizontal travel distance for droplets 18 is limited to 0.31 m. It must be noted that such influence can be very case-specific, as the particle 19 dynamics can also be affected by the temperature distribution, the relative locations between the 20 infection source and susceptible subject, the initial droplet velocity as well as the indoor air flow susceptible subject [39] .

Under the ceiling-return type of airflow, the droplets first transport downward due to gravity and droplets < 45 µm is across the whole room width (2.4 m) [38] , which is much greater than in the 2 case of unidirectional downward flow or still air condition [53] . 3 Under the unidirectional upward air flow, which is also associated with displacement ventilation, 4 the temperature difference between exhaled air and ambient air was smallest in the upward air flow, 5 which caused the exhaled air to rise less vertically than in the mixing ventilation or without 6 ventilation conditions [43] . (1) A considerable number of droplets in both super-micron and sub-micron size ranges are emitted 7 from human respiratory activities, except breathing which produces droplets mainly < 1.0 μm. 8 Droplet size and number as well as the exhaled velocity vary between different respiratory activities. in the coach/tour bus (17%, 35.3%, 44% and 92%) were much higher than in a school bus and 3 transit bus, which might be attributed to the differences in the opening of windows or doors, the 4 co-travel (exposure) time and wearing of face mask. Evidence from New York city indicates that 5 higher subway ridership was associated with increased COVID-19 infection rate and mortality.

The transmission rates in airplanes were 0 -10% (median: 0.36%) and 0.32 % in high-speed trains 

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