Mechanisms of airborne transmission
The COVID-19 pandemic has highlighted controversies and unknowns about how respiratory pathogens unfold between hosts. Historically, it used to be belief that respiratory pathogens unfold between individuals through tremendous droplets produced in coughs and through contact with rotten surfaces (fomites). Nonetheless, just a few respiratory pathogens are known to unfold through tiny respiratory aerosols, which can trudge and tear in air flows, infecting individuals that inhale them at short and long distances from the infected person. Wang et al. overview contemporary advances in determining airborne transmission gained from studying the unfold of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections and other respiratory pathogens. The authors recommend that airborne transmission would be the dominant procure of transmission for just a few respiratory pathogens, alongside with SARS-CoV-2, and that extra determining of the mechanisms underlying an infection from the airborne route will higher whine mitigation measures.
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Publicity to droplets produced in the coughs and sneezes of infected individuals or contact with droplet-rotten surfaces (fomites) were widely perceived as the dominant transmission modes for respiratory pathogens. Airborne transmission is historically defined as involving the inhalation of infectious aerosols or “droplet nuclei” smaller than 5 μm and mainly at a distance of >1 to 2 m away from the infected particular person, and such transmission has been regarded as relevant factual for “uncommon” diseases. Nonetheless, there is sturdy proof supporting the airborne transmission of many respiratory viruses, alongside with severe acute respiratory syndrome coronavirus (SARS-CoV), Heart East respiratory syndrome (MERS)–CoV, influenza virus, human rhinovirus, and respiratory syncytial virus (RSV). The obstacles of passe views of droplet, fomite, and airborne transmission were illuminated at some stage in the COVID-19 pandemic. Droplet and fomite transmission of SARS-CoV-2 alone can now not narrative for the a quantity of superspreading events and variations in transmission between indoor and outdoors environments noticed at some stage in the COVID-19 pandemic. Controversy surrounding how COVID-19 is transmitted and what interventions are important to abet an eye on the pandemic has revealed a severe need to higher perceive the airborne transmission pathway of respiratory viruses, which can allow for higher-educated options to mitigate the transmission of respiratory infections.
Respiratory droplets and aerosols would per chance presumably successfully be generated by varied expiratory activities. Advances in aerosol size options, much like aerodynamic and scanning mobility particle sizing, dangle proven that nearly all of exhaled aerosols are smaller than 5 μm, and an fantastic share are <1 μm for many respiratory activities, alongside with those produced at some stage in respiratory, speaking, and coughing. Exhaled aerosols happen in more than one dimension modes that are linked with varied expertise websites and manufacturing mechanisms in the respiratory tract. Even supposing 5 μm has been old historically to distinguish aerosols from droplets, the scale distinction between aerosols and droplets must be 100 μm, which represents the very most attention-grabbing particle dimension that can live suspended in composed air for better than 5 s from a high of 1.5 m, assuredly reach a distance of 1 to 2 m from the emitter (depending on the rate of airflow carrying the aerosols), and would per chance presumably successfully be inhaled. Aerosols produced by an infected particular person would per chance presumably maintain infectious viruses, and reviews dangle proven that viruses are enriched in tiny aerosols (<5 μm). The transport of virus-laden aerosols is tormented by the physicochemical properties of aerosols themselves and environmental components, alongside with temperature, relative humidity, ultraviolet radiation, airflow, and air whisk alongside with the circulation. As soon as inhaled, virus-laden aerosols can deposit in varied parts of the respiratory tract. Bigger aerosols are inclined to be deposited in the upper airway; alternatively, smaller aerosols, though they'd presumably successfully be deposited there, can penetrate deep into the alveolar effect of the lungs. The solid stop of air whisk alongside with the circulation on transmission, the definite incompatibility between indoor and outdoors transmission, successfully-documented long-range transmission, the noticed transmission of SARS-CoV-2 with out reference to the usage of masks and detect safety, the excessive frequency of indoor superspreading events of SARS-CoV-2, animal experiments, and airflow simulations present solid and unequivocal proof for airborne transmission. Fomite transmission of SARS-CoV-2 has been chanced on to be a ways much less atmosphere friendly, and droplets are very most attention-grabbing dominant when persons are within 0.2 m of each other when speaking. Even supposing both aerosols and droplets would per chance presumably successfully be produced by infected individuals at some stage in expiratory activities, droplets drop speedy to the ground or surfaces within seconds, leaving an enrichment of aerosols over droplets. The airborne pathway likely contributes to the unfold of other respiratory viruses whose transmission used to be previously characterised as droplet driven. The World Neatly being Organization (WHO) and the US Centers for Disease Withhold watch over and Prevention (CDC) dangle officially acknowledged the inhalation of virus-laden aerosols as a predominant transmission mode in spreading COVID-19 at both short and long ranges in 2021.
Airborne transmission of pathogens has been vastly underappreciated, mostly thanks to an inadequate determining in regards to the airborne habits of aerosols and never much less than in part thanks to the misattribution of anecdotal observations. Given the shortcoming of proof for droplet and fomite transmission and the an increasing form of solid proof for aerosols in transmitting a quantity of respiratory viruses, we must acknowledge that airborne transmission is a ways more prevalent than previously acknowledged. Given all that we’ve discovered about SARS-CoV-2 an infection, the aerosol transmission pathway must be reevaluated for all respiratory infectious diseases. Extra precautionary measures must be applied for mitigating aerosol transmission at both short and long ranges, with particular consideration to air whisk alongside with the circulation, airflows, air filtration, UV disinfection, and conceal fit. These interventions are severe tools for ending the hot pandemic and preventing future outbreaks.
Virus-laden aerosols (<100 I1/4m) are first generated by an infected individual through expiratory activities, through which they are exhaled and transported in the environment. They may be inhaled by a potential host to initiate a new infection, provided that they remain infectious. In contrast to droplets (>100 I1/4m), aerosols can linger in air for hours and tear beyond 1 to 2 m from the infected particular person that exhales them, inflicting modern infections at both short and long ranges.
CREDIT: N. CARY/SCIENCE
The COVID-19 pandemic has revealed severe knowledge gaps in our determining of and a necessity to change the passe uncover about of transmission pathways for respiratory viruses. The long-standing definitions of droplet and airborne transmission develop now not narrative for the mechanisms in which virus-laden respiratory droplets and aerosols tear during the air and lead to an infection. In this Overview, we focus on contemporary proof in the case of the transmission of respiratory viruses by aerosols—how they are generated, transported, and deposited, apart from to the components affecting the relative contributions of droplet-spray deposition versus aerosol inhalation as modes of transmission. Improved determining of aerosol transmission triggered by reviews of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) an infection requires a reevaluation of the predominant transmission pathways for other respiratory viruses, which can allow higher-educated controls to diminish airborne transmission.
Over the past century, respiratory viruses were regarded as unfold mainly through tremendous respiratory droplets, produced in the coughs and sneezes of infected individuals that deposit on the mucous membranes of the eyes, nose, or mouth of potential hosts (droplet transmission) or that deposit on surfaces that are then touched by potential hosts and transferred to mucous membranes (fomite transmission). Such droplets are belief to drop to the ground within 1 to 2 m of the infectious person—a key assumption old by most public health companies in recommending a protected distance from individuals infected with respiratory viruses. Regarded as much less traditional, airborne transmission refers to the inhalation of infectious aerosols or “droplet nuclei” (droplets that evaporate in the air), in total defined to be smaller than 5 μm and traveling distances of >1 to 2 m away from the infected particular person. Aerosols are diminutive liquid, real, or semisolid particles that are so tiny that they live suspended in air. Respiratory aerosols are produced at some stage in all expiratory activities, alongside with respiratory, speaking, singing, shouting, coughing, and sneezing from both wholesome individuals and individuals with respiratory infections (1–4).
The historical definition of airborne transmission ignores the likelihood that aerosols would per chance presumably successfully be inhaled at conclude range to an infected person, where publicity is more likely as a outcome of exhaled aerosols are more concentrated nearer to the person emitting them. Furthermore, rather than the worn definition of 5 μm, it has now not too long ago been suggested that the scale distinction between aerosols and droplets must be up-to-the-minute to 100 μm, as this distinguishes between the two on the belief of their aerodynamic habits (5–7). Particularly, 100 μm represents the very most attention-grabbing particles that live suspended in composed air for >5 s (from a high of 1.5 m), tear beyond 1 m from the infectious person, and would per chance presumably successfully be inhaled. Even supposing droplets produced by an infectious particular person through coughing or sneezing would per chance presumably lift an infection at short distances (<0.5 m), the amount and viral load of aerosols produced through speaking and other expiratory activities are important better than those of droplets (8–10). Aerosols are tiny ample to linger in air, procure in poorly ventilated spaces, and be inhaled at both short and long ranges, calling for an urgent need to encompass aerosol precautions in contemporary respiratory disease abet an eye on protocols. Right during the COVID-19 pandemic, controls dangle focused mainly on preserving in opposition to droplet and fomite transmission, whereas the airborne route has required plan more proof sooner than controls would per chance presumably successfully be added to guard in opposition to it.
Debates surrounding the relative importance of assorted transmission modes in spreading respiratory disease dangle spanned centuries. Sooner than the 20th century, infectious respiratory diseases were belief to unfold by “pestilential particles” released by infected individuals (11, 12). This uncover about of airborne transmission used to be brushed aside in the early 1900s by Charles Chapin, who claimed that contact used to be the manager route for respiratory disease transmission, with spray-borne (droplet) transmission being an extension of contact transmission (13). Chapin used to be troubled that citing transmission by air would fright individuals into exclaim of no process and displace hygiene practices. Chapin erroneously equated infections at conclude range with droplet transmission—neglecting the truth that aerosol transmission also occurs at short distances. This unsupported assumption became fashioned in epidemiological reviews (14), and mitigation options for controlling respiratory virus transmission dangle since occupied with limiting droplet and fomite transmission (15). These kinds of options are also in part efficient for limiting aerosol transmission, main to the fake conclusion that their efficacy proved droplet transmission.
Despite the assumed dominance of droplet transmission, there is sturdy proof supporting the airborne transmission of many respiratory viruses, alongside with measles virus (16–18), influenza virus (19–24), respiratory syncytial virus (RSV) (25), human rhinovirus (hRV) (9, 26–28), adenovirus, enterovirus (29), severe acute respiratory syndrome coronavirus (SARS-CoV) (30, 31), Heart East respiratory syndrome coronavirus (MERS-CoV) (32), and SARS-CoV-2 (33–36) (Table 1). Airborne transmission has been estimated to narrative for roughly half of the transmission of influenza A virus in a single look of a family surroundings (20). A human topic look on rhinovirus transmission concluded that aerosols were likely the dominant transmission mode (26). SARS-CoV-2 an infection of hamsters and ferrets has been proven to transmit through air in experimental configurations designed to exclude contributions from screech contact and droplet transmission (33, 37, 38). Diagnosis of respiratory emissions at some stage in an infection with influenza virus, parainfluenza virus, RSV, human metapneumovirus, and hRV has revealed the presence of viral genomes in a quantity of aerosol sizes, with the best quantity detected in aerosols <5 μm rather than in larger aerosols (39). SARS-CoV-2 RNA has been detected and infectious virus has been recovered in aerosols ranging from 0.25 to >4 μm (34, 35, 40–44). Influenza virus RNA has also been detected in both handsome (≤5 μm) and coarse (>5 μm) aerosols exhaled from infected individuals, with more viral RNA contained in the handsome aerosol particles (23). Laboratory reviews dangle chanced on that aerosolized SARS-CoV-2 has a half-lifetime of ~1 to a pair of hours (45–47). The World Neatly being Organization (WHO) and the US Centers for Disease Withhold watch over and Prevention (CDC) officially acknowledged inhalation of virus-laden aerosols as a predominant mode in spreading SARS-CoV-2 at both short and long ranges in April and Will also of 2021, respectively (48, 49).
Table 1 Airborne transmission of respiratory viruses.
Representative proof of airborne transmission for just a few respiratory viruses and their traditional reproduction quantity. Cells with dashes showcase now not applicable.
Mathematical modeling of publicity to respiratory pathogens helps that transmission is dominated by short-range aerosol inhalation at most distances within 2 m of the infectious person, and droplets are very most attention-grabbing dominant when persons are within 0.2 m when speaking or 0.5 m when coughing (50). Anecdotal observations of measles virus (16–18) and Mycobacterium tuberculosis (51, 52) an infection in conclude proximity, previously attributed totally to droplets, encompass transmission by aerosols at short range. Further reviews are warranted for respiratory diseases whose transmission has previously been characterised as droplet driven as a outcome of it is miles plausible that airborne transmission is important and even dominant for many of them.
Early in the COVID-19 pandemic, it used to be assumed that droplets and fomites were the predominant transmission routes on the belief of the quite low traditional reproduction quantity (R0) in contrast with that of measles (53–55) (Table 1). R0 is the moderate selection of secondary infections ended in by a predominant infected particular person in a homogeneously susceptible inhabitants. This argument used to be built on a protracted-standing perception that all airborne diseases must be highly contagious. Nonetheless, there’s no such thing as a scientific foundation for such an assumption as a outcome of airborne diseases whine a range of R0 values that can now not be successfully represented by a single moderate worth, which is dependent on a quantity of components. For instance, tuberculosis (R0, 0.26 to 4.3) is an obligate airborne bacterial an infection (56), but it for sure is much less transmissible than COVID-19 (R0, 1.4 to 8.9) (57–59). The components affecting airborne transmission encompass viral load in varied-sized respiratory particles, the stability of the virus in aerosols, and the dose-response relationship for every virus (the likelihood of an infection given publicity to a obvious selection of virions through a particular publicity route). Furthermore, R0 is a median, and COVID-19 is a superb deal overdispersed, meaning that, below obvious prerequisites, it would per chance presumably successfully be highly contagious. Epidemiological reviews dangle chanced on that 10 to 20% of infected individuals narrative for 80 to 90% of subsequent infections for SARS-CoV-2, highlighting the heterogeneity in secondary assault rates (the percentage of exposed individuals who turn into infected) (60–63).
A rising physique of research on COVID-19 provides plentiful proof for the predominance of airborne transmission of SARS-CoV-2. This route dominates below obvious environmental prerequisites, in particular indoor environments that are poorly ventilated (6, 34, 35, 41, 42, 45, 50, 64–68), an observation that implicates totally aerosols as a outcome of very most attention-grabbing aerosols—and never tremendous droplets or surfaces—are tormented by air whisk alongside with the circulation. Furthermore, the marked incompatibility between rates of indoor and outdoors transmission can very most attention-grabbing be defined by airborne transmission, as a outcome of tremendous droplets, whose trajectories are tormented by gravitational settling but now not air whisk alongside with the circulation, behave identically in both settings (69). Diversified combos of epidemiological analyses; airflow model simulations; tracer experiments; and diagnosis and modeling of superspreading events in ingesting areas (36), in meatpacking vegetation (70), on a cruise ship (71), at some stage in singing at a choir rehearsal (64), and the long-distance transmission at a church (72) all implicate aerosols as the per chance mode of transmission over fomites and droplets. It’s miles highly now not likely that nearly all individuals at any of those events all touch the an analogous rotten surface or are exposed to droplets constituted of the cough or sneeze of an infectious person at conclude range and bump into sufficient virus load to trigger an infection. Nonetheless, the one traditional ingredient for all individuals at these indoor events is the shared air they inhale in the an analogous room. Commonalities among superspreading events encompass indoor settings, crowds, publicity durations of 1 hour or more, unfortunate air whisk alongside with the circulation, vocalization, and scarcity of neatly frail masks (36). Provided that droplet transmission dominates very most attention-grabbing when persons are within 0.2 m when speaking (50) and that transmission of SARS-CoV-2 through rotten surfaces is much less likely (73–75), superspreading events can very most attention-grabbing be defined by alongside with aerosols as a mode of transmission.
To avoid wasting efficient steering and policies for preserving in opposition to airborne transmission of respiratory viruses, it is miles important to higher perceive the mechanisms eager. For airborne transmission to happen, aerosols must be generated, transported through air, inhaled by a susceptible host, and deposited in the respiratory tract to open an infection. The virus must take dangle of its infectivity at some stage in these processes. In this Overview, we focus on the processes mad by the expertise, transport, and deposition of virus-laden aerosols, apart from to the critical parameters that affect these processes, that are severe to informing efficient an infection abet an eye on measures (Fig. 1).
Phases mad by the airborne transmission of virus-laden aerosols encompass (i) expertise and exhalation; (ii) transport; and (iii) inhalation, deposition, and an infection. Every section is influenced by a combination of aerodynamic, anatomical, and environmental components. (The sizes of virus-containing aerosols are to now not scale.)
Technology of virus-laden aerosols
Expiratory activities originate aerosols from varied websites in the respiratory tract through definite mechanisms. Aerosols produced by activities much like respiratory, speaking, and coughing whine varied aerosol dimension distributions and airflow velocities (76, 77), which in turn govern the kinds and hundreds of viruses that every aerosol particle would per chance presumably lift, the location time in air, the gap traveled, and finally the deposition websites in the respiratory tract of a person that inhales them (78). Aerosols released by an infected particular person would per chance presumably maintain viruses (39, 79–81) apart from to electrolytes, proteins, surfactants, and other parts in the fluid that lines respiratory surfaces (82, 83) (Fig. 2).
The habits and fate of virus-laden aerosols are inherently governed by their attribute properties, alongside with physical dimension, viral load, infectivity, other chemical parts in the aerosol, electrostatic worth, pH, and the air-liquid interfacial properties.
Internet sites of aerosol formation
Respiratory aerosols would per chance presumably successfully be labeled into alveolar, bronchiolar, bronchial, laryngeal, and oral aerosols, in step with the websites where they are produced (3, 84, 85). Bronchiolar aerosols are fashioned at some stage in traditional respiratory (3). Right through exhalation, the liquid film lining the lumenal surfaces of the bronchioles ruptures to originate tiny aerosols. Such aerosols are generated by shear forces that destabilize the air-liquid or air-mucous interface. Respiratory airflows are in total turbulent below excessive airflow velocities, in particular in the tremendous lumens of the upper airways, which transition to laminar whisk alongside with the circulation in the bronchi and bronchioles (76, 86–88). Laryngeal aerosols are generated through vocal fold vibrations at some stage in vocalization (3). The apposition of vocal folds kinds liquid bridges, which burst into aerosols at some stage in exhalation. Against this, droplets (>100 μm) are essentially constituted of saliva in the oral cavity (3). Aerosol emission rates manufacture bigger with airflow velocity and speech quantity at some stage in activities much like singing and shouting (9, 89, 90).
Number and dimension distributions
The scale of exhaled aerosols is one of the crucial influential properties governing their fate, as a outcome of dimension now not very most attention-grabbing determines their aerodynamic characteristics but also their deposition dynamics and the positioning of an infection. Measurement distributions of respiratory aerosols were investigated for the reason that 1890s the utter of assorted approaches, alongside with optical microscopy, excessive-run photography, and, more now not too long ago, laser-based fully detection options (1, 2, 91). Early reviews old measuring options and analytical options that were unable to detect aerosols <5 μm (1, 92), but contemporary devices, much like aerodynamic and scanning mobility particle sizing systems, dangle enabled the detection of smaller aerosols. Respiratory aerosols originate a multimodal dimension distribution, with peaks around 0.1 μm, 0.2 to 0.8 μm, 1.5 to 1.8 μm, and 3.5 to 5.0 μm, each representing a varied expertise location, manufacturing route of, and expiratory utter (2, 8, 9, 85, 91, 93). The smaller the modal dimension, the deeper the aerosols manufacture in the respiratory tract. A better mode centered at 145 μm for speaking and 123 μm for coughing originates mainly from the oral cavity and lips (3). In terms of quantity, the majority of exhaled aerosols are <5 μm, and an fantastic share are <1 μm for many respiratory activities, alongside with those produced at some stage in respiratory, speaking, and coughing (8, 9). Overall, speech produces 100 to 1000 times the selection of aerosols <100 μm in size for every droplet that is >100 μm (3).
Customary respiratory has been proven to say to 7200 aerosol particles per liter of exhaled air (9, 93). The selection of virus-laden aerosols expelled by individuals whereas respiratory varies widely between individuals and is dependent on disease stage, age, physique mass index, and preexisting health prerequisites (94, 95). Young individuals assuredly originate fewer virus-laden aerosols than adults as a outcome of their lungs are composed surroundings up and dangle fewer bronchioles and alveoli in which aerosols can procure (96). The processes mad by aerosol formation, in particular the properties of fluid lining the airways that dangle an affect on its propensity to interrupt up to procure aerosols, plays a critical characteristic in the selection of aerosols exhaled (94). One look confirmed that 1 min of speaking would per chance presumably originate now not much less than 1000 aerosols (97). Even supposing coughing can originate more aerosols in a transient time length, it is miles a ways more sporadic than real respiratory and speaking, especially for infected individuals who showcase no scientific symptoms. Which potential fact, respiratory, speaking, and other real vocalization by infected individuals will likely open more complete virus-laden aerosols total than much less-frequent coughing.
Viral exclaim material of aerosols
The viral load of aerosols is a key ingredient in determining the relative contribution of airborne transmission. Nonetheless, sampling and detecting airborne viruses is difficult thanks to their low concentrations in air and susceptibility to destruction and inactivation at some stage in sampling. Air samples are in total analyzed for the presence of viral genomes by quantitative polymerase chain reaction (qPCR) or quantitative reverse transcription PCR (qRT-PCR) options, that are highly soft. However, the presence of genetic topic cloth alone doesn’t showcase whether or now not the virus is infectious. The viability of viruses is dependent on the integrity and characteristic of their genomic topic cloth, nucleoprotein, capsid, and/or envelope. Even supposing some reviews dangle tried and didn’t culture viruses from air, the usage of more gentle options, much like a liquid condensation sequence utility, has enabled the detection of a quantity of viable respiratory viruses, alongside with influenza viruses and SARS-CoV-2 in aerosols (35, 40, 98).
Many viruses were remoted from breath and indoor air samples, alongside with adenovirus (29, 99), coxsackievirus (100), influenza viruses (22, 23, 98, 101), rhinovirus (9, 26–28), measles virus (16, 17), RSV (25, 102), SARS-CoV (31), MERS-CoV (32, 103), and SARS-CoV-2 (34, 35, 40–44) (Table 1). The concentration of SARS-CoV-2 in the air of a scientific institution room with two COVID-19 sufferers used to be between 6 and 74 TCID50 per liter (median tissue culture infectious dose per liter) (35). The distribution of virions across varied sizes of aerosol particles is linked to their location of craftsmanship, the manufacturing mechanism, and the severity of an infection at the expertise location, which varies among varied viruses (104). It’s miles over and over assumed that viral concentrations in scientific samples (e.g., sputum or saliva) translate on to the concentration in droplets and aerosols generated from respiratory fluid—i.e., that viral load scales with the preliminary quantity of droplets and aerosols (50, 55, 71). Nonetheless, dimension-segregated samples of aerosols restful in the exhaled breath of individuals infected with influenza A or B viruses, parainfluenza virus, coronaviruses, hRV, or RSV and air restful in varied settings whine that viruses are enriched in smaller aerosols (10). In samples restful from influenza sufferers whereas respiratory, speaking, and/or coughing, better than half of the viral RNA used to be chanced on in aerosols <4 to 5 μm (23, 104, 105). A look of just a few respiratory viruses chanced on viral RNA more over and over in tiny (<5 μm) than in tremendous aerosols (39). The distribution of influenza virus and RSV in ambient aerosols measured in a scientific sanatorium revealed that 42% of influenza A virus RNA, but very most attention-grabbing 9% of RSV RNA, used to be in aerosols ≤4 μm (102). In a look that restful aerosols in a health sanatorium, childcare center, and airplanes, better than half of influenza A virus RNA used to be chanced on in aerosols <2.5 μm (106). A study found that a subset of COVID-19 patients release up to 105 to 107 SARS-CoV-2 genome copies per hour in exhaled breath, whereas others do not exhale detectable virus (107). Large interpersonal variability in both the number of aerosols produced and their viral load may contribute to overdispersion in COVID-19 transmission, a crucial component in superspreading events (108).
Although infectious viruses are enriched in small aerosols, the dose-response relationship that governs the probability of infection given exposure to a certain number of virions, remains to be determined. In a susceptible host, the minimum infectious dose varies on the basis of virus type and deposition site within the respiratory tract, such that the inhalation of smaller aerosols that deposit deeper in the lungs could require less virus to initiate infection. Studies on influenza virus have shown that the dose required to initiate infection in humans, in terms of plaque-forming units (PFU), is, for the inhalation of aerosols, about a hundredth the size of the dose for intranasal inoculation (101). Improved characterization of the viral load and distribution of infectious virions in individual aerosols as a function of particle size, for different people and stages of disease, will greatly contribute to our understanding of airborne transmission of respiratory viruses.