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Speech droplets generated by asymptomatic carriers of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are increasingly considered to be a likely mode of disease transmission. Highly sensitive laser light scattering observations have revealed that loud speech can emit thousands of oral fluid droplets per second. In a closed, stagnant air environment, they disappear from the window of view with time constants in the range of 8 to 14 min, which corresponds to droplet nuclei of ca. 4 μm diameter, or 12- to 21-μm droplets prior to dehydration. These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.
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The airborne lifetime of small speech droplets and
their potential importance in SARS-CoV-2 transmission
Valentyn Stadnytskyi
a
, Christina E. Bax
b
, Adriaan Bax
a,1
, and Philip Anfinrud
a,1
a
Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0520;
and
b
Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104
Edited by Axel T. Brunger, Stanford University, Stanford, CA, and approved May 4, 2020 (received for review April 10, 2020)
Speech droplets generated by asymptomatic carriers of severe
acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are in-
creasingly considered to be a likely mode of disease transmission.
Highly sensitive laser light scattering observations have revealed
that loud speech can emit thousands of oral fluid droplets per
second. In a closed, stagnant air environment, they disappear from
the window of view with time constants in the range of 8 to
14 min, which corresponds to droplet nuclei of ca. 4μm diameter,
or 12- to 21-μm droplets prior to dehydration. These observations
confirm that there is a substantial probability that normal speak-
ing causes airborne virus transmission in confined environments.
COVID-19
|
speech droplet
|
independent action hypothesis
|
respiratory
disease
|
disease transmission
It has long been recognized that respiratory viruses can be
transmitted via droplets that are generated by coughing or
sneezing. It is less widely known that normal speaking also
produces thousands of oral fluid droplets with a broad size dis-
tribution (ca. 1μm to 500 μm) (1, 2). Droplets can harbor a
variety of respiratory pathogens, including measles (3) and in-
fluenza virus (4) as well as Mycobacterium tuberculosis (5). High
viral loads of severe acute respiratory syndrome coronavirus 2
(SARS-CoV-2) have been detected in oral fluids of coronavirus
disease 2019 (COVID-19)positive patients (6), including
asymptomatic ones (7). However, the possible role of small
speech droplet nuclei with diameters of less than 30 μm, which
potentially could remain airborne for extended periods of time
(1, 2, 8, 9), has not been widely appreciated.
In a recent report (10), we used an intense sheet of laser light
to visualize bursts of speech droplets produced during repeated
spoken phrases. This method revealed average droplet emission
rates of ca. 1,000 s
1
with peak emission rates as high as
10,000 s
1
, with a total integrated volume far higher than in
previous reports (1, 2, 8, 9). The high sensitivity of the light
scattering method in observing medium-sized (10 μm to 100 μm)
droplets, a fraction of which remain airborne for at least 30 s,
likely accounts for the large increase in the number of observed
droplets. Here, we derive quantitative estimates for both the
number and size of the droplets that remain airborne. Larger
droplets, which are also abundant but associated with close-
proximity direct virus transfer or fomite transmission (11), or
which can become resuspended in air at a later point in time
(12), are not considered here.
According to Stokeslaw, the terminal velocity of a falling
droplet scales as the square of its diameter. Once airborne,
speech-generated droplets rapidly dehydrate due to evaporation,
thereby decreasing in size (13) and slowing their fall. The
probability that a droplet contains one or more virions scales
with its initial hydrated volume, that is, as the cube of its di-
ameter, d. Therefore, the probability that speech droplets pass
on an infection when emitted by a virus carrier must take into
account how long droplet nuclei remain airborne (proportional
to d
2
) and the probability that droplets encapsulate at least one
virion (proportional to d
3
), the product of which is proportional
to d.
The amount by which a droplet shrinks upon dehydration
depends on the fraction of nonvolatile matter in the oral fluid,
which includes electrolytes, sugars, enzymes, DNA, and rem-
nants of dehydrated epithelial and white blood cells. Whereas
pure saliva contains 99.5% water when exiting the salivary
glands, the weight fraction of nonvolatile matter in oral fluid falls
in the 1 to 5% range. Presumably, this wide range results from
differential degrees of dehydration of the oral cavity during
normal breathing and speaking and from decreased salivary
gland activity with age. Given a nonvolatile weight fraction in the
1 to 5% range and an assumed density of 1.3 g·mL
1
for that
fraction, dehydration causes the diameter of an emitted droplet
to shrink to about 20 to 34% of its original size, thereby slowing
down the speed at which it falls (1, 13). For example, if a droplet
with an initial diameter of 50 μm shrinks to 10 μm, the speed at
which it falls decreases from 6.8 cm·s
1
to about 0.35 cm·s
1
. The
distance over which droplets travel laterally from the speakers
mouth during their downward trajectory is dominated by the
total volume and flow velocity of exhaled air (8). The flow ve-
locity varies with phonation (14), while the total volume and
droplet count increase with loudness (9). Therefore, in an envi-
ronment of stagnant air, droplet nuclei generated by speaking
will persist as a slowly descending cloud emanating from the
speakers mouth, with the rate of descent determined by the
diameter of the dehydrated speech droplet nuclei.
The independent action hypothesis (IAH) states that each
virion has an equal, nonzero probability of causing an infection.
Validity of IAH was demonstrated for infection of insect larvae
by baculovirus (15), and of plants by Tobacco etch virus variants
that carried green fluorescent protein markers (16). IAH applies
to systems where the host is highly susceptible, but the extent to
which IAH is valid for humans and SARS-CoV-2 has not yet
been firmly established. For COVID-19, with an oral fluid av-
erage virus RNA load of 7 ×10
6
copies per milliliter (maximum
of 2.35 ×10
9
copies per milliliter) (7), the probability that a
50-μm-diameter droplet, prior to dehydration, contains at least
one virion is 37%. For a 10-μm droplet, this probability drops to
0.37%, and the probability that it contains more than one virion,
if generated from a homogeneous distribution of oral fluid, is
negligible. Therefore, airborne droplets pose a significant risk
only if IAH applies to human virus transmission. Considering
that frequent person-to-person transmission has been reported
in community and health care settings, it appears likely that IAH
Author contributions: C.E.B., A.B., and P.A. designed research; V.S., A.B., and P.A. per-
formed research; V.S. analyzed data; and C.E.B., A.B., and P.A. wrote the paper.
The authors declare no competing interest.
This open access article is distributed under Creative Commons Attribution License 4.0
(CC BY).
Data deposition: Movies that show the experimental setup and the full 85-minute obser-
vation of speech droplet nuclei have been deposited at Zenodo and can be accessed at
https://doi.org/10.5281/zenodo.3770559.
1
To whom correspondence may be addressed. Email: bax@nih.gov or philip.anfinrud@
nih.gov.
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applies to COVID-19 and other highly contagious airborne re-
spiratory diseases, such as influenza and measles.
Results and Discussion
The output from a green (532 nm) Coherent Verdi laser oper-
ating at 4-W optical power was transformed with spherical and
cylindrical optics into a light sheet that is 1 mm thick and
150 mm tall. This light sheet passed through slits centered on
opposite sides of a cubic 226-L enclosure. When activated, a
40-mm, 12-V muffin fan inside the enclosure spatially homoge-
nizes the distribution of particles in the enclosure. A movie
showing the arrangement is available (17). Movie clips of speech
droplet nuclei were recorded at a frame rate of 24 Hz with high-
definition resolution (1,920 ×1,080 pixels). The camera lens
provided a horizontal field of view of 20 cm. Therefore, the
volume intercepted by the light sheet and viewed by the camera
is 30 cm
3
. The total number of particles in the enclosure can be
approximated by multiplying the average number of particles
detected in a single movie frame by the volume ratio of the en-
closure to the visualized sheet, which is 7,300. Slow convection
currents, at speeds of a few centimeters per second, remained for
the duration of the recording. These convection currents are at-
tributed to a 0.5 °C temperature gradient in the enclosure (bottom
to top) that presumably is due to heat dissipated by the iPhone11
camera, which was attached to the front side of the enclosure. Since
the net air flux across any horizontal plane of the enclosure is zero,
this convection does not impact the average rate at which droplet
nuclei fall to the bottom of the enclosure.
With the internal circulation fan turned on, the enclosure was
purged with HEPA-filtered air for several minutes. Then, the
purge shutter was closed, the movie clip was started, the speaker
port was opened, and the enclosure was filledwith speech
droplets by someone repeating the phrase stay healthyfor 25 s.
This phrase was chosen because the thphonation in the word
healthywas found to be an efficient generator of oral fluid
speech droplets. The internal fan was turned off 10 s after speech
was terminated, and the camera continued recording for 80 min.
The movie clip was analyzed frame by frame to determine the
number of spots/streaks whose maximum single-pixel intensity
exceeded a threshold value of 30. Fig. 1 charts the time-
dependent decrease in the number of scattering particles de-
tected. We are not yet able to quantitatively link the observed
scattered light intensity to the size of the scattering particle be-
cause the light intensity varies across the sheet. However, the
brightest 25% were found to decay more quickly than the dim-
mer fraction, with the two curves reasonably well described by
exponential decay times of 8 and 14 min, respectively (Fig. 1A).
These fits indicate that, near time 0, there were, on average,
approximately nine droplet nuclei in the 30-cm
3
observation
window, with the larger and brighter nuclei (on average) falling
to the bottom of the enclosure at faster speeds than the smaller
and dimmer ones.
With the assumption that the contents of the box are ho-
mogenized by the muffin fan at time 0, the average number of
droplets found in a single frame near time 0 corresponds to ca.
66,000 small droplets emitted into the 226-L enclosure, or ca.
2,600 small droplet nuclei per second of speaking. If the particle
size distribution were a delta function and the particles were
uniformly distributed in the enclosure, the particle count would
be expected to remain constant until particles from the top of the
enclosure descend to the top of the light sheet, after which the
particle count would decay linearly to background level. The
observation that the decay profiles are approximately exponen-
tial points to a substantial heterogeneity in particle sizes, even
after binning them into two separate groups.
The weighted average decay rate (0.085 min
1
) of the bright
and dim fractions of particles (Fig. 1A) translates into a half-life
in the enclosure of ca. 8 min. Assuming this half-life corresponds
to the time required for a particle to fall 30 cm (half the height of
the box), its terminal velocity is only 0.06 cm·s
1
, which corre-
sponds to a droplet nucleus diameter of 4μm. At the relative
humidity (27%) and temperature (23 °C) of our experiment, we
expect the droplets to dehydrate within a few seconds. A dehy-
drated particle of 4 μm corresponds to a hydrated droplet of ca.
12- to 21-μm diameter, or a total hydrated volume of 60 nL
to 320 nL for 25 s of loud speaking. At an average viral load of
7×10
6
per milliliter (7), we estimate that 1 min of loud speaking
generates at least 1,000 virion-containing droplet nuclei that
remain airborne for more than 8 min. These therefore could be
inhaled by others and, according to IAH, trigger a new SARS-CoV-2
infection.
The longest decay constant observed by us corresponds to
droplets with a hydrated diameter of 12 μm when exiting
the mouth. The existence of even smaller droplets has been
AB
Fig. 1. Light scattering observation of airborne speech droplet nuclei, generated by a 25-s burst of repeatedly speaking the phrase stay healthyin a loud
voice (maximum 85 dB
B
at a distance of 30 cm; average 59 dB
B
). (A) Chart of particle count per frame versus time (smoothed with a 24-s moving average), with
the red curve representing the top 25% in scattering brightness and the green curve representing the rest. The bright fraction (red) decays with a time
constant of 8 min, and the dimmer fraction (green) decays with a time constant of 14 min. Both exponential decay curves return to their respective back-
ground level of ca. 0 (red horizontal dashed line) and 0.4 (green dashed line) counts per frame. Time 0corresponds to the time the stirring fan was turned
off. The 25-s burst of speaking started 36 s before time 0. The black arrow (at 0.5 min) marks the start of the exponential fits. (B) Image of the sum of 144
consecutive frames (spanning 6 s) extracted shortly after the end of the 25-s burst of speaking. The dashed circle marks the needle tip used for focusingthe
camera. The full movie recording is available in ref. 17, with time 0in the graph at time point 3:38 in the movie.
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established by aerodynamic particle sizer (APS) measurements
(2). APS is widely used for detecting aerosol particulates and is
best suited for particles in the 0.5- to 5-μm range. Morawska
et al. (2) detected as many as 330 particles per second in the 0.8-
to 5.5-μm range upon sustained aahvocalization. Considering
the short travel time (0.7 s) between exiting the mouth and the
APS detector, and the high relative humidity (59%) used in that
study, droplet dehydration may have been incomplete. If it were
75% dehydrated at the detector, an observed 5.5-μm particle
would have started as an 8.7-μm droplet when exiting the mouth,
well outside the 12- to 21-μm range observed above by light
scattering. This result suggests that APS and light scattering
measurements form a perfect complement. However, we also
note that, even while the smallest droplet nuclei effectively re-
main airborne indefinitely and have half-lives that are dominated
by the ventilation rate, at a saliva viral load of 7 ×10
6
copies per
milliliter, the probability that a 1-μm droplet nucleus (scaled
back to its originally hydrated 3-μm size) contains a virion is
only 0.01%.
Our current setup does not detect every small particle in each
frame of the movie, and our reported values are therefore con-
servative lower limit estimates. We also note that the saliva viral
load shows large patient-to-patient variation. Some patients have
viral titers that exceed the average titer of Wölfel et al. by more
than two orders of magnitude (7, 18), thereby increasing the
number of virions in the emitted droplets to well over 100,000
per minute of speaking. The droplet nuclei observed in our
present study and previously by APS (2, 9) are sufficiently small
to reach the lower respiratory tract, which is associated with an
increased adverse disease outcome (19, 20).
Our laser light scattering method not only provides real-time
visual evidence for speech droplet emission, but also assesses
their airborne lifetime. This direct visualization demonstrates
how normal speech generates airborne droplets that can remain
suspended for tens of minutes or longer and are eminently ca-
pable of transmitting disease in confined spaces.
Data Availability Statement. All raw data used for analysis are
available in ref. 17.
ACKNOWLEDGMENTS. We thank Bernhard Howder for technical support,
Clemens Wendtner, William A. Eaton, Roland Netz, and Steven Chu for in-
sightful comments. This work was supported by the Intramural Research Pro-
gram of the National Institute of Diabetes and Digestive and Kidney Diseases.
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As of June1st 2021, more than 17 crore people have been infected with COVID-19 across the globe, and almost 3 crore people have been infected in India. The virus can spread through even normal actions like talking with particle emission rates inversely correlating with word frequency and volume, which can be reduced by covering the mouth. However, there is debate concerning the effectiveness of the various face mask types in preventing respiratory infections. Many have reported that wearing a mask is uncomfortable, especially when worn for long hours and while performing strenuous activities. Another disease that has raised its head is mucormycosis. However, COVID-19 can be a serious infection in many, with many fatalities. It is not yet clear how much protection vaccines give, and in a hugely populated country like India, it may be very difficult to vaccinate the whole population. Moreover, the vaccination for pediatric groups has just started. So, it is imperative to wear masks that can be protective against infection. However, some people believe that a straightforward cotton mask is insufficient. We set out to analyze the efficacy of masks through this investigation. According to the results of this systematic review, there are no studies that give conclusive evidence that using face masks as recommended by current public health guidelines will stop this condition. This is a significant discovery that should be communicated to the scientific community and calls into question the rationale for inconsistent and differing public health recommendations.
... ➢ Accurate droplet size measurement is needed ➢Conflicting result in speech/cough droplet size measurement has been reported [4][5][6][7][8] ▪ Need for standardisation in droplet size measurement ▪ Metrology support is needed Wang ➢ Various type of liquids have been used for droplet generation and evaporation study [13][14][15][16][17][18] ➢ We decided to use 6.5% & 18.1% (w/w) saline solutions and artificial saliva (with mucin, stabilized) ...
Technical Report
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COVID-19 virus has been reported to be spread in part by infectious droplets and aerosols produced when infected individuals speak or cough. The size of the droplet will decide the transport distance in air, its life span, risk of virus infection, safety distance, etc. Several published papers have shown conflicting results in measured size distribution of such droplets. Thus, it is important to investigate the droplet size measurement accuracy for more effective infection control. We have proposed this project in response to the COVID-19 and future pandemics. This project is overseed by NMC, A*STAR, with five participating APMP member institutes (NMC, CMS/ITRI, NIM, KRISS, HSA). The main tasks include selection of suitable device to be used for droplet generation and droplet test chamber development. We also develop droplet size measurement methodology, circulate the droplet-generator with test chamber among 4 labs, make droplet size measurement by commercial instruments and evaluate their deviations. We have developed a test chamber for droplet size measurement. 18.1% and 6.5% (w/w) saline and artificial saliva were used to produce droplets via an ULV fogger. We have analysed the composition of artificial saliva with good accuracy. The droplet volume equivalent diameter (VED) measurements have been made by commercial particle sizers: aerodynamic particle sizer (APS) or aerosol spectrometer (AS) in 4 metrology labs (NMC, CMS, NIM, KRISS). This project has investigated saline and artificial saliva droplet size measurement (by AS and APS) with well-designed experiment, for the first time among all national metrology institutes. Based on the measurements of 4 laboratories, our study has found relatively large size deviation from mean droplet VED size (around 4 μm): up to 22.7% for artificial saliva droplets. The deviations of 4 labs from mean VED size are below 11.6% for saline droplets VED measurement. We have found that new reference size standard for artificial saliva and saline droplet size measurement is needed, due to material density effect on APS sizing. More research work is needed to develop a droplet size reference standard to establish traceability for saliva and saline droplet size measurement, and to calibrate commercial instruments. (APMP Response Program against COVID-19, Webinar talk, Nov 2022)
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The spread of COVID-19 resulted in the utilization of preventive measures, such as face masks. However, there is currently a lack of attitude measures available to examine an individual’s face mask attitudes. The purpose of this study was to develop an instrument to measure face mask wearing attitudes. Participants (N = 447) responded to an item pool of 173 positive and negative statements about face masks. Factor analysis resulted in a one-factor 16-item scale solution. Analyses also revealed significant group differences in attitudes. Caucasians had significantly less positive attitudes toward face masks than African Americans. Republicans had significantly less positive attitudes toward face masks than Democrats, those identifying with neither party, and those identifying as having no political affiliation. Heterosexuals had significantly less positive attitudes toward face masks than LGBTQ+ identifying individuals. Implications for research are discussed.
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Droplet impact on a flexible substrate is a prevalent phenomenon in nature and various advanced technologies such as soft bio-printing, tissue engineering, smart biomaterials and flexible electronics. Recent rapid advancement in new functional surfaces, ultra-high-speed imaging, nanotechnology, deep learning, advanced computational strength and the relation between fluid dynamics and interfacial science have intensified the physical understanding of droplet impact on soft materials. Once a droplets impacts on a solid surface, it deposits, spreads, rebounds or splashes. Given the importance of the droplet impact onto soft substrates in biotechnology, medicine and advanced flexible electronics, a deep physical understanding of such complex phenomenon is vital. This review initially presents the liquid-solid interaction physics and relevant interfacial science. Next, this review discusses the physics of droplet impact on soft materials with different physical and interfacial characteristics. Moreover, this review presents advancements in droplet impact on elastic materials relevant to new technologies such as soft electronics, elastic smart biomaterials, tissue engineering and the fight against COVID-19 pandemic. Finally, this review lays out future research directions related to current problems in such complex physical phenomenon.
Chapter
Citizens are nowadays being flooded with huge amounts of information, which will keep growing as the physical spaces become more intelligent, with the proliferation of sensors (e.g., pollution sensors, traffic sensors, etc.), mobile apps, and information services of different types (e.g., malls providing offers and other kinds of information to nearby customers). To actually become resilient modern citizens, people need to be able to handle all this highly-dynamic information and act upon it by taking suitable decisions. In this context, the development of suitable data management techniques to help citizens in their daily life plays a major role. Motivated by this, we focus on the design of novel data management techniques for mobile users (pedestrians) and for drivers, which are two key areas in the daily life of citizens. More specifically, we consider the problem of recommending relevant items to pedestrians (e.g., tourists) and the challenges of drivers when they try to find an available parking space. As evaluating data management strategies in a real environment in a large-scale is very challenging, in this paper we propose suitable simulation approaches that facilitate the evaluation task. Through simulations, we obtain some initial experimental results that show the additional difficulties that appear when we want to satisfy additional constraints such as the desire to minimize the risk of virus spread in a COVID-19 scenario.
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The ongoing COVID-19 outbreak has spread rapidly on a global scale. While the transmission of SARS-CoV-2 via human respiratory droplets and direct contact is clear, the potential for aerosol transmission is poorly understood1–3. This study investigated the aerodynamic nature of SARS-CoV-2 by measuring viral RNA in aerosols in different areas of two Wuhan hospitals during the COVID-19 outbreak in February and March 2020. The concentration of SARS-CoV-2 RNA in aerosols detected in isolation wards and ventilated patient rooms was very low, but it was elevated in the patients’ toilet areas. Levels of airborne SARS-CoV-2 RNA in the majority of public areas was undetectable except in two areas prone to crowding, possibly due to infected carriers in the crowd. We found that some medical staff areas initially had high concentrations of viral RNA with aerosol size distributions showing peaks in submicrometre and/or supermicrometre regions, but these levels were reduced to undetectable levels after implementation of rigorous sanitization procedures. Although we have not established the infectivity of the virus detected in these hospital areas, we propose that SARS-CoV-2 may have the potential to be transmitted via aerosols. Our results indicate that room ventilation, open space, sanitization of protective apparel, and proper use and disinfection of toilet areas can effectively limit the concentration of SARS-CoV-2 RNA in aerosols. Future work should explore the infectivity of aerosolized virus.
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Coronavirus disease 2019 (COVID-19) is an acute respiratory tract infection that emerged in late 20191,2. Initial outbreaks in China involved 13.8% cases with severe, and 6.1% with critical courses³. This severe presentation corresponds to the usage of a virus receptor that is expressed predominantly in the lung2,4. By causing an early onset of severe symptoms, this same receptor tropism is thought to have determined pathogenicity, but also aided the control, of severe acute respiratory syndrome (SARS) in 2003⁵. However, there are reports of COVID-19 cases with mild upper respiratory tract symptoms, suggesting the potential for pre- or oligosymptomatic transmission6–8. There is an urgent need for information on body site-specific virus replication, immunity, and infectivity. Here we provide a detailed virological analysis of nine cases, providing proof of active virus replication in upper respiratory tract tissues. Pharyngeal virus shedding was very high during the first week of symptoms (peak at 7.11 × 10⁸ RNA copies per throat swab, day 4). Infectious virus was readily isolated from throat- and lung-derived samples, but not from stool samples, in spite of high virus RNA concentration. Blood and urine never yielded virus. Active replication in the throat was confirmed by viral replicative RNA intermediates in throat samples. Sequence-distinct virus populations were consistently detected in throat and lung samples from the same patient, proving independent replication. Shedding of viral RNA from sputum outlasted the end of symptoms. Seroconversion occurred after 7 days in 50% of patients (14 days in all), but was not followed by a rapid decline in viral load. COVID-19 can present as a mild upper respiratory tract illness. Active virus replication in the upper respiratory tract puts the prospects of COVID-19 containment in perspective.
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Mechanistic hypotheses about airborne infectious disease transmission have traditionally emphasized the role of coughing and sneezing, which are dramatic expiratory events that yield both easily visible droplets and large quantities of particles too small to see by eye. Nonetheless, it has long been known that normal speech also yields large quantities of particles that are too small to see by eye, but are large enough to carry a variety of communicable respiratory pathogens. Here we show that the rate of particle emission during normal human speech is positively correlated with the loudness (amplitude) of vocalization, ranging from approximately 1 to 50 particles per second (0.06 to 3 particles per cm3) for low to high amplitudes, regardless of the language spoken (English, Spanish, Mandarin, or Arabic). Furthermore, a small fraction of individuals behaves as “speech superemitters,” consistently releasing an order of magnitude more particles than their peers. Our data demonstrate that the phenomenon of speech superemission cannot be fully explained either by the phonic structures or the amplitude of the speech. These results suggest that other unknown physiological factors, varying dramatically among individuals, could affect the probability of respiratory infectious disease transmission, and also help explain the existence of superspreaders who are disproportionately responsible for outbreaks of airborne infectious disease.
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Although short-range large-droplet transmission is possible for most respiratory infectious agents, deciding on whether the same agent is also airborne has a potentially huge impact on the types (and costs) of infection control interventions that are required. The concept and definition of aerosols is also discussed, as is the concept of large droplet transmission, and airborne transmission which is meant by most authors to be synonymous with aerosol transmission, although some use the term to mean either large droplet or aerosol transmission. However, these terms are often used confusingly when discussing specific infection control interventions for individual pathogens that are accepted to be mostly transmitted by the airborne (aerosol) route (e.g. tuberculosis, measles and chickenpox). It is therefore important to clarify such terminology, where a particular intervention, like the type of personal protective equipment (PPE) to be used, is deemed adequate to intervene for this potential mode of transmission, i.e. at an N95 rather than surgical mask level requirement. With this in mind, this review considers the commonly used term of ‘aerosol transmission’ in the context of some infectious agents that are well-recognized to be transmissible via the airborne route. It also discusses other agents, like influenza virus, where the potential for airborne transmission is much more dependent on various host, viral and environmental factors, and where its potential for aerosol transmission may be underestimated.
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Significance Lack of human data on influenza virus aerosol shedding fuels debate over the importance of airborne transmission. We provide overwhelming evidence that humans generate infectious aerosols and quantitative data to improve mathematical models of transmission and public health interventions. We show that sneezing is rare and not important for—and that coughing is not required for—influenza virus aerosolization. Our findings, that upper and lower airway infection are independent and that fine-particle exhaled aerosols reflect infection in the lung, opened a pathway for a deeper understanding of the human biology of influenza infection and transmission. Our observation of an association between repeated vaccination and increased viral aerosol generation demonstrated the power of our method, but needs confirmation.
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