The airborne lifetime of small speech droplets and
their potential importance in SARS-CoV-2 transmission
, Christina E. Bax
, Adriaan Bax
, and Philip Anfinrud
Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0520;
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.
independent action hypothesis
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
with peak emission rates as high as
, 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 Stokes’law, 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
) and the probability that droplets encapsulate at least one
virion (proportional to d
), the product of which is proportional
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
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
to about 0.35 cm·s
distance over which droplets travel laterally from the speaker’s
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
speaker’s 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
copies per milliliter (maximum
of 2.35 ×10
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
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
To whom correspondence may be addressed. Email: firstname.lastname@example.org or philip.anfinrud@
www.pnas.org/cgi/doi/10.1073/pnas.2006874117 PNAS Latest Articles
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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
. 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 “filled”with speech
droplets by someone repeating the phrase “stay healthy”for 25 s.
This phrase was chosen because the “th”phonation in the word
“healthy”was 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
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
) 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
, 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
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
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
Fig. 1. Light scattering observation of airborne speech droplet nuclei, generated by a 25-s burst of repeatedly speaking the phrase “stay healthy”in a loud
voice (maximum 85 dB
at a distance of 30 cm; average 59 dB
). (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 “0”corresponds 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 “0”in 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 “aah”vocalization. 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
milliliter, the probability that a 1-μm droplet nucleus (scaled
back to its originally hydrated 3-μm size) contains a virion is
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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