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1. INTRO
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potential
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*Author for c
†Present addr
State Univers
Electronic sup
1098/rsif.2009
One contribution of 10 to a Theme Supplement ‘Airborne
transmission of disease in hospitals’.
J. R. Soc. Interface (2009) 6, S727–S736Received 17 J
Accepted 14 SFurther, a high-speed schlieren video of a single cough is analysed by a computerized method
of tracking individual turbulent eddies, demonstrating the non-intrusive velocimetry of the
expelled airflow. Results show that human coughing projects a rapid turbulent jet into the
surrounding air, but that wearing a surgical or N95 mask thwarts this natural mechanism
of transmitting airborne infection, either by blocking the formation of the jet (N95 mask),
or by redirecting it in a less harmful direction (surgical mask).
Keywords: schlieren; visualization; mask; coughing; airflow;
infection control
DUCTION AND LITERATURE
W
rne infection in hospitals
nical infection control is performed in indoor
ts, mainly those of healthcare institutions
pitals, clinics and long-stay residential or nur-
. These buildings, by nature of containing sick
e the potential to be major sources of various
smitted infections, some of which may be
multiple antimicrobial agents and which
e transported out into the wider community.
de various viruses (e.g. chickenpox, measles
za), bacteria (e.g. tuberculosis,Pseudomonas
Staphylococcus aureus, including MRSA
-resistant Staphylococcus aureus)) and
Aspergillus species, especially their spores).
Vulnerable patients, both inpatients and ou
may be directly infected by such agents in su
care institutions during their admission, or m
cross-infected by visiting friends and relatives
infected from the community or from the same
healthcare institutions on previous visits.
Hence, it is important to control the trans
such infections at the source, within these indo
care environments. Hand-washing is a recog
important part of infection control for agents tr
by direct contact, but aerosol-transmitted infec
become more important and much debated
severe acute respiratory syndrome (SARS) ou
2003 (Wong et al. 2004; Yu et al. 2004, 2005
2005a,b), and during the preparedness for t
influenza pandemic (Tellier 2006, 2007; Brank
2007; Lemieux et al. 2007; Tang & Li 2007).
Several infections have been shown to be tr
via short-range and long-range aerosols in c
and healthcare settings, including tubercul
chickenpox and measles (Cole & Cook 1998
et al. 2004; Wong & Leung 2004; Tang et
Many natural human activities also have the
orrespondence (gss2@psu.edu).
ess: Applied Research Laboratory, The Pennsylvania
ity, University Park, Pennsylvania, USA.
plementary material is available at http://dx.doi.org/10.
.0295.focus or via http://rsif.royalsocietypublishing.org.N95 masks. The object is to characterize the exhaled airflows and evaluate the effect of thesefor aerosol inf
Julian W. Tang1, Thomas J.
and Gary
1Department of Laboratory Medicine, N
2Gas Dynamics Laboratory, Departmen
The Pennsylvania State University,
Various infectious agents are known to be
produced by infected patients. Such aerosols
breathing, talking, coughing and sneezing. T
mostly in engineering and physics, can be ef
human subjects in such indoor situations, n
tracer gas or airborne particles. It accomplis
gradients owing to real-time changes in air te
are obtained of human volunteers coughing wuly 2009
eptember 2009 S727tudy of the human
out wearing masks
ction control
iebner2, Brent A. Craven2,†
Settles2,*
nal University of Singapore, Singapore
Mechanical and Nuclear Engineering,
iversity Park, Pennsylvania, USA
nsmitted naturally via respiratory aerosols
ay be produced during normal activities by
schlieren optical method, previously applied
tively used here to visualize airflows around
intrusively and without the need for either
this by rendering visible the optical phase
erature. In this study, schlieren video records
and without wearing standard surgical and
doi:10.1098/rsif.2009.0295.focus
Published online 8 October 2009This journal is q 2009 The Royal Society

compared with the case of natural breathing (Hui
owing to turbulent entrainment (Settles 2005, 2006).
S728 Schlieren imaging for aerosol infection control J. W. Tang et al.et al. 2006a,b, 2007, 2009; Ip et al. 2007).
Many of the cited studies have involved a mannequin
as a patient simulator, or a lung model into which tracer
or ‘smoke’ particles have been introduced to track the
airflows produced. This has been mainly because such
tracer particles may be irritating or toxic to live
human volunteers and patients. It is, however, difficult
to extrapolate results from a mannequin with simulated
respiratory airflows to a live human patient.
1.2. The fluid dynamics of airborne
infection spread
Airflows generated by the human body include those due
to thermal effects, motion of the bodyand natural human
respiratory activities, e.g. breathing, talking, coughing,
sneezing, etc. The temperature difference between
human skin and the surrounding air drives a natural-
convection boundary layer vertically upwards on the
body surface of a standing person, eventually separating
from the head and shoulders to form the human thermal
plume (Lewis et al. 1969; Clark & Edholm 1985;
Craven & Settles 2006). The strong vertical transport
of air in this plume can convey floor-level contaminants
to the breathing zone, as well as entrain the surrounding
air along with any accompanying particle burden.
At walking speeds above approximately 0.2 m s21,
the human thermal plume gives way to the human aero-
dynamic wake (Edge et al. 2005), an unsteady turbulent
airflow that follows a moving person and mixes strongly
with the surrounding air owing to entrainment. Thus,
hospital staff and patients in motion can transport
airborne infectious agents from stationary patients to
others by way of the bulk air motion in their aerody-
namic wakes. Likewise, aircraft passengers moving in
the aisles can spread contamination along the lengthmay be a particularly effective means of transmitting
airborne infection (Cole & Cook 1998; Edwards et al.
2004; Wong & Leung 2004). More recently, there have
been several inter-disciplinary reviews involving both
microbiologists and engineers (Tang et al. 2006; Li
et al. 2007), as well as cross-disciplinary studies invol-
ving both physicians and engineers examining the
risks of airborne transmission from droplet dispersion
and using airflow visualization in different clinical scen-
arios, including some simulated environments (Qian
et al. 2006; Xie et al. 2006, 2007) and others involving
‘real-life’ operating theatres and the workplace
(Kim & Flynn 1991; Maynard et al. 2000; Pan et al.
2003; Brohus et al. 2006).
Similarly, artificial respiratory assist devices, such as
the oxygen masks and nebulizers used in many hospital
wards and the various forms of mechanically assisted
ventilation often used on intensive care units, have a
greater potential to disseminate airborne infection
because of high supply rates of air or oxygen that may
be required. These supply rates can be as high as
10–15 l min21 for an oxygen mask or nebulizer,
depending on the needs of the patient. With the use
of such masks the exhaled, potentially infectious air is
expelled under a greater pressure than normal and
may thus reach greater distances from the patientJ. R. Soc. Interface (2009)Originating from an infected respiratory tract, it can
expose others in close quarters to infection.
The airflow profile of a cough has been studied
(Piirila¨ & Sovija¨rvi 1995; McCool 2006), has been
measured quantitatively by Khan et al. (2004)
(figure 1a), and has been imaged at high speed by
Tang & Settles (2008). The first phase is the strong
horizontal expulsion of air from the mouth at a flow
rate of up to 8 l s21 during the first 0.1 s. This is fol-
lowed by a second jet of air directed downwards at
about 308 with respect to the first, and with a visible
nasal contribution. The final phase of the cough is
once again a horizontal expulsion of air, slowly tapering
off to conclude the cough with a total duration of just
less than 1 s. The total volume of air expelled by the
cough, based on figure 1a, is approximately 2 l. For
some subjects the cough begins with a downward orien-
tation rather than horizontally, projecting the expelled
air jet towards the feet of bystanders (figure 1b). Such
cough orientation probably varies between individuals
as a matter of personal habit and routine.
Given a peak velocity of 13 m s21 (Khan et al. 2004)
and an effective initial diameter of roughly 2 cm, the
Reynolds number of the human cough is perhaps
18 000, more than high enough to ensure turbulent
flow. Such a turbulent jet spreads after leaving the
cougher’s mouth by entraining the surrounding air.
The accumulated knowledge of a wide range of round
turbulent jets—of which the cough jet is a member—
indicates that they spread conically with a total included
angle of approximately 248 (e.g. Pope 2000).
The main impetus for this study is that, despite the
potential for airborne infection, very little attention has
been devoted to characterizing the behaviour of the air-
flow produced by a cough, once it exits the mouth. Here
we investigate the cough using the real-time, non-invasive
schlieren optical technique with human volunteers.
1.3. The schlieren optical technique applied
to biomedical imaging
The optical technique known as schlieren has been used
for many years in science and engineering to visualize
transparent phenomena non-intrusively, especially the
flows of gases and liquids (Settles 2001). The physical
principle involved is that transparent phenomena
often refract, or bend, a light beam projected through
them. Air temperature differences, produced for
example by body heat or a cough, are one source of
such refraction. The resultant bending of light raysof the cabin despite the aircraft’s ventilation system
(Poussou et al. 2008).
As first photographed by Jennison (1942), oral
liquids are atomized during speech, coughing and sneez-
ing by the outflow of air from the lungs, generating
aerosols that can contain viruses and bacteria. The
abrupt release of the air in the lungs by coughing or
sneezing projects an impulsively started turbulent jet
of air from the mouth or nose with considerable momen-
tum. This aerosol-laden jet, led by a characteristic
vortex ring, can penetrate an impressive distance into
the surrounding ambient air before finally mixing out