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MICROBIOLOGY AND MOLECULAR BIOLOGY REVIEWS, Sept. 2008, p. 413–444 Vol. 72, No. 3
1092-2172/08/$08.00⫹0 doi:10.1128/MMBR.00002-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Methods for Sampling of Airborne Viruses
Daniel Verreault,
1
Sylvain Moineau,
2,3
and Caroline Duchaine
1,2
*
Centre de Recherche, Hoˆpital Laval, Institut Universitaire de Cardiologie et de Pneumologie de l’Universite´ Laval,
2725 Chemin Ste.-Foy, Quebec City, Quebec, Canada G1V 4G5
1
;De´partement de Biochimie et de Microbiologie,
Faculte´ des Sciences et de Ge´nie, Universite´ Laval, Pavillon Alexandre-Vachon, 1045 Avenue de la Medecine,
Quebec City, Quebec, Canada G1V 0A6
2
; and Groupe de Recherche en Ecologie Buccale (GREB)
and Fe´lix d’He´relle Reference Center for Bacterial Viruses, Faculte´deMe´decine Dentaire,
Universite´ Laval, Pavillon de Medecine Dentaire, 2420 rue de la Terrasse,
Quebec City, Quebec, Canada G1V 0A6
3
INTRODUCTION .......................................................................................................................................................413
CONTACT VERSUS AEROSOLS............................................................................................................................413
EPIDEMIOLOGICAL EVIDENCE OF AIRBORNE SPREAD OF VIRUSES ..................................................413
COMMON SOURCES OF AIRBORNE VIRUSES ...............................................................................................414
SIZE DISTRIBUTION OF VIRUS-LADEN PARTICLES ....................................................................................415
FACTORS AFFECTING THE RECOVERY OF AIRBORNE VIRUSES BY SAMPLING ..............................416
AIR SAMPLING METHODOLOGIES....................................................................................................................417
AIR SAMPLING FOR VIRUS RECOVERY...........................................................................................................417
Solid Impactors .......................................................................................................................................................417
Liquid Impactors ....................................................................................................................................................418
Filters .......................................................................................................................................................................419
Electrostatic Precipitators .....................................................................................................................................439
Other Sampling and Detection Methods.............................................................................................................440
ASSESSING THE EFFICIENCY OF AIRBORNE VIRUS SAMPLERS BY USE OF TRACERS..................440
LABORATORY STUDIES OF AIRBORNE VIRUSES .........................................................................................440
SURROGATE VIRUSES............................................................................................................................................440
CONCLUSIONS .........................................................................................................................................................441
ACKNOWLEDGMENTS ...........................................................................................................................................441
REFERENCES ............................................................................................................................................................441
INTRODUCTION
Any microorganism, including viruses, can become airborne.
Contaminated material can be aerosolized in many different
ways, ranging from wind to human and animal activities such as
sneezing, mechanical processes, etc. If the aerodynamic size of
an infectious particle is appropriate, it can remain airborne,
come into contact with humans or animals, and potentially
cause an infection. The probability of an airborne microorgan-
ism-laden particle causing an infection depends on its infec-
tious potential and its ability to resist the stress of aerosoliza-
tion.
Airborne microorganisms can represent major health and
economic risks to human and animal populations. Appropriate
preventive actions can be taken if the threat posed by such
microorganisms is better understood. Authorities need to be
aware of the nature, concentration, and pathogenicity of air-
borne microorganisms to better control them. This informa-
tion can be obtained by using various air sampling methods,
each of which has its particular advantages and disadvantages.
Many types of samplers and analytical methods have been used
over the years (Fig. 1). The purpose of this review is to present
the principles underlying viral aerosol sampling methods, with
their advantages and pitfalls.
CONTACT VERSUS AEROSOLS
The route of transmission of infections is not always easily
determined in an environment with undefined parameters. In-
fection by direct contact can occur when infected hosts are in
close proximity with a susceptible population. On the other
hand, infected hosts can transmit the disease without direct
contact. Moreover, many microorganisms, including viruses
(110), can remain infectious outside their hosts for pro-
longed periods of time, and this can lead to infections by
indirect contact. For example, a surface can become con-
taminated by deposited infectious droplets and eventually
cause the infection of susceptible hosts coming into contact
with it. The probability of airborne transmission of an in-
fectious disease can be determined by conducting epidemi-
ological studies (145) and/or by analyzing the microbiolog-
ical content of air samples.
EPIDEMIOLOGICAL EVIDENCE OF AIRBORNE
SPREAD OF VIRUSES
Studies on the aerobiology of infectious diseases, including
viral diseases, have been rather limited (115). This is due
mainly to the difficulty in collecting and analyzing airborne
biological contaminants, which is an even greater problem for
viruses. This technical challenge has made epidemiological
* Corresponding author. Mailing address: Centre de Recherche,
Hoˆpital Laval, Institut Universitaire de Cardiologie et de Pneumolo-
gie, 2725 Chemin Ste.-Foy, Quebec City, Quebec, Canada G1V 4G5.
Phone: (418) 656-8711, ext. 5837. Fax: (418) 656-4509. E-mail: caroline
.duchaine@bcm.ulaval.ca.
413
studies particularly useful. While data inferred from epidemi-
ological studies using computer-based analytical methods are
more equivocal than those from air sampling coupled with
microbial analyses, epidemiological studies can provide very
valuable information.
Many epidemiological studies have proposed that viruses
can spread from one host to another by using air for transport.
The capacity of the foot-and-mouth disease (FMD) virus to
spread by air has been studied and reviewed (36) over the years
and is now being investigated using computer models. One of
these models predicted that in a “worst-case scenario” of an
FMD outbreak, cattle could be infected as far as 20 to 300
kilometers away from an infectious source (37). Dispersion
models based on meteorological data and information on the
spread of FMD at the beginning of the 1967–1968 epidemic in
the United Kingdom strongly suggested that the infection may
have spread by the airborne route over a distance of 60 km
(59). Airborne transmission of FMD was also reported to have
occurred during the 1982–1983 epidemic in Denmark. In the
latter case, an analysis of epidemiological dynamics using mo-
lecular methods coupled with meteorological data concluded
that the infection had spread by air over a distance of 70 km
(27). Similarly, the results of a Canadian study on an FMD
epidemic reported that airborne viruses may have traveled 20
km downwind from the contaminated source (29). Neverthe-
less, a recent study on the O/UKG/2001 strain of FMD virus
indicated that it does not spread efficiently between sheep by
the airborne route. However, other strains may behave differ-
ently (134).
In 2001, a Norwalk-like virus outbreak in a school in the
United Kingdom was believed to have been caused by airborne
transmission (89). A similar occurrence has also been reported
for a hotel restaurant (88). A retrospective cohort study con-
ducted after a severe acute respiratory syndrome (SARS) ep-
idemic in Hong Kong in 2003 suggested that airborne spread
may have played an important role in the transmission of the
disease (146). The same mode of transmission was also hypoth-
esized in other studies of SARS (87, 104, 145). Aerosols may
also be responsible for the transmission of other viral diseases
(63, 83, 113).
COMMON SOURCES OF AIRBORNE VIRUSES
A virus can multiply only within a host cell. Infected cells can
spread viruses directly into the surrounding air (primary aero-
solization) or to fluids and surfaces, which can become sources
for airborne transmission (secondary aerosolization). Second-
ary aerosolization can occur for any virus, predominantly when
air displacements or movements around contaminated surfaces
or fluids disperse the viruses into the air. It can also occur by
liquid splashes, which can aerosolize viruses in liquids or on
surfaces. In fact, almost any kind of disturbance of infected
organisms or materials, even the bursting of bubbles in seawa-
ter (9), can produce airborne, virus-laden particles.
The most important aerosol source representing a risk for
human health is humans themselves. Since the interspecies
barrier is not a factor in the transmission of infections from
human to human, aerosol-mediated infections from human
sources can occur in everyday situations. Human infections
through viral aerosol sources have been studied in various
environments, including office buildings (102), hospitals (3, 10,
11, 13, 19, 41, 92, 95, 117, 126), restaurants (88), and schools
(89). The mechanisms of dispersion of infectious aerosols orig-
inating from humans are described in detail in a recent review
FIG. 1. Airborne virus sampling studies, according to date and analysis method.
414 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
(98). It is important to recognize that viruses can be spread by
airborne particles released by humans but also by other means.
Simply flushing a toilet containing infectious particles can
aerosolize significant concentrations of airborne viruses (14,
136). Wastewater treatment plants (24, 51) and sewage sprin-
klers (97, 125) can also produce viral aerosols.
Farm animals have also been studied for their emission of
airborne viruses. The FMD virus, which is one of the most
widely studied airborne animal viruses, has been detected in air
contaminated by infected pigs and ruminants (7, 8, 38, 39, 56,
60, 121) in both laboratory settings and farm environments.
This single-stranded RNA (ssRNA) virus of the Picornaviridae
family is excreted in all body fluids of infected animals (100)
and can become airborne directly from the animals or from the
secondary aerosolization of deposited viruses or virus-laden
particles. Other suspected sources of airborne viruses, such as
burning carcasses of infected animals (26), have not yet been
identified formally as true sources because additional investi-
gations are needed.
Poultry farms are also potential producers of virus-laden
airborne particles. The exotic Newcastle disease virus
(Paramyxoviridae family) was probably the first virus isolated
from a naturally contaminated environment (35) of poultry
houses sheltering infected birds. This 150-nm-diameter ssRNA
virus was detected in air samples from two farms during an
outbreak in Southern California in 2002–2003 (74). Air sam-
ples in and around broiler poultry houses have also been stud-
ied for the presence of viruses such as Escherichia coli bacte-
riophages, which are a fecal contamination tracer (50). Other
animals, such as bats (rabies virus) (144), rabbits (rabbit pox-
virus) (128, 141), and mice (polyomavirus) (94), have been
studied as sources of bioaerosols. These viruses can be released
into the air directly from animals by their breathing, coughing,
and sneezing or by secondary aerosolization. It should be
noted that the means of aerosolization has a critical impact on
the aerodynamic size and, thus, on the behavior of the airborne
particles.
SIZE DISTRIBUTION OF VIRUS-LADEN PARTICLES
For humans, most particles larger than 10 m will not pass
the upper airways; while smaller particles will travel more
easily toward the lungs, the particles will be trapped at different
proportions in the head airways and the tracheobronchial and
alveolar regions (75). The particle size determines whether or
not it can be inhaled and retained in the respiratory tract.
Given that virus-laden particles are a complex mixture of
various components (salts, proteins, and other organic and
inorganic matter, including virus particles), it is essential to
realize that the size of the viral particle itself does not rule the
airborne particle size. The influence of viruses alone on the
granulometric distribution of aerosols is likely negligible com-
pared to that of the remainder of the aerosol. To support this,
it was demonstrated that the particle size distribution of arti-
ficially produced submicrometer and ultrafine aerosols of cul-
ture media is not affected by the presence of bacteriophages
(76).
Infectious bioaerosols spontaneously released by sick ani-
mals are composed of variously sized particles. The smaller
size limit of a viral aerosol is limited to the virus diameter itself,
which can be as small as 20 to 30 nm, while the larger limit
depends on the size of the particle with which it is associated.
Size also dictates the capacity of a particle to remain airborne.
A study investigating the natural excretion of the FMD virus
(25 to 30 nm in diameter) into the air by infected pigs, using a
multistage liquid impinger sampler, showed that 65% to 71%
of the virus-laden particles were over 6 m in diameter, 19% to
24% ranged from 3 to 6 m in diameter, and 10% to 11% were
under 3 m in diameter (121). Similar results were also ob-
tained with infected sheep (56). The same type of bioaerosol
sampler was also used to establish a link between the concen-
tration and the size of infectious particles, using artificially and
naturally produced aerosols of FMD virus. This study reported
that over 85% of the particles in the artificially produced aero-
sols were less than 3 m in diameter, whereas the size distri-
butions of the natural aerosols were similar in all three stages
of the sampler (39). Another study investigating pigs infected
with Aujeszky’s disease virus (Herpesviridae family; approxi-
mately 150 nm in diameter; double-stranded DNA [dsDNA]
virus) found that the infectivity of the aerosols collected in
each stage of the three-stage impinger varied over time. The
investigators reported that the size distributions of the aerosols
in the three stages were comparable on day 2 of the infection
but that there was an increase in infectivity associated with
larger particles on days 3 and 4 (40). Nevertheless, no clear
association has been made between aerosol infectivity and a
particular size range (60).
While single virus particles exist in the air (76), they tend to
aggregate rapidly. Aggregation speed depends on the size dis-
tribution of the airborne particles, the concentration of the
aerosol, and the thermodynamic conditions (142). Many fac-
tors influence the size distribution of both naturally and arti-
ficially produced viral aerosols. Artificially produced aerosols
are normally used in controlled environments where there are
no other aerosols to which the nebulized particles can bind.
They are thus influenced only by the size of the original droplet
created by the nebulizer and by the solute concentration in
the droplet. When a droplet evaporates (Fig. 2), its final size (the
droplet nucleus) depends on the relative humidity (RH) in the
chamber. To some extent, this phenomenon can also be ob-
served with natural aerosols. For example, infectious droplets
exhaled by animals shrink rapidly with the lower humidity
outside the respiratory airway, creating smaller aerosols. How-
ever, the size distribution of such naturally generated bioaero-
sols depends on the sizes of the particles to which the micro-
organisms bind. This binding may occur by diffusion,
impaction, interception, or electrostatic attraction (98). If
mostly large particles are encountered in the air of a given
environment, then the particles making up the infectious aero-
sol will also tend to be large. Interestingly, larger particles may
be relatively less hazardous than smaller ones. It has been
shown on pig farms that a visually clean environment may be
more contaminated by bioaerosols than a visually dirty one
(43). This may be due to the fact that larger particles tend to
settle faster than smaller particles do; the settling velocity of
0.001-m particles is 6.75E⫺09 m/s, while 10-m particles
settle at 3.06E⫺03 m/s and 100-m particles settle at
2.49E⫺01 m/s (75). Airborne particles in a “clean” environ-
ment are more likely to remain small and inhalable by animals
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 415
and humans than are particles in a dirty environment, which
tend to grow larger by sticking to other airborne particles.
FACTORS AFFECTING THE RECOVERY OF AIRBORNE
VIRUSES BY SAMPLING
Organic and inorganic materials in viral aerosols can affect
the size of the aerosolized particles and their infectious poten-
tial. Many factors, such as RH, temperature, radiation, aero-
solization medium, exposure period, chemical composition of
the air, and sampling methods, can affect the infectivity of
airborne viruses. Each virus reacts in its own particular way to
each factor or combination of factors, depending on the struc-
tural composition of the virus and its interactions with other
aerosol components. However, the structural composition of
airborne viruses alone cannot be used to predict survival under
different environmental conditions (78).
RH is the most widely studied of the factors that affect
airborne virus infectivity (Table 1). Depending on the virus,
optimal preservation of infectivity may require a low RH (un-
der 30%), an intermediate RH (30% to 70%), or a high RH
(over 70%). Influenza virus (65, 118), Semliki Forest virus (17),
Japanese B encephalitis virus (86), porcine reproductive and
respiratory syndrome virus (72), Newcastle disease virus, and
vesicular stomatitis virus (122), all of which are enveloped, are
most stable at low RH, while rhinovirus (79, 84), poliovirus (65,
79, 81), T3 coliphage (45, 122), rhinotracheitis virus (122),
picornavirus (5), and viruses of the Columbia SK group (4),
which are nonenveloped (with the exception of the rhinotra-
cheitis virus), are most stable at high RH. Human coronavirus
229E (79), pseudorabies virus (119), and rotavirus (81, 82, 116)
are most stable at intermediate RH. The first two are envel-
oped, while mature rotaviruses are usually nonenveloped. RH
has no effect on the stability of airborne St. Louis encephalitis
virus under the conditions tested (112).
Seasonal variations in indoor RH have also been correlated
with fluctuations in the morbidity of influenza (low RH) and
poliomyelitis (high RH) viruses, with the highest morbidity
occurring at the optimal RH for each virus (68, 69). Seasonal
variations have also been observed with measles virus (34) and
respiratory syncytial virus (147). An intriguing study comparing
the effect of RH on the stability of an airborne picornavirus to
that on its genomic RNA (5) indicated that the inactivation of
airborne picornaviruses by low RH levels is not due to the
instability of the RNA but, rather, to structural damage to the
virion (5). The findings of these studies indicate that there is no
absolute correlation between RH and the preservation of viral
FIG. 2. Evaporation of a liquid droplet (left) to a droplet nucleus (right). As the liquid evaporates, the nonevaporative content concentrates
until a droplet nucleus is obtained.
TABLE 1. Effects of RH on infectivity of a selection of airborne viruses
Virus Optimal RH for
maximum infectivity Family Genetic material Size (nm) Envelope
Influenza virus Low Orthomyxoviridae ssRNA (⫺) 80–120 Yes
Newcastle disease virus Low Paramyxoviridae ssRNA (⫺) 150 Yes
Vesicular stomatitis virus Low Rhabdoviridae ssRNA (⫺)60⫻200 Yes
Japanese encephalitis virus Low Flaviviridae ssRNA (⫹) 40–60 Yes
Porcine reproductive and respiratory
syndrome virus
Low Arteriviridae ssRNA (⫹) 45–60 Yes
Semliki Forest virus Low Togaviridae ssRNA (⫹) 70 Yes
Human coronavirus 229E Mid-range Coronaviridae ssRNA (⫹) 120–160 Yes
Rotavirus Mid-range Reoviridae dsRNA 100 No
Pseudorabies virus Mid-range Herpesviridae dsDNA 200 Yes
Rhinovirus High Picornaviridae ssRNA (⫹) 25–30 No
Poliovirus High Picornaviridae ssRNA (⫹) 25–30 No
Picornavirus High Picornaviridae ssRNA (⫹) 25–30 No
Columbia SK group High Picornaviridae ssRNA (⫹) 25–30 No
T3 coliphage High Podoviridae dsDNA 60 (capsid) No
Rhinotracheitis virus High Herpesviridae dsDNA 200 Yes
St. Louis encephalitis virus All Flaviviridae ssRNA (⫹) 40–60 Yes
416 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
infectivity in aerosols and that the impact of RH should be
determined for each virus. However, it appears that low RH
tends to preserve the infectivity of enveloped viruses, while the
stability of nonenveloped viruses is best preserved at high RH.
Temperature can also have an impact on the infectivity of
airborne viruses. For example, the stability of certain infectious
airborne viruses (47, 65, 72, 79, 119, 122) is improved at low
temperatures but does not depend on RH. UV radiation is
another factor that influences survivability. UV germicidal
lamps, for instance, can be used to inactivate airborne micro-
organisms, including viruses, in indoor settings (53). However,
in certain cases, RH must be taken into consideration. For
example, vaccinia virus is more susceptible to UV radiation at
low RH than at high RH (93).
Interestingly, aerosolization can inactivate some viruses to a
certain extent, depending upon the nature of the spray fluid,
the temperature, and the RH (80). This was reported for the
recovery of bovine parainfluenza virus (47) and infectious bo-
vine rhinotracheitis virus, where various combinations of these
factors generated different results (48). Certain chemicals also
have diverse effects on the stability of airborne viruses. For
example, adding salt to a spray suspension reduces the recov-
ery of airborne infectious Semliki Forest virus at high RH in a
controlled chamber (17). On the other hand, polyhydroxy com-
pounds (17, 118) and peptones (69) are protective. Similarly,
adding dextrose to the spray fluid significantly enhances the
recovery of coliphage T3 at mid-range RH, but spermine, sper-
mine-phosphate, thiourea, galacturonic acid, and glucosaminic
acid have no effect on virus recovery (45). Mid-range RH and
fecal matter as a spray fluid have been shown to enhance the
recovery of a strain of human rotavirus (82). Organic matter
and chemical compounds probably exert their protective effect
by reducing desiccation and other environmental stresses.
Lastly, the gas composition of the air can also have an
influence on viruses, as ozone has been shown to inactivate
airborne viruses (96). In fact, virus susceptibility to ozone is
much higher than those of bacterial and fungal bioaerosols
(133). However, the ozone efficacy will vary from virus to virus.
For example, phage X174 is more susceptible to ozone than
are phages MS2 and T7 (133). Ions in the air can also reduce
the recovery rate of certain viruses, such as aerosolized T1
bacteriophage, with positive ions having the most detrimental
effect (64).
AIR SAMPLING METHODOLOGIES
Most air sampling technologies depend on the aerodynamic
diameter of the airborne particles, the adhesion properties of
airborne particles, Brownian motion, thermal gradients, and
the inertia of the particles. Aerosolized particles attach to any
surface with which they come into contact (75). Adhesive
forces such as van der Waals forces, electrostatic forces, and
surface tension partly explain this adhesion. Most of the sam-
pling methodologies presented in this review are based on this
principle.
Airborne particles with aerodynamic diameters on the order
of 100 nm or less are prone to a particular way of moving,
mainly due to the billions of collisions they encounter with gas
molecules. This is called Brownian motion, and the smaller the
particle, the greater the movement and the more likely that the
particle will diffuse, come into contact with a surface, and
adhere to it. When this happens, the other suspended particles
occupy the space left vacant by the particle that has adhered to
the surface. This phenomenon is the basis for the efficient
removal of very small particles by filtration, particularly when
the distance between two surfaces of the filter is sufficient for
the particles to pass through.
Larger particles with aerodynamic diameters on the order of
a micrometer or more are less influenced by Brownian motion
but have greater inertia. Gravitational attraction has a signif-
icant impact on these particles and causes them to settle on
surfaces. These particles are also more easily diverted from a
gas streamline, leading to impaction on surfaces, especially at
high velocity and when the angle of the airflow is drastically
altered. Very small particles have less inertia and will more
likely follow the streamline.
AIR SAMPLING FOR VIRUS RECOVERY
Various sampling devices can be used to recover airborne
viruses, and some are illustrated in Fig. 3. The most common
are liquid and solid impactors as well as filters. Electrostatic
precipitators have also been tested. Table 2 presents an exten-
sive compilation of studies on the recovery of viral particles.
The history of use of air samplers for viral aerosols is summa-
rized in Fig. 1.
Solid Impactors
Solid impactors, such as Andersen samplers, slit samplers,
and cyclone samplers, are usually more efficient at capturing
large particles. Andersen and slit samplers accelerate the par-
ticles through narrow holes or slits. The streamline moves
toward a solid surface and abruptly changes direction. The
inertia of the particles deviates them from the airflow and
impacts them on the surface, which usually holds a petri dish
with a culture medium. The medium is either washed to collect
the particles or used directly for plaque assays. Andersen sam-
plers contain a number of stages, each of which traps particles
of a specific aerodynamic size range, and is often used to
determine the sizes of virus-laden particles (54, 79, 132, 133).
The multistage configuration is designed to accelerate the in-
coming particles. The first stage induces moderate acceleration
so that only the largest particles deviate from their trajectory.
The second stage accelerates the smaller particles a little more,
and so on. A six-stage Andersen sampler recovers particles
ranging from 0.65 m in diameter on the lowest stage to 7.5
m and over on the top stage. Single-stage Andersen samplers
can also be used to capture particles. The lower recovery limit
of these samplers is a function of the diameter of the holes
through which the particles are accelerated.
Slit samplers are used mostly to determine aerosol concen-
trations of bacteria as a function of time. The accelerated
particles are impacted onto a rotating petri dish containing a
culture medium. This makes it possible to determine the time
when each particle was sampled. Time-dependent results can
be obtained only if the samples are grown directly on the solid
culture medium. Obviously, the use of a liquid medium or
buffer on top of a solid medium compromises this function.
Nevertheless, slit samplers have been used with a liquid layer
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 417
on the culture medium to recover viruses. The particles that
impact the solid surface are immediately resuspended in a
liquid medium to maximize the recovery of both infective vi-
ruses and viral nucleic acids. This method was used successfully
in Toronto, Canada, during the 2003 SARS outbreak (19). In
fact, two air sampling methods were used during this outbreak,
namely, a modified high-resolution slit sampler system and
polytetrafluoroethylene (PTFE) membrane filters with 0.3-m
pores. The samples were tested for viruses by reverse tran-
scriptase PCR (RT-PCR) and culture assays. The cultures
were all negative, and only 2 of the 10 slit samples were PCR
positive. These results might be explained by the absence or a
very low concentration of airborne viruses.
Centrifugal forces have also been used to sample artificially
generated airborne influenza viruses. The samples collected by
centrifugation within an hour after aerosolization caused in-
fluenza in inoculated ferrets (140). The centrifugal sampler
recovered close to 100% of 2.3-m-diameter particles and 50%
of 0.77-m-diameter particles at 4,500 rpm, which corresponds
to an airflow of 1.44 to 1.90 cubic feet/min (40 to 54 liters/min)
(108). Errington and Powell developed small and large cyclone
separators. The small cyclone separator has a flow rate of 75
liters/min, and the large one has a flow rate of 350 liters/min.
Both cyclones accelerate the air by using a centrifugal vortex,
pushing the airborne particles into contact with a solid surface
by using the inertia of the particles. A scrubbing liquid is
constantly injected into the cyclone and collected in a bottle at
its base. The concentration of the aerosol in the liquid depends
on the air sampling and liquid injection rates. The smaller
sampler can, for example, concentrate 100 liters of air in 1 ml
of liquid (49). This first generation of cyclone separators in-
spired the development of similar apparatuses, which can sam-
ple the air at various rates.
A 170-liters/min flow rate (2- and 20-min samples) has been
used to sample air contaminated with FMD virus released by
infected pigs (7) as well as by sheep and heifers (8). In another
study, a 300-liters/min flow rate (15-min sample) was success-
fully used to sample the air of isolated units housing pigs
infected with Aujeszky’s disease virus. However, the sampler
was unable to detect low levels of airborne virus (20). Cyclone
samplers have also been used at high flow rates, ranging from
700 to 1,000 liters/min (5- to 30-min samples), to recover air-
borne viruses (38–40, 60, 62). A recirculating liquid cyclone-
style air sampler operating at 265 liters/min (8-h sample), com-
bined with culture and RT-PCR, has been used to detect exotic
Newcastle disease virus in naturally contaminated commercial
poultry flocks (74). The capacity of cyclones to concentrate
aerosols in large volumes of air over long, uninterrupted peri-
ods is one of the major advantages of this type of sampler.
However, some investigators have reported that cyclone sam-
plers are much less efficient than other samplers at recovering
low concentrations of airborne viruses (20). This may be due to
the physical stress caused by cyclone samplers, which may
cause structural damage to the viruses and thus decrease their
infectivity (20).
Liquid Impactors
All-glass impingers (AGIs) (Fig. 3), also called Porton im-
pingers, and AGI-like samplers are the most often used sam-
FIG. 3. Diagrams of six different bioaerosol samplers. Red lines and arrows represent the airflow into the sampler. Blue arrows represent
airflow out of the sampler. These drawings are simplified representations.
418 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
plers for the capture of airborne viruses (Fig. 1; Table 2). The
liquid impinger, which was first described by May and Harper
(91), works by accelerating airborne particles through a narrow
orifice placed at a fixed distance from the bottom of a flask
containing a liquid. A pressure drop is created in the flask and
forces the air to enter through the inlet of the impinger. The
air enters horizontally through a glass tube, which curves to a
vertical position, forcing the air to change direction and flow
downward. The diameter of the tubing abruptly narrows and
acts as a critical flow orifice, accelerating the air passing
through it to sonic velocity. The flow remains constant as long
as there is at least half an atmosphere of suction in the im-
pinger. The curve in the tube is intended to trap the larger
particles by inertial impaction and mimics the airway of the
human nose. The largest particles entering the flask through
the critical flow orifice are impacted onto the liquid. The for-
mation of small bubbles in the liquid of the impinger can also
help to sample very small particles by diffusion. However, the
reaerosolization of particles due to the scavenging properties
of the air bubbles can be a problem, especially for hydrophobic
particles. The liquid prevents desiccation and facilitates the
extraction of genetic material for subsequent analysis. The
AGI-4 sampler (the number refers to the distance, in millime-
ters, separating the tip of the critical orifice from the bottom of
the flask) and the AGI-30 sampler, also called a raised im-
pinger, are often used as standard reference samplers (71).
Multistage liquid impingers are also available. Particles im-
pinge into liquids in successive stages as a function of their
aerodynamic size. This type of sampler, like the Andersen
sampler, is used mainly to determine the size distribution of
infectious particles (39, 40).
According to Harstad (66), who compared the sampling
efficiencies of two types of liquid impingers, two types of filters,
and a fritted bubbler, using submicrometer aerosols of a sus-
pension of bacteriophage T1 (a tailed bacterial virus) with a
radioactive tracer, liquid impingers are the least destructive
samplers, with a relative efficiency, as determined by culture,
superior by 18% to that of the next best sampler, although 30%
to 48% of the sample was physically lost. Harstad also reported
that filters are very destructive for this bacterial virus but are
the most efficient at collecting submicrometer particles and
that the fritted bubbler is the least efficient sampler, with a
physical loss of over 80% of the sample (66). These differences
in the recovery rates of AGI samplers and filters were con-
firmed in a later study (64). The gentler sampling process
leading to better recovery of infective viruses seems to be the
main reason for the wide use of AGI samplers in aerovirology.
Many studies involving airborne virus sampling have been con-
ducted using the AGI-30 or AGI-4 sampler as the main sam-
pling device (4, 10, 17, 42, 45–48, 57, 58, 65, 67, 78, 81, 82, 84,
86, 105, 116, 118, 119, 122, 129–131, 138). Most were done to
determine the effects of various factors on the recovery rates of
airborne viruses. Although some studies indicate that the AGI
has a lower recovery potential than other samplers, such as the
large-volume sampler (LVS) (94, 144), the Andersen sampler
(127), and the slit sampler (126), other studies suggest that the
AGI recovers concentrations of viruses that are equivalent to
those with the LVS (121), greater than those with the
Andersen sampler (15), and greater than (30) or equal to (85)
those with the slit sampler.
A recently developed impinger model, the “swirling aerosol
collector” (Fig. 3) (143), commercialized as the BioSampler,
has also been used to study viral aerosols in the same way as
AGIs (22, 72, 124). This newer impinger works much like the
AGI, with a curved inlet tube and a vacuum in the flask to force
the air through the sampler. The major difference is the num-
ber and positions of nozzles. Instead of forcing air at sonic
speed through a single nozzle directed toward the base of the
flask, as with the AGI, the BioSampler has three tangential
sonic nozzles. The collection liquid in the flask moves in a
swirling motion during sampling. The sampling procedure is
less violent and less destructive than that with the AGI-30
sampler. Hermann et al. (73) studied the BioSampler and
reported that it, as well as the AGI-30 sampler, collects signif-
icantly more aerosolized porcine reproductive and respiratory
syndrome viruses than the AGI-4 sampler does. They also
reported that the collection efficiency of the BioSampler is
significantly greater than that of the AGI-30 sampler after 15
and 20 min of sampling (73). However, both the AGI-30 and
the BioSampler (as well as the frit bubbler) are surprisingly
inefficient at recovering submicrometer and ultrafine virus
aerosols, with collection efficiencies of ⬍10% for all three
samplers for the 30- to 100-nm particle size range (76).
Prehumidifying aerosols by using a humidifier bulb in com-
bination with an AGI-30 sampler can have both positive and
negative effects on the recovery of infectious viruses from air-
borne material, depending on the virus. The AGI-30 with hu-
midifier bulb has been shown to increase the recovery of air-
borne coliphages T3 (67, 137), T2, and T7 (16), as well as
Pasteurella pestis bacteriophages (67). Recovery increases five-
fold at high RH and up to 1,000-fold at low RH when a
peptone solution or saliva is used as the spray medium. How-
ever, adding NaCl to the spray medium has no effect on the
recovery of T3 (131). Prehumidification has no effect on the
recovery of mengovirus 37A (Picornaviridae family; ssRNA
virus) or vesicular stomatitis virus (Rhabdoviridae family;
ssRNA virus) but increases the recovery of bacteriophage S13
(Microviridae family; ssDNA virus) at mid-range RH (137).
One possible explanation for the beneficial effect of prehu-
midification may be that the median size of the particles is
increased at high RH by the condensation of the water vapor
on the airborne particles. The condensation may also have a
negative effect by dissolving the particle nuclei, exposing the
viruses to high concentrations of solutes, which may structur-
ally damage the virus, leading to a loss of infectivity. No de-
finitive explanation has yet been proposed to explain the effect
of prehumidification on viral infectivity.
Filters
Since most samplers cannot efficiently trap particles with an
aerodynamic size of ⬍500 nm, filters are frequently used to
sample airborne viruses. Filter efficiency is based on the fol-
lowing five basic mechanisms: (i) interception, (ii) inertial im-
paction, (iii) diffusion, (iv) gravitational settling, and (v) elec-
trostatic attraction (75). While each mechanism depends on
the aerodynamic diameter of the particle, interception also
depends on particle radius. Interception occurs when a particle
follows the streamline going around an obstacle but, because
of its size, comes into contact with and is intercepted by the
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 419
TABLE 2. Summary of virus aerosol sampling studies
Sampler type Air sampling rate
or capacity
Sampling
environment Virus and/or tracer Particle size
(m) Analytical method Comments Aerosol source Reference Yr of
publication
Wells air centrifuges 200-ft
3
chamber Influenza virus strain
Puerto Rico 8
Inoculation of ferrets All ferrets inoculated
with material
collected from the
air contracted
influenza when the
sampling was done
within an hour after
the material was
suspended
Atomizer 140 1936
Sampling atomizers 540 and 1,080 liters Poultry houses Pneumoencephalitis
virus
Inoculation of chick
embryos
Air samples contained
sufficient viruses to
infect chick embryos
Poultry 35 1948
Impingers 11 liters/min for 15
to 120 s
Modified Henderson
apparatus and
rotating stainless
steel drum
Vaccinia virus, influenza
A virus strain PR8,
Venezuelan equine
encephalomyelitis
(VEE) virus,
poliomyelitis virus
type I (Brunhilde),
formalin-killed
Pasteurella tularensis
cells labeled with
32
P
for VEE virus and
virus suspension
labeled with
32
P for
the other three
viruses
Inoculation of eggs
or mice, culture
Airborne viruses were
recovered up to 23 h
after aerosolization
at different RH and
temperatures
Collison atomizer 65 1961
Slit sampler (solid
gelatin medium)
1ft
3
/min for 1 h 1,500-liter Plexiglas
chamber
Bacteriophage T3 0.5–1 (96% of
particles)
Culture Slit sampler with 12%
gelatin medium
recovered ⬃75% of
phages compared to
AGI-30 sampler
Vaponefrin nebulizer 30 1961
AGI-30 12.8 liters/min for
15 min
1,500-liter Plexiglas
chamber
Bacteriophage T3 0.5–1 (96% of
particles)
Culture Slit sampler with a
12% gelatin medium
recovered ⬃75% of
phages compared to
AGI-30 sampler
Vaponefrin nebulizer 30 1961
Slit sampler 1 ft
3
/min for 4 and
15 min
Modified Henderson
apparatus
VEE virus Inoculation of mice,
titration with
embryonated hen’s
eggs, and culture
Results with the slit
sampler and the
AGI-30 sampler
were comparable
Collison generator 85 1961
AGI-30 12.5 liters/min for 4
and 15 min
Modified Henderson
apparatus
VEE virus Inoculation of mice,
titration with
embryonated hen’s
eggs, and culture
Results with the slit
sampler and the
AGI-30 sampler
were comparable
Collison generator 85 1961
Porton impinger 10 to 20 liters/min
for 0.5 and 1 min
14-ft
3
aerosol
chamber with fan
Bacteriophage T3 Culture The electrostatic
precipitator was very
efficient; ozone can
be produced at high
RH with the
electrostatic
precipitator; a 0.1%
peptone solution can
protect phage T3
from ozone
Collison spray 99 1961
420 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
Electrostatic
precipitator
5 to 40 liters/min
for 5 min
14-ft
3
aerosol
chamber with fan
Bacteriophage T3 Culture The electrostatic
precipitator was very
efficient; ozone can
be produced at high
RH with the
electrostatic
precipitator; a 0.1%
peptone solution can
protect phage T3
from ozone
Collison spray 99 1961
Glass sampler
containing
tightly packed
dry cotton
20 to 18,000 liters Hospital Variola virus Inoculation of chick
embryos
Viruses were recovered
in 1 of 38 samples
Humans 95 1961
Modified capillary
impingers
11 liters/min for
10 s
4-m
3
conditioned
room
Bacteriophage T5,
influenza virus,
poliomyelitis virus
5–6 (mean
size 5 cm
from the
outlet)
Culture Recovery depended on
the aerosolization
medium and the RH,
but the effect of RH
did not depend on
the composition of
the medium; the
survival curves at
different RH were
not the same for
influenza and
poliomyelitis viruses
All-glass indirect-
type spray
69 1962
AGI-30 12.5 liters/min for 1
min
1,600-liter Plexiglas
chamber with fan
Bacteriophage T3 0.5–5.0 (peak
at 2.0 m)
Culture The best recovery was
at high RH
Hartman atomizer 45 1964
Short-stem AGI 12.0 liters/min for 5
to 20 min
Hospital rooms of
infected patients
Viruses associated with
respiratory diseases
Culture Viruses were recovered
in 1 of 23 trials
Humans 10 1964
Liquid impingers
AGI-4 12.5 liters/min for 5
min
Aerosol chamber Bacteriophage T1,
32
P Mass median
diameter,
0.2
Culture Recovered the most
viable phages, but
major sample loss
(30 to 40%)
Dautreband D301
aerosol generator
66 1965
Capillary impinger 2.5 L/min for 5 min Aerosol chamber Bacteriophage T1,
32
P Mass median
diameter,
0.2
Culture Recovered the most
viable phages, but
major sample loss
(30 to 40%)
Dautreband D301
aerosol generator
66 1965
Filters
Chemical Corps
type 6
1 liter/min (8 cm/s)
for 5 min
Aerosol chamber Bacteriophage T1,
32
P Mass median
diameter,
0.2
Culture Very destructive but
most efficient
collection of
submicrometer
airborne particles
Dautreband D301
aerosol generator
66 1965
Glass filter paper
(MSA 1106BH)
1 liter/min (8 cm/s)
for 5 min
Aerosol chamber Bacteriophage T1,
32
P Mass median
diameter,
0.2
Culture Very destructive but
most efficient
collection of
submicrometer
airborne particles
Dautreband D301
aerosol generator
66 1965
Fritted bubbler 1 liter/min for 5
min
Aerosol chamber Bacteriophage T1,
32
P Mass median
diameter,
0.2
Culture Important sample loss
(⬎80%)
Dautreband D301
aerosol generator
66 1965
Porton impinger 10 liters/min for 5
to 10 min
Hospital rooms Smallpox virus Inoculation of chick
embryos
Large particles from
patients’ bed clothes
seemed to be mostly
responsible for
contamination of the
air in the vicinity of
patients
Humans 41 1965
Continued on following page
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 421
TABLE 2—Continued
Sampler type Air sampling rate
or capacity
Sampling
environment Virus and/or tracer Particle size (m) Analytical method Comments Aerosol source Reference Yr of
publication
Petri dish (settling
plates)
5 to 10 min Hospital rooms Smallpox virus Inoculation of chick
embryos
Large particles from patients’
bed clothes seemed to be
mostly responsible for
contamination of the air in
the vicinity of patients
Humans 41 1965
Top part of an
Andersen
sampler
10 liters/min for 5
to 10 min
Hospital rooms Smallpox virus Inoculation of chick
embryos
Large particles from patients’
bed clothes seemed to be
mostly responsible for
contamination of the air in
the vicinity of patients
Humans 41 1965
Capillary
impingers
4-m
3
conditioned
room
Measles virus 5 (average diameter) Culture Recovery depended on RH 34 1965
LVS 1,785 ft
3
for 5 min 1,440-ft
3
Army
hospital room
Adenovirus type 4 Culture Viable viruses were
recovered at very low
concentrations
Humans 11 1966
LVS 10,000 liters/min for
3.5 min
32,800-liter room
with atomized
virus
suspension
Coxsackie virus A type
21, sodium
fluorescein
1–15 Culture LVS consistently recovered
more fluorescein than the
AGI-30 did
University of
Chicago Toxicity
Laboratories
atomizer
54 1966
AGI-30 12.5 liters/min for 1
min
32,800-liter room
with atomized
virus
suspension
Coxsackie virus A type
21, sodium
fluorescein
1–15 Culture LVS consistently recovered
more fluorescein than the
AGI-30 did
University of
Chicago Toxicity
Laboratories
atomizer
54 1966
Electrostatic
precipitator
Rooms containing
infected rabbits
Rabbit pox virus strain
Utrecht and
Rockefeller Institute
strain
Culture Low concentrations were
recovered with the
electrostatic precipitator,
and none was recovered
with the impinger
Rabbits 141 1966
Raised glass
impinger
Rooms containing
infected rabbits
Rabbit pox virus strain
Utrecht and
Rockefeller Institute
strain
Culture Low concentrations were
recovered with the
electrostatic precipitator,
and none was recovered
with the impinger
Rabbits 141 1966
AGI-4 12.5 liters/min for 5
min
Aerosol chamber Bacteriophage T1 ⬍1 Culture The AGI-4 sampler
recovered more airborne
viruses than type 6 filter
paper did; ions affected the
stability of submicrometer
T1 phage
Dautrebande aerosol
generator
64 1966
Chemical Corps
type 6 filter
paper
5 min at 1.0 liter/
min
Aerosol chamber Bacteriophage T1 ⬍1 Culture The AGI-4 sampler
recovered more airborne
viruses than type 6 filter
paper did; ions affected the
stability of submicrometer
T1 phage
Dautrebande aerosol
generator
64 1966
AGI-30 12.5 liters/min for 5
min
Rotating drum Columbia SK group
viruses
Culture Inactivation of the airborne
viruses depended on RH
Modified Wells
refluxing atomizer
4 1966
AGI 12 liters/min (3-liter
samples)
140-liter
aluminum drum
Newcastle disease
virus, infectious
bovine
rhinotracheitis virus,
vesicular stomatitis
virus, T3 phage,
rhodamine B
Culture Best recovery at low RH for
Newcastle disease virus
and vesicular stomatitis
virus and at high RH for
bovine rhinotracheitis virus
and T3
De Vilbiss no. 40
nebulizer
122 1967
LVS 10,000 liters/min for
6or7min
1,440-ft
3
Army
hospital room
Adenovirus Culture Recovery of one viral unit
per 204 to 1,970 ft
3
of air
in 10 of 14 samples
Humans 13 1967
422 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
AGI-4 10 liters/min for 10
min
Frio Cave, TX Rabies virus Inoculation of
animals
Rabies virus was isolated
from four of eight samples
collected with the LVS L
and from none of the five
AGI-4 samples
Bats 144 1968
LVS model L 10,000 liters/min for
10 to 30 min
Frio Cave, TX Rabies virus Inoculation of
animals
Rabies virus was isolated
from four of eight samples
collected with the LVS L
and from none of the five
AGI-4 samples
Bats 144 1968
Impinger 2 min 500-liter rotating
toroid drum
Mengovirus 37A Culture Inactivation of the virus was
due to damage to the
virion structure
Modified Wells
refluxing atomizer
5 1968
LVS with added
preimpactor
357 ft
3
for 1 min or
full room for 3
min
986-ft
3
room Adenovirus type 4 Depended on
individual subjects
Culture Recovery of one viral unit
per 86 to 448 ft
3
of air in 3
of 11 samples; infectious
particles were present in
both small- and large-
particle aerosols
Humans 12 1968
LVS model M 1,000 liters/min for
1h
3.65- by 3.35- by
3.05-m loose
boxes
Four strains of FMD
virus (01 Lombardy,
01 Swiss 1/66, 01
BFS 1860, and 02
Brescia)
Culture and
inoculation of
unweaned mice
The amt of virus recovered
by the impinger was similar
to that recovered by the
LVS
Cattle, sheep, and
pigs
121 1969
Multistage liquid
impinger
55 L/min for 45 min 3.65 ⫻3.35 ⫻
3.05 m loose
boxes
Four strains of FMD
virus (01 Lombardy,
01 Swiss 1/66, 01
BFS 1860, and 02
Brescia)
Culture and
inoculation of
unweaned mice
The amt of virus recovered
by the impinger was similar
to that recovered by the
LVS
Cattle, sheep, and
pigs
121 1969
AGI-30 Dual-aerosol
transport
apparatus
T3 and S13 coliphages,
mengovirus-37A,
and vesicular
stomatitis virus
Culture While the recovery of some
viruses from aerosols was
increased by
prehumidification, no
generalization could be
drawn
Modified Wells
reflux atomizer
137 1969
AGI-30 with a
humidifier
bulb
Dual-aerosol
transport
apparatus
T3 and S13 coliphages,
mengovirus-37A,
and vesicular
stomatitis virus
Culture While the recovery of some
viruses from aerosols was
increased by
prehumidification, no
generalization could be
drawn
Modified Wells
reflux atomizer
137 1969
AGI-30 12.5 liters/min Two 500-liter
rotating drums
Bacteriophage T3 Culture Virus recovery was best at
higher RH
Atomizer designed
by the lab (similar
to the Wells
atomizer)
138 1969
Raised impingers Semliki Forest virus,
washed Bacillus
subtilis spores
Culture Best recovery at low RH,
with good protective effect
of polyhydroxy compounds
at low RH
Collison spray 17 1969
AGI-30, with or
without a
humidifier
bulb
Two 500-liter
rotating drums
Bacteriophage T3 and
Pasteurella pestis
bacteriophage
1–5 Culture Prehumidification of the air
samples significantly
enhanced the recovery of
airborne viruses
Atomizer designed
by the lab (similar
to the Wells
atomizer)
67 1969
AGI-30 1 min 500-liter rotating
toroid drum
Bacteriophages S13
and MS2
Culture RH and aerosol composition
had a major impact on
viral recovery
Modified Wells
reflux atomizer
42 1970
AGI-30 with
humidifier
bulb
1 min 500-liter rotating
toroid drum
Bacteriophages S13
and MS2
Culture RH and aerosol composition
had a major impact on
viral recovery
Modified Wells
reflux atomizer
42 1970
Continued on following page
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 423
TABLE 2—Continued
Sampler type Air sampling rate
or capacity
Sampling
environment Virus and/or tracer Particle size
(m) Analytical method Comments Aerosol source Reference Yr of
publication
Slit sampler with
adhesive
surface petri
dishes
Room containing
infected rabbits
(4,500 ft
3
)
Rabbit poxvirus strain
Utrecht
Culture More viruses were recovered in
the top stages of the
Andersen sampler than in the
lower stages; both the slit
samplers and the Andersen
sampler successfully
recovered airborne viruses
Rabbits 128 1970
Automated slit
sampler with
adhesive
surface petri
dishes
1 h (60 ft
3
) Room containing
infected rabbits
(4,500 ft
3
)
Rabbit poxvirus strain
Utrecht
Culture More viruses were recovered in
the top stages of the
Andersen sampler than in the
lower stages; both the slit
samplers and the Andersen
sampler successfully
recovered airborne viruses
Rabbits 128 1970
Andersen sampler
with adhesive
surface petri
dishes
1 h (60 to 120 ft
3
) Room containing
infected rabbits
(4,500 ft
3
)
Rabbit poxvirus strain
Utrecht
Culture More viruses were recovered in
the top stages of the
Andersen sampler than in the
lower stages; both the slit
samplers and the Andersen
sampler successfully
recovered airborne viruses
Rabbits 128 1970
Modified Andersen
sampler
1ft
3
/min Aerosol apparatus
(Henderson)
Vaccinia virus,
poliovirus
Culture The modified Andersen sampler
gave the best percentage
recovery, followed by the
impinger and the slit sampler;
the adhesive surface sampling
method indicated the number
of virus-bearing particles
Spray 127 1970
Slit sampler with
sucrose,
glycerol, and
bovine serum
albumin
(SGB)
medium
1ft
3
/min for 0.5
to 10 min
Aerosol apparatus
(Henderson)
Vaccinia virus,
poliovirus
Culture The modified Andersen sampler
gave the best percentage
recovery, followed by the
impinger and the slit sampler;
the adhesive surface sampling
method indicated the number
of virus-bearing particles
Spray 127 1970
Porton impinger 11.5 liters/min Aerosol apparatus
(Henderson)
Vaccinia virus,
poliovirus
Culture The modified Andersen sampler
gave the best percentage
recovery, followed by the
impinger and the slit sampler;
the adhesive surface sampling
method indicated the number
of virus-bearing particles
Spray 127 1970
AGI 6 liters/min 650-liter aerosol
chamber
VEE virus 1.5 (median
diameter)
Culture Sodium fluorescein affected the
recovery of VEE virus in the
presence (or not) of
simulated solar radiation,
depending on RH
FK-8 gun 18 1971
Porton raised
impingers
120-liter rotating
drum
Semliki Forest virus,
Langat virus,
poliovirus Sabin
type I strain, T7
coliphage,
32
P-
labeled T7
coliphage or
radioactive sodium
phosphate
Culture Salts in the aerosolization
medium influenced the
recovery of some viruses at
different RH;
prehumidification enhanced
the recovery of T7 coliphage
and poliovirus
Three-jet Collison spray 16 1971
424 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
May’s subsonic
impinger
120-liter rotating
drum
Semliki Forest virus,
Langat virus,
poliovirus Sabin
type I strain, T7
coliphage,
32
P-
labeled T7
coliphage or
radioactive sodium
phosphate
Culture Salts in the aerosolization
medium influenced the
recovery of some viruses at
different RH;
prehumidification enhanced
the recovery of T7 coliphage
and poliovirus
Three-jet Collison spray 16 1971
AGI 12.9 liters/min Chamber (aerosol
inoculator)
Type A influenza
virus strain PR8,
sodium fluorescein
50% Egg infective
dose
No significant difference was
noted in the physical tracer or
viral recovery of the two
samplers
Collison atomizer 55 1971
Shipe impinger 10 liters/min Chamber (aerosol
inoculator)
Type A influenza
virus strain PR8,
sodium fluorescein
50% Egg infective
dose
No significant difference was
noted in the physical tracer or
viral recovery of the two
samplers
Collison atomizer 55 1971
Raised impingers 11 liters/min 10-ft by 10-ft by 10-ft
rooms, poultry
houses with
infected chickens,
and a modified
Henderson
apparatus with a
500-liter rotating
stainless drum
Three strains of
Newcastle disease
virus (Herts ’33/56,
Eastwood ’67, and
Essex ’70), Bacillus
globigii spores
Inoculation of
eggs
Viruses were recovered with all
the samplers, but under
different sampling conditions
Poultry or Collison
atomizer
77 1973
LVAS 1,000 liters/min 10-ft by 10-ft by 10-ft
rooms, poultry
houses with
infected chickens,
and a modified
Henderson
apparatus with a
500-liter rotating
stainless drum
Three strains of
Newcastle disease
virus (Herts ’33/56,
Eastwood ’67, and
Essex ’70), Bacillus
globigii spores
Inoculation of
eggs
Viruses were recovered with all
the samplers, but under
different sampling conditions
Poultry or Collison
atomizer
77 1973
Cascade impactor 17 liters/min 10-ft by 10-ft by 10-ft
rooms, poultry
houses with
infected chickens,
and a modified
Henderson
apparatus with a
500-liter rotating
stainless drum
Three strains of
Newcastle disease
virus (Herts ’33/56,
Eastwood ’67, and
Essex ’70), Bacillus
globigii spores
Inoculation of
eggs
Viruses were recovered with all
the samplers, but under
different sampling conditions
Poultry or Collison
atomizer
77 1973
Multistage liquid
impinger
55 liters/min 10-ft by 10-ft by 10-ft
rooms, poultry
houses with
infected chickens,
and a modified
Henderson
apparatus with a
500-liter rotating
stainless drum
Three strains of
Newcastle disease
virus (Herts ’33/56,
Eastwood ’67, and
Essex ’70), Bacillus
globigii spores
Inoculation of
eggs
Viruses were recovered with all
the samplers, but under
different sampling conditions
Poultry or Collison
atomizer
77 1973
AGI-30 1 min 500-liter rotating
drum
Simian virus 40 2 (mean
diameter)
Culture The viruses were stable at all
RH tested at 21°C but were
inactivated at mid-range RH
at 32°C
Collison three-jet
atomizer
6 1973
Raised Porton
impinger
11.5 liters/min for
1 min
2,000-liter double-
walled static
system with fan
Bacteriophage MS2 2 (before
evaporation)
Culture The composition of the
aerosolization medium can
affect virus recovery
Direct spray apparatus
(FK8)
130 1973
Continued on following page
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 425
TABLE 2—Continued
Sampler type Air sampling rate
or capacity Sampling environment Virus and/or tracer Particle size (m) Analytical method Comments Aerosol source Reference Yr of
publication
Lower stage of a
multistage
liquid impinger
275 liters in 5 min 2,000-liter static air
cabinet with fan
Poliovirus type I strain
LSc2ab, fluorescein
Culture The infectivity of the
virus depended on
the RH, but the
infectivity of the
RNA remained
unchanged
FK8 direct-type
nebulizer
32 1973
Multistage impinger 30 min Loose boxes Swine vesicular disease
virus
Culture Viruses were recovered
in the top, middle,
and bottom stages of
the multistage
impinger
Pigs 120 1974
May’s multistage
liquid impinger
275 liters in 5 min 2,000-liter double-
walled stainless steel
tank
Encephalomyocarditis
virus
Culture with intact
viruses or
infectious RNA,
hemagglutination
activity, and
antibody-
blocking activity
Viruses were recovered
from the air; the
infectivity of the
virus decreased, but
the viral RNA was
unaffected
FK-8 direct-type spray
gun
33 1974
Porton impinger 2,000-liter static system Bacteriophage T3 Culture Viruses were recovered
with and without
prehumidification of
the aerosols
FK-8 spray gun 131 1974
Slit sampler with
SGB medium
1ft
3
/min for 30 to
60 min
Wards of smallpox
isolation hospital
Variola virus Inoculation of
hen’s eggs,
culture
The slit sampler and
sedimentation plates
gave positive results;
the impinger samples
were all negative
Humans 126 1974
Porton impinger 11.5 liters/min for
15 min
Wards of smallpox
isolation hospital
Variola virus Inoculation of
hen’s eggs,
culture
The slit sampler and
sedimentation plates
gave positive results;
the impinger samples
were all negative
Humans 126 1974
Sedimentation
plates with
SGB medium
Many hours at a
time
Wards of smallpox
isolation hospital
Variola virus Inoculation of
hen’s eggs,
culture
The slit sampler and
sedimentation plates
gave positive results;
the impinger samples
were all negative
Humans 126 1974
LVS 1,000 liters/min
for 30 to 120
min
Field in proximity to
wastewater
treatment plants
Bacteriophages of E.
coli strains C3000
and K-12 HfrD
Culture (most-
probable-number
and plaque
counts)
Both the multislit
impinger and the
LVS successfully
recovered airborne
bacteriophages
Wastewater treatment
facilities
51 1976
Multislit impinger 1,000 liters/min
for 35 min
Field in proximity to
wastewater
treatment plants
Bacteriophages of E.
coli strains C3000
and K-12 HfrD
Culture (most-
probable-number
and plaque
counts)
Both the multislit
impinger and the
LVS successfully
recovered airborne
bacteriophages
Wastewater treatment
facilities
51 1976
Membrane filter
(0.45-m pore
size)
14 liters/min for 3
to 30 min in the
chamber and 23
to 25 liters/min
for 30 to 45
min in the
hemodialysis
center
Aerosol chamber and a
20-bed hemodialysis
center
Hepatitis B virus
surface antigen
(HBsAg), Bacillus
subtilis var. niger (in
chamber)
Radioimmunoassay HBsAg was detected in
chamber aerosols but
not in the samples
from the
hemodialysis center
Airflow directed on
dust reservoir,
nebulizer, or
humans
107 1976
426 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
AGI-30 12.5 liters in 1
min
Two 208-liter chambers Influenza A virus
strain WSNH
Inoculation of
embryonated
eggs, culture
Differences were noted
in percentages of
viral recovery
depending on RH
Wells refluxing
atomizer
118 1976
Raised Porton
impinger
1 or 5 min Chamber (50 or 2,000
liters)
Bacteriophage X174,
Bacillus globigii
spores, or no tracer
Culture Bacteriophage
infectivity decreased
in the presence of
ozone
Three-jet Collison
nebulizer or spray
gun (FK-8 type)
96 1977
LVS 1,100 liters/min 18-m
3
animal
laboratory housing
infected mice
Polyomavirus Mouse antibody
production tests
and culture
Airborne viruses were
detected with the
LVS in four of six
samples; no airborne
viruses were
recovered with the
AGI-4 sampler
Infected mice 94 1978
AGI-4 12.3 liters/min 18-m
3
animal
laboratory housing
infected mice
Polyomavirus Mouse antibody
production tests
and culture
Airborne viruses were
detected with the
LVS in four of six
samples; no airborne
viruses were
recovered with the
AGI-4 sampler
Infected mice 94 1978
Large-volume
aerojet-general
liquid scrubber
15 to 20 min at
600 liters/min
In the vicinity of an
effluent-irrigated
field
Enteric viruses Culture Four of 12 samples
taken 40 m
downwind from the
aerosol source were
positive for echovirus
7
Sewage sprinklers 125 1978
AGI-30 18 s 200-liter stainless steel
rotating drum
Parainfluenza virus
type 3, bovine
adenovirus type 3,
rhodamine
Culture Viruses were recovered
with different
efficiencies that
depended on
nebulization
medium, RH, and
ambient temperature
Devilbiss 40 nebulizer 47 1979
LVS 1,000 liters/min
for 30 min (6 to
8 samples from
each sampler
were pooled;
eight samples
were operated
simultaneously)
Field in proximity to a
source of spray
irrigation of
wastewater
Enteric viruses Culture Low concentrations of
coliphages,
poliovirus, and
Coxsackie virus were
recovered
Sewage sprinklers 97 1979
AGI-30 18 s 200-liter stainless steel
rotating drum
Infectious bovine
rhinotracheitis virus
strain Cooper,
rhodamine B
Under 5 (diameter)
(over 88% of the
particles)
Culture The decay rate
depended on RH
and the
aerosolization
medium
Devilbiss 40 nebulizer 48 1979
Membrane filter
(0.45-m pore
size)
15 liters/min for
the duration of
the treatment
(3 to 13 min)
Dental unit, while
treating infected
patients
HBsAg Radioimmunoassay None of the 40
samples was positive
for HBsAg
Humans 106 1979
AGI-30 12.5 liters/min for
1 min
6,200-liter static
aerosol chamber
Japanese B
encephalitis virus
4.0 (median
diameter)
Culture Viral recovery was
inversely related to
RH
FK-8 atomizer 86 1980
AGI 2 to 5 min 1,000-liter stainless
steel dynamic
aerosol toroid with
mixing chamber
Reovirus, Bacillus
subtilis var. niger
spores
Mean diameters of
2 for Collison
nebulizer and 5
for Chicago
atomizer
Culture Reovirus particles were
relatively stable
when airborne; they
were least stable at
mid-range RH
Collison three-jet
nebulizer and
Chicago atomizer
1 1982
Continued on following page
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 427
TABLE 2—Continued
Sampler type Air sampling rate
or capacity Sampling environment Virus and/or tracer Particle size (m) Analytical method Comments Aerosol source Reference Yr of
publication
Andersen viable-
type stacked
sieve
28.3 liters/min 930-liter Plexiglas chamber Coliphage f2 2.0 or 3.9 (median
aerodynamic
particle size)
Culture The Andersen sampler had
an efficiency of 28.2%
compared to the AGI
samplers
All-glass two-fluid
nebulizer for 2.0-
m median
aerodynamic
particle diameter;
spinning-disk
aerosol generator
for 3.9-m particles
15 1982
AGI-30 12.5 liters/min 930-liter Plexiglas chamber Coliphage f2 2.0 or 3.9 (median
aerodynamic
particle size)
Culture The Andersen sampler had
an efficiency of 28.2%
compared to the AGI
samplers
All-glass two-fluid
nebulizer for 2.0-
m median
aerodynamic
particle diameter;
spinning-disk
aerosol generator
for 3.9-m particles
15 1982
LVS model M (i) 1,000 liters/min
for 30 min
(i) 3.6-m by 3.3-m by 3.0-m
loose boxes with
confined infected pigs;
(ii) 610-liter chamber,
one pig at a time
FMD virus type C strain
Noville
Culture The LVS had a higher
viral recovery rate, but
the cyclone sampler was
much easier to use
Pigs 38 1982
All-glass cyclone
separator
700 liters/min for
(i) 30 min or (ii)
15 min
(i) 3.6-m by 3.3-m by 3.0-m
loose boxes with
confined infected pigs;
(ii) 610-liter chamber,
one pig at a time
FMD virus type C strain
Noville
Culture The LVS had a higher
viral recovery rate, but
the cyclone sampler was
much easier to use
Pigs 38 1982
LVS 1,000 liters/min for
30 min
3.65- by 3.35- by 3.05-m
loose boxes for groups of
pigs and 610-liter
chamber for individual
pigs
Four strains of Aujeszky’s
disease virus (UK AD
74/33, Northern Ireland
NIA-2, U 298/81, and
UK AD 82/196)
Culture The cyclone sampler had
a slightly lower
recovery rate than the
LVS; the settling plates
were successful in
recovering viruses; the
three-stage impinger
showed daily variations
in the size distributions
of virus-containing
particles
Pigs 40 1983
All-glass cyclone
sampler
1,000 liters/min for
10 and 30 min
3.65- by 3.35- by 3.05-m
loose boxes for groups of
pigs and 610-liter
chamber for individual
pigs
Four strains of Aujeszky’s
disease virus (UK AD
74/33, Northern Ireland
NIA-2, U 298/81, and
UK AD 82/196)
Culture The cyclone sampler had a
slightly lower recovery
rate than the LVS; the
settling plates were
successful in recovering
viruses; the three-stage
impinger showed daily
variations in the size
distributions of virus-
containing particles
Pigs 40 1983
Square settling
plates with 20
ml of
collection
fluid
30 min 3.65- by 3.35- by 3.05-m
loose boxes for groups of
pigs and 610-liter
chamber for individual
pigs
Four strains of Aujeszky’s
disease virus (UK AD
74/33, Northern Ireland
NIA-2, U 298/81, and
UK AD 82/196)
Culture The cyclone sampler had a
slightly lower recovery
rate than the LVS; the
settling plates were
successful in recovering
viruses; the three-stage
impinger showed daily
variations in the size
distributions of virus-
containing particles
Pigs 40 1983
428 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
Three-stage liquid
impinger
55 liters/min for 15
min
3.65- by 3.35- by 3.05-m
loose boxes for groups of
pigs and 610-liter
chamber for individual
pigs
Four strains of Aujeszky’s
disease virus (UK AD
74/33, Northern Ireland
NIA-2, U 298/81, and
UK AD 82/196)
Culture The cyclone sampler had a
slightly lower recovery
rate than the LVS; the
settling plates were
successful in recovering
viruses; the three-stage
impinger showed daily
variations in the size
distributions of virus-
containing particles
Pigs 40 1983
AGI 5.6 liters in 1 min 300-liter stainless steel
rotating drum
Rotavirus SA11,
rhodamine
Culture Best survival of the virus at
medium (50% ⫾5%)
RH; high (80% ⫾5%)
humidity was the least
favorable
Six-jet Collison
nebulizer
116 1984
AGI 5.6 liters/min for 2
min
300-liter stainless steel
rotating drum
Human rotavirus strain
Wa, uranine
Culture Best recovery at 50%
⫾5% RH and 6 ⫾1°C;
recovery was enhanced
when feces were used in
the aerosolization
medium
Six-jet Collison
nebulizer
82 1985
AGI 5.6 liters/min for 2
min
300-liter stainless steel
rotating drum
Calf rotavirus strain C-
486, poliovirus type 1
(Sabin), uranine
Culture The best survival rates
were at 50% ⫾5% RH
for the rotavirus and
80% ⫾5% RH for the
poliovirus
Six-jet Collison
nebulizer
81 1985
Impinger 5.6 liters/min for 2
min
300-liter stainless steel
rotating drum
Human coronavirus 229E,
poliovirus type 1,
uranine
Culture The half-life of aerosolized
coronavirus was
determined under
different temperature
and RH conditions
Six-jet Collison
nebulizer
78 1985
AGI 300-liter rotating drum Rhinovirus type 14,
uranine
⬍5 in diameter
(theoretically;
⬎90% of
particles)
Culture Best viral recovery at high
(80% ⫾5%) RH
Six-jet Collison
nebulizer
84 1985
Aerosol collection
device with
Filterite
filters (pore
size, 0.4 m)
moistened
with glycine
buffer
100 liters/min for
10 to 15 s before
and 2 min after
flushing
Sampling device placed
over a toilet bowl seeded
with poliovirus
Poliovirus Culture Recovery of viruses from
the air was possible,
but virus adsorption to
the filter was hampered
if the filter became dry;
large volumes of dry air
would be problematic
Toilet flush 136 1985
Three-stage liquid
impinger
55 liters/min for 10
min
610-liter chamber FMD virus type O1 strain
BFS 1860
Culture The three-stage liquid
impinger recovered
smaller amounts of virus
than the Porton
impingers did
Pigs 56 1986
Porton raised
AGI
11 to 13 liters/min
for 10 min
610-liter chamber FMD virus type O1 strain
BFS 1860
Culture The three-stage liquid
impinger recovered
smaller amounts of virus
than the Porton
impingers did
Pigs 56 1986
Continued on following page
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 429
TABLE 2—Continued
Sampler type Air sampling rate or
capacity
Sampling
environment Virus and/or tracer Particle size (m) Analytical
method Comments Aerosol source Reference Yr of
publication
Three-stage liquid
impinger
55 liters/min for 2 to
5 min
610-liter chamber FMD virus strains O1 and
SAT 2
⬍3 with spinning-
top aerosol
generator
Culture Artificial aerosols were found
mostly in the lower stage
of the three-stage
impinger; natural aerosols
were found in equal
amounts in all three stages;
the minimal infectious
dose of the airborne virus
was determined with the
Porton impinger; no
viruses were recovered
from the cyclone sampler
Modified May spinning-
top aerosol generator
or pigs
39 1987
Porton raised AGI 11 to 13 liters/min
for 1 min 47 s to
2 min 40 s
610-liter chamber FMD virus strains O1 and
SAT 2
⬍3 with spinning-
top aerosol
generator
Culture Artificial aerosols were found
mostly in the lower stage
of the three-stage
impinger; natural aerosols
were found in equal
amounts in all three stages;
the minimal infectious
dose of the airborne virus
was determined with the
Porton impinger; no
viruses were recovered
from the cyclone sampler
Modified May spinning-
top aerosol generator
or pigs
39 1987
Glass cyclone
sampler
700 liters/min Corridor FMD virus strains O1 and
SAT 2
⬍3 with spinning-
top aerosol
generator
Culture Artificial aerosols were found
mostly in the lower stage
of the three-stage
impinger; natural aerosols
were found in equal
amounts in all three stages;
the minimal infectious
dose of the airborne virus
was determined with the
Porton impinger; no
viruses were recovered
from the cyclone sampler
Modified May spinning-
top aerosol generator
or pigs
39 1987
AGI 5.6 liters/min for 1
min
300-liter stainless
steel rotating
drum
Simian rotavirus SA-11 strain
H-96, human rotavirus
subgroup 2 strain Wa, bovine
rotavirus C-486, mouse
rotavirus, bovine rotavirus
Campton UK isolate,
poliovirus type I Sabin strain,
human coronavirus strain
229E, rhinovirus type 14/75Se
(for radiolabeling),
rhodamine B, or uranine
1.0–3.3 (over 87%
of the
infectious
viruses were
collected in the
last three
stages of the
Andersen
sampler)
Culture The Andersen sampler was
used to determine the size
distribution of the
aerosolized particles; both
samplers successfully
recovered viruses; uranine
is safer as a tracer than
radiolabeling and, unlike
rhodamine B, does not
affect viral infectivity
Six-jet Collison nebulizer 79 1987
Andersen sampler
with 3%
gelatin
medium
28 liters/min for 1
min
300-liter stainless
steel rotating
drum
Simian rotavirus SA-11 strain
H-96, human rotavirus
subgroup 2 strain Wa, bovine
rotavirus C-486, mouse
rotavirus, bovine rotavirus
Campton UK isolate,
poliovirus type I Sabin strain,
human coronavirus strain
229E, rhinovirus type 14/75Se
(for radiolabeling),
rhodamine B, or uranine
1.0–3.3 (over 87%
of the
infectious
viruses were
collected in the
last three
stages of the
Andersen
sampler)
Culture The Andersen sampler was
used to determine the size
distribution of the
aerosolized particles; both
samplers successfully
recovered viruses; uranine
is safer as a tracer than
radiolabeling and, unlike
rhodamine B, does not
affect viral infectivity
Six-jet Collison nebulizer 79 1987
430 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
AGI 24 liters in 2 min 500-liter stainless
steel rotating
drum
Pseudorabies virus, Bacillus
subtilis spores
Culture Recovery was best at 55%
RH and 4°C
Nebulizer 119 1990
Stainless steel
cyclone
sampler
300 liters/min for 15
min
60-m
3
isolated
units (six units
with eight pigs
each)
Aujeszky’s disease virus
strain 75V19
Culture Air sampling was less
sensitive than nasal swab
sampling; however, virus
concentrations in the air
were closely related to
those in the nasal cavity
Pigs 20 1992
AGI 300-liter rotating
drum
Bovine rotavirus UK isolate,
murine rotavirus, uranine
Culture Both viruses were recovered
from the air
Collison nebulizer 80 1994
Cellulose filter
(0.45-m pore
size)
2.5 to 9.4 liters/min
for 15 min to 24 h
Hospital rooms of
patients with
active varicella-
zoster virus
(VZV)
infections
VZV PCR VZV DNA was detected in
64 of 78 air samples
Humans 117 1994
Surface air system 0.9 m
3
of air Near aeration tank
of an activated
sludge treatment
plant
Coliphages and enteroviruses Culture Coliphages and enteroviruses
were recovered from air
samples, but no
relationship was found
between the two
Aeration tank of an
activated sludge
treatment plant
24 1995
Polycarbonate
membrane
filter (0.1-m
pore size)
1.9 liters/min for 6 h Rooms of patients
with active and
latent
cytomegalovirus
(CMV) infection
Human CMV PCR CMV DNA was detected in
the rooms of all three
patients
Humans 92 1996
AGI-30 15 min at 12.5 liters/
min
Exposure room Aujeszky’s disease virus Culture A virus-containing aerosol
was recovered from the
breath of only one pig;
viruses were recovered
more easily from the
nebulized aerosol; the
sampler inactivated the
virus, making the method
less sensitive
Pigs or a DeVilbiss
ultrasonic nebulizer
(model 99)
57 1996
AGI-30 1 or 2 liters/min 80-liter aluminum
chamber
St. Louis encephalitis virus
strain MS1–7, Bacillus
subtilis var. niger
Culture Collison spray 109 1997
Cellulose filters
(0.45-m pore
size)
2.0 liters/min for ⬍6
h, ⬎18 h, or 24 h
Hospital rooms Respiratory syncytial virus
(RSV)
PCR-based
detection
methods
RSV DNA was detected in
17 of the 27 rooms housing
infected patients and in 32
of the 143 samples
Humans 3 1998
Surface air system
agarized
terrain
impactor
1,800 liters indoors
and 3,000 liters
outdoors
Urban sewage
plants
Reovirus and enterovirus Culture Both viruses were detected in
some samples
Urban sewage treatment
plants
25 2000
AGI-30 28-m
3
isolation
rooms
Aujeszky’s disease virus Culture No virus was detected Pigs 58 2000
AGI-30 12.5 liters/min for 10
min
Exhaust air from
an infected barn
Porcine reproductive and
respiratory syndrome virus
(PRRSV)
PCR and
culture
No virus was detected in the
air samples by PCR or by
culture
Pigs 105 2002
All-glass cyclone
sampler
170 liters/min for 20
min
Animal rooms FMD virus O UKG 34/2001 Culture Viruses were recovered with
both samplers
Sheep and heifers 8 2002
Three-stage liquid
impinger
55 liters/min for 5
min
610-liter cabinet FMD virus O UKG 34/2001 Culture Viruses were recovered with
both samplers
Sheep and heifers 8 2002
All-glass cyclone
sampler
170 liters/min for 2
min
Chamber FMD virus strain O1
Lausanne Sw/65
Culture FMD virus was recovered
from the air samples, but
the efficiency of the
samplers was not
compared
Pigs 7 2002
Continued on following page
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 431
TABLE 2—Continued
Sampler type Air sampling rate
or capacity Sampling environment Virus and/or tracer Particle size
(m) Analytical method Comments Aerosol source Reference Yr of
publication
Porton AGI 10 to 13 liters/min
for2or5min
Chamber FMD virus strain O1
Lausanne Sw/65
Culture FMD virus was recovered
from the air samples,
but the efficiency of the
samplers was not
compared
Pigs 7 2002
Three-stage liquid
impinger
55 liters/min for 5
min
Chamber FMD virus strain O1
Lausanne Sw/65
Culture FMD virus was recovered
from the air samples,
but the efficiency of the
samplers was not
compared
Pigs 7 2002
Modified SAS-100 100 liters/min for
2.5 min
Within and around
poultry broiler
houses
Male-specific coliphages Culture Coliphages were
recovered from air
samples when a
premoistened cellulose
ester filter collection
medium was used for
sampling by impaction
Poultry 50 2002
PTFE filters (2.0-m
pore size)
10 min Chamber with UV light Rhinovirus 16 strain
11757 from ATCC,
uranine
2 (mass-median
diameter of
droplets)
Seminested RT-PCR The detection limit was
1.3 50% tissue culture
infective doses/filter for
aerosolized virus
Six-jet Collison
nebulizer (CN-38)
101 2003
MD8 air sampler with
sterile gelatin
membrane filter
(3-m pore size)
100 liters in 1 or 2
min
Dairy factory, close
proximity to a
running whey
separator
Lactococcus lactis
bacteriophages
Culture The MD8 and AirPort
MD8 results were very
similar; the phage
recovery rates for the
MAS-100 (impaction)
sampler were 1 to 5%
that for the MD8
(filtration) sampler
Whey separator in a
dairy factory
103 2003
AirPort MD8 sampler
with sterile
gelatin
membrane filters
(3-m pore size)
100 liters in 2 min Dairy factory, close
proximity to a
running whey
separator
Lactococcus lactis
bacteriophages
Culture The MD8 and AirPort
MD8 results were very
similar; the phage
recovery rates for the
MAS-100 (impaction)
sampler were 1 to 5%
that for the MD8
(filtration) sampler
Whey separator in a
dairy factory
103 2003
MAS-100 device (with
five different
setups)
100 liters in 1 min Dairy factory, close
proximity to a
running whey
separator
Lactococcus lactis
bacteriophages
Culture The MD8 and AirPort
MD8 results were very
similar; the phage
recovery rates for the
MAS-100 (impaction)
sampler were 1 to 5%
that for the MD8
(filtration) sampler
Whey separator in a
dairy factory
103 2003
AGI-30 12.5 liters/min for
10 min
Exhaust air from an
infected barn
PRRSV PCR, culture, and
pig bioassay
All 168 air samples were
negative by PCR, culture,
and pig bioassay
Pigs 129 2004
PTFE filters (2.0-m
pore size)
Average of 47 h,
from 9 a.m. to
5 p.m. at 4
liters/min
Office buildings Picornaviruses
(rhinovirus and
enteroviruses)
Nested RT-PCR and
sequencing
Fifty-eight (32%) of 181
filters were positive for
picornavirus
Humans 102 2004
Bubbling sampler 4 liters/min for 5
min
400-liter aerosol
chamber
Influenza virus A/Aichi/
2/68 (H3N2), vaccinia
virus strain LIVP
(C0355 K0602),
uranine
0.5–2.2
(majority of
particles)
Culture and titration
on chicken
embryos
The average recovery rate
was 20% for the
influenza virus and 89%
for the vaccinia virus
Collison nebulizer 2 2005
432 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
Portable, single-sieve,
MicroBio MB1
impactor
100 to 600 liters Domestic toilet in a
2.6-m
3
room
Bacteriophage MS2 Culture Bacteriophages were
recovered with both
sampling methods; the
impactor sampler
recovered phages from
the air 60 min after
toilet flushing
Toilet flush 14 2005
Settling plates 30 min Domestic toilet in a
2.6-m
3
room
Bacteriophage MS2 Culture Bacteriophages were
recovered with both
sampling methods; the
impactor sampler
recovered phages from
the air 60 min after
toilet flushing
Toilet flush 14 2005
High-resolution slit
sampler system
30 liters/min for
18 min (10
sampling heads
for a total of
180 min)
Hospital rooms of
patients with SARS
SARS coronavirus RT-PCR,
quantitative PCR,
culture, and DNA
sequencing
Two of 10 samples from
the room of a
recovering SARS
patient collected using
the slit sampler were
PCR positive but
culture negative; the
PTFE membrane filters
used in the other
rooms were all PCR
and culture negative;
low concentrations (or
absence) of airborne
viruses may explain the
negative results
Humans 19 2005
PTFE membrane
filters (0.3-m
pore size)
2 liters/min for
10.5 to 13 h
Hospital rooms of
patients with SARS
SARS coronavirus RT-PCR,
quantitative PCR,
culture, and DNA
sequencing
Two of 10 samples from
the room of a
recovering SARS
patient collected using
the slit sampler were
PCR positive but
culture negative; the
PTFE membrane filters
used in the other
rooms were all PCR
and culture negative;
low concentrations (or
absence) of airborne
viruses may explain the
negative results
Humans 19 2005
Portable air sampler 450 liters/min 10.16-cm-diameter
polyvinyl chloride
pipe of 3 to 150 m
long attached to a
blower
PRRSV strain MN 30-
100
Quantitative PCR
and culture
Viruses were recovered
from a distance of up
to 150 m
Cooking oil spritzer 31 2005
Cyclone samplers 700 (⫾50) liters/
min for 5 min
Chamber Bacteriophage MS2 Culture Viruses were recovered One or two three-
jet or six-jet
Collison
nebulizers
62 2005
Wetted-wall cyclone-
style air sampler
265 liters/min for
8h
Commercial poultry
flocks
Exotic Newcastle
disease virus
Real-time RT-PCR
(RRT-PCR),
inoculation of
eggs, culture, and
sequencing
Low concentrations of
virus particles were
detected by RRT-PCR;
culture was apparently
more sensitive than
RRT-PCR for
detecting viruses
Poultry 74 2005
Continued on following page
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 433
TABLE 2—Continued
Sampler type Air sampling rate
or capacity
Sampling
environment Virus and/or tracer Particle size
(m) Analytical method Comments Aerosol source Reference Yr of
publication
AGI-30 Set 2 experiments,
12.5 liters/min
Closed system Bacteriophages MS2
and T3
Particle size was
influenced
mainly by the
properties of
the liquid
medium and
the method of
aerosolization,
not by the
physical size
of the viruses;
the particles
studied were
under 300 nm
in diameter
Set 4 experiments,
culture
The capture efficiency
for particles in the
30- to 100-nm size
range was 10% or
lower; the efficiency
increased for
particles smaller
than 30 nm and
larger than 100 nm;
all three samplers
exhibited low
capture efficiencies
for ultrafine
particles that varied
over time, as well
as a potential loss
of virus viability
during sampling
Constant output
atomizer
76 2005
SKC BioSampler Set 2 experiments,
12.5 liters/min
Closed system Bacteriophages MS2
and T3
Particle size was
influenced
mainly by the
properties of
the liquid
medium and
the method of
aerosolization,
not by the
physical size
of the viruses;
the particles
studied were
under 300 nm
in diameter
Set 4 experiments,
culture
The capture efficiency
for particles in the
30- to 100-nm size
range was 10% or
lower; the efficiency
increased for
particles smaller
than 30 nm and
larger than 100 nm;
all three samplers
exhibited low
capture efficiencies
for ultrafine
particles that varied
over time, as well
as a potential loss
of virus viability
during sampling
Constant output
atomizer
76 2005
Frit bubbler Set 2 experiments,
12.5 liters/min
Closed system Bacteriophages MS2
and T3
Particle size was
influenced
mainly by the
properties of
the liquid
medium and
the method of
aerosolization,
not by the
physical size
of the viruses;
the particles
studied were
under 300 nm
in diameter
Set 4 experiments,
culture
The capture efficiency
for particles in the
30- to 100-nm size
range was 10% or
lower; the efficiency
increased for
particles smaller
than 30 nm and
larger than 100 nm;
all three samplers
exhibited low
capture efficiencies
for ultrafine
particles that varied
over time, as well
as a potential loss
of virus viability
during sampling
Constant output
atomizer
76 2005
434 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
SKC BioSampler 12.5 liters/min for
20 min
Field downwind from
a biosolid spray or
seeded
groundwater
Bacteriophage MS2 Culture Bacteriophages were
recovered
downwind from the
seeded groundwater
spray but not from
the biosolid spray
Spray tanker 124 2005
Andersen one-
stage
impactor; a
six-stage
impactor
was also
used for size
distribution
28.3 liters/min 29-cm-diameter, 32-
cm-high chamber
Bacteriophages X174,
MS2, T7, and 6
1.23–1.25 (mean
aerodynamic
diameter;
⬎95% of
PFU
recovered
with the six-
stage
Andersen
sampler were
⬍2.1 min
diameter)
Culture The capture efficiency
for infectious
viruses was highly
dependent on the
properties of the
viruses and the RH;
viral recovery was
very low with the
Nuclepore filter
Three-jet Collison
nebulizer
132 2005
AGI-30 12.5 liters/min for
5 min
29-cm-diameter, 32-
cm-high chamber
Bacteriophages X174,
MS2, T7, and 6
1.23–1.25 (mean
aerodynamic
diameter;
⬎95% of
PFU
recovered
with the six-
stage
Andersen
sampler were
⬍2.1 min
diameter)
Culture The capture efficiency
for infectious
viruses was highly
dependent on the
properties of the
viruses and the RH;
viral recovery was
very low with the
Nuclepore filter
Three-jet Collison
nebulizer
132 2005
Gelatin filter
(3.0-m
pore size)
30 liters/min for 5
min
29-cm-diameter, 32-
cm-high chamber
Bacteriophages X174,
MS2, T7, and 6
1.23–1.25 (mean
aerodynamic
diameter;
⬎95% of
PFU
recovered
with the six-
stage
Andersen
sampler were
⬍2.1 min
diameter)
Culture The capture efficiency
for infectious
viruses was highly
dependent on the
properties of the
viruses and the RH;
viral recovery was
very low with the
Nuclepore filter
Three-jet Collison
nebulizer
132 2005
Nuclepore filter
(polycarbonate
membrane;
0.4-m pore
size)
2 liters/min for 20
min
29-cm-diameter, 32-
cm-high chamber
Bacteriophages X174,
MS2, T7, and 6
1.23–1.25 (mean
aerodynamic
diameter;
⬎95% of
PFU
recovered
with the six-
stage
Andersen
sampler were
⬍2.1 min
diameter)
Culture The capture efficiency
for infectious
viruses was highly
dependent on the
properties of the
viruses and the RH;
viral recovery was
very low with the
Nuclepore filter
Three-jet Collison
nebulizer
132 2005
Continued on following page
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 435
TABLE 2—Continued
Sampler type Air sampling rate
or capacity
Sampling
environment Virus and/or tracer Particle size
(m) Analytical method Comments Aerosol source Reference Yr of
publication
AGI-30 12.5 liters/min Glass chamber PRRSV (North
American prototype)
and swine influenza
virus strain A/Swine/
Iowa/73 (H1N1)
Culture and
quantitative RT-
PCR
The BioSampler and
the AGI-30 sampler
collected more
viruses than the AGI-
4 sampler at 10, 15,
and 20 min; the
BioSampler collected
more viruses than the
AGI-30 and AGI-4
samplers at 15 and
20 min
24-Jet Collison
nebulizer
73 2006
AGI-4 12.5 liters/min Glass chamber PRRSV (North
American prototype)
and swine influenza
virus strain A/Swine/
Iowa/73 (H1N1)
Culture and
quantitative RT-
PCR
The BioSampler and
the AGI-30 sampler
collected more
viruses than the AGI-
4 sampler at 10, 15,
and 20 min; the
BioSampler collected
more viruses than the
AGI-30 and AGI-4
samplers at 15 and
20 min
24-Jet Collison
nebulizer
73 2006
SKC BioSampler 6 liters/min Glass chamber PRRSV (North
American prototype)
and swine influenza
virus strain A/Swine/
Iowa/73 (H1N1)
Culture and
quantitative RT-
PCR
The BioSampler and
the AGI-30
sampler collected
more viruses than
the AGI-4 sampler
at 10, 15, and 20
min; the
BioSampler
collected more
viruses than the
AGI-30 and AGI-4
samplers at 15 and
20 min
24-Jet Collison
nebulizer
73 2006
Andersen six-stage
sampler (for size
distribution)
23-liter exposure
chamber
Bacteriophages MS2,
X174, 6, and T7
0.5–3.0 (⬎95%
of virus-
containing
particles
were ⬍2.1
m)
Culture The surviving fraction
of airborne viruses
decreased
exponentially as the
ozone
concentration
increased; viruses
with more complex
capsid architectures
were less
susceptible to
ozone inactivation
Three-jet Collison
nebulizer
133 2006
436 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
Andersen one-stage
sampler
28.3 liters/min for
0.5 to 5 min
23-liter exposure
chamber
Bacteriophages MS2,
X174, 6, and T7
0.5–3.0 (⬎95%
of virus-
containing
particles
were ⬍2.1
m)
Culture The surviving fraction
of airborne viruses
decreased
exponentially as the
ozone concentration
increased; viruses
with more complex
capsid architectures
were less susceptible
to ozone inactivation
Three-jet Collison
nebulizer
133 2006
AGI-30 12.5 liters/min for
10 min or less
Stainless steel
closed-loop wind
tunnel
Transmissible
gastroenteritis virus,
avian pneumovirus,
and fowlpox virus
Culture Viruses were detected
upstream but not
downstream from
the filter
500 ELSD nebulizer 52 2006
Porton sampler 55 liters in 5 min 610-liter air
sampling cabinet
(for each pig) and
a loose box (for
six pigs)
(approximately 4
by4by3m)
FMD virus O UKG
FMD 34/2001
0.015–20 (no
clear
association
of viable
virus with a
particular
size range)
Culture and RRT-
PCR
The number of
infectious FMD
virus and RNA
copies was
independent of the
sampling method
Pigs 60 2007
May sampler 165 liters in 5 min 610-liter air
sampling cabinet
(for each pig) and
a loose box (for
six pigs)
(approximately 4
by4by3m)
FMD virus O UKG
FMD 34/2001
0.015–20 (no
clear
association
of viable
virus with a
particular
size range)
Culture and RRT-
PCR
The number of
infectious FMD
virus and RNA
copies was
independent of the
sampling method
Pigs 60 2007
Cyclone sampler 3,900 liters in 5
min
610-liter air
sampling cabinet
(for each pig) and
a loose box (for
six pigs)
(approximately 4
by4by3m)
FMD virus O UKG
FMD 34/2001
0.015–20 (no
clear
association
of viable
virus with a
particular
size range)
Culture and RRT-
PCR
The number of
infectious FMD
virus and RNA
copies was
independent of the
sampling method
Pigs 60 2007
25-mm filters with
an SKC Button
inhalable
aerosol sampler
Gelatin filter (3-
m pore size)
4 liters/min Chamber Bacteriophage MS2 10–80 nm Particle counters
(airborne particle
concentrations
were measured
downstream and
upstream from the
filters)
Physical efficiency was
over 96%
Six-jet Collison-type
air-jet nebulizer
23 2007
PC filter (0.4-m
pore size)
4 liters/min Chamber Bacteriophage MS2 10–80 nm Particle counters
(airborne particle
concentrations
were measured
downstream and
upstream from the
filters)
Left aside due to a
high pressure drop
Six-jet Collison-type
air-jet nebulizer
23 2007
PC filter (1-m
pore size)
4 liters/min Chamber Bacteriophage MS2 10–80 nm Particle counters
(airborne particle
concentrations
were measured
downstream and
upstream from the
filters)
Physical collection
efficiency was 68%
Six-jet Collison-type
air-jet nebulizer
23 2007
Continued on following page
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 437
TABLE 2—Continued
Sampler type Air sampling rate
or capacity
Sampling
environment Virus and/or tracer Particle size
(m) Analytical method Comments Aerosol source Reference Yr of
publication
PC filter (3-m
pore size)
4 liters/min Chamber Bacteriophage MS2 10–80 nm Particle counters
(airborne particle
concentrations
were measured
downstream and
upstream from the
filters)
Physical collection
efficiency was 27%
Six-jet Collison-type
air-jet nebulizer
23 2007
PTFE filter (0.5-
m pore size)
4 liters/min Chamber Bacteriophage MS2 10–80 nm Particle counters
(airborne particle
concentrations
were measured
downstream and
upstream from the
filters)
More than 96%
physical collection
efficiency
Six-jet Collison-type
air-jet nebulizer
23 2007
PTFE filter (1-m
pore size)
4 liters/min Chamber Bacteriophage MS2 10–80 nm Particle counters
(airborne particle
concentrations
were measured
downstream and
upstream from the
filters)
More than 96%
physical collection
efficiency
Six-jet Collison-type
air-jet nebulizer
23 2007
PTFE filter (3-m
pore size)
4 liters/min Chamber Bacteriophage MS2 10–80 nm Particle counters
(airborne particle
concentrations
were measured
downstream and
upstream from the
filters)
More than 96%
physical collection
efficiency
Six-jet Collison-type
air-jet nebulizer
23 2007
Preloaded in three-
part 37-mm
plastic cassettes
PTFE filter (0.3-
m pore size)
2 liters/min Chamber Bacteriophage MS2 10–80 nm Particle counters
(airborne particle
concentrations
were measured
downstream and
upstream from the
filters)
More than 96%
physical collection
efficiency
Six-jet Collison-type
air-jet nebulizer
23 2007
SKC BioSampler
impingers
12.5 liters/min for 1
min
133-liter stainless
steel dynamic
aerosol toroid
PRRSV, rhodamine B Microinfectivity
assays and
quantitative RT-
PCR
Temperature had a
greater effect than
RH on the stability
of PRRSV
24-Jet Collison
nebulizer
72 2007
Bioaerosol personal
sampler
4 liters/min for 5
min
Aerosol chamber Influenza virus A strain Culture and RRT-
PCR
Viruses were detected
in aerosol samples
Three-jet Collison
nebulizer
111 2007
37-mm gelatin filter 28.3 liters/min for 5
or 10 min
3.0-m by 4.6-m by
3.0-m chamber
with upper-room
UV germicidal
irradiation
Vaccinia virus (WR
strain)
2.5 (median
diameter)
Culture Concentrations of
infective vaccinia
virus can be
diminished by
upper-room UV
germicidal
irradiation
Six-jet Collison
nebulizer
53 2007
37-mm gelatin filter
in a polystyrene
air sampling
cassette
Aerosol chamber
with UV light
Vaccinia virus (WR
strain)
Culture Inactivation of viruses
by UVC decreased
at high RH
Six-jet Collison
nebulizer
93 2007
Andersen sampler
with gelatin
filters
Aerosol chamber
with UV light
Vaccinia virus (WR
strain)
Culture Inactivation of viruses
by UVC decreased
at high RH
Six-jet Collison
nebulizer
93 2007
438 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
obstacle. This is the only mechanism that does not depend on
particles being diverted from the streamline. Inertial impaction
occurs when a particle impacts an obstacle when the streamline
changes direction. The inertia of the particle forces it to divert
from the streamline and to impact a surface. As mentioned
previously, only very small particles are affected by the diffu-
sion mechanism based on Brownian motion. Gravitational set-
tling affects mostly particles of much larger aerodynamic di-
ameter by pushing them downward due to gravity. The
importance of this force depends on the other forces affecting
the particle in various directions. Lastly, electrostatic forces
also influence the trajectory of particles. This mechanism de-
pends on the size and charge of the particle and the charge
difference with the filter.
Given that small particles are highly governed by the diffu-
sion phenomenon and that larger particles have a tendency to
impact and to be intercepted, it was shown that both types of
behaviors are at their lowest at 0.3 m (75). Thus, filters are
least efficient at removing 0.3-m particles. Filtration efficiency
improves with increasing and decreasing particle size. This is
why filter efficiency is based on the 0.3-m benchmark.
Many different types of filters have been used to sample
airborne viruses. They differ mainly in composition, pore size,
and thickness. To our knowledge, the first filters used to sam-
ple airborne viruses were made out of tightly packed cotton
and were used to sample variola virus (Poxviridae family;
dsDNA virus) in a hospital (95). Cellulose filters (0.45-m
pore size) have also been used to sample hospital air; PCR
analysis of these samples permitted the detection of naturally
produced aerosols of varicella-zoster virus (Herpesviridae fam-
ily; dsDNA virus; 200 nm) (117) and respiratory syncytial virus
(Paramyxoviridae family; ssRNA virus; 150 nm) (3). PTFE fil-
ters (2.0-m pore size) have been used to collect artificial
rhinovirus (Picornaviridae family; ssRNA virus; 25 to 30 nm)
aerosols in a small aerosol chamber (101) and naturally pro-
duced rhinovirus aerosols in office buildings (102). In both
cases, PCR was used to detect the viruses. While polycarbon-
ate filters are much less efficient than gelatin or PTFE filters
(23), 0.1-m polycarbonate filters have been used in combina-
tion with PCR to detect human cytomegalovirus (Herpesviridae
family; dsDNA virus; 200 nm) in samples of naturally produced
aerosols (92). The low filtration efficiency of polycarbonate
filters may be due to the structure of the filter. The contact
area of filters with uniform cylindrical pores, such as polycar-
bonate filters, is much smaller than that of filters with a com-
plex structure, such as PTFE filters, where the probability of
adherence is greater because airborne particles are exposed to
a greater surface area.
However, filters are not commonly used to sample airborne
viruses because they can cause structural damage. In addition,
the desiccation of the samples that occurs during sampling can
interfere with culture analysis of the samples. While modern
molecular biology tools do not require infectious particles to
detect viruses, studies investigating the effects of environmen-
tal factors on viral infectivity, for example, require the collec-
tion of infectious viruses. Gelatin filters can be used because
they do not appear to significantly affect viral infectivity. For
example, MD8 air samplers equipped with 80-mm gelatin
membrane filters with a pore size of 3 m in combination with
culture techniques have been used successfully to detect Lac-
tococcus lactis tailed bacteriophages in a cheese factory (103).
Gelatin filters, as well as the Andersen sampler and the
AGI-30 impinger, are 10 times more efficient than polycarbon-
ate filters at collecting active bacteriophages (132). The phys-
ical collection efficiencies of both gelatin and PTFE filters,
calculated by placing particle counters before and after the
filters, exceed 96% (23). While gelatin filters can be very useful
for sampling functional viruses, their use can be limited by
environmental conditions. Low humidity can cause them to dry
out and break, while high humidity or water droplets can cause
them to dissolve. On the positive side, this property can be
used to recover viruses or virus-laden particles by dissolving
the filters in water. Nevertheless, 0.3-m PTFE filters appear
to be the best option for long-term sampling of 10- to 900-nm-
diameter virus-laden particles (23).
Filter materials mounted on three-piece cassettes all have
the same limitation. These cassettes are hollow cylinders made
out of plastic or metal, with an inlet or outlet hole at the center
of the base of each cylinder. A filter is deposited on a porous
support pad (cellulose, plastic, or metal) on part one, the base.
The second part, the cylinder, is placed on the base to seal the
edge of the filter and ensure that the air pumped from the
outlet of the cassette passes through the filter. If only the first
two parts are used, it is considered an open cassette. The third
part is placed on top to close the cassette. The pressure used to
seal the cassettes can influence the outcome of an experiment.
If the cassette is not properly sealed, aerosol slippage can
occur. The airflow goes around the filter and into the pump,
preventing most of the airborne particles from being captured
by the filter and significantly contaminating the pump in the
process. On the other hand, if the cassette is sealed too tightly,
the filter can be damaged or weakened, especially with fragile
filters such as gelatin membranes, and aerosol slippage can also
occur (135). If premounted cassettes are available with the
desired filters, their efficiency should be compared to that of
laboratory-prepared cassettes.
Standardized filters are still available but are often left aside
because of the damage they can cause to viruses. In compari-
son studies, filters often provide the best physical recovery of
nanoscale particles, but the filtering process can damage vi-
ruses and complicate the analysis, especially if culture is used
to assess virus levels. Comparative studies using culture as an
analytical tool have shown that filters are less efficient than
other, less destructive methods, such as liquid impingers, for
recovering airborne infectious viruses (64, 66, 132).
Electrostatic Precipitators
LVS use electrostatic precipitation to sample air. One ex-
ample is the LVS designed for the U.S. army in the 1960s (54).
This device can draw up to 10,000 liters of air per minute
through a high-voltage corona, where the particles are charged
before being precipitated onto a grounded rotating disc. A
recirculating fluid is used to wash off and concentrate the
precipitated particles. LVS have been used to recover airborne
adenoviruses (Adenoviridae family; nonenveloped dsDNA vi-
rus; 70 to 90 nm in diameter) in a military hospital, where
50,545 liters of air were sampled in 5 minutes, with the partic-
ulate content collected in 180 ml of fluid (11). LVS have also
been used in other circumstances to recover low concentra-
VOL. 72, 2008 SAMPLING OF AIRBORNE VIRUSES 439
tions of airborne adenoviruses and picornaviruses (12, 13, 38,
40, 51, 77, 94, 97, 121, 144). These samplers are commonly
used with a preimpactor attached to the air inlet to capture
particles of over 15 m in diameter. Smaller particles are
subsequently collected by electrostatic precipitation (12). Al-
though LVS recover slightly more infective viruses than does
the cyclone separator described by Errington and Powell (49),
LVS are more complicated to operate (38). In addition, the
production of ozone at high RH in the presence of the intense
electric field may damage viruses (28).
Other Sampling and Detection Methods
Other methods can be used to detect airborne viruses with-
out any actual air sampling. Swabbing the surfaces of air pu-
rifier filters (44, 123) can be used to assess the viral composi-
tion of air. Settling dishes (14, 40, 41, 126) can also be used, but
this method is more suitable for larger droplets that settle by
gravity on a petri dish. While cumbersome, sentinel animals
can be used to detect the presence of viruses (21, 57, 105, 129).
Susceptible host animals can be used as air samplers and cul-
ture support systems (90) by exposing the animals to poten-
tially contaminated air and then noting symptoms or perform-
ing laboratory tests. Sufficient concentrations of airborne virus
and appropriate conditions are required for this approach to
detect viruses.
ASSESSING THE EFFICIENCY OF AIRBORNE VIRUS
SAMPLERS BY USE OF TRACERS
In order to study the efficiency of samplers for airborne virus
sampling, a variety of strategies have been used. It is important
to note the difference between the capture efficiency of a sam-
pler and its efficiency for viral recovery. The capture efficiency
(or total physical efficiency) is based on the rate of recovery of
different particle sizes and is measured with methods indepen-
dent of the integrity of the viruses, while the efficiency of viral
recovery is, in most studies, an indicator of the remaining
integrity and infectivity of the sampled viruses.
Tracers can be used to measure the total capture efficiency
of samplers. P-32 (16, 65, 66), uranine (fluorescein sodium salt)
(2, 32, 54, 55, 78, 81, 82, 84, 101), rhodamine B (46–48, 72, 116,
122), and Bacillus spores (17, 77, 119) have all been used as
tracers to detect airborne viruses. Uranine remains the most
popular tracer because it is safer than radiolabeling for aerosol
studies. In addition, unlike rhodamine B, it does not affect viral
infectivity (79). New molecular methods have led to alternative
tracers for determining sampler efficiency. For example, viral
genetic material can be used to estimate the total number of
viruses sampled. Genomic tracers do not depend on viral in-
fectivity or, to a certain degree, viral integrity. However, the
lack of information on the degradation of viral genetic material
in aerosols makes the replacement of physical tracers by quan-
titative PCR premature (72). The major weakness of the quan-
titative PCR method is the detection limit. Lastly, particle
counters can be installed upstream and downstream from the
sampler to determine the total number of particles trapped by
the sampler. However, this method cannot be used to deter-
mine the proportion of captured particles that can be extracted
for further analysis.
LABORATORY STUDIES OF AIRBORNE VIRUSES
To our knowledge, the oldest study on the sampling of air-
borne viruses was performed with a laboratory setup using a
chamber and an artificially produced aerosol of influenza virus
(140). Since then, many chamber setups have been used to
study artificially produced infectious aerosols (61, 70, 139) and
airborne viruses. Rotating drum or dynamic aerosol toroids
(61) have been used to study the biological decay rates of
airborne viruses under different temperature and/or RH con-
ditions (4, 5, 16, 42, 46–48, 65, 67, 72, 78, 80–82, 84, 116, 119,
122, 138). These devices make it possible to study aerosols
under controlled atmospheric conditions for extended periods,
with little loss of airborne particles to gravitational settling. A
variety of chambers and other types of closed and/or controlled
systems, some inspired by previous technologies, have been
used to study artificially and naturally produced aerosols (2, 7,
8, 15, 20, 23, 30–33, 38, 39, 45, 54–58, 60, 62, 64, 66, 73, 76, 86,
99, 101, 107, 109, 118, 120, 121, 130–133, 137). Most setups for
viral aerosol studies are handmade for specific purposes. The
aerosol source or generator, temperature, RH, radiation, time
of exposure, aerosolization medium, sampling method, viruses,
tracers, and analytical methods are rarely the same. As such,
even controlled studies are difficult to compare.
Aerosol generators are most often used to study the behav-
ior of airborne viruses. For the generation of submicrometer
aerosols, neutralizers are placed between the generator and
the chamber to prevent uncommon aerosol behavior by remov-
ing charges on particles created during the nebulization pro-
cess. Desiccators are also often used to shrink particles by
evaporation. The median size of aerosol particles is controlled
by the intensity of the aerosolization process and by preimpac-
tors to stop larger particles. Since the concentration of the
nonevaporative solutes in the nebulization medium determines
the size of the droplet nuclei, low solute concentrations can be
used to produce small particles and high solute concentrations
can be used for large particles. The concentration of the aero-
sol in the chamber can be modified by adding clean dilution air.
The size of the droplet nuclei can be calculated using equations
that take into account the initial droplet diameter and the
volume fraction of solid material (75).
Many types of devices can be used to determine the concen-
tration of particles in the air. Spectrometers equipped with
various technologies can count and size airborne particles in
real time or near real time. However, they are often limited to
measuring particles of ⬎300 nm to 500 nm in diameter. Scan-
ning mobility particle sizers can be used to count and measure
smaller particles. These devices neutralize airborne particles
before separating them based on electrical mobility (charge
and size). The particles are then passed through a condenser to
increase their size so they can be detected by photometry.
SURROGATE VIRUSES
While aerobiological studies using hazardous viruses can
provide valuable information, they can also expose personnel
to unnecessary risks. Surrogate viruses, such as bacteriophages
of E. coli and other bacteria, can be used to mimic the behavior
of pathogenic viruses. Bacteriophages of the order Caudovi-
rales, such as T7-like (16, 30, 42, 45, 67, 76, 99, 122, 131–133,
440 VERREAULT ET AL. MICROBIOL.MOL.BIOL.REV.
137, 138), T1-like (64, 66), and T5-like (69) bacteriophages,
were the first surrogates used. The genomic material of these
bacteriophages consists of dsDNA, the capsid is nonenveloped,
and the presence of a tail for host recognition renders the
viruses susceptible to physical damage. They are thus unreli-
able for infectivity assays. In addition, the morphological char-
acteristics of these bacteriophages do not resemble those of
any mammalian viruses. As such, they are used more rarely
nowadays and have largely been replaced by other surrogate
viruses. Bacteriophage MS2 (Leviviridae family) has been used
as a surrogate virus in many studies (14, 23, 42, 62, 76, 130, 132,
133). It is a nonenveloped ssRNA coliphage with a very small
(25 to 27 nm) icosahedral capsid. It has no tail and is morpho-
logically similar to members of the Picornaviridae family, which
includes many pathogenic viruses, such as poliovirus, rhinovi-
rus, and FMD virus. Another surrogate virus is bacteriophage
X174 (Microviridae family), which possesses a ssDNA ge-
nome and a morphology similar to that of MS2. Bacteriophage
X174 has been compared to the most resistant human-patho-
genic viruses, such as polioviruses and parvoviruses (114), and
has been used in aerobiology (96, 132, 133) along with another
microvirus, S13 (42, 137). Bacteriophage 6(Cystoviridae fam-
ily; dsRNA virus; 85 nm) can be used as a surrogate for small
enveloped viruses (132, 133).
Since every virus has a unique response to environmental
factors, no surrogate is perfect. Nonetheless, nonpathogenic
models can greatly simplify virus studies, especially when aero-
sols are used.
CONCLUSIONS
Sampling techniques have been improved greatly over the
years, and we are definitely better equipped today to tackle the
important health issue of airborne viruses. However, the lack
of standardization has to be addressed, as it limits the devel-
opment of general recommendations for sampling of airborne
viruses. Given the wide range of aerodynamic properties of
airborne viruses, which can be nanometer- to micrometer-sized
particles, the issue of standardization is of the utmost impor-
tance. The detection of viruses in air samples depends on the
type of aerosol and the sampling and analytical methodologies.
Studies to date have rarely included quantitative analyses of
total viral load. While culture is often used to determine virus
concentrations, most sampling methods affect viral infectivity,
making culture inadequate for calculating the true concentra-
tions of infectious airborne viruses. Technologies such as PCR
can be used to detect viruses in air samples even when they are
no longer infectious. While filters cause more damage to vi-
ruses than other methods do, they are more efficient for de-
termining viral loads in aerosols. Lastly, viruses are an aston-
ishingly diverse group of microorganisms, so this diversity has
to be taken into consideration to select the most appropriate
sampling devices. Whatever techniques or recommendations
are proposed for sampling virus aerosols, it is important to
keep in mind that a representative sample should contain
nanoparticles together with larger airborne particles. Future
studies will contribute to better predicting the potential risk of
infection by airborne viruses.
ACKNOWLEDGMENTS
This study was funded by a concerted grant from FQRNT-
NOVALAIT-MAPAQ and by Agriculture and Agri-Food Canada.
S.M. and C.D. also received funding from the Natural Sciences and
Engineering Research Council of Canada (NSERC). C.D. is the
recipient of an FRSQ Junior 2 scholarship.
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