Indoor Air. 2021;00:1–14.
1 | INTRODUCTION
The SARS- CoV- 2 virus is transmitted between humans via aerosols
and droplets that are expelled during breathing, talking, sneezing,
and coughing. Indoor environments are especially known for their
high risk of infection transmission.1 Respiratory aerosols and drop-
lets disperse in the room, shrink due to evaporation, and remain sus-
pended in the air for several hours. The risk of airborne infection
strongly depends on the amount and the duration of time in which an
individual is exposed to the virus load. Thus, the analysis of the prop-
erties and behavior of the virus in the air are of great interest and the
spread of airborne infectious particles in indoor environments has
been the subject of several research studies.1– 7 While the virus size
is approximately 100 nm and the mass is around 1 fg, the respiratory
aerosol and droplet distribution emitted during breathing, talking,
sneezing, and coughing is in the range of 0.3– 100 µm.
Sputum consists of 94.5% water, 0.9% sodium chloride (NaCl),
and 4.6% carbohydrate, protein, lipids, and DNA.8 Therefore, re-
spirator y droplets and aerosols evaporate slower than pure water
droplets and result in solid droplet nuclei of approximately 30% of
their original diameter.3,4,9,10 Respiratory viruses are embedded and
distributed homogeneously in this nuclei.10
After the emission, droplets and aerosols behave differently
depending on their mass. Larger droplets (>100 µm) settle to the
Received: 13 December 2020
Revised: 14 April 2021
Accepted: 6 May 2021
DOI : 10.1111/ina.1 2860
Novel measurement system for respiratory aerosols and
droplets in indoor environments
Michael Lommel1 | Vera Froese1 | Moritz Sieber2 | Marvin Jentzsch2 |
Tim Bierewirtz1 | Ümit Hasirci1 | Tim Rese1 | Josef Seefeldt2 | Sebastian Schimek2 |
Ulrich Kertzscher1 | Christian Oliver Paschereit2
This is an open access ar ticle under the terms of the Creative Commons Attribution- NonCommercial- NoDerivs License, which permits use and distribution in
any medium, provided the original work is properly cited, the use is non- commercial and no modific ations or adaptations are made.
© 2021 The Authors. Indoor Air published by John Wiley & Sons Ltd.
Michae l Lommel and Vera Fro ese should be co nsidered joint f irst author.
1Biofluid Mechanics Laborato ry, Institute
for Imaging Science and Computational
Modelling in Cardiovascular Medicine,
Charité – Universitätsmedizin Berlin,
Augustenburger Platz 1, Berlin, Berlin
2Instit ute of Fluid Dynamics and Technical
Acoustics, Hermann- Föttinger- Institute,
Chair of Fluid Dynamics, TU Berlin, Straße
des 17. Juni, 135, Berlin, Berlin 10623,
Michael Lommel, Biofluid Mechanics
Laboratory, Institute for Imaging
Science a nd Comput ational Mod elling
in Cardiovascular Medicine, Charité
– Universitätsmedizin Berlin, Berlin,
The SARS- CoV- 2 pandemic has created a great demand for a better understanding
of the spread of viruses in indoor environments. A novel measurement system con-
sisting of one portable aerosol- emitting mannequin (emitter) and a number of port-
able aerosol- absorbing mannequins (recipients) was developed that can measure the
spread of aerosols and droplets that potentially contain infectious viruses. The emis-
sion of the virus from a human is simulated by using tracer particles solved in water.
The recipients inhale the aerosols and droplets and quantify the level of solved tracer
particles in their artificial lungs simultaneously over time. The mobile system can be
arranged in a large variety of spreading scenarios in indoor environments and allows
for quantification of the infection probability due to airborne virus spreading. This
study shows the accuracy of the new measurement system and its ability to compare
aerosol reduction measures such as regular ventilation or the use of a room air purifier.
aerosol, infection transmission, measurement system, respiratory droplets, virus spread
LOMMEL Et aL .
ground within a shorter time due to gravity. The middle- sized (20–
100 µm) and smaller droplets (5– 20 µm), as well as aerosols (<5 µm),
remain airborne over a longer period of time and evaporate quickly,
depending on environmental conditions like humidity or tempera-
ture and their initial diameter.2,3,5– 7 The evaporation time of aero-
sols with an initial diameter of 1 µm is 1– 2 ms; for small droplets
of 10 µm, it is 250– 550 ms; and for large droplets of 100 µm, it is
5– 30 s.3 ,7, 9 After evaporation, the average size distribution of the
solid droplet nuclei of sputum is 0.0786– 26.2 µm 4 and can linger in
still air for 20– 60 min.6
Because of the higher mass of large- and middle- sized droplets,
the resulting viral load is significantly higher than in aerosols and
small droplets. Additionally, there is an increased chance of survival
for viruses of this magnitude.9– 11 Thus, after evaporation, the orig-
inally middle- sized droplets become small, high- risk droplet nuclei,
carrying a great number of active viruses.4,10 In their review, Mao
et al.4 combine this finding with studies on the deposition probability
as a function of aerosol size in the upper respiratory tract and the
alveolar region.12- 17 They conclude that aerosols and droplets with
a size of 2– 10 µm carry the highest risk of infection. Therefore, it
is important to investigate aerosol and droplet dispersion at these
orders of magnitude, which are mostly responsible for infectious
Especially in scenarios with poor ventilation, the ambient air
can become enriched with viruses up to a critical concentration.
All persons in the environment are simultaneously exposed to
a high concentration of viruses and the risk of so- called super-
spreading events emerges. Currently, there are no experimental
studies that quantitatively determine viral spreading in everyday
environments. Most studies use particle size analyzers to evalu-
ate the dispersion and spread of aerosols and droplets in rooms.
Noti et al.5 measured the spread of viruses in an Aerosol E xposure
Simulation Chamber using a breathing and a coughing simulator.
They used Madin- Darby canine kidney cells and influenza viruses
to determine the influence of air humidity on the risk of infection.
The amount of infectious viruses was determined by real- time
qPCR analysis and a viral plaque assay (VPA). In another study,
Lindsley et al.18 examined the efficiency of a face shield as pro-
tective equipment in the same scenario. The measuring method
is very well suited for the direct measurement of virus spread
via aerosols. Alsved et al.19 determined the size of the aerosols
and droplets during singing and talking by using this method.
However, measurements with a particle size analyzer are limited
to a deter mined range of drop let a nd ae rosol sizes. M oreover, the
direct measurement of the virus spread or aerosol and droplet
measurements with a particle size analyzer can only be done with
much effort in clean room environments. Thus, these measure-
ments are not applicable in everyday environments.
Another important factor is the experimental substitute for spu-
tum, which is used to track the aerosol spread. Most studies use
water as a single component droplet model to analyze the evap-
oration and dispersion.2,19 – 22 However, as stated above, the com-
position of sputum is much more complex than that of pure water
and the evaporation differs strongly.4,5,8 Another substitute that
has been used in experimental studies is NaCl- water solution.3,7, 9
Although the evaporation time of NaCl- water droplets is more simi-
lar to water than to sputum, the resulting droplets and aerosols also
shrink to a solid nuclei that stays airborne.9,23
To consider and investigate a larger range of parameters that in-
fluence the spread of aerosols and droplets, several studies simulate
the dispersion of droplets in closed rooms numerically.2, 6, 7,9, 24 The
simulations allow for the analysis of the aerosol dispersion at any
time and position in a room. Important parameters like the veloc-
ity during exhalation, humidity, temperature, ventilation pattern or
droplet nuclei size (solid phase in the aerosol or droplet), and ventila-
tion rate can be included.2
Nevertheless, a limitation of these simulations is that the airflow
in a specific room is influenced by many factors like ventilation slits
or door and window columns that cannot be covered by simplified
boundary conditions in the simulation.9 In most studies, the back-
ground flow is solely driven by coughing or sneezing or by defined
ventilation. Additionally, turbulent fluctuations are not considered at
all or only in a limited way. Therefore, it is necessary to validate the
simulation results with experimental measurements.
This study presents an experimental measurement method,
which is suitable to measure the spread of aerosols in everyday situ-
ations and reproduce the spread of viruses between humans. For this
purpose, an emitter was developed that simulates the human droplet
and aerosol emission. It releases a NaCl- water solution. The NaCl
serves as a tracer embedded in small- and middle- sized droplets or
aerosols that evaporate, while the NaCl nuclei remain in the air and
follow the room airflow. The size of the NaCl nuclei is in the range
of the size of the sputum nuclei after evaporation. Additionally, re-
cipients were developed which simulate inhaling persons at several
positions in different spreading scenarios. They are able to quan-
tify the amount of inhaled tracer over time. Thus, the measurement
method simulates the tracer inhalation simultaneously at several po-
sitions in a specific scenario. It allows for the evaluation of various
protective measures like passive or active ventilation or the variation
of distances between the emitting and the absorbing persons. The
aim of this study was the experimental and analytical validation of
the novel measurement method. To validate the measuring results,
• Novel mobile measurement system for the quantitative
determination of aerosol and droplet transmission be-
tween humans in everyday situations.
• Examination of the effects of protective measures like
active or passive ventilation.
• Validation tool for flow simulations of aerosol and drop-
let dispersion: simultaneous measurement at different
positions in the environment.
LOMMEL Et aL .
an analytical model was set up which calculates the time- dependent
concentration of droplets and aerosol depending on the distance to
the emitter. The model was also used to determine the effect of the
emission mode on the measurement results. In addition, a field study
was conducted to demonstrate the suitability of the new system to
measure the effect of aerosol reduction measures of ventilation and
indoor air purification in indoor environments.
2 | MATERIALS AND METHODS
The setup of the aerosol measurement system consists of one port-
able emitter and a number of portable recipients that can be posi-
tioned in different spatial arrangements with respect to one another.
The system is designed to measure the transmission of tracer parti-
cles in aerosols and droplets from the emitter to the recipients and
to evaluate and compare the infection risk of different configura-
tions with and without protective measures.
2.1 | Emitter
The emitter developed for this study enables the repeatable emis-
sion of aerosols and droplets. A siphon fed two- fluid nozzle (XA- SR
050; BETE Fog Nozzle, Inc.) is used to disperse the tracer solution
into an aerosol and droplet spray. The emitted fluid is a solution
of distilled water with a certain amount of NaCl (sodium chloride
, p.a., ACS, ISO, Carl Roth). During dispersion, the NaCl is
released and acts as a tracer particle dissolved in aerosols and drop-
lets or cr ystallized as a solid nucleus after evaporation. The tracer
solution is kept in a water reservoir with a constant liquid level that
is aligned with the outlet of the spray nozzle to avoid hydrostatic
pressure differences. This prevents a back flow or fluid leaking from
the system between the emissions, which would affect the meas-
urement accuracy. Furthermore, the tracer solution is placed on a
microscale (FC- 2000, GRAM Group) to measure the total amount of
the emitted fluid.
Through the two channels, compressed air and tracer solution
are fed into the nozzle and are mixed at the outlet (Figure 1A). A by-
pass flow (sheath flow), which is injected through four radial inlets,
is integrated and positioned circularly around the nozzle to adjust
the exhaled volume of air as well as the momentum of the emitted
aerosol. The volume flow through the nozzle and bypass system
is controlled independently by pressure regulators (DR021- 01- 3,
Landefeld GmbH, Germany) and solenoid valves (SLP15E4, E.MC
Machinery Co., Ltd.). The schematic diagram of the nozzle is shown
in Figure 1B. As described in the following chapters, the droplet size
distribution and outlet velocity are a function of the mass flow and
applied pressure to the nozzle and bypass. An exemplary emission is
shown in Figure 2. The emitter can be operated in a pulsed or con-
tinuous emission mode. The number and frequency of the emissions
and the duration of the pauses in between are set by an in- house
built pulse generator. The head of the emitter is 3D- printed from
polylactide (PLA). The basic head geometry is obtained from free3d.
com.25 It is extended by an elliptical mouth opening with an average
surface of a woman's mouth during coughing (3.37 cm2).26 The noz-
zle is placed at the back of the head so that the emitted aerosol exits
through the mouth (see Figure 1A). Except for the mouth opening,
the human appearance of the emitter is only for representative pur-
poses and future studies on face masks.
2.2 | Phase Doppler anemometry
In order to characterize the dependence of emitted aerosols and
droplets on the supply pressure and air mass flow, a phase Doppler
anemometry (PDA) system (SprayE xplorer, Dantec Dynam ics GmbH)
is used.27 The measurement point of the focused laser beams is
placed centered and at a distance of
to the mouth open-
ing of the emitter. The laser wavelength measures
scattering angle is
, and the focal length of the transmitter
and receiver yields
. The PDA system is able to detect
particle diameters in the range of 1– 150 µm. Each measurement is
conducted until the information of 200 000 particles is collected.
FIGURE 1 (A) The emitter with a
two- fluid nozzle. The tracer- fluid- air
mixture exits the nozzle as a spray due to
the applied pressure. (B) Schematic (P&I)
diagram of the emitter
LOMMEL Et aL .
The PDA measurements are conducted with continuous air sup-
ply to the nozzle. The supplied air mass flow is controlled using a
pressure regulator valve with manometer (DR021- 01– 3, Landefeld
GmbH, Germany) and is monitored using a Coriolis mass flow meter
(Promass A, Endress+Hauser AG).
2.3 | Recipients
The aerosol- absorbing recipients (Figure 3) inhale the surround-
ing air that is enriched with the tracer. The design of the portable
recipients is as follows: A vacuum pump (ATTIX 30- 01, NILFISK
GmbH, Brøndby, Denmark) creates a vacuum in a sealed glass cyl-
inder that is partly filled with 200 ml double deionized water. Due
to the vacuum, air with NaCl nuclei, aerosols, or droplets (tracer
particles) is led through a tube (trachea) from the mouth of the face
model into the water of the glass cylinder. This part of the recipi-
ent is called the water lung. The volume of the aspirating airflow
is controlled by two flowmeter sensors (flowmeter DK46, Krohne
GmbH) with a variable flow rate of 0 L min−1 to 23.2 L min−1. The
inhalation is continuous for the presented investigation. A fine fil-
ter paper (Cytiva grade 589/1, Thermo Fisher Scientific) is located
at the end of the trachea, completely surrounded by the deionized
water. It functions as an atomizer and thus splits the air stream
into fine bubbles to dissolve the tracer particles in the water of
the glass cylinder, which increases its electrical conductivity. The
fluid is pumped continuously by a roller pump (GROTHEN DC 12 V,
Aibecy) through a parallel circuit in which the probe of a conductiv-
ity meter (HI98192, Hanna Instruments) with a resolution of 0.01
µS and a measure accuracy of ±0.01 µS is implemented. It hereby
FIGURE 2 Image of the emit ter as it
emits the droplets and aerosols in the
scenario of a cough.
FIGURE 3 Display of a recipient: the
aerosol is sucked through the trachea into
the water lung by a vacuum. There, a filter
dissolves the tracer in the measuring fluid.
The measuring fluid is pumped through
the sensor by a roller pump. The airflow is
adjusted by a flowmeter
LOMMEL Et aL .
measures the increase of the conductivity over time and, thus, the
NaCl concentration. These measurement data are used to deter-
mine the amount of droplets and aerosols transported from the
emitter to the recipient's lungs in relation to the total amount of
emitted droplets and aerosols. The face and body of the recipients
are used solely for representational purposes and for easier posi-
tioning on chairs in a room. The physiological human heat emission
can be simulated by heating pads. This influence of the buoyancy
effects is not considered in this study to simplify the experimental
conditions. This facilitates the repeatability of experimental condi-
tions for the verification of the measurement technique.
2.4 | Measurement procedure
The recipients are placed at defined positions in the room. The fre-
quency and duration of the emission and the inhalation flow rate of
the recipients are set. The measurement starts with the recording of
the current state: The recipients measure the increase in conductiv-
ity without any aerosol emission of the emitter (lead- in time). Thus,
in the first minutes of the measurement, the background concentra-
tion of particles in the room is measured. This slight increase of the
conductivity caused by room specific conductive aerosols or due to
aerosols of the previous measurements is possible in every room and
has to be eliminated from the actual measurement. Therefore, this
background concentration is subtracted as an offset from the meas-
ured values during the post- processing. This procedure also shows
whether the environment was sufficiently vented after previous
measurements. After the lead- in time, the emitter starts exhaling
and aerosols and droplets are released in a predetermined amount
of ejection cycles (emission period). After the emission stops, the
measurement is continued up to a predetermined point in time or
until no more increase in conductivity can be measured (lead- out
time). During the entire measurement procedure (lead- in, emission,
lead- out), the conductivity measurement of each individual recipient
is recorded in real time on the measuring device as well as monitored
and stored on the measuring computer. The conductivity sensor
measures the temperature in the water lung during the entire meas-
urement. The dependence of the conductivity on the temperature is
compensated by the device. The room temperature and humidity are
2.5 | Verification measurements
To determine the sensitivity, a 0.9% NaCl solution (sodium chloride
solution 0.9%, CELLPURE®, Carl Roth) was pipetted into the meas-
urement system of four recipients. The detectable amount of NaCl
was determined by the increase in conductivity.
For the determination of the ability of the water lung to dissolve
the tracer from the aspirated air, two water lungs were connected
in series. The filter efficiency was determined from the ratio of the
increase in conductivity in the two lungs.
The verification measurement for the determination of the
measurement accuracy and the comparison with the analytical
model was performed in a sealed room at the university with
100 m3 air volume and negligible room air circulation and air ex-
change rate. Four recipients were positioned around the emitter
at a distance of 1.5 m (Figure 4). Due to the specific setup, it was
possible to model the dispersion analytically using a spherical
model to compare the experimental with the theoretical results
(see following paragraph). Moreover, the data obt ained from each
recipient were compared to the others to determine the accuracy
of the measurement method.
The verification measurement followed the measurement proce-
dure stated above. A lead- in time of 300 s was chosen to sufficiently
measure the background concentration. The aerosol was released
with low momentum (see Table 1) and within an emission period of
500 s with 100 ejection cycles to ensure a good signal to noise ratio.
The lead- out time was predetermined to 1300 s. Therefore, the en-
tire measurement time was 2100 s.
A 5% NaCl solution was used as the tracer solution. This partic-
ular concentration was selected, because it corresponds to the size
reduction of sputum after evaporation.4 It results in a tracer parti-
cle size of 29% of the original droplet diameter after evaporation,
which results from the proportion of the solid phase. The density
of 2.16 kg m−3 of the salt spheres is higher than the density of the
remaining sputum nuclei which consists of further components with
lower density like proteins, lipids, carbohydrates, and DNA.9 The
duration of one cycle of exhalation was set to 2.5 s with a pause
of 2.5 s. Thus, the exhalation to pause ratio was 1:1. The inhalation
flow rate of the recipients was set to the average flow rate during
inhalation of 15 l min−1. The measurement was repeated four times.
Between the measurements, the room air was fully ventilated until
the conductivity did not further increase and the background con-
centration of particles was detected anew for each measurement.
2.6 | System response to aerosol
To further evaluate the new system and to verify its responsiveness
to the aerosol reduction measures venting and room air purification,
a field study was conducted. The measurements were performed
in a conference room with a volume of 100 m3 and two windows
(3 m2 opening area) on one side of the room (see Figure 4). During
the measurement procedures, the room was fully closed with sealed
windows and doors to minimize the air exchange rate. The approach
was the same as described in the measurement procedure and veri-
fication measurements. Settings of the emitter, recipients, position-
ing, lead- in time (300 s), emission period (500 s with 100 ejection
cycles), and lead- out time (1300 s) were performed accordingly.
Three different measures were conducted, and each measurement
was repeated three times:
• A measurement without any aerosol- reducing mechanisms.
LOMMEL Et aL .
• A measurement by using a room air purifier (Philips AC2882/10)
with HEPA filter. The device was positioned centrally near one
wall, that has no windows or doors, at a distance of 3 m from the
emitter. It was started at the beginning of the lead- in time with an
air exchange rate of 333 m3 h−1.
• A measurement with ventilation. The room was ventilated by
opening both windows two times for 120 s after 480 s and 800 s
after the beginning of the measurement.
2.7 | Particle migration modeling
The aerosol propagation and the increase of tracer concentration in
the air during the verification measurements were investigated ana-
lytically. A simplified model was chosen without taking convective
transport into account, which was negligible in the verification meas-
urements. The model is utilized to validate the time- dependent aerosol
absorption of the recipients and to evaluate the influence of different
emission m odes, distance s between the emi tter and the recip ients, and
different measurement duration of the experimental setup. Therefore,
it provides a basis for the generalization of the measured aerosol trans-
mission beyond a specific investigated scenario.
The transport of aerosols and droplets in the air is modeled
by diffusion and dissipation in a spherical coordinate system. The
spatio- temporal distribution of the aerosol concentration is de-
scribed by the following differential equation
where c denotes the concentration, v the diffusion rate, and
sipation rate. The equation describes the diffusion in spherical coor-
dinates assuming a perfect rotational symmetr y. The radius r denotes
the distance from the emitter located at the origin. The diffusion pro-
cess approximates the dispersion of aerosols due to turbulent mixing
and molecular diffusion. The dissipation approximates the dilution due
to air exchange and settlement of aerosols on the ground.
The equation is approximated in space by a second- order finite
difference scheme, and the propagation in time is exactly solved by
an exponential integrator of the discretized system. To avoid singu-
larities at the origin, the system is solved for the variable
The scaling of the absorbed concentration with increasing dis-
tance is further characterized by the protection factor (PF), which
is given as 28
It indicates the possible reduction of absorbed aerosols over
time at any point r in relation to the point of emission
a PF of two indicates half the amount of absorbed aerosols.
The diffusion and dissi pation equation (1) is simul ated in a domain
from r = 0.01 m to r = 15 m for 1800 s, where (aerosol) concentration
is added to the domain at the origin with a constant rate for the first
500 s. The grid is refined up to the point in which changes in the
simulated concentrations were below 1%. This results in 600 grid
points for the simulated conditions. The diffusion rate is varied in the
range of v = 0.0 01 m2 s−1 to 0.01 m2 s−1 , which is two orders of mag-
nitude larger than typical molecular diffusion rates in air at normal
FIGURE 4 Image of the setup and schematic of the positioning in the verification measurement (A) and of the setup in the measurements
with ventilation and room air purification (B). In both scenarios, the transmitter is positioned in the center of the four recipients, each of
which is located at a distance of 1.5 m from the emitter. At the end of the conference room (B) is the room air purifier.
TABLE 1 PDA parameters and mean results
Mass flow [kg
Average diameter after
evaporation [µm] D(4,3) [µm]
0.38 0.84 4.37 1. 27 35.02 10.16 1.48
0.95 1. 41 3.29 0.95 13.89 4.03 2.55
1.65 2.0 2.87 0.83 9.31 2.70 3.52
2.4 2.6 2.52 0.73 8.75 2.54 4.38
Abbreviation: PDA, phase Doppler anemometry.
LOMMEL Et aL .
conditions. Therefore, the values of the diffusion rate, which are ad-
justed to replicate the time series observed in measurements, corre-
spond mainly to turbulent diffusion. The dissipation rate is varied in
the range of µ = 0.0001 s−1 to 0.1 s−1 which corresponds to a halving
of the concentration in two hours and one minute, respectively.
The experimental results were compared against the analytical
model. Therefore, the model was simulated in a sphere of 3 m radius
to approximate the volume of the investigated room. The diffusion
rate and dissipation rate of the model were calibrated to minimize
the sum of squared differences between measurement and simu-
lation. A delay of 10 s in the measurement signal is considered that
accounts for the measuring delay of the recipients. For each recipi-
ent, the analytical model is compared with the measurement results
averaged over the four repetitions and normalized to the maximum
conductivity for each case.
3 | RESULTS
3.1 | Aerosol characterization
With the two- fluid nozzle, a wide range of output possibilities could
be achieved by varying the parameters (see Appendix 1). It was pos-
sible to create aerosol emission conditions similar to those of breath-
ing, talking, sneezing, and coughing. The bypass flow allows high
ejection speeds such as sneezing and coughing. The aerosol output
with low momentum for breathing and speaking can be simulated
without bypass flow. The mass flow, average size, and velocity emis-
sion can be adjusted by the pressure applied to the nozzle. With an
increasing pressure head of the air, the volume flow and the velocity
increase, while the average particle size distribution shifts to smaller
particle diameters. A summary of the investigated parameters and
the results of the PDA measurements is given in Table 1.
An average diameter of 2.52– 4.37 µm was measured. The De
Brouckere mean diameter (D(4,3)), the mean of the particle size distri-
bution weighted by the volume, was 8.75– 35.02 µm for the tested
parameters. Furthermore, a mass flow of 0.84 kg h−1 to 2.6 kg
h−1 and a velocity of 1.48 m s−1 to 4.38 m s−1 of the aerosols and
droplets were measured without the bypass flow. Figure 5 shows
the relative droplet and aerosol size distribution in relation to the
particle number and aerosol volume percentage of the verification
measurement. The pressure was set to 1.65 bar, resulting in a mass
flow of 2 kg h−1 , a velocity of 3.52 m s−1, and an average diameter
of 2.87 µm. The resulting D(4, 3) for this configuration was 9.31 µm.
After evaporation, the calculated remaining average tracer par ticle
diameter was 0.83 µm and D(4, 3) was 2.7 µm.
3.2 | Measurement accuracy and experimental
results of the verification measurement
The encoder resolution of the conductivity meter was 0.01 µS.
Thus, the smallest increase detectable by the device corresponds to
2.113 µg (±0.11 µg) per 0.01 µS. The detection limit of a 5% tracer
solution in the water lung is 40.9 µL. This corresponds to
droplets with a diameter of 10 µm. A filter efficiency of 97.38%
(±0.97%) was determined with the measurement of the conductivity
increases of two water lungs connected in series.
For the determination of the measurement accuracy, four mea-
surements with four recipients were performed in a room with neg-
ligible air convection. After a lead- in time of 300 s, the emission
started for 800 s (100 exhalations, emission 2.5 s, pause 2.5 s, ratio
1:1), followed by a lead- out time of 1300 s. The overall sampling du-
ration was 2100 s. An aerosol and droplet quantity of 6.36 g ± 0.38 g
was emitted. Thus, for each measured increase in value (0.01 µS),
an average of 0.0033% of the overall amount of tracer particles re-
leased within a single exhalation could be detected. The ratio of the
power of the signal to the power of the background noise (signal to
noise ratio) in this setup was 7.95 (±0.922). The temperature was
20°C ± 1°C, and the humidity was 36 (±2%). The average increase
in conductivity of the four verification measurements is shown in
Figure 6. In this figure, the background concentration during the
lead- in time is shown. This concentration increase is always sub-
tracted from the overall increase in post- processing.
During the lead- in time, a slight increase in conductivity was
measured. At 300 s, the aerosol emission was started. With a delay
300 s, the tracer absorption of the recipients in 1.5 m distance
increased over time. At 800 s, the aerosol emission was stopped. In
the lead- out time, the tracer particle absorption increased further
over time. 350– 400 s after the emission was stopped, an inflection
FIGURE 5 Spray characterization
in the verification measurement at a
pressure of 1.65 bar and mass flow of
2.0 kg h−1
LOMMEL Et aL .
occurred and the tracer absorption decreased slightly over time until
the end of the measurement. The overall average increase over the
four experiments is 0.936 µS with a standard deviation of ±1.46%.
This increase in conductivity refers to an absorbed tracer solution
mass of 198, which corresponds to 0.003% of the overall emitted
aerosol. The measurement accuracy among the four recipients
within a single experiment was ±7% (Figure 7).
3.3 | Analytical modeling of aerosol dispersion
The time series at selected conditions is displayed in Figure 8 indi-
cating se veral characte ristics of the inves tigated problem . It is visible
that the start of concentration increase and the de cay after stopping
the emission are delayed, depending mainly on the diffusion rate.
The maximum of the observed concentration scales directly with
the diffusion rate and inversely with the dissipation rate. For large
dissipation rates, steady- state conditions can be reached, where
the concentration approaches a constant value during the emission.
Similarly, a complete decay of the concentration in the investigated
time frame is only obser ved for large dissipation rates.
To further describe the scaling of the absorbed concentration with
an increasing distance, the PF (2) is computed for the entire simulation
= 0 s and
= 1800 s, see Figure 8). The scaling of the PF is
foun d to be exponential with the distance as indicated in Figure 9A. To
relate this exponential trend to the simulation parameters, the radial
dependency is approximated by an exponential function
in the range from
. The PF scaling exponent α is found
to be almost perfectly proportional to
as displayed in Figure 9B.
The observed deviations from this proportionality are related to very low
𝜇<0.001 s −1
, where the simulated time is too short to
account for the slow time scale of the dissipation process. The increase of
the simul ation time (not displaye d) collapses all th e points to a strai ght line.
The PF and the related scaling exponent are found to be inde-
pendent of the emission duration and magnitude, given that the ob-
served time frame is long enough to capture the time scale of the
process. The same proportionality is found if a short impulsive emis-
sion at the beginning is considered or a continuous emission for the
entire observation time. The investigations show that no matter how
the aerosol emission is timed and how much aerosol is emitted, the
relative decrease of aerosol inhalation with distance is always the
same, given that the observation time is long enough. However, the
absolute amount of inhaled aerosol strongly depends on the emis-
sion mode and the duration of the exposure.
3.4 | Agreement of the verification measurement
with the analytical model
An excellent agreement could be found between the verification
measurement and the analytical model. Two exemplary graphs are
shown in Figure 10. The calibrated model parameters for the four
FIGURE 6 Average increase in
conductivity for four verification
measurements within the same setup:
mean values of four recipients, negligible
airflow, distance 1.5 m, lead- in time:
300 s, aerosol emission until 800 s, lead-
out time 130 0 s, and overall sampling
duration 2100 s. The dashed lines for each
measurement represent the basic increase
during the lead- in, which are subtracted
from the measurement results in the post-
FIGURE 7 Verification measurement s: the overall average
increase in conductivity after a sampling duration of 2100 s
and the st andard deviation of the four recipients is shown. Four
measurements with four recipients were performed. The distance
between the recipients and the emitter was 1.5 m. As tracer
solution a 5% NaCl solution was used. A total of 100 exhalations
with a duration of 2.5 s and a pause of 2.5 s were performed,
during which a total of 6.36 g ± 0.38 g tracer solution was emitted.
Thus, the exhalation to pause ratio was 1:1
LOMMEL Et aL .
recipients are presented in Table 2. Among the four recipients, the
average dif fusion rate is 0 .647 × 10−3 m2 s−1 (± 0.052 × 10−3 m2 s−1). The
average diss ipation rate is 0.9125 × 10−3 m2 s−1 (±0.0 87 × 10−3 m2 s−1).
Thus, both show a standard deviation of <10%.
3.5 | Response of the system to ventilation and
Two different aerosol reduction measures were compared to a ref-
erence measurement without any measures in the same room (see
Figure 11). All three measurements were repeated three times with
four recipients, respectively.
3.5.1 | Reference measurement
The reference measurement showed very similar results and curve
progression to the verification measurement described above (see
Figure 6). The tracer absorption of the recipients in 1.5 m dis-
tance began to increase over time
300 s after the emitter was
started. In the lead- out time (after 80 0 s), the tracer particle ab-
sorption increased further over time. The curve showed a slight
turning point 350– 400 s after the emission was stopped, and the
tracer absorption decreased slightly over time until the end of the
The overall average increase over the three experiments
in the measurement without any measures was 0.76 µS with a
FIGURE 8 Simulated time series
displayed at a distance of r = 1 m from the
origin for discussion rates v = 0.001 m2 s−1
(A and C) and v = 0.01 m2 s−1 (B and D).
The graphs (A) and (B) show concentration
for different dissipation rates µ. The
graphs (C) and (D) display the integral
of the concentration given in graphs (A)
and (B), respectively, which is measured
in the experiments as the increase of
the conductivity. The dashed vertical
line indicates the stopping time of the
0510 15 20 25 30
0510 15 20 25 30
0510 15 20 25
1(B) =0.001 s-1
0510 15 20 25
FIGURE 9 (A) Protection factor (PF) at
selected conditions and the approximated
exponential trend indicated by the dashed
lines. (B) The exponential scaling of the
PF(r) ≈ exp(αr) is displayed for all simulated
conditions (diffusion rate v = 0.001 m2 s−1
to 0.01 m2 s−1 and dissipation rate
µ = 0.0001 s−1 to 0.1 s−1).
FIGURE 10 Verification measurements
and simulation results calibrated to the
-5 0510 15 20 25 30
-5 0510 15 20 25 30
LOMMEL Et aL .
standard deviation of 1.87%. This increase in conductivity refers
to an absorbed tracer solution mass of 160.16 µg. The temperature
was 17.7°C ± 1°C, and the humidity was 35% (2%).
3.5.2 | Air purifier measurement
During the emission period (300– 800 s), the aerosol absorption
with the air purifier increased significantly faster, than within the
other two measurements. During the emission phase, a strong in-
crease of the conductivity and thus the aerosol absorption could
be measured. After stopping the emitter, a turning point occurred
and the aerosol concentration in the room continued to be low-
ered and the slope of the curve is decreasing until the end of the
measurement. Overall, it provided a fundamentally lower increase
in the absorption curve up to 0.53 µS with a standard deviation
of ±9.27% over the three experiments. Thus, an average mass of
112.04 µg of the tracer solution has been absorbed, which is 30%
less than without any measures. The temperature during the three
repeated experiments was 17.5°C±1°C, and the humidity was 36
3.5.3 | Ventilation measurement
In the beginning of the emission of the ventilation measurement
(from 300 s to 480 s), almost no increase of conductivity could
be measured. This equals the curve progression of the measure-
ment without any measures. About halfway of the first ventilation
480– 600 s, the aerosol absorption suddenly increases sharply.
Subsequently, while the windows were closed and the emitter was
still generating aerosol (after 600– 800 s), a similar increase as in
the measurement with the room air purifier occurred. During the
second ventilation from 800 s to 920 s, the emission phase is al-
ready over and the curve flattens. Accordingly, a flatter curve with a
lower aerosol uptake compared to the reference measurement can
be observed thereafter. The concentration in the room remained
almost constant and decreased only very slightly over time. Thus,
the recipients continued to absorb a constant amount of aerosol.
The overall average increase over the three experiments in this
measurement was 0.51 µS with a standard deviation of 2.8%. After
2100 s, a solution mass of 106.77 µg of the overall emitted aerosol
has been absorbed, 33% less than in the measurement without any
measures. The temperature during the three repeated experiments
was 17.6°C ± 1.5°C, and the humidity was 36 (±2%).
4 | DISCUSSION AND LIMITATIONS
4.1 | Verification of the novel measurement system
The novel measurement system enables the quantification of the
spread of tracer particles in aerosols and droplets in indoor environ-
ments. Due to the high sensitivity and low measurement fluctua-
tions, the system can detect small amounts of aerosol (40.9 nl of a
TABLE 2 Results of the diffusion rate and dissipation rate for the
Diffusion rate v
1 0.640 × 10−3 1.016 × 10−3
2 0.732 × 10−3 0.927 × 10−3
3 0.623 × 10−3 0.775 × 10 −3
40.594 × 10−3 0.932 × 10−3
FIGURE 11 The overall average increase in conductivit y after a sampling duration of 2100 s is shown. The dashed vertical lines mark
the start 300 s and end 800 s of aerosol emission of each measurement. Each curve is based on three measurements with four recipients,
respectively. The yellow curve represents the measurement with no aerosol- reducing measures. The measurement shown by the red curve
has been conducted with a room air purifier, filtering the air with a HEPA filter from the start to the end of the measurement. During the
ventilation measurement (blue curve), the room was ventilated two times (after 480 s and 800 s) for 120 s (blue highlighted sections).
LOMMEL Et aL .
5% tracer solution per measurement value of 0.01 µS). A very high
measuring accuracy of the average tracer absorption among the four
verification experiments could be demonstrated with a standard
deviation of only ±1.46%. Thus, measurements over long distances
with small aerosol quantities are possible. The water lungs are ca-
pable of dissolving and detecting 97.38% (±0.97%) of the incoming
tracer, which reaches the measurement fluid in form of solid nuclei
or in dissolved form in droplets and aerosols.
The system has been compared to an analytical model which
provides realistic estimations of how the emission mode effects the
results of the measurement. It concludes that if the observation time
is long enough, the relative aerosol inhalation with distance is inde-
pendent of the emission mode. The fitted model shows a diffusion
rate and a dissipation rate with time scales in the order of 1 × 10−3 s.
This indicates that the measurement duration of 1800 s was chosen
long enough to capture all effects sufficiently. However, the abso-
lute amount of inhaled aerosol strongly depends on the emission
mode and the duration of the exposure.
The modeling of the aerosol uptake in the verification measure-
ment shows an excellent agreement with the measured values. The
modeling of the individual recipients shows a small standard devi-
ation of the diffusion rate and dissipation rate of under <10%. The
setup of the analytical model is only a coarse simplification that does
not consider convective transport and thermal stratification of the
environment, which are both very relevant for everyday environ-
ments. Nevertheless, the investigation of Mittal et al.28 shows that
even in realistic scenarios with convection and stratification, a sim-
ple scaling of the PF can be found.
Most measurement methods only quantify the evaporation
and dispersion of aqueous droplets and aerosols.2,1 9- 22 However,
the liquid part of the aerosols and droplets evaporates in the range
of milliseconds to seconds, depending on humidity and tempera-
ture.4 In this case, dried, potential virus- containing sputum nuclei
remain airborne, follow the airflow, and are potentially infectious
for hours.4,29 Measurement metho ds that only spread and analyze
aqueous droplets neglect this aspect, whereas this novel measur-
ing method is able to reproduce this process due to the applied
The reaction time of the measurement setup is 5– 20 s. Changes
in the tracer concentration in the recipients’ water lungs can be de-
tected with relatively low measuring delay. Thus, during a measure-
ment process, the effect of different changes in the environment like
ventilation or air filtration can be quantified in real time.
The size of the tracer nuclei depends on the concentration of the
tracer solution. The higher the amount of the tracer concentration,
the larger the diameter of the crystals that remain in the air after
the water is evaporated.23 Thus, a concentration should be chosen,
which results in droplet nuclei in the same magnitude as the modeled
aerosols and droplets after evaporation and increases the conduc-
tance in the water lung enough to exceed the measurement noise.
Therefore, in environments with very low transmission of aerosols
(eg, rooms with strong ventilation), it can be necessary to increase
the concentration and the amount of emitted tracer solution. A
tracer solution of 5% was chosen in the measurements, because
it creates crystals with sizes in the same order of magnitude (see
Figure 3) as the distribution in a human cough after evaporation.4
An almost perfect ability to follow the airflow is assumed for all
particles smaller than 20 µm after evaporation.30
In the measurements, a higher mass flow is emitted than during
speaking. This increases the humidity in the immediate vicinity of
the emitter. It is assumed that this does not influence the results
of the measurements significantly due to the distance of 1.5 m be-
tween emitter and recipients.
In the measurements, the emitted air has not been heated, so the
trajectory deviates from body heated exhaled air. In addition, the air-
flow caused by the plume was not mimicked, so the exposure of the
recipient could be underestimated. The error depends on the princi-
ple of room air distribution. The errors can be prevented by further
improvements of the measurement system. The electrostatic charge
of the materials has not been checked. It is therefore possible that
charged NaCl particles have deposited on the walls. The aerosol up-
take may therefore have been underestimated.
4.2 | The novel measurement system in different
In the system response measurements, the measurement without
aerosol reduction measures proceeds similar to the verification
measurements. With the room air purifier, the aerosol absorption
starts earlier, which can be explained by the increased convection
of the room air. After the aerosol emission, the aerosol absorp-
tion decreases steadily over time, which can be explained by the
continuous reduction of the aerosol concentration. This resulted
in a 30% reduction in total aerosol absorption over the measure-
ment period. In the ventilation measurement, the first opening
also causes convection of the room air and therefore an earlier
increase in aerosol absorption. Both ventilation processes caused
a reduced slope and therefore a reduced aerosol concentration in
the room air. During the periods without ventilation, the aerosol
absorption remains almost constant over time, which indicates a
constant concentration. The short- term ventilation procedures
reduced the total aerosol absorption by 33%. The effect of the
measures and the reduction of the total absorption could be re-
corded repeatably with the novel measuring system. The results
are in line with expectations.
As shown in this field study, the new system is able to display
changes in the environment such as the opening of windows or the
switching on of a room purifier within seconds. It is therefore very
well suited for testing various measures to reduce aerosol disper-
sion in a wide variety of rooms. Instead of applying general safety
measures, specific precautions can be tailored toward the risk of
infection in different environments. The high fluctuations during
ventilation did not influence the system accuracy and the standard
deviation was only 2.8%. The increased convection due to the room
air purifier resulted in a standard deviation of 9.27%.
LOMMEL Et aL .
A major advantage of the new measurement system with tracer
particles is its broad applicability. In contrast to other methods, it is
not limited to clean rooms. The measurement can be performed in
various scenarios of indoor environments, because it was not dis-
turbed by dirt or dust in any of the environments investigated so
far. It is not harmful, since it does not use toxic components, dyes,
or laser beams or causes biological contamination like the real- time
qPCR methods.5 A restriction might only evolve in environments
where the air contains a similarly high concentration of conductive
particles as the emitted tracer solution. Nonetheless, the back-
ground concentration of particles is detected anew for each mea-
surement. If a room has been fully ventilated after a measurement or
if there is any background concentration, can be derived by a change
of increase of the conductivity in the lead- in time, which than will be
subtracted from the overall increase. In Figure 4, in the verification
measurement, the increase due to the background concentration is
represented by the dashed lines, whereas in the system response
measurement (Figure 11), it has already been subtracted from the
entire cur ves respectively. The measurement system is portable and
can be placed on seats or chairs in a room, and the positioning can be
changed easily within minutes.
In order to obtain the most expressive and comprehensive mea-
surement output possible, the positioning of the recipients in the
room or environment of the emitter is essential. Whether a tracer
will reach a certain recipient in a certain time mainly depends on the
flow field. For the application in rooms with complex airflow, it is
therefore helpful to perform a smoke spread analysis 31 prior to the
measurements. This gives a good first impression of the global flow
in the room. Afterward, especially critical areas can be analyzed by
the emitter and the recipients.
4.3 | Outlook
Currently, the spread of viruses in everyday situations is mainly
evaluated by flow simulations. However, the flow simulations make
strong simplifications and cannot take the many disturbing influ-
ences into account, such as local temperature fluctuations, con-
vection through window gaps and door slits, and evaporation of
droplets. Therefore, it is important to validate the results of the
simulations. This system is very well suited for this purpose, as it
simultaneously delivers time- resolved measurement data at several
positions in the environment.
For this study, the human appearance of the recipients and the
emitter is for representative purposes and for an easier positioning
only. The physiological human heat emission can be simulated in
future studies by heating pads. Furthermore, in a current redesign
we plan for heating the liquid phase of the aerosol before atomiza-
tion in order to prevent a temperature drop due to the evaporation.
However, the realistic representation of human plume in experimen-
tal investigations is beyond the focus of the current paper. The re-
cipients and physiological human breathing differ from each other,
since the inhalation of the recipients is continuous. This is sufficient
for the examination of the spreading of aerosols and droplets, as
long as a transport of aerosols and droplets mainly driven by large
flow structures can be assumed. The local pulsatile flow dynamics
due to in- and exhaling processes would not play a significant role in
this context. However, for the future potential evaluation of masks,
it would be more appropriate to include an inhalation and exhalation
rhythm. In this context, the flow dynamics in the direct vicinity of
the individuals become more important. The system will be further
developed for these applications, and the human appearance will
be necessary for a realistic fit of the masks. Medical face masks are
currently tested according to the DIN EN standard on requirements
and test procedures for medical face masks. Herein, the tests are
performed with a par ticle size of 0.65– 7 µm (DIN EN 14683:2019- 1).
With the emitter, it is possible to generate the corresponding droplet
and aerosol distribution and evaluate the masks accordingly.
The system could be suitable to evaluate schools or offices as
well as event locations, cinemas or public transport. An individual
protection strategy could be developed, and the efficiency of pro-
tective measures could be quantified. In addition to the measure-
ments of this study, the measurement system has already been
tested in a large concert hall, a train, and a dentist's examination
room with promising results. Initial measurements outdoors are also
in progress. The results of these additional measurements in indoor
and outdoor environments will be presented in future studies.
As long as a representation of airborne salt crystals is valid in
terms of size, the model can also be used to recreate and analyze
the propagation processes of other bioaerosols like bacteria, fungi,
or pollen and aerosols consisting of inorganic harmful substances.
5 | CONCLUSION
The novel measurement system allows for the quantitative de-
termination of aerosol and droplet transmission between humans
in everyday situations. Due to its portability, it can be used on-
site and measure in real time with a relatively short measurement
delay. The experimental in situ characterization allows for the ex-
amination of the effects of protective measures like active or pas-
sive ventilation and face masks without underlying assumptions
or constraints like clean room environments of several other ex-
perimental measurement techniques. Moreover, it is a useful tool
to validate flow simulations of aerosol and droplet dispersion at
different positions in the environment experimentally. The emit-
ter is able to simulate aerosol and droplet emission with the same
characteristics as a human during breathing, talking, sneezing, and
coughing. The tracer can be measured with a resolution of 40.9 nl
of a 5% tracer solution per measurement value. The recipients de-
tect 97.38% (±0.97%) of the incoming tracer. In the verification
measurements of this study, a very high measurement accuracy
of the average tracer absorption among the four verification ex-
periments was demonstrated with a standard deviation of ±1.46%.
Thus, the measurement system has proven to be reliable due to its
high sensitivity and low measurement fluctuation. This has also
LOMMEL Et aL .
been demonstrated in the system response measurement. The
two different measures ventilation and room air purifying could be
represented and evaluated very well by the measurements, where
both processes were able to reduce aerosol uptake by about one-
third after 2100 s. Despite the expected high fluctuations during
ventilation, the standard deviation was only 2.8%. In conclusion,
this method is suitable to assess the respiratory hazards of every-
day situations in private and public indoor environments, enabling
the evaluation of ventilation strategies, air purification technolo-
gies, and further protective measures.
The valuable and inspiring support of Joshua Gray is gratefully
The peer review history for this article is available at https://publo
ns.com/publ o n/10 .1111/ina.12860.
Michael Lommel https://orcid.org/0000-0003-1788-276X
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Novel measurement system for respiratory aerosols and
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LOMMEL Et aL .
TABLE A1 CRediT
acquisition Investigation Methodology
administration Resources Software Supervision Validation Visualization
x x x x x x x x x x x x
Vera Froese x x x x x x x x
Moritz Sieber x x x x x x x x x x
x x x x x x
Tim Bierewirtz x x
Ümit Hasirci x x x x
Tim Rese x x x
Josef Seefeldt x x x
x x x x x
x x x x x
x x x x x