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A comparative study of walking-induced dust resuspension using a
consistent test mechanism
Abstract Human walking influences indoor air quality mainly by resuspending
dust particles settled on the floor. This study characterized walking-induced
particle resuspension as a function of flooring type, relative humidity (RH),
surface dust loading, and particle size using a consistent resuspension
mechanism. Five types of flooring, including hardwood, vinyl, high-density cut
pile carpet, low-density cut pile carpet, and high-density loop carpet, were tested
with two levels of RH (40% and 70%) and surface dust loading (2 and 8 g/m
2
),
respectively. Resuspension fraction r
a
(fraction of surface dust resuspended per
step) for house dust was found to be varied from 10
7
to 10
4
(particle size:
0.4–10 lm). Results showed that for particles at 0.4–3.0 lm, the difference in
resuspension fraction between carpets and hard floorings was not significant.
For particles at 3.0–10.0 lm, carpets exhibited higher resuspension fractions
compared with hard floorings. Increased RH level enhanced resuspension on
high-density cut pile carpet, whereas the opposite effect was observed on hard
floorings. Higher surface dust loading was associated with lower resuspension
fractions on carpets, while on hard floorings the effect of surface dust loading
varied with different RH levels.
Y. Tian
1
, K. Sul
2
, J. Qian
2
,
S. Mondal
3
, A. R. Ferro
2
1
Institute for a Sustainable Environment, Clarkson
University, Potsdam, NY, USA,
2
Department of Civil and
Environmental Engineering, Clarkson University,
Potsdam, NY, USA,
3
Department of Mathematics,
Clarkson University, Potsdam, NY, USA
Key words: Resuspension; House dust; Mechanical foot;
Indoor air quality; Relative humidity; Surface dust
loading.
Y. Tian
Institute for a Sustainable Environment
Clarkson University
8 Clarkson Ave
Potsdam NY 13699
USA
Tel.: +1-315-267-6361
Fax: +1-315-268-7985
e-mail: tian@clarkson.edu
Received for review 30 August 2013. Accepted for
publication 1 March 2014.
Practical Implications
The results from this study validate the recommendation that people sensitive to allergens could select hard floorings
to reduce exposure and related adverse health outcomes. The results can also be applied to exposure models to deter-
mine the overall impact of exposure to resuspension as compared with other particle sources.
Introduction
Resuspension of settled dust caused by human activi-
ties contributes a considerable portion of human
particulate matter (PM) exposure indoors (Ferro et al.,
2004; Thatcher and Layton, 1995; Yakovleva et al.,
1999). House dust is a heterogeneous mixture of parti-
cles generated from both indoor and outdoor sources,
which typically contain many toxicants, carcinogens,
and allergens (Lioy et al., 2002; Mercier et al., 2011).
Inhalation of dust particles is linked to several acute
and chronic detrimental respiratory health effects. For
instance, studies have shown that exposure to some
indoor pollutants and allergens can trigger and exacer-
bate asthma symptoms in susceptible adults and
children (Ahluwalia and Matsui, 2011; McCormack
et al., 2011). Mitigating resuspension of inhalable par-
ticles can be an effective way to improve indoor air
quality in residences, thereby reducing adverse health
effects.
Previous research has shown that estimated particle
resuspension rates from human activities vary over sev-
eral orders of magnitude. Although some of this vari-
ability is attributed to different methods and
assumptions used in various studies, some is stemmed
from different flooring types, particle sizes, and other
factors, such as surface dust loading and relative
humidity (Thatcher and Layton, 1995; Buttner et al.,
2002; Gomes et al., 2007; Manthena and Ferro, 2009;
Qian and Ferro, 2008; Rosati et al., 2008; Shaughnessy
592
Indoor Air 2014; 24: 592–603 ©2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
wileyonlinelibrary.com/journal/ina
Printed in Singapore. All rights reserved INDOOR AIR
doi:10.1111/ina.12107
and Vu, 2012). The dependence of resuspension rates
to flooring type indicates that the reduction of dust
resuspension and resulting human exposure in resi-
dence can be achieved through flooring material selec-
tion. However, the optimized setting that minimizes
resuspension remains unclear due to the complex nat-
ure of walking-induced resuspension.
Several studies have compared particle resuspension
induced by human walking on different floorings. For
hard floorings, Goldasteh et al. (2013) investigated
house dust particle removal from common flooring
surfaces under turbulent flow in a wind tunnel and
observed higher resuspended fraction from hardwood
than linoleum. Their physicostochastic model, which
included effects of surface roughness, corroborated
these results. For carpets, Lewis and Breysse (2000)
reported that pile density (tightness of fiber packing)
has a significant effect on dust recovery from carpet.
Buttner et al. (2002) indicated that carpet construction
(cut pile or loop) may affect allergen resuspension by
studying the flooring effect on fungal spore resuspen-
sion caused by human walking with RH ranging from
22% to 33%. For comparison between hard floor and
carpet, Buttner et al. (2002) reported that cut pile car-
pet was associated with the highest airborne fungal
spore concentration compared with loop pile carpet
and vinyl tile. Likewise, Qian and Ferro (2008) noted
that significantly higher resuspension rates were
obtained from level-loop carpet than from vinyl tile,
under 30–50% relative humidity (RH) with 20 g/m
2
surface particle loading. Ultrafine Arizona test dust
was used as a surrogate for house dust in their work.
Conducting laboratory experiments with real house
dust, Manthena and Ferro (2009) found that level-loop
carpet was associated with higher resuspension rates
compared with vinyl tile under 30–40% and 60–70%
RH, while the opposite flooring effect was observed
under 50–55% RH. The opposite flooring effects
observed under different RH levels indicate that the
possibility of flooring type and RH interaction.
Shaughnessy and Vu (2012) showed that dust resuspen-
sion rates from vinyl tile were greater than loop carpet
under 50% RH, which agrees with Manthena and
Ferro (2009) results under the same RH level. In addi-
tion, Shaughnessy and Vu (2012) reported that resus-
pension rates associated with carpets were influenced
by surface dust loading (SDL), while Rosati et al.
(2008) demonstrated that SDL had no effect on resus-
pension emission factors. These findings indicate that
interaction effects of flooring type, surface dust load-
ing, and RH may play a considerable role in particle
resuspension.
The influences of surface dust loading and RH
should be taken into account when comparing the
effect of flooring type on particle resuspension. How-
ever, a systematic characterization of the influential
factors mentioned previously has not been established
yet. Qian and Ferro (2008) attempted to compare envi-
ronmental factors on resuspension with a large cham-
ber study using human participants. However, they
discovered that person-to-person variability attributed
to different walking styles is a large confounding fac-
tor, which indicates that a consistent test mechanism is
necessary for the determination of effects on resuspen-
sion due to different flooring type, surface dust loading
and RH. Distinct from the mechanical resuspension
mechanisms used in previous studies (Eisner et al.,
2010; Gomes et al., 2007; Kildeso et al., 1999; Kubota
and Higuchi, 2013), the one used in this work was able
to mimic the heel–toe rotation motion of the human
footstep and provide continuous footsteps with con-
trolled pressure loading and stepping rate.
In this study, the normalized dimensionless term
resuspension fraction, r
a
, which is defined as the frac-
tion of particle surface concentration resuspended per
shoe-flooring contact, was selected to quantify resus-
pension phenomena. The term resuspension fraction r
a
was developed by Karlsson et al. (1999), who verified
that r
a
is independent of stepping rate. This was deter-
mined when they obtained similar r
a
from one-person
walking (75 steps/min) and four-person walking
(300 steps/min). Thus, r
a
multiplied by the stepping
rate is the fraction of particles in the stepping area
emitted per time, and the resuspension fraction
obtained in a laboratory-scale chamber could be used
to estimate emission rate of walking-induced resuspen-
sion in large indoor environments with multiple people
and various stepping rates.
The primary goal of this study was to characterize
walking-induced dust resuspension as a function of
particle size, flooring type, surface dust loading and rel-
ative humidity. The consistent test mechanism was
used instead of human participants to eliminate the
impact of varied walking style, improve experimental
reproducibility, and facilitate a systematic comparison
of the influencing factors mentioned above. The sec-
ondary goal was to provide size-resolved resuspension
fractions for each flooring type, RH, and surface dust-
loading combination.
Materials and methods
Experimental design
Five types of flooring, including two types of hard
flooring and three types of carpet, were tested with two
levels of RH (30–40% and 70–80%) and surface dust
loading (2 and 8 g/m
2
), respectively. The choice of sur-
face dust-loading levels was justified in the Supporting
Information. As both categorical (flooring type) and
numerical (RH and SDL) variables were involved, a
general factorial design that examined all possible com-
binations was used to determine the main effects of each
variable as well as their interactions on resuspension.
593
Characterization of walking-induced dust resuspension
This design yields 60 experiments including three repli-
cates for each possible combination (5 9292=20;
20 93=60). The order of experimental runs was ran-
domized to reduce the impacts from extraneous fac-
tors.
Resuspension mechanism
In this study, experiments were conducted using a
mechanical resuspension device to improve experi-
mental reproducibility and eliminate the variability
caused by walking style on resuspension. Developed
at the Lawrence Berkeley National Laboratory, the
resuspension mechanism is comprised of heel and toe
plates controlled by electric actuators to simulate
human footstep (See Figure S1). Stepping rate and
pressure loading are controlled by the actuators and
the compressed air supplied to the pistons, respec-
tively. A U.S men’s size 10 tennis shoe was fitted to
the toe and heel plates, and the resuspension mecha-
nism was programmed to step down at a constant fre-
quency of 0.55 0.03 Hz. The time between the heel
peak pressure and the toe peak pressure was
0.47 0.05 s. The rotation speed was approximately
2 rad/s when toe area contacts the floor. Consistent
and comparable heel and toe pressure loadings were
obtained on various flooring materials. The resuspen-
sion mechanism was surrounded with a 61 9v
38 953 cm acrylic chamber (Volume: 0.12 m
3
), with
inner surfaces treated with antistatic spray to reduce
electrostatic charge. Two small fans were located in
the bottom corners of the chamber and directed
toward the center of the chamber to provide a well-
mixed condition inside the chamber and facilitate the
use of a mass balance model to determine r
a
. The
increased turbulence generated by the mixings fans
had little impact on the detachment of particles
because detachment is determined by the characteris-
tics of the jet flow formed between the shoe and the
floor during the gait cycle (Eisner et al., 2010; Gomes
et al., 2007; Kubota and Higuchi, 2013; Zhang et al.,
2008). However, the increased turbulence helped to
enhance the transport of detached particles and
reduce the spatial variation of airborne particle con-
centrations. We assumed that the increased turbulence
did not significantly contribute to the de-aggregation
of resuspended aggregates.
Air-exchange rates (ACH) at three different loca-
tions were measured by the decay of sulfur hexafluo-
ride (SF
6
) concentration after a pulse release. The
ACH inside the chamber was measured at 1.38 0.05/h
(SF
6
decay method) and kept consistent during the
experimental period. The <5% coefficient of variation
(C.V.) of multilocation measurements indicates a well-
mixed condition inside the chamber. The mixing condi-
tion of the particle phase was tested, and the result
showed that a reasonable well-mixed condition was
achieved. Two magnets were installed at each edge of
the front door, which facilitated sample exchange and
minimized the infiltration of outdoor air by securing
the seal. Passed through a set of humidifiers, filtered air
was supplied from the top of the chamber to compen-
sate for the negative pressure caused by the sampling
instruments, while maintaining the RH level inside the
chamber. The resuspension mechanism and acrylic
chamber were located in a temperature and RH-con-
trolled 25 m
3
indoor air chamber at Clarkson Univer-
sity in Potsdam, NY.
Flooring materials
Two types of hard flooring commonly used in resi-
dences, vinyl and hardwood, were selected to test the
difference between hard floorings. The vinyl sheet
flooring used in this study has a CleanSweep
â
(Lancas-
ter, PA, USA) no-wax wear surface. The selected hard-
wood flooring is made of oak boards with Ultimate
TM
Urethane finish. To obtain a better understanding of
the effect of carpet, the following three types of nylon
carpets that were commonly used in residences were
selected: low-density cut pile carpet (Carpet LD), high-
density cut pile carpet (Carpet HD), and high-density
loop carpet (Carpet Loop). The descriptions for the
selected carpets are listed in Table S3. Although carpet
wear affects particle resuspension (Rosati et al., 2008),
only new flooring materials were tested in this study.
The flooring materials were cut into 0.15 90.3 m size
pieces and then stored in a covered container before
use. Flooring samples were vacuumed with a 12 amp
canister vacuum before seeding with house dust.
Dust Preparation
Dust used in this study was collected from vacuum
bags obtained in 18 houses in St. Lawrence County,
NY. The house dust was sieved to 44 lm with a series
of U.S. standard sieves. Determined by a Sunset Lab
thermal-optical carbon analyzer (Tigard, OR), the
organic carbon/elemental carbon ratio of the bulk dust
is 36:1. To reduce agglomeration during the seeding
process, 1% (by weight) Cab-0-Sil fumed-silica was
mixed with the sieved dust. Particle size distributions
of the bulk dust (preseeding) were characterized by the
Malvern Mastersizer 2000 (Worcestershire, UK) using
laser diffraction measurement. The number size distri-
bution had a mode at 0.6 lm and more than 99% of
the dust particles were smaller than 10 lm (See Figure
S2). The volume size distribution had a mode at
25 lm, and approximately, 15% of the volume was
attributed to particles smaller than 10 lm. The number
and volume size distributions differ substantially
because the volume is proportional to the cubic of
diameter, which means larger particles contribute
much more to volume than smaller particles.
594
Tian et al.
Flooring dust seeding
Two surface dust-loading levels were tested. The target
values for the low and high levels were determined to
be 2 and 8 g/m
2
, respectively. To minimize the vari-
ance in particle loading, a reproducible seeding system
was developed and validated. The seeding system com-
prised of a dust-feeding system (a stainless steel funnel
and a venturi mini-vacuum) and an acrylic deposition
chamber with dimensions 53 948 961 cm and elec-
trically grounded inner surfaces (See Figure S3).
House dust was fed into the mini-vacuum through the
funnel and then aerosolized by compressed air sup-
plied at 40 psi. Four computer fans were mounted on
the ceiling of the deposition chamber, blowing to its
center point (the location of the dust inlet) to generate
adequate turbulence to create a well-mixed condition.
The flooring samples were placed on the bottom of the
deposition chamber, and up to three flooring samples
were seeded at once. To evaluate the dust loading and
spatial distribution of the dust, nine 37-mm filters were
evenly placed on the chamber bottom as weighing cou-
pons. The dust loading was estimated using gravimet-
ric analysis. Results from four test runs showed that
the intra-run spatial variability and the difference in
mass loading inter-run were both approximately
10%. Actual dust loadings were 1.9 0.2 and
7.8 0.4 g/m
2
for the low and high levels, respec-
tively. To evaluate the change in particle size distribu-
tion during the seeding process, dust was collected
from the bottom surface of the deposition chamber
and then characterized by Malvern Mastersizer 2000.
According to Figure S2, particle size distribution
changed marginally after seeding as the postseeding
modes shifted slightly to the right. The difference
between preseeding and postseeding results may be
attributed to particle agglomeration as well as differen-
tial settling due to particle size on surfaces within the
chamber.
Adopted from Lewis et al. (1999), a 6.3-kg steel
roller was dragged back and forth for 30 strokes on
carpet samples to embed particles along fibers after
seeding. Particle loss during this embedding process
was estimated using the method described in ASTM
D7144-05a (2005). For cut pile carpets, the losses on
the roller were <1% of the total seeded amount, while
for loop carpet, the losses were 14%. According to
Qian (2007) and Causer et al. (2010), an even distribu-
tion of particles along the carpet fiber can be achieved
with the embedding process. The embedding procedure
was not applied to hard flooring samples. After seed-
ing, flooring samples were transferred immediately to
one of two temperature and RH-controlled condition-
ing chambers for 24 h. The relative humidity levels in
the conditioning chambers were maintained at 40%
and 70%, respectively. The conditioning chambers
were slightly pressurized and air supplied to the
chambers was filtered to avoid contamination during
conditioning.
Experimental procedure
The real-time, size-differentiated PM concentration
inside the resuspension chamber was monitored by a
Grimm Technologies (Douglasville, GA, USA)
model 1.108 portable laser aerosol spectrometer
(Grimm 1.108) with sampling inlet 5 cm away from
the center of the chamber wall. The Grimm 1.108
has 15 size channels providing real-time size distribu-
tions of particles ranging from 0.3 to 20 lm and lar-
ger. HOBO H08 data loggers (Onset Computer
Corporation, Waltham, MA, USA) were used to
record the temperature and relative humidity in the
resuspension chamber and the two conditioning
chambers.
Following the 24-h conditioning process, resuspen-
sion experiments were conducted following the proce-
dure described below:
•The front door of the resuspension chamber was
opened. The shoe bottom and the resuspension
mechanism were cleaned with wet Kimwipe to avoid
cross contamination between runs.
•The Grimm 1.108 and the two mixing fans were
turned on. The humidity in the resuspension cham-
ber was adjusted to the designed value.
•The flooring sample was placed under the shoe as
disturbance as possible. Two-sided carpet tape was
attached to the back of the flooring to eliminate slid-
ing.
•With the front door of the resuspension chamber
closed, the chamber was left still for 90 min for the
particles to settle.
•The resuspension mechanism was operated for 30
continuous steps. This resuspension event lasted for
approximately 55 s. During the method develop-
ment, a 100-step resuspension event (~3 min) was
conducted. The airborne particle concentrations
increased within the first ~55 s then remained rela-
tively steady in the final 2 min. Therefore, the 55-s
30-step resuspension event was selected.
•The chamber was left still for 45 min for particles to
decay.
Data analysis
A two-compartment material balance model was used
to quantify resuspension fraction r
a
. The resuspension
chamber was well sealed, and the supplied air was fil-
tered by a HEPA filter; hence, the influences of the air-
borne concentration outside the resuspension chamber,
indoor-outdoor penetration, other emission sources,
and particle track-in attributed to the particles
attached to the shoe sole during previous steps were
595
Characterization of walking-induced dust resuspension
neglected. Particle coagulation was considered negligi-
ble in this study because the particle concentrations
measured were relatively low (<10
7
# particles/m
3
dur-
ing the resuspension period). Airborne particle number
concentrations inside the resuspension chamber
obtained by the Grimm 1.108 were combined into five
particle size ranges, including 0.4–0.5, 0.5–1.0, 1.0–3.0,
3.0–5.0 and 5.0–10.0 lm. For particle size range j,r
a
was estimated by the change in airborne number con-
centration between each time step Dtand the current
surface dust loading L
j
(t), using the following equa-
tions:
rajðtþDtÞ
¼V
AsfsLjðtÞ
CjðtþDtÞCjðtÞ
DtþðaþkjÞCjðtÞ
ð1Þ
LjðtþDtÞ
¼LjðtÞ V
Asfs
CjðtþDtÞCjðtÞ
DtþaCjðtÞ
ð2Þ
where a=Air-exchange rate (h
1
). Here,
a=1.38 0.05/h. It is notable that the resuspension
fraction estimated using the two-compartment material
balance model is independent of the air-exchange rate
used in the experiment, as the air-exchange rate is
taken into account as a loss term.
A
s
=Contact area per step (m
2
). Here,
A
s
=0.021 m
2
, which is the measured area of the
shoe bottom mounted on the resuspension mecha-
nism.
C
j
=Concentration inside the resuspension chamber
(# particles/m
3
).
f
s
=Stepping rate (# steps/h). Here, f
s
=1980 steps/
h (0.55 Hz).
k
j
=Deposition rate (h
1
), which represents particle
loss rate due to all mechanisms besides air exchange.
Here, k ranged from 10
0
to 10
2
/h based on particle size.
We expect that the large surface to volume ratio and
the turbulence created by the mixing fans, which were
used to create a well-mixed condition, enhanced the
deposition to surfaces.
L
j
=Surface dust concentration (# particles/m
2
).
raj=Resuspension fraction ().
Dt=Time step. Here, Dt=6 s, the sampling inter-
val of the particle concentration measurements.
V=Mixing volume (m
3
). Here, V=0.12 m
3
, the
geometric volume inside the resuspension chamber.
The value (a+k
j
) was determined by the slope of
ln C
j
of the exponential decay period after the 30-step
resuspension event.
According to the SEM images reported by Goldasteh
et al. (2012), most house dust particles are irregular in
shape and include fibers and mineral crystal forma-
tions. However, we are unaware of any direct studies
that provide a suitable shape factor to use for house
dust. Therefore, we assumed that all dust particles were
spherical. L
j
(0) was estimated from the initial mass con-
centration Lmj(g/m
2
) using the equation given below:
Ljð0Þ¼ Lmj
qð
p
6D3
pjÞð3Þ
Lmj¼Ltot VFjð4Þ
where qis the density of dust particle which is
assumed to be 2.5 g/cm
3
,Dpjis the geometric mean
diameter, L
tot
is the total dust loading (g/m
2
) and VF
j
is the postseeding volume fraction (%). Postseeding
volume fractions for particles at 0.4–0.5, 0.5–1.0, 1.0–
3.0, 3.0–5.0 and 5.0–10.0 lm are 0.1%, 1.2%, 3.1%,
3.0% and 7.4%, respectively. House dust is a hetero-
geneous mixture of particles with both inorganic and
organic components. Rather than the density of 1.6–
1.7 g/cm
3
used for ambient particulate matter, the
density of soil dust (2.5 g/cm
3
) is commonly used for
dust density in indoor resuspension studies because
soil dust has been found to be the largest component
of house dust. Accordingly, we chose to use a density
of 2.5 g/cm
3
. According to Equations 1 and 3, an
increase in density would result in an increase in the
estimated resuspension fraction. When conducting an
inter-study comparison, one should pay attention to
the density used.
The other commonly used term is the resuspension
rate coefficient r(h
1
), which is defined as the fraction
of particle surface concentration resuspended per unit
time. This can be easily derived from r
a
using Equa-
tion 5 listed below, where A
r
is the resuspension area.
In this study, the resuspension fraction r
a
can be con-
verted to resuspension rate coefficient rby multiplying
a factor of A
s
9f
s
/A
r
=924/h, assuming A
r
is equal to
the total flooring area, A
tot
, of 0.045 m
2
. The emission
rate S(mg/h), which refers to the resuspended mass per
unit time, can be calculated with either ror r
a
as shown
in Equation 6.
rj¼Asfsraj
Ar
ð5Þ
Sj¼fsAsLjraj¼rjArLjð6Þ
Particle deposit structures of the two levels of sur-
face dust loading (2 and 8 g/m
2
) were characterized by
the method reported in the studies of Friess and Yadig-
aroglu (2002). They evaluated particle deposit structure
based on the mass median diameter of deposited parti-
cles, D(lm) and the approximate height of the deposit,
d(lm):
596
Tian et al.
d6m0
pqð1eÞð7Þ
where eis the porosity of the particle deposit. Accord-
ing to Friess and Yadigaroglu (2002), the porosity of
deposits on floorings is assumed to be 0.75. For the
dust used in this study, Dis estimated to be 21 lm.
The classification criteria presented by Boor et al.
(2013a) was used to determine the type of particle
deposit: the particle deposit is considered as monolayer
if d≤D, as intermediate between a monolayer and
multilayer if D≤d≤2D, or as multilayer if d≥2D.
The deposit heights dof low-loading condition (2
g/m
2
) and high-loading condition (8 g/m
2
) were esti-
mated to be 6 lm and 24 lm, respectively.
Results and discussion
Following the experimental design, 60 experiments
were conducted using the resuspension mechanism.
Figure 1 illustrates the airborne particle concentration
time series of one representative experiment. During
the 1.5-h background period, the particle concentra-
tions inside the resuspension chamber dropped to
background level. Hence, we concluded that the turbu-
lence generated by the mixing fans was not strong
enough to overcome the adhesive forces and resuspend
particles from the surface. The last 10 min of the back-
ground period showed in the Figure 1 indicated that
90 min was sufficient for the particles reaerosolized
during cleaning and sample placement to settle. Coin-
ciding with the 30-step resuspension event, concentra-
tions for particles ranging from 0.4 to 10 lm rose
sharply to orders of magnitude higher than the back-
ground value. This observation agrees with Thatcher
and Layton (1995) and Qian and Ferro (2008), who
reported walking-induced resuspension of 0.3–25 lm
particles and 0.4–10 lm particles, respectively. After
the resuspension event, airborne concentrations
decayed to the background levels within 5–30 min for
different particle size ranges. The rapid decay was
caused by a high surface to volume ratio as well as the
increased turbulence generated by the two mixing fans
inside the 0.12 m
3
resuspension chamber.
Multiple estimations of the resuspension fraction r
a
,
obtained from successive time steps during the resus-
pension event, were computed for each of the 60 runs
and then averaged, generating 300 data points in total
with five particle size bins involved per run. During
each run, the estimated resuspension fractions
remained relatively steady with small fluctuations, sup-
porting the selection of the 30-step resuspension per-
iod. Average size-resolved resuspension fractions for
each combination of factors range from 10
7
to 10
4
for 0.4–10 lm particles with an intra-run C.V. of
18 12% (See Table S4).
The present results are within, but on the low end of,
the range of previous studies (10
7
to 10
2
) based on
the study described by Qian et al. (2014) review. This is
likely due to the interstudy differences of flooring type,
flooring condition, surface dust loading and algorithm
used to estimate the resuspension term. In addition,
one of the assumptions made in this study was that the
amount of dust lost due to adhesion to the shoe outsole
during the resuspension event was negligible, which
may result in overestimation of surface dust loading
and underestimation of resuspension fraction. To date,
there is little information available regarding the floor-
shoe outsole particle transfer rate as a function of
flooring type, surface dust loading, pressure loading,
and contact frequency. The confined stepping motion
of the resuspension mechanism may be another reason
for the difference, as it cannot mimic the wake and the
buoyant plume generated by a walking person which
enhances the transport of the detached particles (Edge
et al., 2005). Despite these limitations, the consistency
of the results provided by the resuspension mechanism
allow for comparison of effects from environmental
factors that have not previously been determined.
Statistical analysis
Statistical analysis in this study was accomplished by
Minitab
â
16 Statistical Software (Minitab Inc., State
College, PA, USA). The original data failed the Ander-
son-Darling normality test, indicating that the data did
not fit a normal distribution. Determined by Box–Cox
method, a natural log transformation was used to nor-
malize the data, allowing the use of parametric statis-
tics. A three-way analysis of variance (ANOVA) was
conducted with flooring type, surface dust loading, and
relative humidity as the three variables and the resus-
pension fractions for five particle size bins as five
response variables. The results of the main effects and
Fig. 1 Particle concentration versus time profile of one represen-
tative experiment (Flooring type: Vinyl; RH: 40%; SDL: 8 g/m
2
)
597
Characterization of walking-induced dust resuspension
two-factor interactions for particle size bin with confi-
dence level of 90% (a=0.1) are reported in Table S5.
For the main effects, the smaller than 0.1 P-values
indicate that flooring type had significant effects on
resuspension fraction for all particle size ranges
concerned, which means that at least one of the floor-
ing types was significantly different than the others for
each size range. RH and SDL were influential only
for certain size particles. For interactions, the p-values
for flooring type and RH interaction are smaller than
0.1 among most particle size ranges concerned, reveal-
ing that RH may have different effects on different
flooring types. Similarly, the flooring type and SDL
interaction had significant effects on 1.0–10.0 lm parti-
cles, suggesting that the effect of SDL varies with floor-
ing type for supermicron size particles. The p-values
show that the RH and SDL interaction had a signifi-
cant effect for all five ranges of particles.
Because not all data passed the normality test, a
nonparametric Wilcoxon rank sum test was used to
investigate which of the levels of each factor differed
significantly from others. Two-tailed Wilcoxon rank
sum test was conducted first to evaluate the equality of
two factor levels’ population medians (a=0.1), fol-
lowed with a one-tailed Wilcoxon rank sum test if sig-
nificant difference was observed. No transformation of
data was applied in this step. The results are provided
in Table S6–S18.
Flooring type
As demonstrated by the ANOVA results, flooring type
was the most influential factor on particle resuspension
in comparison with RH and surface dust loading. To
interpret the main effect of flooring type, Figure 2
shows the size-resolved resuspension fractions for the
five flooring types among all tested conditions. For all
five types of flooring tested, resuspension fractions
increased with the increase in particle size for particles
at 1–10 lm, while resuspension fractions of particles at
0.4–1.0 lm did not change significantly. Overall, sub-
micron particles are associated with substantially lower
resuspension fractions than supermicron particles. This
observation is consistent with previous studies
(Thatcher and Layton, 1995; Manthena and Ferro,
2009; Qian and Ferro, 2008; Rosati et al., 2008; Mukai
et al., 2009).
Comparing carpet and hard flooring, median resus-
pension fractions of the two categories of floorings
showed no significant difference for submicron parti-
cles (0.4–1.0 lm). For particles 3.0–10 lm, cut pile car-
pets (Carpet LD and Carpet HD) exhibited
significantly higher resuspension fractions than hard-
wood and vinyl. For particles ranging from 5 to
10 lm, dust particles were more readily resuspended
from all carpet types than hard floorings. This finding
is consistent with Qian and Ferro (2008), who found
out that level-loop carpets were associated with higher
resuspension rate than vinyl tile. Mukai et al. (2009)
investigated the effect of airflow characteristics on
resuspension and found that carpet was associated
with the higher relative resuspension fractions than
linoleum under the same condition. For particles 1.0–
10 lm, Carpet LD was associated with the highest
median resuspension fractions among the five flooring
tested.
The difference between carpet and hard flooring may
be attributed to differences in surface roughness and
composition. Carpets might have higher microscale
surface roughness than that of hard flooring which
affects resuspension. Unlike hard flooring that can be
considered flat surfaces, the carpet tuft also has larger
macroscale surface roughness that may affect the air
turbulence around the surface (Mukai et al., 2009).
The bounce of carpet fibers during the footfall may
also enhance particle resuspension on carpets. Also,
the surface composition affects the magnitude of the
electrostatic force, which impacts particles adhesion.
For the two types of hard flooring, hardwood and
vinyl, there was no statistical difference observed
between the median resuspension fractions for the size
range tested. Among the three types of carpets, Carpet
LD was always associated with the highest resuspen-
sion fractions for the size range tested. The average
resuspension fractions for Carpet LD were 1.5–2.5 and
1.5–4.5 times higher than Carpet HD and Carpet
Loop, respectively. This finding indicates that carpet
density had a significant effect on dust resuspension,
and lower pile density was associated with higher resus-
pension fractions. This finding is likely due to the fiber
orientation during the stepping activity. When stepped
Fig. 2 Size-resolved resuspension fractions for different types of
floorings. The symbols represent the median values for all tested
conditions (n=60, replicates =12). The upper and lower ends
of the error bars represent the 75th and 25th percentiles, respec-
tively
598
Tian et al.
on, carpet tuft of high-density carpets was observed to
hold the vertical orientation better, while the tuft of
low-density carpet tended to bend more readily. Along
the bent fibers of low-density carpet, more dust
particles may have been exposed to the shoe, hence
increasing resuspension. Carpet construction also
affected resuspension. Having the same density,
Carpet HD (cut pile) was associated with significantly
higher resuspension fractions than Carpet Loop for
3.0–10 lm particles, but the difference is not signifi-
cant for particles at 0.4–3.0 lm. This observation is in
agreement with Buttner et al. (2002) who suggested that
greater vertical compression was exhibited on cut pile
carpet in comparison with loop carpet, which enhanced
resuspension by increasing physical disturbance.
Relative humidity
Figure 3 illustrates the resuspension fractions of each
flooring type at 40% and 70% RH. Hardwood exhib-
ited significantly lower resuspension fractions under
the 70% RH level than under the 40% RH level among
all particle ranges tested. For vinyl, higher resuspen-
sion fractions were associated with a lower RH (40%)
for particles at 1–5lm, while no significant difference
was found between the two RH levels for other particle
size ranges. RH exhibited no significant effect on Car-
pet LD and Carpet Loop for all particle size ranges
considered. For Carpet HD, relative humidity showed
greater impact on larger particles. Resuspension frac-
tions obtained from Carpet HD under 70% RH were
significantly higher than the fractions obtained under
40% RH for particles at 3–10 lm, while for particles at
0.4–3lm, no statistical difference in median resuspen-
sion fractions at 40% and 70% RH levels was detected.
These findings are in agreement with the study of
human-induced resuspension by Rosati et al. (2008),
which found a higher RH was associated with higher
resuspension for new medium-pile carpets. The carpet
construction and pile height (0.01 m) were the same
between this study and Rosati et al. (2008).
Because RH showed opposite effects on Carpet HD
and hardwood, the effect of flooring type was re-evalu-
ated for these two floorings at each RH level. For par-
ticles at 1–3lm, hardwood was associated with higher
resuspension fractions as compared to Carpet HD at
40% RH, while the opposite conclusion was drawn at
70% RH. The effects of RH did not change with differ-
ent surface dust-loading levels.
The different RH effects observed on different floor-
ings may be explained by the adhesive forces on the
particles. Several forces are fundamental for micron-
scale particle adhesion, including the van der Waals,
electrostatic and capillary forces (Hinds, 1999; Walton,
2008; Gradon, 2009). At high relative humidity, extra
water adsorbed by the surface increases the capillary
force by forming meniscuses between particles and sur-
face asperities (Ibrahim et al., 2004; Rabinovich et al.,
2002). The increased adhesion makes particles more
difficult to detach and therefore suppresses resuspen-
sion. However, the opposite effect is expected when the
initial adhesion is dominated by the electrostatic force,
as the extra water increases the leak-off rates of charges
built on particles, resulting in reduction of adhesion
forces and prevention of charge accumulation (Rosati
et al., 2008; Walton, 2008). The results of this study
suggest that electrostatic forces play an important role
on the adhesion of particles on Carpet HD, increasing
adhesive force under the lower RH conditions.
Hygroscopic growth due to the water absorbed by
the particles may also contribute to the RH effect.
Although a semi-empirical model is available to quan-
tify hygroscopic growth (Dua and Hopke, 1996), it is
difficult to apply the model to the current study with-
out compositional analysis of the house dust used.
House dust is a complex heterogeneous mixture of par-
ticles and the composition may vary from study to
study. In addition, Rosati et al. (2008) studied the
effect of RH (40% and 80%) on resuspension using
Arizona Test Dust as a surrogate of house dust.
Although enhanced resuspension was observed with
the increased RH level, they found no significant
change in resuspended particle size distribution with
different RH levels associated with new medium-pile
carpet.
Surface dust loading
The statistical analysis results reveal that the interac-
tion between surface dust loading and relative humid-
ity had a significant effect on resuspension. Figure 4
Fig. 3 Size-resolved resuspension fractions associated with dif-
ferent levels of relative humidity. The symbols represent the
median values for all tested conditions (n=60, replicates =6).
The upper and lower ends of the error bars represent the 75th
and 25th percentiles, respectively
599
Characterization of walking-induced dust resuspension
(a) and (b) illustrate the resuspension fractions of each
flooring type with different SDL at 40% RH and 70%
RH, respectively. The three types of carpets generally
exhibited lower resuspension fractions with higher sur-
face dust loading regardless of the relative humidity
level, while for hard floorings the effect of surface dust
loading varied with different RH values. At 40% RH,
resuspension fractions of hardwood and vinyl both
increased with the increase in surface dust loading for
particles at 1.0–10 lm, while the opposite trend was
observed for particles at 0.4–0.5 lm and no significant
difference was detected for particles at 0.5–1.0 lm. It is
notable that under 40% RH and a 8 g/m
2
surface dust-
loading condition, vinyl exhibited significantly higher
resuspension fractions as compared with Carpet Loop
for particles at 0.5–5lm. This observation is consistent
with the studies of dust resuspension conducted by
Manthena and Ferro (2009) and Shaughnessy and Vu
(2012), whose experiments were conducted under com-
parable RH level and surface dust loading. This finding
indicates that when comparing loop carpet and vinyl,
environmental conditions should be considered. At
70% RH, resuspension from vinyl flooring decreased
with the increase in surface dust loading for particles at
0.4–5lm, and the same trend was observed on hard-
wood for particle at 0.4–1.0 lm. No significant differ-
ence in resuspension fractions was detected on vinyl
and hardwood with 2 and 8 g/m
2
for particles at 5–10
and 1–10 lm, respectively.
On flat surfaces, the structure of particle deposits for
low-loading conditions (2 g/m
2
) was estimated to be
monolayer, while for high-loading conditions was (8
g/m
2
), the deposit structure was estimated to be inter-
mediate between a monolayer and multilayer, where
some formation of particle clusters and a partial multi-
layer structure were expected. Lazaridis and Drossinos
(1998) modeled resuspension of multilayer particle
deposits induced by turbulent flow. They reported that
the particles from the upper layer resuspended at lower
friction velocity as compared with particles from the
bottom layer, indicating that higher surface dust load-
ing would be linked to higher resuspension. Boor et al.
(2013b) conducted a wind tunnel study to investigate
the effect of particle deposit structure on aerodynamic
particle resuspension on flat surfaces. They found that
particles were significantly easier (lower air velocity
required) to resuspend from multilayer deposits than
monolayer deposits. These findings are consistent with
the observation of higher resuspension fractions with
higher surface loading under 40% RH on hard flooring
in this study.
The surface dust loading and relative humidity inter-
action observed on hard floorings could be explained
by the theories discussed below. As reviewed by Gra-
don (2009), relative humidity (damping coefficient) can
influence the cohesive forces between the interacting
particles forming an agglomerate. Using an eddy fluid
particle model, Gac et al. (2008) noted that when parti-
cle-to-particle cohesive forces dominate, the re-entrain-
ment efficiency is reduced and single particles, versus
particle clusters, are detached from the particle struc-
ture. In this study, we surmise that the increased capil-
lary force at 70% relative humidity enhanced the
particle-to-particle cohesive force, thus reducing resus-
pension due to the presence and strength of the particle
clusters.
For carpet, we hypothesize that the porous nature of
the surface and the embedding process prevented sub-
stantial formation of particle clusters on the fibers of
the carpet. In addition, the stepping down motion of
(a)
(b)
Fig. 4 Size-resolved resuspension fractions for different types of
floorings under different surface dust loading at (a) 40% RH.
(b) 70% RH. The symbols represent the median values for all
tested conditions (n=30, replicates =3). The upper and lower
ends of error bars represent 75th and 25th percentiles, respec-
tively
600
Tian et al.
the resuspension mechanism may push the particles
deeper toward the backing, which made them less
available to be resuspended. Causer et al. (2010) mea-
sured the effect of the dust-embedding process on dust-
mite allergen distribution along cut pile carpet fibers.
They soiled new carpets with house dust using the soil-
ing system developed by Lewis et al. (1999). They
found that the ASTM fixed roller method resulted in
an relatively even distribution of dust-mite allergen,
while running a plate compactor for 2 min on carpet
samples led to a distribution biased toward the carpet
backing. We surmised that the stepping motion of the
resuspension mechanism provided similar effect on
particle distribution along carpet fibers as the plate
compaction method. When the seeded dust loading
increased from 2 to 8 g/m
2
, the amount of readily
resuspend particles did not increase four times as some
were pushed deeper toward the backing. Hence, car-
pets showed a decrease trend of dust resuspension frac-
tions with increased surface dust loading.
Although resuspension fractions decrease with
increase in surface dust loading in general, higher sur-
face dust loading resulted in higher dust emission rates
for all conditions as shown in Figure S4. The emission
rates presented in Figure S4 were derived from resus-
pension fractions using Equation 6. The assumptions
made based on the following: (i) resuspension was the
only loss term for the flooring compartment; and (ii)
the change of surface dust loadings was negligible
because only a small fraction of particles got resus-
pended. The time dependence of the emission rates
derived from the resuspension fractions was found to
be negligible for the 30-step resuspension event.
The estimated emission rates for the present study
(Particles range from 0.4–10 lm: 10
4
–10
1
mg/h) are
lower than those reported by Ferro et al. (2004) (PM
5
:
6–83 mg/h) and Rosati et al. (2008) (Sum of 0.5–
20 lm particles: 10–378 mg/h). The lower emission
rates reported here may be because of the difference in
surface dust loading, flooring types, and environmental
factors among the different studies, differences in the
resuspension mechanisms (mechanical foot versus
human walking) discussed above, and the addition of
particle shedding from clothing during activity for the
field studies.
Conclusions and implications
This study investigated the effects of particle size, floor-
ing type, relative humidity, surface dust loading and
their interactions on dust resuspension by conducting a
systematic experimental comparison. The resuspension
mechanism used in this study provided consistent step-
ping rate and pressure loading which facilitated the
experimental comparison of factors.
For fine particles (0.4–3.0 lm), the difference in
resuspension caused by flooring type is negligible,
while for coarse particles (3.0–10 lm) carpets are asso-
ciated with 2–4 times higher resuspended concentration
in comparison with hard floorings. The coarse particle
fraction of house dust contains many allergens and
asthmagens in indoor environments, including cat,
dog, dust-mite, and cockroach allergens, mold spores
and pollen. In addition, carpeted floors may be associ-
ated with significantly higher surface dust loadings and
allergen concentrations than hard floors (Adgate et al.,
1995; Franke et al., 1997; Tranter, 2005; Causer et al.,
2006). The results from this study validate the recom-
mendation that people sensitive to allergens to select
hard floorings to reduce exposure and related adverse
health outcomes. The results can also be applied to
exposure models, with the limitations and assumptions
assessed carefully, to determine the impact of human
exposure to dust resuspension as a function of flooring
type, particle size, surface dust loading and relative
humidity. The use of condition-specific resuspension
fractions could reduce the model uncertainty. In addi-
tion to the environmental conditions and flooring types
tested in this study, other factors, such as in home shoe
preference (shod or barefoot), shoe bottom material
(e.g., leather, plastic), walking style (child or adult;
slow or fast; heel–toe or scuff) and plantar pressure
loading, may impact the results and are worth testing
to improve the model accuracy.
Acknowledgements
This research material is based upon work supported
by the U.S. Department of Housing and Urban Devel-
opment (HUD) under Grant Number NYLHH0168-
08 and the U.S. National Science Foundation (NSF)
under Grant Number CBET 0846704. The contents of
this article are the views of the authors and do not nec-
essarily reflect the views or policies of HUD, NSF or
the U.S. Government.
The resuspension device was provided to us by the
Lawrence Berkeley National Laboratory and we
acknowledge the original design and development
work done by Dr. Mark Sippola.
Supporting Information
Additional Supporting Information may be found in
the online version of this article:
Figure S1. Resuspension mechanism inside plaxiglass
chamber.
Figure S2. Volume and number size distributions of
house dust measured by Malven Mastersizer (2000).
Figure S3. Photograph of the seeding system.
Figure S4. Size-resolved emission for different types of
floorings with two levels of initial surface dust loading.
Table S1. Surface dust loadings in U.S. homes.
Table S2. Particle size fractions of house dust.
Table S3. Carpet type descriptions.
601
Characterization of walking-induced dust resuspension
Table S4. Average resuspension fraction (n=3 for
each treatment).
Table S5. Summary of ANOVA results.
Table S6. Two-sided Mann–Whitney rank sum test
results for the comparison of resuspension fractions
among different flooring types (n=6, a=0.1).
Table S7. One-sided Mann–Whitney rank sum test
results for the comparison of resuspension fractions
among different flooring types (n=6, a=0.1).
Table S8. Two-sided Mann–Whitney rank sum test
results for the comparison of resuspension fraction
between 40% and 70% relative humidity levels (n=6,
a=0.1).
Table S9. One-sided Mann–Whitney rank sum test
results for the comparison of resuspension fractions
between 40% and 70% relative humidity levels (n=6,
a=0.1).
Table S10. One-sided Mann–Whitney rank sum test
results for the comparison of resuspension fractions
between 40% and 70% relative humidity levels (n=6,
a=0.1).
Table S11. Two-sided Student t-test results for the com-
parison of resuspension fraction between 2 and 8 g/m
2
surface dust loadings on carpets(n=6, a=0.1).
Table S12. One-sided Mann–Whitney rank sum test
results for the comparison of resuspension fraction
between 2 and 8 g/m
2
surface dust loadings on carpets
(n=6, a=0.1).
Table S13. Two-sided Mann–Whitney rank sum test
results for the comparison of resuspension fraction
between 2 and 8 g/m
2
surface dust loadings for hard
floorings under 40% RH (n=3, a=0.1).
Table S14. One-sided Mann–Whitney rank sum test
results for the comparison of resuspension fraction
between 2 and 8 g/m
2
surface dust loadings for hard
floorings under 40% RH (n=3, a=0.1).
Table S15. One-sided Mann–Whitney rank sum test
results for the comparison of resuspension fraction
between 2 and 8 g/m
2
surface dust loadings for hard
floorings under 70% RH (n=3, a=0.1).
Table S16. Two-sided Mann–Whitney rank sum test
results for the comparison of resuspension fraction
between 2 and 8 g/m
2
surface dust loadings for hard
floorings under 70% RH (n=3, a=0.1).
Table S17. One-sided Mann–Whitney rank sum test
results for the comparison of resuspension fraction
between 2 and 8 g/m
2
surface dust loadings for hard
floorings under 70% RH (n=3, a=0.1).
Table S18. One-sided Mann–Whitney rank sum test
results for the comparison of resuspension fraction
between 2 and 8 g/m
2
surface dust loadings for hard
floorings under 70% RH (n=3, a=0.1).
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Characterization of walking-induced dust resuspension
