Ann. Occup. Hyg., Vol. 56, No. 2, pp. 207–220, 2012
Ó The Author 2011. Published by Oxford University Press
on behalf of the British Occupational Hygiene Society
doi:10.1093/annhyg/mer089
Performance Study of Personal Inhalable Aerosol
Samplers at Ultra-Low Wind Speeds
DARRAH K. SLEETH1* and JAMES H. VINCENT2
1
Rocky Mountain Center for Occupational & Environmental Health, Department of Family &
Preventative Medicine, University of Utah, Salt Lake City, UT 84108, USA; 2Department of
Environmental Health Sciences, University of Michigan, Ann Arbor, MI 48108, USA
Received 31 March 2011; in final form 15 August 2011; published online 10 October 2011
The assessment of personal inhalable aerosol samplers in a controlled laboratory setting has
not previously been carried out at the ultra-low wind speed conditions that represent most
modern workplaces. There is currently some concern about whether the existing inhalable
aerosol convention is appropriate at these low wind speeds and an alternative has been suggested. It was therefore important to assess the performance of the most common personal
samplers used to collect the inhalable aerosol fraction, especially those that were designed
to match the original curve. The experimental set-up involved use of a hybrid ultra-low speed
wind tunnel/calm air chamber and a rotating, heating, breathing mannequin to measure the
inhalable fraction of aerosol exposure. The samplers that were tested included the Institute
of Occupational Medicine (IOM), Button, and GSP inhalable samplers as well as the closedface cassette sampler that has been (and still is) widely used by occupational hygienists in many
countries. The results showed that, down to 0.2 m s21, the samplers matched the current inhalability criterion relatively well but were significantly greater than this at the lowest wind
speed tested. Overall, there was a significant effect of wind speed on sampling efficiency, with
lower wind speeds clearly associated with an increase in sampling efficiency.
Keywords: aerosols; dust sampling conventions; inhalable dust; low wind speed; personal samplers; wind tunnel
ticles that will enter into the nose and/or mouth during breathing—is increasingly replacing what used
to be referred to as ‘total aerosol’ and therefore continues to be of considerable interest in relation to
aerosols found in many workplace environments.
Several sampling instruments have been proposed
as being applicable to the collection of inhalable
aerosols. The first such instrument was the IOM
sampler, named after the Institute of Occupational
Medicine in Edinburgh, Scotland, where it was developed (Mark and Vincent, 1986). This sampler
was designed in the 1980s specifically to collect particles according to the criterion that had been proposed at the time, which was based on experiments
with breathing mannequins in wind tunnels for wind
speeds upwards of 0.5 m s–1. In the years that have
followed, this sampler has become widely regarded
as providing a good measure of inhalable aerosol exposure and has been commercially available for
INTRODUCTION
Personal sampling is widely accepted as providing
the most representative measure of human exposure
to airborne contaminants. For effective assessment
of exposure to aerosols, it is essential to properly understand the performance characteristics of the various sampling instrument options so that the choice
may be made to provide the best match with the
aerosol fraction of interest. For industrial hygienists,
these fractions of interest include the health-related
inhalable, thoracic, and respirable fractions, for
which widely agreed particle size-selective criteria
now exist (ACGIH, 1985; CEN, 1992; ISO, 1992).
The coarser inhalable fraction—consisting of all par*Author to whom correspondence should be addressed.
Tel: þ1-801-585-3587; fax: þ1-801-581-7224;
e-mail: [email protected]
207
208
D. K. Sleeth and J. H. Vincent
many years. In the meantime, several other samplers
have been proposed to fill this need and they too are
commercially available. These include the Button
sampler and the GSP sampler (which is equivalent
to the CIS or conical inlet sampler). Meanwhile,
the plastic closed-face cassette (CFC), nominally intended to collect ‘total aerosol’, is still widely used
by occupational hygienists, especially in the USA.
These are the samplers that will be the subject of
the work described below.
Comprehensive laboratory studies of the personal
aerosol samplers listed above have been performed
in both relatively fast-moving air (wind speeds
.0.5 m s–1) (Kenny et al., 1997; Görner et al,
2010) and under calm air conditions (essentially zero
wind speed) (Kenny et al., 1999; Görner et al., 2010).
However, it is now known that most indoor working
environments are characterized by ambient wind
speeds in the range generally ,0.2 m s–1, intermediate between the conditions pertaining to the work described above (Berry and Froude, 1989; Baldwin and
Maynard, 1998). Previous work from our laboratory
to investigate human inhalability at such low wind
speeds showed significant differences from what
had been found earlier at the higher wind speeds
(Sleeth and Vincent, 2009; Sleeth and Vincent,
2011). With that in mind, it was important to test
the samplers in question under these more realistic
conditions as well. Only limited such research has
been previously carried out, inhibited by the inherent
difficulty in generating a uniform test aerosol under
such conditions, especially for particles toward the
upper end of the size range of interest. For those sizes,
the effect of gravitational settling on aerosol behavior
becomes increasingly influential. However, with respect to the effects of wind speed, it is noted that
one recent experimental effort suggested that the
IOM, GSP, and Button samplers generally performed
similarly at 0.5 and 1 m s–1, with minimal dependence
on the external air velocity (Aizenberg et al., 2001).
As already indicated in reference to previous work,
human aspiration efficiency within the ultra-low
range of wind speeds of interest was significantly different from what had been found at the higher wind
speeds (Sleeth and Vincent, 2011). Specifically, human aspiration efficiency for aerosols in the range
of particle aerodynamic diameter from 7 lm up
to 90 lm, as assessed using a breathing mannequin,
was found to be significantly greater at the ultra-low
wind speed of 0.10 m s–1 compared to the higher
values of 0.24 and 0.42 m s–1. With those results in
mind, it is therefore important to determine whether
personal samplers like those mentioned might reflect
corresponding performance characteristics. Specifi-
cally, we were interested to know whether these sampling devices might still provide fair measures of
what is inhaled, even at reduced wind speeds. In
other words, are the observed differences in human
aspiration efficiency based on wind speed matched
by the samplers in question?
This paper describes an experimental study that was
conducted to answer that question, namely to carefully evaluate the effectiveness of the various aerosol
samplers listed above for collecting inhalable aerosols
at ultra-low wind speeds. This work also represents an
important extension of our previously reported work
to examine changes in the particle size-dependent inhalable fraction at such low wind speeds.
EXPERIMENTAL METHODS
Wind tunnel and mannequin set-up
The experimental set-up consisted of an ultra-low
speed wind tunnel and life-sized heated, breathing
mannequin system that has been described in detail
elsewhere (Schmees et al., 2008a). In brief, the novel
wind tunnel design combined a conventional aerosol
wind tunnel and a calm air aerosol chamber in terms
of both the principles and the modes of operation. In
this way, it was able to generate a spatially uniform
distribution of aerosols—with respect to both aerosol concentration and particle size distribution—at
the low wind speeds of interest. The mannequin,
consisting of head, torso, and upper arms, was attached to a computer-controlled breathing machine
that enabled the simulation of a representative range
of breathing minute volumes and various combinations of breathing modes (i.e. nose or mouth breathing or combinations of the two if desired). Separate
pathways were designed for inhalation and exhalation to prevent the possibility of re-entrainment of
particles already deposited inside the mannequin
nose or mouth. A 47-mm filter holder was situated
along the inhalation pathway inside the mannequin
head for the collection of particles during inspiration. Any other material that deposited prior to
reaching the filter was also recovered by wiping
the inside walls with cotton balls impregnated with
alcohol. In this way, all particulate material that
passed through the opening of the breathing orifice
was measured and so the inhalable aerosol to which
the samplers were exposed was measured directly.
Inhalable aerosol samplers
The four different personal sampling devices mentioned earlier were those that were tested, i.e. the
IOM, Button, GSP/CIS, and CFC. The first three
Performance study of personal inhalable aerosol samplers
samplers were chosen because they are increasingly
being used around the world to measure personal exposures to the inhalable aerosol fraction and are
commercially available to industrial hygienists.
The fourth sampler, the CFC, is not specifically described as an inhalable aerosol sampler, but it is
the sampler most commonly used by occupational
hygienists in many countries for collecting ‘total
aerosol,’ and therefore, it is of considerable interest
to determine the extent to which it too might collect
inhalable aerosol under low wind speed conditions.
The IOM inhalable aerosol sampler (SKC, Inc.,
Eighty-four, PA, USA) was designed with specific
regard to the inhalability criterion, which was already widely accepted at the time of its development
(Mark and Vincent, 1986). The version of this sampler used in the experiments described here is shown
mounted to the mannequin in Fig. 1 (in the far right
of the picture). It included a stainless steel cassette
insert that held a 25-mm glass fiber filter, and the
whole cassette (including the filter) was analyzed
gravimetrically. For this, the entire cassette was
placed in a desiccator overnight prior to weighing
to reduce contributions from moisture uptake, which
included both before the experiment was carried out
Fig. 1. Experimental set-up for assessing personal sampler
performance at ultra-low wind speeds, showing the mannequin
with all four personal samplers tested—from left to right: CFC,
GSP, Button, and IOM. Not shown is the bag situated on the
back of the mannequin torso that held the four sampling
pumps.
209
and after the sample was obtained. Fitted caps were
placed over the inlet when the samples were inside
the desiccator—to prevent dust from settling onto
the loaded filters—and care was taken to wipe off
the outside of the cassette with a dry cloth prior to
weighing. The IOM sampler required a sampling
pump operated at a flow rate of 2 l min–1 and the
fully assembled sampler was clipped to the lab coat
of the mannequin so that the inlet pointed directly
outwards from the body, as shown in the picture.
The Button sampler (SKC, Inc.) is a newer
addition to the group of samplers that have been
suggested as matching the inhalable aerosol convention (Aizenberg et al., 2000). It features a hemispherical, multi-orifice inlet configuration and is shown
on the center right side of the mannequin picture in
Fig. 1. For this sampler, a 25-mm glass fiber filter,
which had been desiccated overnight, was weighed
on its own before and after sampling. The O-ring that
held the filter in place tended to be quite snug against
the filter, and so extra care was taken when removing
it from atop the filter to prevent sample loss. The
pump was operated at 4 l min–1 and the sampler itself
was attached to the lab coat, again with the inlet
pointing outwards from the body.
The GSP conical inlet sampler (BGI, Inc., Waltham, MA, USA) is shown attached to the mannequin lab coat in the center left portion of the
photograph in Fig. 1. In this case, the sampler utilized was the plastic CIS version, which is no longer
commercially available, but it was expected to perform similarly to the metal version that is currently
available. It featured a removable plastic cassette
holding a 37-mm glass fiber filter. For these experiments, the filter—again desiccated overnight—was
removed from the cassette and weighed separately
before and after sampling. The pump connected to
this sampler was operated at 3.5 l min–1 and the sampler was clipped to the lab coat with the inlet pointing directly away from the body.
Finally, the 37-mm CFC (SKC, Inc.) is shown to
the far left of Fig. 1, draped over the mannequin
shoulder in the mode of its most common use (i.e.
at 45° from horizontal). The version of the CFC
sampler used for these experiments consisted of
three polypropylene stages that fitted snugly together. A 37-mm glass fiber filter was weighed individually and placed on top of a supportive pad inside
the cassette. All pieces of the sampler were kept in
the desiccator overnight with the filter before and after sampling to control moisture uptake. Again, care
was taken when removing the filter from the sampler
to avoid potential sample loss due to mishandling.
The sampling pump was operated at 2 l min–1,
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D. K. Sleeth and J. H. Vincent
consistent with the general practical use of this instrument. One concern with the CFC sampler, as
noted previously, was that it had been consistently
shown to suffer from high levels of internal wall deposits, which are not analyzed when only the filter is
weighed (Puskar et al., 1991; Demange et al., 2002).
There are ways in which to collect these deposits,
such as wipes or specially designed inserts, as is often done by the Occupational Safety and Health Administration (OSHA) and hygienists in some
industries. However, it was decided that, for this
study, the analysis of the CFC would be for the filter
only. In general, this is the way it has historically
been done by the majority of industrial hygienists
as general practice, and so the results presented here
may be informative for interpretation of historical
data. Additionally, there is no standard method for
collection of wall deposits; for example, there is no
mention in the US National Institute of Occupational
Safety & Health (NIOSH) Method 0500 (Particles
Not Otherwise Regulated, Total).
Sampling and analytical procedures
During each experiment, all four personal samplers
were operating concurrently with the breathing mannequin, as was a thin-walled cylindrical tube operating as an isokinetic reference sampler placed 0.75 m
upstream of the mannequin. The body was always
heated to skin temperature (33°C) and continuously
rotated reciprocally (i.e. through a full 360° and then
reversing back again) at 2 r.p.m. in order to achieve
orientation-averaged sampling. In order to eliminate
any possible biases associated with position, the samplers were moved to the opposite coat lapel for repeat
tests. Otherwise, initial determination of sampler
placement, that is, on which lapel they were attached,
was arbitrary in the actual experiments.
The parameters that were studied are listed in Table 1. The first of these is particle size, with aerosols
generated from narrowly graded fused alumina
(powder grades F1200, F800, F500, F400, F280,
and F240; Washington Mills, Niagara, NY, USA)
providing test aerosols with mass median aerodynamic diameter from 9.3 to 89.5 lm. For most of
the powder grades, the particle size distribution was
measured in the wind tunnel directly using a modified
Marple-type cascade impactor (Wu and Vincent,
2007) under each set of experimental conditions.
The exception was for the two coarsest powder
grades (F280 and F240) at 0.10 m s–1, where a nominal particle size was used (Mark et al., 1985). Wind
speed covered the ultra-low range of interest (0.10,
0.24, and 0.42 m s–1), and several different mannequin breathing patterns (expressed as the combination of minute volume and mode of breathing) were
used (6 l min–1 mouth, 20 l min–1 mouth, 6 l min–1
nose, 6 l min–1 nose–mouth, and 20 l min–1 nose–
mouth). For each combination of the experimental
conditions tested, at least two repeats were performed, for a total of 180 experiments, with 20 min
per sampling period.
Each sampler was operated with its own individual personal sampling pump (Model XR5000, SKC
Inc.). In a typical occupational hygiene survey in
an actual workplace, a personal pump of this type
would be clipped to the worker’s belt, usually positioned at the lower back. In our experiments, it was
more convenient to place all the pumps together in
a small backpack located at the back of the mannequin torso, secured by a single strap that rode across
the mannequin chest. This allowed easy rotation of
the mannequin with all equipment fully contained.
All samplers used either 25-mm (IOM and Button)
or 37-mm (GSP and CFC) glass fiber filters which, as
already mentioned, were conditioned overnight in
a desiccator to reduce moisture content, both before
and after sampling. As indicated, the IOM was used
with a stainless steel cassette insert that was weighed
along with the filter; for all other samplers, only
the filter was analyzed. With the exception of the
disposable CFC samplers, the IOM, Button, and
GSP samplers were washed with soap and water,
Table 1. Parameters assessed for inhalable sampler performance study.
Samplers
Powder grades (MMADa, lm)
Wind speeds (m s1)
Breathing patterns
IOM
F1200 (9.6, 9.5, 9.3)
0.10
6 l min1 mouth
Button
F800 (13.9, 12.8, 12.4)
0.24
20 l min1 mouth
GSP
F500 (28.8, 32.7, 28.7)
0.42
6 l min1 nose
CFC
F400 (37.7, 44.3, 40.0)
b
F280 (74.0 , 62.4, 66.9)
6 l min1 nose–mouth
20 l min1 nose–mouth
b
F240 (89.5 , 60.1, 63.0)
a
Mass median aerodynamic diameter measured directly for each combination of powder grade and wind speed. Listed values are
for wind speeds 0.10, 0.24, and 0.42 m s–1, respectively.
b
MMAD is a nominal value from Mark et al. (1985).
Performance study of personal inhalable aerosol samplers
rinsed and dried, and reused for subsequent experiments. Blanks were obtained for each sampler type
once per day, usually during the first experiment of
that day, alongside cotton, and filter blanks prepared
separately for the mannequin and isokinetic reference sampler. All the recorded sampled mass values
were blank corrected. In general, most such corrections were very small, so contributed little to the
overall recorded collected mass. These steps basically follow the procedures adopted by most occupational hygienists in practical aerosol sampling.
The different pump flow rates required for the personal samplers (i.e. between 2 and 4 l min–1) were
calibrated using a primary flow meter both before
each test and then again after sampling was completed. If the change in flow rate was .–5% for
a given sampler, that sample was deemed unacceptable and not included in subsequent analyses. The
average of the two flow rates was used to determine
the total volume of air sampled. Taking the change in
filter weight (fm2 – fm1) less the change in filter
blank weight (fb2 – fb1) and dividing that by the
pump flow volume (V) provided the concentration
of sampled material (CS), calculated using
CS 5
½ðfm2 fm1 Þ ðfb2 fb1 Þ
:
V
ð1Þ
This value was then compared to the reference
sampler concentration (CR) measured by the isokinetic sampler. That value was calculated in a similar
way but included the addition of wall deposits in the
sampling tube, which were determined by taking the
change in weight of the cotton wipe (cm2 – cm1) less
the change in cotton blank weight (cb2 – cb1). The
ultimate calculation for the reference sampler concentration was therefore
CR 5
211
by the mannequin (CM), again calculated in a similar
manner to equation (2)—including deposits from
along the inhalation pathway—to estimate sampling
efficiency of the ‘inhalable’ aerosols (AI) as estimated by what was inhaled by the mannequin, thus
AI 5
CS
:
CM
Ultimately, a total of 735 personal samples were
taken (not including the mannequin aspiration efficiency measurements), with just seven rejected for
being unacceptable due to unexplained inconsistency
but most likely due to equipment failure or operator
error.
RESULTS
IOM inhalable aerosol sampler
Figure 2 shows the results for the aspiration efficiency of the IOM sampler as a function of particle
aerodynamic diameter (dae). The graph represents
all experimental data averaged across all mannequin
breathing conditions and separated out into each
wind speed (0.10, 0.24, and 0.42 m s–1, respectively).
Each data point represents the mean of at least 10 experiments performed for a single particle size at the
same wind speed, with error bars representing 1 SD.
The current inhalability convention is also included
for the purpose of comparison.
Analysis of variance (ANOVA) results, comparing
the IOM efficiency based on wind speed, showed
that significant differences existed, with sampling efficiency decreasing as wind speed increased (P-value ,
0.0001). There were no statistically significant differences based on the breathing pattern of the mannequin (P-value 5 0.1900) or for just the mode of
½ðfm2 fm1 Þ ðfb2 fb1 Þ þ ½ðcm2 cm1 Þ ðcb2 cb1 Þ
:
V
Combining equations (1) and (2) provided an estimate of the sampling efficiency (AS) using
AS 5
CS
:
CR
ð3Þ
The sampler filter mass concentration was also
compared to the inhaled concentration as measured
ð4Þ
ð2Þ
breathing (P-value 5 0.2712). However, sampling efficiency was significantly different based on mannequin
breathing rate (P-value 5 0.0180). Such differences
could be associated with the increased mixing in front
of the mannequin at lower wind speeds and higher
breathing flow rates, resulting from the significant
and persistent air disturbances that have been seen to
be present in the air approaching the mannequin under
such conditions (Schmees et al., 2008b). However, we
212
D. K. Sleeth and J. H. Vincent
do not have enough information to make any firm conclusions in this regard.
From Fig. 2, it can also be seen that, at the lowest
wind speed (0.10 m s–1), the IOM sampler consis-
Fig. 2. Mean aspiration efficiency of the IOM sampler (AIOM)
as a function of particle aerodynamic diameter (dae) for three
different ultra-low wind speeds when attached to a heated,
breathing, and rotating mannequin. Error bars represent 1 SD
and the current inhalability convention is also shown.
tently indicated sampling efficiencies that lay significantly above the original inhalability curve. At the
two higher wind speeds (0.24 and 0.42 m s–1), the
IOM followed the criterion somewhat better, but even
so, most of those data still appeared to fall above the
curve. Paired t-tests confirmed that, for each wind
speed (0.10, 0.24, and 0.42 m s–1), the IOM sampler
indicated significantly greater values for sampling efficiency in relation to the convention (P-values:
,0.0001, ,0.0001, and 0.0147, respectively).
Next, Fig. 3 shows the IOM sampling efficiency
compared directly with what was inhaled by the
mannequin. This, after all, is the most appropriate
comparison in terms of the relevance of the IOM
sampler to inhalable aerosol and it removes any
biases that might have been introduced through the
reference measurement. In this figure, the solid line
represents perfect agreement (AMannequin 5 AIOM)
and the various dashed lines—all passing through
the origin—represent simple linear regressions of
the data for each wind speed separately. Results
show that, for a given set of conditions, the IOM
sampler consistently collected more than what was
Fig. 3. Comparison of the aspiration efficiency for the IOM sampler (AIOM) to the mannequin aspiration efficiency (AMannequin),
for all concurrent experiments at wind speeds of 0.10 m s1 (black circles), 0.24 m s1 (grey squares), and 0.42 m s1 (white
triangles). The solid line shows perfect agreement and the various dotted lines are linear regressions for 0.10 m s1 (long dashes),
0.24 m s1 (dashes and dots), and 0.42 m s1 (dotted).
Performance study of personal inhalable aerosol samplers
inhaled by the mannequin, a tendency that was supported by a t-test comparison (all P-values ,
0.0001). Numerical values for each of those relationships (including values for all other samplers tested
as well) are summarized in Table 2.
Button inhalable aerosol sampler
Figure 4 shows the results for the mean sampling
efficiency of the Button sampler as a function of dae
(in this case, only the filter was analyzed and so the
results are a measure of ‘sampling’ efficiency instead
of ‘aspiration’ efficiency, with the terminology reTable 2. Slope factor for linear regression (forced through
the origin) shown as the ratio of sampling efficiency to
mannequin aspiration efficiency (AI), for each wind speed
tested.
Wind speed (m s1)
Sampler
0.10
IOM
1.52 (0.36)
Button
1.48a (0.38)
GSP
1.40a (0.51)
CFC
0.35 (0.15)
0.24
0.42
Slope Factor (R2)
IOM
1.30 (0.14)
Button
GSP
1.12 (n/a)
1.10a (0.48)
CFC
0.27a (0.19)
IOM
1.18 (n/a)
Button
1.00 (n/a)
GSP
0.92a (0.27)
CFC
0.26a (0.09)
a
Linear regression that is not forced through the origin has
intercept that is not significantly different from zero.
Fig. 4. Mean sampling efficiency of the Button sampler
(AButton) as a function of particle aerodynamic diameter (dae)
for three different ultra-low wind speeds when attached to
a heated, breathing, and rotating mannequin. Error bars
represent 1 SD and the current inhalability convention is also
shown.
213
flecting this difference). These data are organized
in the same manner as Fig. 2, separated into the different wind speeds studied, with each data point then
representing the average efficiency across all mannequin breathing conditions at that particle size. ANOVA results indicated that the Button sampling
efficiency was significantly different based on the
wind speed inside the wind tunnel (P-value ,
0.0001), with sampling efficiency increasing with
decreasing wind speed. This is reflected in Fig. 4,
which clearly shows that the Button sampler typically indicated higher sampling efficiencies at 0.10
m s–1 compared to 0.24 and 0.42 m s–1. There were
no significant differences (at a 5 0.05) with respect
to the mannequin breathing pattern, breathing flow
rate or mode of breathing.
As was seen for the IOM sampler, the lowest wind
speed was consistently associated with sampling efficiencies that were greater than the current inhalability convention, with better agreement for the
two higher wind speeds. In a general sense, therefore, performance was similar to that of the IOM.
However, the results show that agreement between
the Button sampler and the inhalable convention
was better for the highest wind speed in the range
tested here. Although the two lower wind speeds revealed significant differences to the existing convention based on paired t-tests (P-values: ,0.0001 and
0.0021, respectively), at the highest wind speed the
Button was not statistically different from that standard (P-value 5 0.1308).
Figure 5 shows the relationship between what was
collected by the mannequin and by the Button sampler for all concurrent experiments. Overall, a paired
t-test showed that the two data sets were significantly different (P-value , 0.0001). After separation based on wind speed, however, it emerged
that there was no significant difference at the higher
wind speed tested here (U 5 0.42 m s–1) (P-value 5
0.2317). This is reflected in Table 2, where it is seen
that, on average, the Button sampler collected 48%
more than the mannequin at 0.10 m s–1 and 12%
more at 0.24 m s–1. At 0.42 m s–1, however, there
was no detectable difference between the Button
and the mannequin.
GSP conical inlet sampler
Figure 6 shows the results for the mean sampling
efficiency of the GSP conical inlet sampler at the
various wind speeds indicated. That figure is organized in the same way as previously, with the inhalability convention again included for comparison.
ANOVA results showed that the sampling efficiency
of the GSP was significantly different based on the
214
D. K. Sleeth and J. H. Vincent
Fig. 5. Comparison of the sampling efficiency for the Button sampler (AButton) to the mannequin aspiration efficiency (AMannequin),
for all concurrent experiments at wind speeds of 0.10 m s1 (black circles), 0.24 m s1 (grey squares), and 0.42 m s1 (white
triangles). The solid line shows perfect agreement and the various dotted lines are linear regressions for 0.10 m s1 (long dashes),
0.24 m s1 (dashes and dots), and 0.42 m s1 (dotted). (In this case, the regression for U 5 0.42 m s1 exactly matches the
mannequin and so overlaps with the solid line).
Fig. 6. Mean sampling efficiency of the GSP sampler (AGSP)
as a function of particle aerodynamic diameter (dae) for three
different ultra-low wind speeds when attached to a heated,
breathing, and rotating mannequin. Error bars represent 1 SD
and the current inhalability convention is also shown.
wind speed in the wind tunnel (P-value , 0.0001),
with sampling efficiency again increasing with decreasing wind speed. There were no statistically sig-
nificant differences (at a 5 0.05) for the sampling
efficiency of the GSP based on any of the mannequin
breathing parameters tested.
Similar to what was seen for the performance of the
other inhalable aerosol samplers already discussed,
the GSP sampling efficiency was generally greater
than the inhalability convention at the lowest wind
speed as well. For the higher wind speed, however,
the GSP performed similarly to the Button, more so
than the IOM sampler. Statistically, the GSP performance was significantly different from the current
convention for each wind speed (P-values: ,0.0001,
0.0029, ,0.0001, respectively), but here, it was observed that, at the highest wind speed (0.42 m s–1),
the GSP under-sampled relative to the convention.
Figure 7 shows the direct relationship between
the GSP sampling efficiency and the mannequin aspiration efficiency for each of the three ultra-low wind
speeds tested. Overall, a paired t-test showed a statistically significant difference (P-value , 0.0001).
However, again, when separated out by wind speed,
the results for the highest wind speed (0.42 m s–1)
Performance study of personal inhalable aerosol samplers
215
Fig. 7. Comparison of the sampling efficiency for the GSP sampler (AGSP) to the mannequin aspiration efficiency (AMannequin), for
all concurrent experiments at wind speeds of 0.10 m s1 (black circles), 0.24 m s1 (grey squares), and 0.42 m s1 (white
triangles). The solid line shows perfect agreement and the various dotted lines are linear regressions for 0.10 m s1 (long dashes),
0.24 m s1 (dashes and dots), and 0.42 m s1 (dotted).
showed no significant difference. Looking at the values in Table 2, the same trend is observed as for the
other samplers already discussed—that is, better
agreement between the mannequin and sampler is
seen with increasing wind speed. Specifically, at
0.10 m s–1, the GSP collected, on average, 42% more
than the mannequin, at 0.24 m s–1 it collected 10%
more, and at 0.42 m s–1 it collected 8% less.
It is worth recalling that, as indicated previously,
the plastic cassette that held the filter inside the sampler was not included for gravimetric analysis in the
present set of experiments. However, during the experiments, visible deposits of particulate material
were observed on the inside walls of the cassette,
and these were not evaluated. It is expected, therefore, that inclusion of the cassette in the gravimetric
analysis (as was done for the IOM sampler) would
generally result in higher sampling efficiency than
has been reported here.
CFC sampler
Figure 8 shows the results for the CFC sampler,
with data again organized as for the other samplers
tested. ANOVA results indicated that filter-only sampling efficiency was significantly different based on
wind speed (P-value , 0.0001) but not significant
(at a 5 0.05) based on any of the mannequin breathing
parameters. It is clear from that figure that the sampling efficiency of the CFC dropped off quickly with
increasing particle size, approaching zero for particles with aerodynamic diameter larger than 30
lm. The only conditions for which the CFC filter provided a sampling efficiency approaching the current
inhalability convention were for the smallest particles
(aerodynamic diameter below 10 lm) and at the
lowest wind speed (0.10 m s–1). These observations
were consistent with many previous studies of this device indicated previously, both in the lab and in the
field. Overall, the CFC sampler significantly underestimated the inhalable fraction of aerosols—as defined by the currently accepted convention—at all
three ultra-low wind speeds (all P-values , 0.0001).
It was only slightly better at the lowest wind speed
compared to the highest wind speed.
Figure 9 shows the direct relationship between
what was collected by the CFC filter and the
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D. K. Sleeth and J. H. Vincent
mannequin, respectively. As expected, a t-test comparison of those data revealed statistically significant
differences, across all wind speeds as well as for
each wind speed separately (all P-values ,
0.0001). In contrast to the other samplers tested, differences associated with the CFC were in the opposite direction, indicating that it consistently collected
much less than was inhaled by the mannequin. From
Table 2, it can be seen that, on average, the CFC typically collected only between 26 and 35% of what
the mannequin inhaled.
DISCUSSION
Fig. 8. Mean sampling efficiency of the CFC sampler (ACFC)
as a function of particle aerodynamic diameter (dae) for three
different ultra-low wind speeds when attached to a heated,
breathing, and rotating mannequin. Error bars represent 1 SD
and the current inhalability convention is also shown.
The primary objective of this personal sampler
performance study was to assess the effectiveness
of these aerosol samplers for collecting the inhalable
aerosol fraction at ultra-low wind speeds considered
to be realistic of a large proportion of workplaces.
Therefore, it will be useful to include: (i) a further
discussion about the limitations of these results,
(ii) a discussion about the relationship between their
sampling efficiencies and the current inhalability criterion (which is currently the benchmark for what is
expected of inhalable aerosol samplers); and (iii) assessment of how these results relate to the
Fig. 9. Comparison of the sampling efficiency for the CFC sampler (ACFC) to the mannequin aspiration efficiency (AMannequin), for
all concurrent experiments at wind speeds of 0.10 m s1 (black circles), 0.24 m s1 (grey squares), and 0.42 m s1 (white
triangles). The solid line shows perfect agreement and the various dotted lines are linear regressions for 0.10 m s1 (long dashes),
0.24 m s1 (dashes and dots), and 0.42 m s1 (dotted).
Performance study of personal inhalable aerosol samplers
applicability of a modified convention for defining
inhalability at low wind speeds (Aitken et al.,
1999; Sleeth and Vincent, 2011).
Study limitations
First, the results for the sampling efficiency of the
IOM, Button, and GSP were much larger than expected for the smallest particles in the lowest wind
speed. In those experiments, it would be expected
that the efficiency would be at or below one. The
possibility exists that some experimental bias was introduced to skew those results, inherent in the wind
tunnel design or in the sampling and analytical methods used. Considering the consistent shift in results,
it is also possible that the isokinetic reference sampler was producing erroneous results. It is uncertain
if this was in fact the case, but it should be pointed
out that the experiments were carried out randomly
for each particle size. In other words, all the experiments for the smallest particles at the lowest wind
speed were not carried out in succession. Therefore,
if there were an experimental bias, it would most
likely affect the other results as well. An additional
possibility is that, at the lowest wind speed, there
was a greater amount of deposition (as opposed to
actual aspiration) inside the sampler due to the presence of aerosols being injected from overhead. On
the other hand, the uniformity was previously tested
for those conditions (i.e. smallest particles and lowest wind speed) and shown to be within –5%
throughout the wind tunnel, so we are left with the
results as they are.
Relatedly, an important detail to note in Figs. 2, 4,
and 6 is the magnitude of the standard deviations,
which appear to be greatest for the largest particle
sizes at the lowest wind speed. It is likely that this
may be associated with the fact that the spatial distribution of aerosol concentration and wind speed was
less uniform under these conditions, as had been
noted during our initial set-up and calibration tests
(see Schmees et al., 2008a). This may simply represent the physical limitations of studying large aerosols at low wind speeds—similarly high variability
in sampling efficiency for large aerosols has been
seen by others as well (e.g. Aizenberg et al.,
2001). It implies that, not surprisingly, repeatability
was more difficult under these conditions, but it
may also limit the ability to generate firm conclusions based on these results.
Next, as seen in Figs. 3, 5, 7, and 9, it was decided
that the linear regressions should be forced through
the origin. However, it is interesting to note that,
when the regression line was not forced through
217
the origin (data not shown), the relationship is close
to the desired 1:1 for many of the inhalable aerosol
samplers. This indicates that, at ultra-low wind
speeds, increases in mannequin aspiration efficiency
were matched by corresponding similar increases in
sampling efficiency. But, that said, it is the more acceptable assumption here to force the line to go
through the origin, based simply on the undeniable
fact that, when no aerosol is present, both the mannequin and the sampler should collect zero particulate
material. In the absence of any direct evidence for an
offset in the performance of these samplers (e.g. an
artifact leading to a consistent bias), the only acceptable approach is the simple linear regression leading
to the numbers presented in Table 2. However, for
completeness, Table 2 also provides information
about those samplers for which the intercept of a linear regression not forced through the origin was not
significantly different from zero.
And finally, regarding the CFC sampler, a few further remarks are required. It is worth noting some results of other researchers showing that, if worn with
the inlet pointing directly outwards (as opposed to
facing in the customary 45° downward direction),
there was some increase in the sampling efficiency
of the CFC sampler (Buchan et al., 1986; Görner
et al., 2010). In addition, inclusion of wall deposits
inside the plastic cassette of this sampler will increase the overall estimate of efficiency and so provide improved performance in relation to inhalable
aerosol. Indeed, the US OSHA have recently revised
several aerosol sampling recommended methods,
notably for metals analysis, to require that such deposits be included as part of the analyzed sample
(Hendricks et al., 2009). It has therefore been suggested that wall deposits might be large enough that,
if included routinely in the analysis, would provide
better agreement with the inhalability convention.
However, it should be pointed out that, from the
point of view of the physics of the sampling process
(see Vincent, 2007), there is no reason to expect that
the CFC should aspirate aerosol with the same
efficiency as, say, the IOM sampler over the entire
range of inhalable aerosol particle sizes (i.e. up to
100 lm).
In the present work, wall deposits were not routinely analyzed and therefore, only speculations
can be made regarding their importance. Ultimately,
taking into account all that is known, including what
has been learned from the present work, we must
conclude that, when used in the traditional manner
as it was in this study and in many historical exposure assessments, the CFC does not accurately sample the inhalable aerosol fraction.
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D. K. Sleeth and J. H. Vincent
Comparison of personal sampling data to current
inhalability criteria
As discussed previously, one of the primary applications of the existing inhalable aerosol convention
(or ‘inhalability curve’) is to provide a benchmark
against which inhalable aerosol samplers may be examined, with respect to how accurately they actually
sample the inhalable fraction of aerosols. Therefore,
integration of the results from the personal samplers
presented here relative to the existing convention is
of great interest.
First, it will be remembered that all the inhalable
aerosol samplers—IOM, Button, and GSP—showed
greater sampling efficiencies relative to the mannequin at the lowest wind speed used in this study.
Therefore—assuming this mannequin provides an
accurate estimate of the inhalable fraction—each
sampler would be expected to be greater than the convention under those wind speed conditions as well.
However, the performances of the Button and GSP
samplers were actually quite similar to the mannequin at the highest wind speed and would therefore
be expected to follow the inhalability curve in a similar manner under those conditions. Finally, the sampling efficiency of the CFC was shown to be far too
low for it to be of use for sampling the inhalable fraction at ultra-low wind speeds and would therefore not
be expected to match the current criteria.
In fact, the quantitative comparison between the
sampler results and the current convention, as discussed separately for each sampler previously, generally produced these expected relationships.
Specifically, each of the inhalable aerosol samplers
over-sampled relative to the convention at 0.10 m
s–1 and showed increasing agreement with increasing wind speed. Notably, the Button and GSP samplers did not appear to be statistically different
from the inhalable convention for the higher wind
speeds. In contrast, the CFC significantly under-sampled relative to the convention, based on the statistical analysis discussed previously.
Keeping in mind the relatively high intrinsic variability in this work, this new collection of data
implies that at ultra-low wind speeds, these commonly used personal aerosol samplers may not always be appropriate for estimating the inhalability
convention as defined by the currently accepted criteria. On the one hand, at the highest wind speed
tested here, the inhalable samplers do appear adequate for those purposes. However, at 0.10 m s–1,
and up to 0.24 m s–1, an alternative inhalability criterion, against which inhalable aerosols samplers
may be compared, may be advisable.
Comparison of personal sampling data to proposed
low wind inhalability criteria
As suggested by previous work, a practical transition between ‘calm’ and ‘moving’ air might lie
somewhere in the region of 0.2 m s–1 (Sleeth and
Vincent, 2011). Therefore, it will be instructive to
compare the new personal sampler data at 0.10 and
0.24 m s–1 to the low wind/calm air criterion proposed by Aitken et al. (1999).
At 0.10 m s–1, it was shown that all the nominally
inhalable aerosol samplers provided greater sampling efficiency relative to the proposed calm air criterion, with the CFC sampler providing lower
sampling efficiency relative to that criterion. Again
keeping in mind the high variability in the results
at this wind speed, it appears that all those samplers
were significantly different from the target suggested
by the calm air model (IOM, GSP, and CFC: P-values , 0.0001 and Button: P-value 5 0.0002). That
indicates that, although the proposed calm air criterion might provide a better definition of true human
inhalability at 0.10 m s–1, the personal samplers currently used to measure the inhalable fraction did not
tend to match that criterion particularly well at that
wind speed in this study.
At 0.24 m s–1, agreement with that proposed criterion improved for all the inhalable aerosol samplers
but remained poor for the CFC sampler. Although
the GSP and Button samplers appear to provide moderately good fits to the calm air model, these data
were in fact quite widely dispersed and so a paired
t-test found the sampling efficiency of both those
samplers to be significantly different from the calm
air criterion (P-values , 0.0001 and 0.0035, respectively). On the other hand, the IOM sampler, which
also provided somewhat variable data, did match
reasonably well with the proposed criterion for calm
air at this wind speed (P-value 5 0.7488), despite the
fact that it over-estimated exposures relative to the
mannequin at that same wind speed. Again, however,
the variability in the data does limit the strength of
the conclusions.
CONCLUSIONS
The results shown here should help fill a gap in
knowledge about the performance of personal samplers under low wind speed conditions. It is an important finding that the wind speed in the wind tunnel
was a significant factor influencing the performance
of all four samplers tested (IOM, Button, GSP, and
CFC). In contrast, previous studies have suggested
that, for air velocities of 0.5 and 1 m s–1, sampling efficiency was not dependent on wind speed
Performance study of personal inhalable aerosol samplers
(Aizenberg et al., 2001). In general, the results presented here appear to indicate that each of the inhalable aerosol samplers (IOM, Button, and GSP)
provide an increasingly accurate estimate of the inhalable aerosol fraction as wind speed increases from
0.1 up to 0.42 m s–1—a range which is more representative of actual workplaces.
It was also shown that the breathing pattern of the
mannequin (i.e. the combination of minute volume
and mode of breathing) and the breathing mode on
its own (i.e. nose, mouth, or nose–mouth breathing)
had no significant impact on the sampling efficiency
of any personal samplers attached to the body.
Where the breathing minute volume was looked at
as a separate factor, only the IOM sampler was significantly different, although that difference appeared to be only minimal. As a whole, these
results suggest that differences in human aspiration
do not have any substantial effect on the performance of personal samplers attached to the body.
However, it is possible that mounting personal samplers much closer to the nose or mouth (e.g. behind
a welder’s face shield as was carried out by Lidén
and Surrakka, 2009) could increase the likelihood
that such factors may become influential.
The performance of each sampler was also assessed relative to the mannequin aspiration efficiency obtained during the same experiments.
Again, keeping in mind the large spread in the data,
both the Button and the GSP were statistically similar to the mannequin at the highest wind speed (0.42
m s–1) but the IOM remained different from the mannequin at each wind speed. This suggests that at the
higher end of the ultra-low wind speeds used in this
work, these aerosol samplers may provide reasonably accurate measurements of the inhalable fraction
of aerosols. In contrast, at both lower wind speeds
(0.10 and 0.24 m s–1), the inhalable samplers were
only somewhat more adequate at estimating the inhalable aerosol fraction—as measured here directly
by the mannequin. The CFC, however, remained statistically significantly different from the mannequin
at each wind speed.
All these conclusions presume that the mannequin
aspiration efficiency measured here provided an accurate estimate of the inhalable fraction. However, a direct comparison of the results for these personal
samplers to the currently accepted inhalability convention—as well as to proposed calm air criteria—was
also instructive for more practical purposes. Those results indicate that, at 0.10 m s–1, each of the three inhalable aerosol samplers tested here consistently
provided greater sampling efficiency relative to the
current inhalable aerosol convention. In contrast, at
219
the highest wind speed of 0.42 m s–1, agreement with
the criterion appeared to improve for all three inhalable aerosol samplers, indicating that they may still
be appropriate for use at such wind speeds. For the
CFC sampler, it was not surprising—based on the results of many other studies discussed previously, as
well as what is known about the physics of the aspiration process—that it had poor agreement with the inhalability convention.
On the other hand, the alternative criterion suggested by Aitken et al. (1999) for use in calm air/
low air movement environments appeared to provide
good agreement with measured sampling efficiency
at a wind speed of 0.24 m s–1. However, the personal
samplers were still significantly different from that
criterion at the lowest wind speed (0.10 m s–1). That
suggests that, if current samplers are to be used in ultra-low wind speed environments ,0.24 m s–1, physical modifications to better correlate those sampler
measurements to the inhalable aerosol fraction may
be necessary. However, the large amount of variability in sampler performance may also explain some of
the discrepancy in the data and so attempts to reproduce these results would be advisable.
Ultimately, for the most accurate application of
these samplers in low wind speed environments,
a modified criterion that is specific for the low wind
speed regime against which to compare those measurements would need agreement and adoption. In
fact, such discussions have become more formalized
in recent years (Lidén and Harper, 2006; Sleeth and
Vincent, 2011). Regardless, the results presented here
indicate that these samplers may generally measure
higher aerosol concentrations than what would actually be inhaled by humans at ultra-low wind speeds.
In that case, the proposed calm air criterion appears
to provide a somewhat more accurate, yet still conservative benchmark for use at ultra-low wind speeds.
As a whole, the experiments just described represent an important addition to the knowledge and understanding of common personal samplers in use
today. In general, all these findings are consistent
with what has been learned about the relative performances of these samplers in other studies, both
in laboratory and in field tests. But laboratory assessment of inhalable aerosol samplers at ultra-low wind
speeds between 0.05 and 0.5 m s–1—being more representative of typical working environments—had
not been performed previously. Recognizing the large
variability in the data, future studies utilizing this
wind tunnel system will attempt to improve the
uniformity and spatial distribution of test aerosols.
Ultimately, these results suggest that, as wellestablished inhalable aerosol samplers with
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D. K. Sleeth and J. H. Vincent
consistent agreement to mannequin inhalability, the
IOM, Button, and GSP may be useful at some ultralow wind speeds. On the other hand, the CFC sampler is not recommended for measuring the inhalable aerosol fraction at ultra-low wind speeds
without modifications, such as inclusion of wall deposits and altering the inlet configuration. Such results should be informative not only for the
industrial hygienists performing workplace assessments but also for the standards-setting community
concerned with sampling methodologies.
Finally, while we have acknowledged repeatedly
that the conclusions to be drawn from this work
are softened by the considerable variability in the
experimental data, it is important to note that experiments like those described were extremely difficult,
at the edge of what aerosol scientists have long considered to be nearly impossible. It is difficult to
imagine laboratory experiments for the conditions
of interest that can achieve lower variability than
was achieved here. So, we are pleased to present this
work as a significant step forward in our knowledge
about sampling for the inhalable aerosol fraction under realistic workplace conditions.
FUNDING
National Institute for Occupational Safety and
Health (5-RO1-OH002987-09).
Acknowledgements—This research was conducted while one
of the authors (D.K.S.) was at the University of Michigan,
where we are grateful to the Department of Environmental
Health Sciences and the School of Public Health for the space
and resources that were required to perform the work described
in the paper. We are especially grateful to the National Institute
for Occupational Safety and Health for its support in the form
of grant 5-RO1-OH002987-09.
REFERENCES
Aitken RJ, Baldwin PEJ, Beaumont GC et al. (1999) Aerosol
inhalability in low air movement environments. J Aerosol
Sci; 30: 613–26.
Aizenberg V, Choe K, Grinshpun SA et al. (2001) Evaluation
of personal aerosol samplers challenged with large particles.
J Aerosol Sci; 32: 779–93.
Aizenberg V, Grinshpun SA, Willeke K et al. (2000) Performance characteristics of the button personal inhalable aerosol sampler. Am Ind Hyg Assoc J; 61: 398–404.
ACGIH. (1985) Particle size selective sampling in the workplace. Report of the Technical committee on Air Sampling
Procedures. Cincinnati, OH: American Conference of Governmental Industrial Hygientists.
Baldwin PE, Maynard AD. (1998) A survey of wind speeds in
indoor workplaces. Ann Occup Hyg; 42: 303–13.
Berry RD, Froude S. (1989) An investigation of wind conditions in the workplace to assess their effect on the quantity
of dust inhaled. U.K. Health and Safety Executive Report
IR/L/DS/89/3. London, UK: Health and Safety Executive.
Buchan RM, Soderholm SC, Tillery MI. (1986) Aerosol sampling efficiency of 37-mm filter cassettes. Am Ind Hyg Assoc J; 47: 825–31.
CEN. (1992) Workplace atmospheres: size fraction definitions
for measurement of airborne particles in the workplace. European Standard EN 481. Brussels, Belgium Comité Européen de Normalization.
Demange M, Görner P, Elcabache JM et al. (2002) Field comparison of 37-mm closed-face cassettes and IOM samplers.
Appl Occup Environ Hyg; 17: 200–8.
Görner P, Simon X, Wrobel R et al. (2010) Laboratory study of
selected personal inhalable aerosol samplers. Ann Occup
Hyg; 54: 165–87.
Hendricks W, Stones F, Lillquist D. (2009) On wiping the interior walls of 37-mm closed-face cassettes: an OSHA perspective. J Occup Environ Hyg; 6: 732–4.
ISO. (1992) Air quality-particle size fraction definitions for
health-related sampling. Technical Report ISO/TR/7708–
1983 (E), Revised version. Geneva, Switzerland: International Organization for Standardization.
Lidén G, Harper M. (2006) Analytical performance criteria:
the need for an international sampling convention for inhalable dust in calm air. J Occup Environ Hyg; 3: D94–101.
Lidén G, Surrakka J. (2009) A headset-mounted mini sampler
for measuring exposure to welding aerosol in the breathing
zone. Ann Occup Hyg; 53: 99–116.
Kenny LC, Aitken RJ, Baldwin PEJ et al. (1999) The sampling
efficiency of personal inhalable aerosol samplers in low air
movement environments. J Aerosol Sci; 30: 627–38.
Kenny LC, Aitken RJ, Chalmers C et al. (1997) A collaborative European study of personal inhalable aerosol sampler
performance. Ann Occup Hyg; 41: 135–53.
Mark D, Vincent JH. (1986) A new personal sampler for airborne total dust in workplaces. Ann Occup Hyg; 30: 89–102.
Mark D, Vincent JH, Gibson H et al. (1985) Applications of
closely graded powders of fused alumina as test dusts for
aerosol studies. J Aerosol Sci; 16: 125–31.
Puskar MA, Harkins JM, Moomey JD et al. (1991) Internal
wall losses of pharmaceutical dusts during closed-face,
37-mm polystyrene cassette sampling, Am Ind Hyg Assoc
J; 52: 280–6.
Schmees DK, Wu Y-H, Vincent JH. (2008a) Experimental
methods to determine inhalability and personal sampler performance for aerosols in ultra-low wind speed environments. J Environ Monit; 10: 1426–36.
Schmees DK, Wu Y-H, Vincent JH. (2008b) Visualization of
the air flow around a life-sized, heated, breathing mannequin
at ultra-low wind speeds. Ann Occup Hyg; 52: 351–60.
Sleeth DK, Vincent JH. (2009) Inhalability for aerosols at ultra-low wind speeds. J Phys Conf Ser; 151: 9012062.
Sleeth DK, Vincent JH. (2011) Proposed modification to the inhalable aerosol convention applicable to realistic workplace
wind speeds. Ann Occup Hyg; 55: 476–84.
Vincent JH. (2007) Aerosol sampling: science, standards, instrumentation and applications. Chichester, UK: Wiley & Sons.
Wu Y-H, Vincent JH. (2007) A modified Marple-type cascade
impactor for assessing aerosol particle size distributions in
workplaces. J Occup Environ Hyg; 4: 798–807.