Analysis of particulate contamination in personal dust monitor 

sampling 

J. C. Volkwein 

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania, USA 

ABSTRACT: The National Institute for Occupational Safety and Health (NIOSH) has continued study of the 

Personal Dust Monitor (PDM) developed by Rupprecht & Patashnick Co. Inc. through a CDC contract and 

currently being commercialized by Thermo Fisher Scientific. This report examines the effect of oversize particles, 

cyclone inversion, and cleaning procedures. Clean and exposed filters were examined using light microscopy and 

particulates counted before and after various instrument manipulations. The data presented show that the dry 

cleaning procedures do not cause significant (greater than limit of detection) sample contamination by respirable 

or oversize particulates and that the cyclone is resistant to particulate carry over during inversion. 

1 Introduction 

The personal dust monitor (PDM) is a continuous 

measuring dust sampler incorporated into a miners cap 

lamp. The PDM represents a new way to determine 

personal respirable coal mine dust exposure in a 

continuous way. The timely data and electronic records 

produced will provide significantly more information to 

miners than currently possible using conventional dust 

samplers. This new technology will inevitably raise 

questions regarding the meaning and interpretation of the 

data. The EPA has gone though similar processes in 

converting filter type air pollution measurements to 

continuous monitoring systems measurements. The 

original data remains, but new ways to understand the 

impact of the additional information had to be developed. 

This understanding comes from use of the instruments. 

Published data on the PDM confirm that it measures 

personal dust exposure of miners plus many other 

parameters. Our understanding and interpretation of these 

findings is on the rapidly rising portion of the learning 

curve. This report addresses questions regarding the 

appearance and significance of large particles on PDM 

filters, the handling of filters, instrument cleaning and 

their impacts on measurement accuracy. 

2 Oversize Particulates 

The 1995 NIOSH criteria document contains two 

definitions of relevance to respirable dust. The first is the 

definition of respirable coal mine dust being: “That 

portion of airborne dust in coal mines that is capable of 

entering the gas-exchange regions of the lungs if inhaled; 

by convention, a particle-size-selective fraction of the 

total airborne dust; includes particles with aerodynamic 

diameters less than approximately (emphasis added) 10 

μm.” The document also defines the respirable 

convention to be: “The target sampling curve for 

instruments approximating the respirable fraction.” The 

definition continues with the theoretical mathematical 

equations. 

MSHA, by convention, defines oversize particulate 

as being greater than 10μm in diameter. When more than 

30 such particles are found by microscopic examination 

of approximately 2.5 mm2

 of filter surface area of a 37 

mm sampling filter, then the sample is voided due to the 

presence of oversize. The value of 30 oversize particles 

originated from a calculation that it would take 9615 

particles of 20 μm diameter coal at a density of 1.3 

gram/cm3

 to change the mass on the filter by 0.1 mg. 

2.1 Sources of Oversize Particles on Filters 

The presumed sources of oversize particles in the current 

sampler were from improperly assembled samplers, 

dumping of cyclones,1

or other mishandling of the sample. 

These sources are minimized in the PDM since the 

sampling train undergoes minimal disassembly for 

cleaning purposes and the BGI cyclone design (BGI, Inc., 

Waltham, MA) used in the PDM is resistant to dumping. 

Mishandling of filters is minimized as a source of 

oversize particles since the filter never is exposed to the 

atmosphere outside of the PDM during the time at which 

the filter mass is determined by the electronics of the 

instrument. If the filter area of the sampler is ever opened 

during sampling, an error is displayed on the screen and a 

record of the event occurs in the data file. 

1

 Dumping is defined as the act of inverting the cyclone 

such that oversize particles captured in the cyclone grit 

pot reenter the sampling air stream and become deposited 

on the filter. 

There are then three ways in which oversize particles 

can be deposited onto the PDM filters during sampling: 

First, the actual penetration through the BGI cyclone; 

second, the release of previously collected and 

agglomerated respirable dust in the instrument that was 

not removed during cleaning of the section between the 
 

 

 

 

 

cyclone outlet and the filter; and third, the remote 

possibility of a cyclone dump. 

2.2 Penetration Through a Cyclone 

Figure 1 shows the theoretical lung penetration curves of 

the Mining Research Establishment (MRE) sampler, the 

former American Conference of Governmental Industrial 

Hygienists (ACGIH) Dorr-Oliver cyclone having at a 2 

lpm flow rate, and the International Standards 

Organization (ISO) BGI cyclone flowing at 2.2 lpm. 

MRE samplers use gravity to separate respirable dust 

from an air sample and have a sharp cut off at about 7 μm 

in aerodynamic diameter. Cyclone samplers do not have a 

sharply defined cut off but instead have an asymptotic 

decay in the upper size range. Table 1 shows the fraction 

of particles by aerodynamic size that will be classified as 

respirable mass by the cyclones. This means that a dust 

sample, properly classified by cyclone, will have a small 

but finite number of particles greater than 10 μm in 

diameter. 

12 

THEORETICAL RESPIRABLE CURVES 

0.0 

0.2 

0.4 

0.6 

0.8 

1.0 

1.2 

0 2 4 6 8 10 SIZE, microns 

PENETRATION, %/100 

ISO 

BMRC 

ACGIH 

Figure 1. Classification criteria 

The other consideration when attempting to determine 

the presence of oversize particles on a sample filter is the 

fact that optical microscopic examination of a filter can 

determine only the equivalent circular diameter of a 

particle. Particle aerodynamic diameter is used to measure 

the actual way in which the dust moves in air and 

becomes deposited in the lung. The aerodynamic diameter 

is defined as a particle having the same movement in air 

as a spherical unit-density particle. Low density particles 

may in fact have very large equivalent circular diameters 

yet still be classified as respirable. Measuring equivalent 

circular diameter of a particle on a filter cannot accurately 

determine if a particle is respirable or not without 

measurement of that specific particle’s density and shape 

factor. 

2.3 Release of Agglomerated Respirable Dust 

Another way in which oversize particles may be deposited 

on the filter during sampling is if cyclone-classified 

respirable particles deposit and agglomerate in the 

Table 1. Calculated fraction of particles of a particular 

size that may be deposited on sample filters. 

Aerodynamic 

diameter 

μm 

Percent of 

ISO 

classified as 

respirable 

% 

Percent of 

ACGIH 

classified 

as 

respirable 

% 

Percent of 

MRE 

classified 

as 

respirable 

3.5 62 50 75 

4 50 37 68 

10 1.3 0.48 0 

12 0.39 0.19 0 

14 0.12 0.032 0 

16 0.04 0.01 0 

18 0.01 0.003 0 

20 0.005 0.001 0 

instrument’s internal plumbing between the outlet of the 

cyclone and the filter, are not removed by the current 

cleaning procedures, are not removed during the 30 

minute -- non-sampling -- instrument warm-up period, 

and become dislodged during sampling. RI 9669 

discussed the loss of particulate in this portion of the 

instrument to be a part of the instruments overall 6.6% 

negative bias. It was not determined how much of this 

loss was cleaned from the instrument during each 

cleaning with compressed air between each day of 

sampling. The compressed air cleaning method in the 

laboratory used a laboratory dry and oil free compressed 

air sources or in the field used cans of compressed air. 

The cyclone grit pot and tapered element filter holder 

were removed from the instrument and compressed air 

was sprayed into all exposed openings. The grit pot was 

also cleaned by spraying with compressed air. 

On properly cleaned units, there remains a thin film 

of coal mine dust on the internal surfaces, so the 

possibility exists for some portion of that dust to end up 

on a subsequent sample, either as agglomerated oversize 

or as respirable dust. Experiments by Hinds (1982) show 

that particle attachment to surfaces is proportional to the 

particle diameter (d), while removal forces are 

proportional to d3

. This means that as particle size 

decreases, the force to remove those particles increases as 

the cube root. For practical purposes, particles less 

than10μm in size are not likely to be removed by 

common forces such a vibration or sampling air flows. 

However, if these particles accumulate to the point where 

a layer of 0.1 to 10 mm forms, then they can more readily 

be dislodged as agglomerates. Dust films observed on 

internal sample lines in properly cleaned units did not 

approach this deposit thickness. The NIOSH compressed 

air cleaning method is presumed to remove those thicker 

deposits that could be readily dislodged during sampling. 

2.4 Measurement of Particulates Released by Motion 

Forces 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The question of whether the act of wearing the PDM 

could generate sufficient forces to release oversize or 

respirable dust from a previously collected sample was 

also studied. An apparatus was constructed to 

continuously bend and flex the inlet tubing to the 

instrument while simultaneously raising and dropping one 

end of an upright PDM every 13 seconds for a period of 

eight hours. The height of the drop was about two inches 

onto the laboratory floor. The PDMs used for this test 

were returned from a field project where they each had 

been used for about 144 hours over a four week period. 

The PDMs were maintained by mine personnel and 

cleaned according to the latest NIOSH cleaning 

procedures. In the laboratory, groups of three PDMs at a 

time were tested due to weight limitations of the test 

apparatus. 

New PDM filters were used in this experiment. These 

filters were examined by an optical microscope using 

transmitted light at a magnification of 32X. A minimum 

of 30 optical fields were observed and particles greater 

than 20μm in size were counted as oversize particles. 

Filters counted in this way were called certified. Filters 

were then run in the PDM as described below and were 

post counted according to same procedures by the same 

analyst. Figure 2 shows some examples of filter pre and 

post counts. For each test, a high efficiency particle air 

(HEPA) filter was placed on the PDM inlet. The PDM 

was started and programmed to run for 8 hours. Optically 

certified filters were in place when the units were turned 

on and run during instruments normal 30 minute warmup. At 25 minutes after the start of the warm-up, filters 

were replaced with a second optically counted filter for 

the subsequent sampling period. The PDM self zeros at 

the end of the warm-up period and no mass that may have 

been deposited during warm-up is recorded. 

Pre-test PDM 

filt 

Post-test PDM 

filt 

20 μm diameter 

Figure 2. Typical pre and post count PDM filters with 

examples of oversize particulates observed. 

After the second certified sample filters were installed, 

the PDM was placed on the apparatus and the motion and 

bumping action was begun. At the end of the testing the 

filters were removed and post counted and the PDM data 

file was downloaded. Table 2 contains the results of pre 

and post optical counts of the number of oversize particles 

on the filters. The companion-PDM reported-mass value 

(MASSO TOTAL) was also recorded. The mass data 

indicated that two samples were close to the 0.022 mg 

limit of detection of the PDM. Unfortunately, two of 

PDM’s failed to record mass in this testing, but the optical 

data for this test was still valid. 

The conclusion from the data in Table 2 was that no 

statistically significant difference in oversize particle pre 

and post counts for the 8-hour sampling period was 

observed. There was also no quantifiable mass recorded 

by the PDM. The small mass gain observed is most likely 

a positive filter bias previously observed and reported 

Page et al. (2007). 

2.5 Anecdotal Testing 

At MSHA’s request, NIOSH examined one of their PDM 

field units, using the same procedures as above. The 

recent history of the unit involved at least 96 hours of 

testing in various District 7 mines, some of which 

operated diesel equipment. The warm-up filter contained 

no significant difference in particulates between pre and 

post test counts. 

The mass recorded during the sampling period of this 

test was 0.0084 mg, however, the optical counts of 

particulates greater than 20μm in size were significantly 

more. The pre-test count was nine particles in 34 fields 

versus a post-test count of 240 particles in 34 fields. 

Upon closer examination using variable focusing of the 

post-test particles, it was seen that the particulates were 

low density agglomerates that looked very similar to 

diesel particulate structures (Figure 3). Also of note was 

that the particulates were all about 40 to 55μm in size. 

Researchers conclude that the observed particles were 

large (greater than 40 μm), low density agglomerates that 

were likely released by the flexing of the inlet tube and 

that they were of sufficiently low density to penetrate the 

cyclone, but not to contribute in a substantial way to the 

mass on the filter. This is an anecdotal result and not 

much confidence is placed in the result or conclusion until 

it can be reliably repeated and the density of the 

particulates determined. 

Figure 3. Example type of particulate released from 

MSHA unit #116 during motion testing. 
 

 

 

 

 

 

 

 

Table 2. Results of PDM filters sampling HEPA filtered air and subject to 8-hours of continuous movement and bumping. 

 Pre-test Post-test 

Oversize Oversize MASSO 

TEST PDM # Filter # Fields particles Fields Particles TOTAL 

1 105 PT1AG 36 3 34 4 -0.011 

1 112 PT1AH 38 6 36 4 -0.007 

1 113 1

PT1AI 36 2 36 4 -

 

2 110 PT1AJ 34 18 36 16 -0.004 

2 126 PT1AK 34 2 36 4 0.006 

2 127 PT1AL 32 15 36 18 0.027 

 

3 115 2

PT1AM 32 2 38 10 0.003 

3 132 PT1AN 36 5 34 6 0.028 

3 133 3

PT1AO 32 9 38 9 -

 

 Average 0.006 1

 Damaged filter when removing from TE after the test. 

NOTE Flat line data 

 2

 PDM 115 stopped operating after 6 hours and 43 

minutes mass recorded to that point. 3

PDM133 data periodically reported "TE not detected" from 

11:42AM on. 

3 Cyclone Inversion 

Guidelines for using the current sampler with the DorrOliver cyclone have recommended that the cyclone, with 

close-coupled filter, not be rotated more than 120 degrees 

from vertical. This is to prevent the possibility of oversize 

particles already separated from the air and retained in the 

cyclone grit pot from falling back into the cyclone body 

and being deposited onto the filter. In reality, the 

centrifugal forces generated inside of the cyclone are 

much greater than the force of gravity and should prevent 

gravity from moving oversize particles from the grit pot 

onto the filter. 

The likelihood of cyclone dumping being a source of 

oversize in the current sampler is remote. Nevertheless, it 

is possible and the guidelines for not tipping the current 

cyclone are appropriate. A dumping event with the PDM 

is even more unlikely because the filter is remote and not 

close coupled to the cyclone and no direct path exists 

between the grit pot and the filter. Furthermore, the BGI 

cyclone grit pot is larger and contains a larger lip to 

prevent dumping. Finally, a significant dump of a cyclone 

would result in a rapid, step-change in the mass loading 

data file which could be readily detected in the data file. 

An experiment was conducted where 1 gram of 

monodisperse 20 μm glass spheres were placed into the 

grit pots of PDM and Coal Mine Dust Personal Sampling 

Unit (CMPDSU) Dorr-Oliver cyclones. The density of the 

spheres was 2.5 g/cm3 which results an equivalent 

aerodynamic diameter of 32 μm. The PDM, with an 

optically certified filter, was turned on and programmed 

to run for one hour. During sampling, the PDM was 

inverted along its longitudinal axis for 20 complete 

rotations in each direction. It was then inverted on its 

transverse axis for 20 complete rotations in each 

direction. At the end of the sample period the filters were 

optically examined for oversize particles. The same test 

was conducted for the CMPDSU with its sampling pump 

running. 

Results in Table 3 demonstrate that the PDM filters 

had some level of oversize contamination on the filters 

prior to the test, probably as a result of the way in which 

the filters are packaged. Post test counts, however, were 

not statistically different (p = 0.124) from the pretest 

counts. The MSA filters used in the CMPDSU were very 

clean and showed no oversize particles in either pre or 

post counts. In no cases were 20 μm spherical particles 

detected. 

4	 Observations of Oversize Particulate on Personal 

Samples Taken with A Prototype PDM 

Tests with six prototype PDMs by Volkwein et al. (2003) 

involved underground sampling with side-by-side 

gravimetric filter samples. Inlet shape, length of 

conductive tubing, and BGI-4CP cyclones, identical to the 

PDM configuration, were used to collect gravimetric filter 

samples in parallel with the PDM. Results showed no 

difference in mass collection between the two systems. 

Filters from the PDM, companion gravimetric filters, and 

blank filters from this testing were preserved and stored at 

NIOSH. 
 PDM FILTERS 

PRE-DUMP FILTER DATA POST-DUMP FILTER DATA 

 microscope microscope 

Filter Fields particles Filter fields Particles

PDM PDM 

filter filter 

#1 37 16 #1 36 12

filter filter 

#2 36 4 #2 36 9

filter filter 

#3 32 6 #3 36 5

filter filter 

#4 31 0 #4 38 3

filter filter 

#5 32 10 #5 36 15

SUM 168 36 182 44 

 

CMDPSU FILTERS 

PRE-DUMP FILTER DATA POST-DUMP FILTER DATA 

 microscope microscope 

Filter Fields particles Filter fields particles

MSA MSA 

#2345 62 0 #2345 76 0 

#3656 42 0 #3656 74 0 

#3657 54 0 #3657 72 0 

#3658 55 0 #3658 70 0 

#3659 55 0 #3659 74 0 

 268 0 366 0 

 

 

 

 

 

 

 

 

 

 

Table 3. Filter counts of dumping experiments with glass 

spheres

4.1 Prototype Filter Examination 

Filters from the prototype work were retrieved from 

storage and sent to the MSHA technical support 

laboratory for microscopic analysis of oversize particles 

similar to their standard methods. Optical counting 

techniques were used for the 37 mm gravimetric and 13 

mm PDM filters such that an equivalent surface area 

proportional to each filter diameter was used to count 

oversize particulates on the filters. Both blank control 

filters and filters worn by miners and NIOSH researchers 

were examined. Similar cleaning procedures were used 

for the prototype PDM testing. The prototype PDMs had 

about 200 hours of use with routine cleaning prior to this 

examination. 

4.2 Prototype Results 

Results of the optical examination of the prototype test 

filters for particulates greater than 20 μm are in Table 4. 

None of the gravimetric filters met the MSHA excessive 

oversize criteria of 30 particles in 10 fields. The PDM 

filters showed fewer oversize particles than the 

gravimetric samples. This could occur because the dust 

deposit on the smaller PDM filter must be thicker -- for an 

equivalent mass-- presenting the possibility that oversize 

may have been hidden. If this was true, one would expect 

to see a proportion of 5:32 (proportional to the counting 

areas of 10:64 for PDM vs 37 mm filters) of the number 

seen in the gravimetric filters. When data are normalized 

for the thickness of deposit, only sample M4 showed 

slightly more oversize on the PDM filter than the 

gravimetric. These filters were not expected to be used for 

this examination and no particulate counts were done on 

the filters prior to sampling. However, control filters show 

little contamination with oversize particulates and 

researchers have some confidence that the particles 

observed were a result of the sampling or post sampling 

handling process. The results in Table 4 indicate that no 

oversize particle contamination from the use and cleaning 

of the prototype PDM after 200 hours of use occurred. 

5 Significance of Cross-Sample Contamination 

Putting some perspective into the significance of filter 

contamination, filters were prepared with dust masses less 

than the minimum detection limit and microscopic 

photographs were taken of the filter deposits. To collect a 

deposit below the limit of detection, a known and steady 

dust concentration was created in the Marple chamber. 

Three PDM inlets were placed in the chamber while 

sampling with HEPA filters over the inlets. Using the 

concentration measured by a tapered element oscillating 

microbalance (Model 1400, TSI, Inc. Shoreview, MN) in 

the Marple chamber, the time required to deposit 

nominally 0.010, 0.020 and 0.030 μg of respirable coal 

dust onto the filters was calculated. The PDM HEPA inlet 

filters were removed and the PDMs sampled the 

atmosphere for the calculated time intervals before the 

HEPA filters were replaced. 

Figure 4 shows the overall appearance and Figure 5 

shows the microscopic appearance of the blank and 

exposed filters at various low level mass loadings. 

Comparing these microscopic images to the previous 

photographs of oversize and post cleaning filter results, 

we can conclude that a substantial amount of respirable 

dust is required to reach the 0.022 mg limit of PDM 

detection. This level of contamination was not observed 

in any of this testing. 

6 Conclusion 

The PDM represents a new way to determine personal 

respirable coal mine dust exposure. Many sources of error 

have been eliminated by the PDM warm-up procedure, 

automatically obtaining the tare weight of the filter, and 

measuring the final end-of-shift mass, all performed 

without human intervention. The current design of the 

PDM precludes comprehensive cleaning using detergent 

and ultrasonic techniques, commonly used with the 

CMPDSU. Alternative dry cleaning using compressed air 

has been developed and used in the NIOSH studies. 

Results presented in previously by Volkwein et al. 2006 

demonstrated that the units remained accurate after an 

average of 437 hours of testing using these dry cleaning 

procedures. The data presented above further indicate that 

the dry cleaning procedures do not cause significant 

(greater than limit of detection) 
sample contamination by respirable or oversize 

particulates and do not cause significant (greater than 

limit of detection) response to the final end-of-shift 

personal dust exposure of a miner. 

Oversize Companion 

Particles gravimetric 

Sample PDM filter >20 μm filter 

 No Count No 

Control 77 1 93 

Oversize 

Particles 

>20 μm 

Count

0 

Control 119 0 156 0 

M1 113 0 148 10 

M4 115 4 156 9

M1 71 1 83 7 

Ca 75 4 87 29

Sh 76 0 90 14

xx 99 0 121 11

Control 98 2 (f) 130 0 

Control 112 0 147 0 

P-1 85 0 103 5

P-2 96 1 116 26

P-3 111 3 143 

(f) = fibers 

PDM filters - 10 fields of 0 25mm2 

PVC filters - 64 fields of 0 25mm2 

21

 

Table 4. Prototype PDM and companion gravimetric filter 

oversize particle counts. 

Figure 4. Appearance of full PDM filter 

After sampling low mass loadings of coal dust. 

Blank 

Typically see 0.2 particles 

> 20um size per field 

viewed 

0.026 mg 

0.009 mg 

~40 um 

0.034 mg 

 

 

 

Figure 5. Microscopic appearance of showing the 

numbers of dust particles per area required to achieve 

the indicated mass. 

Acknowledgements 

The author thanks the MSHA Technical Support Staff for 

the oversize particle counting and providing one of their 

field PDM units for testing. 

References 

Hinds, W.C. Aerosol Technology, John Wiley & Sons, 

1982, p. 130. 

Page, S. J. D.P. Tuchman, R.P. Vinson. J. Environ. 

Monit., 2007, 9, 760 – 767 

Volkwein, J. C., R. P. Vinson, L. J. McWilliams, D. P. 

Tuchman, and S. E. Mischler, 2004. Performance of a 

New Personal Respirable Dust Monitor for Mine 

Use. DHHS, CDC RI 9663, pp 20 

Volkwein J. C., R. P. Vinson, S. J. Page, G. J. Joy, S. E. 

Mischler, D. P. Tuchman and L. J. McWilliams, 

Laboratory and Field Performance of a Personal 

Dust Monitor, US Department of Health and Human 

Services, Public Health Services, Centers for Disease 

Control, National Institute for Occupational Safety 

and Health, Report RI 9669, 2006, 47pp.