Evaluation of the SKC® DPM cassette for monitoring diesel 

particulate matter in coal minest 

t Disclaimer: mention of a company name or product does not 

constitute endorsement by the Centers for Disease Control and 

Prevention. 

James D. Noll*a 

a US Department of Health and Human Services, Public Health Service, Centers for Disease 

Control and Prevention, National Institute for Occupational Safety and Health, Pittsburgh 

Research Lab, 626 Cochrans Mill Road, Pittsburgh, PA 15236, USA. 

E-mail: JIN1@cdc.gov; Fax: +1412-386-4917; Tel: +1 412-386-6828 

and Eileen Birchb 

b US Department of Health and Human Services, Public Health Service, Centers for Disease 

Control and Prevention, National Institute for Occupational Safety and Health, Division of 

Applied Research and Technology, 4676 Columbia Parkway, Cincinnati, OH 45226, USA. 

E-mail: MIB2@cdc.gov; Fax: +1 513-841-4500; Tel: +1 513-841-4298 

In a previous study, the efficacy of commercial and prototype impactors for sampling diesel particulate 

matter (DPM) in coal mines was investigated. Laboratory and field samples were collected on quartz-fiber 

filters and analyzed for organic and elemental carbon. Coal dust contributed a minimal amount of elemental 

carbon when commercial cascade impactors and prototype impactors, designed by the University of 

Minnesota (UMN) and the US Bureau of Mines (BOM), were used to collect submicrometer dust fractions. 

Other impactors were not as effective at excluding coal dust. The impactors evaluated in that study were 

either not commercially available or were multi-stage, expensive, and difficult to use for personal 

measurements. A commercial version of the BOM impactor, called the DPM Cassette, was recently 

introduced by SKC®. Tests were conducted to evaluate the performance of the DPM Cassette for 

measuring diesel-source elemental carbon in the presence of coal dust. Bituminous coals from three mines 

in two different coal provinces were examined. The dust particle diameters were small and the coal dust 

contained a high percentage of carbon, thereby giving a worst-case condition for non-anthracite coal mines. 

Results for the DPM Cassette were essentially identical to those obtained by the BOM impactors in a 3 previous study. At a respirable coal dust concentration of 5.46 mg m- , which is 3.8 times the regulatory 

limit, the DPM Cassette collected only 34 !lg m-3 of coal-source elemental carbon. 

Introduction 

Diesel exhaust has been linked to acute health effects 1-3 and is 

considered a potential human carcinogen or similar designation by several organizations.4-6 Underground miners are 

exposed to the highest levels of diesel exhaust in the United 

States. The Mine Safety and Health Administration (MSHA) 

has promulgated rules to reduce the levels of diesel particulate 

matter (DPM) in underground coal and metal/non-metal 

mines 7-9 

In metal/non-metal mines, the MSHA rule regulates the 

limit of personal exposure to DPM. DPM is measured by 

collecting air samples on quartz fiber filters and analyzing the 

filters for elemental carbon (EC) or total carbon (TC) by 

National Institute for Occupational Safety and Health 

(NIOSH) Method 50407 ,8,10 To avoid overloading with mineral dust and any potential interference of graphitic ore, a singlestage impactor having a cut point (diameter of particle 

collected with 50% collection efficacy) of 0.8 !lm at 1.7 L 

IH3 min-I is used to minimize (eliminate) ore dust collection. 

In underground coal mines, the MSHA rule does not regulate 

an exposure limit for DPM. A standard based on EC or TC 

was not considered practical because of coal dust interference. 

Previously, the Bureau of Mines (BOM) developed a sizeselective sampler (impactor) for determination of DPM mass 

concentrations in underground coal mines. I I The impactor cut 

point of 0.8 !lm at 1.7 L min-I, which is based on particle size 

distributions in underground coal mines, was chosen to provide optimal separation between diesel and coal particles. Most 

diesel particles have aerodynamic diameters less than I !lm, 

while most coal dust particles have aerodynamic diameters 

greater than I !lm. Thus, a size-selective sampler can be used to 

separate most of the DPM from coal dust. Based on particle 

mass, the BOM sampler was 90% effective in the exclusion of 

coal dust, so about 10% of the dust was collected by the sample 

filter. 

In a previous study,14 we examined the potential contribution of coal-source Ee. Commercial and prototype impactors, 

including the BOM impactor, were evaluated in field and 

laboratory studies. A commercial, multi-stage cascade impactor (Marple Series 290, Andersen Instruments, Inc., Smyrna, 

GA) having a cut point of 0.93 !lm effectively excluded coal 

dust. In various locations of five non-dieselized coal mines, the 

EC levels were negligible. Results obtained in laboratory tests 

of the BOM impactors (cut point = 0.8 !lm at 1.7 L min-I) 

were similar to those obtained with the commercial impactor. 

When the respirable coal dust concentration was 5 mg m- 3 

(total dust = 12 mg m- 3), the EC concentration was 31 !lg m- 3

. 

Other size-selective samplers did not exclude coal dust as 

efficiently. 

The previous study showed that DPM-EC could potentially 

be measured in the presence of coal dust by using a size 

selective sampler. However, the impactors evaluated in that 

study were either not commercially available or were multistage, expensive, and difficult to use for personal measurements. SKC®, Inc. (863 Valley View Road, Eighty-Four PA 

15330 USA) recently introduced a single stage size selective 

sampler, called the DPM Cassette, based on the original BOM 
impactor design. NIOSH and MSHA have tested the performance of the DPM Cassette for sampling DPM in underground metal/non-metal mines. In this environment, the device 

15 16 effectively collected DPM and excluded ore dUSt. . Based on 

these and other results, MSHA uses a submicron impactor (e.g. 

the DPM Cassette) for DPM compliance sampling in metal/ 

nonmetal mines. 

In the study reported in this paper, we evaluated the 

performance of the DPM Cassette for monitoring DPM (as 

EC) in coal mines. We anticipated performance similar to the 

BOM impactors because the DPM Cassette design is based on 

this design. For our investigations, we first looked at characteristics of coal dusts in mines across the country to determine 

what coal types present the worst-case scenario relative to 

interference. After reviewing the available information, we 

selected a small diameter dust with high carbon content. 

Various quantities of coal dust and DPM were introduced into 

a laboratory chamber to produce a variety of dust and DPM 

concentrations. The collected samples vvere analyzed by 

NIOSH Method 5040 to determine whether coal dust poses a 

significant interference in the measurement of diesel-source EC 

and total carbon (TC). Following this initial evaluation, two 

other coals were also examined. 

Methods 

Coal dust selection 

As mentioned above, properties of coal dusts from across the 

United States were first examined to determine what types 

would present the worst-case scenario. A coal dust with the 

finest particle size distribution and high carbon content would 

most likely pose the worst interference problem. Anthracite 

coals were not considered in this study because they are a very 

small portion of mining production in the United States and 

14 anthracite is not representative of other types of coal.

We examined the mass median diameter (MMD) and the 

geometric standard deviation (GSD) of different coal dusts in 

mines across the country to determine what particle size 

distribution would include the highest fraction of fine particles. 

The MMD is the particle diameter at which 50% (by mass) of 

the particles are less than that value. Finer coal dusts have a 

smaller MMD. The GSD is a measure of the broadness of the 

size distribution. The larger the GSD is, the broader the 

distribution. 

To describe the geographical location of a coal mine, a 

province location is sometimes given for that mine. For 

example, the eastern portion of the United States is considered 

the Eastern Province. The other provinces include: the middle 

or Interior Province, the mid-west or Rocky Mountain Province, the northern mid-west or Northern Great Plains Province, the Gulf Province (near the Gulf of Mexico), and the 

west or the Pacific Coast Province. 

A literature review and analysis of information from a 

database on mines provided some MMDs and GSDs for mines 

located in various provinces. Rubow et al. reported the MMD 

of coal dust in two mines as 7.2 flm. 17 Organiscak and Page 

measured the particle size distribution for three bituminous 

coal mines from different provinces. IS Two mines were located 

in the Eastern Province, and the other mine was located in the 

Rocky Mountain Province. The dust from one of the mines in 

the Eastern Province was 61 % fixed carbon and had a MMD 

of7.5 flm. Less than 10% of the particle mass was under I flm 

in aerodynamic diameter. Coal dust from the other mine in the 

Eastern Province was 65% fixed carbon and had a MMD of 

16.2 flm, with less than 5% under I flm. The dust from the 

mine in the Rocky Mountain Province was 48% fixed carbon. 

The MMD was 27.8 flm, and less than 5% of the dust was 

under 1 flm. 

Fig. 1 shows the MMDs and GSDs of coal dusts collected 

over several days in eleven mines across the country. 

30.00 

25.00 

20.00 

15.00 

1O.00·j ····......·........ ·..··.. ··,·,:;..;>;...............·:.. ·· 


5.00 +-.-.......,;;;~----.--i'r---I't--,.,------;1\:-;;::-;;,,-;:;;p;;:-1 

0.00 LrTT--,-,.-r-,.-,-~_--,--:-r---rT-,--.,-,.-.,-.--,r-r-,.---c...,--,-,-' 

5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 

number 

~MMD 

FiO'. 1 The mass median diameter (MMD) and geometric standard 

de~iation (GSD) for coal dusts at different locations in eleven mines. 

Data are from a NIOSH database. 

Data are 

from a NIOSH Pittsburgh Research Laboratory database. 

Taking all the information into account, we selected the Keystone Mineral Black 325BA (Keystone Fillier and Mfg. Co., 

Muncy, PAl bituminous coal dust, which has a MMD of about 

4.5 flm and a GSD of 3.3. By volume, about 10% of the 

particles have aerodynamic diameters less than I flm, and the 

coal dust is about 75% fixed carbon (information from specification sheet). The Keystone Black is a ground coal dust from 

a mine in the Eastern Province. It represents a worse case 

interference wise because it contains a high percentage of 

carbon (for a non-anthracite coal) and has a particle size 

distribution on the low end of the distribution range for coal 

mines across the United States. The smaller particle size 

presents a greater separation challenge to the DPM Cassettes, 

and the dust that does penetrate has a high, nonvolatile (fixed) 

carbon content. 

Laboratory chamber experiments 

Various concentrations (Table 1) of Keystone Black coal dust 

and DPM were sampled with DPM Cassettes and analyzed to 

determine how much EC is contributed by coal dust. 

Table 1 Targeted respirable dust and DPM-TC concentrations for 

each experiment 

Experiment Mine 

Respirable 

dust/(mg m- 3 DPM-TC!>lg m- 3 

X 0.2 50,400,800 

2 X 2 50, 200, 400, 800 

3 X 5 50, 200, 400, 800 

4 X 0.6 400 

5 X 1.2 500 

6 Y 2.9 100, 200, 400 

7 Z 1.5 200 

8 Z 1.3 200 

An 

aerosol chamber,15,19 capable of dispersing DPM and coal 

dust uniformly throughout its volume with a spatial variation 

ofless than 5%, was used for these evaluations. For each target 

coal dust level, the following procedure was followed. As 

shown in Table 2, twelve DPM Cassettes and three respirable 

dust gravimetric samplers were placed inside the chamber. 

Coal dust was introduced to the chamber by a fluidized bed 

aerosol generator. A tapered element oscillating microbalance 

(TEOM 1400A Mass Measurement Systems, Rupprecht and 

Patashnick Co., Inc., Albany, NY), which measures the ambIent particulate mass in real time, was used to monitor the coal 

dust concentration. Critical orifices were used to control the 
Samplers 

Set A: 3­

respirable 

gravimetric 

Sct B: 3­

SKC DPM 

Cassettes 

Set C: 3­

SKC DPM 

Cassettes 

Set D: 3­

SKC DPM 

Cassettes 

Set E: 3­

SKC DPM 

Cassettes 

Set F: 3­

SKC DPM 

Cassettes 

Set H: 3­

SKC DPM 

Cassettes 

Set I: 3­

SKC DPM 

Cassettes 

Status of pumps for sampler on off 

Experimental steps 

Step I: insert sets A through D 

in chamber 

Step 2: coal dust in chamber x x x x a Step 3: collect about 0.2 mg of x x x x 

respirable coal dust on filters 

Step 4: clear chamber, take out 

Sets A and B, insert sets E through I 

Step 5: DPM in chamber x x x x x x 

Step 6: collect about 200 ~gb x x x x x x 

of DPM on filter 

Step 7: collect about 490 ~g' x x x x x x 

of DPM on filter 

Step 8: collect about 975 ~gd x x x x x x 

of DPM on filter 

Step 9: clear and empty chamber 

flow rate through each of the samplers to 2.0 L min- 1 

Respirable coal dust was sampled until the target equivalent 

8-h time weighted average (8-h TWA) concentration was 

obtained using an estimation from the TEOM real time data. 

The 8-h TWA mass concentrations for coal and DPM samples 

were calculated using the following equation: 

. m x 1000 

TWA (l1g m- 3

) == 480' 

qx mill 

where m is the collected dust mass in I1g, q is the sample flow 

rate in L min-\ and 1000 is the conversion constant (1000 L 

per m3 air). 

Table 2 Steps used to collect coal dust, DPM, and mixed coal-DPM samples 

a 0.2 mg of respirable coal dust (TEOM) is equivalent to collecting about 0.2 mg m -3 respirable coal dust for 8 h. b 200 ~g of DPM (TEOM) is 

equivalent to collecting 167-200 ~g m-3 DPM-TC for 8 h depending on TC content (usually 80-100%) in the DPM. ' 490 ~g of DPM (TEOM) 

is equivalent to collecting 408-510 ~g m- 3 DPM-TC for 8 h depending on TC content (usually 80-100%) in the DPM. d 975 ~lg ofDPM (TEOM) is 

equivalent to collecting 812-1015 ~g m- 3 TC-DPM for 8 h depending on TC content (usually 80-100%) in the DPM. 

After collection of the coal dust samples, the dust was 

cleared from the chamber. Three (forming a triplicate sample) 

of the twelve DPM Cassettes and all three respirable gravimetric samplers were then removed. Nine DPM Cassettes that 

sampled coal dust only remained in the chamber and were later 

used to collect DPM. 

Nine unused DPM Cassettes were added to the chamber, 

and diesel exhaust from a Kubota engine with 80% load was 

introduced. 80% load was used since the DPM created by the 

engine with this load simulated the ECjTC ratios most common in underground metal/nonmetal mines 8 The pumps connected to three of the DPM Cassettes that had sampled coal 

dust only and three unused DPM Cassettes were started. DPM 

was sampled until the DPM mass (monitored by the TEOM) 

on the filter corresponded to a DPM-TC concentration (8-h 

TWA) of about 50 I1g m-3 At this point, the pumps for these 

six DPM Cassettes were shut off. Pumps connected to three of 

the six remaining DPM Cassettes that had sampled only coal 

dust (but had not been turned on yet) and three unused DPM 

Cassettes were then turned on. The six DPM Cassettes collected DPM until the mass of DPM collected corresponded to 

a DPM-TC concentration (8-h TWA) of approximately 400 I1g 

m- 3 The final six DPM cassettes (three exposed to coal and 

three unused) were used to collect DPM-TC concentrations 

(8-h TWA) of approximately 800 I1g m-3 

All DPM Cassettes were analyzed for TC and EC via 

NIOSH Method 5040. The quartz sample filters in the DPM 

Cassettes exposed to only coal dust were analyzed to determine 

the amount of coal dust TC and EC that had passed through 

the impactor to the filter. The DPM Cassettes that collected 

only DPM provided the TC and EC contributed by diesel 

exhaust, while those that collected both coal dust and DPM 

provided the TC and EC contributed jointly by coal and diesel. 

The difference between the carbon found with the DPM 

Cassettes that collected only DPM and those that collected 

both DPM and coal provided a measure of the coal-source TC 

and EC. The respirable dust samplers were used to determine 

(gravimetrically) the total respirable dust concentration. 

This procedure of loading several DPM cassettes with coal 

dust and then using the loaded cassettes to sample different 

concentrations of DPM was repeated for respirable coal and 

DPM-TC concentrations shown in Table 1. Some samples were 

collected at a flow rate of 1.7 L min-I, which is the flow rate at 

which the impactors were designed to operate. The two different flow rates (i.e., 1.7 or 2 L min-I) gave no significant 

differences in the impactor cut points (0.7 11m at 2 L min-I, 

0.8 11m at 1.7 L min-l) or carbon results. 

Through the experiments described above, we determined 

the amount of coal-source carbon collected by the DPM 

Cassette at different concentrations of respirable coal dust. 

The set of respirable coal dust concentrations generated provided a range around the current regulatory level for respirable 

coal dust (8-h TWA = 1.45 mg m-3, or 2 mg m- 3 as the MRE 

equivalent). (Note: an MRE equivalent is used by MSHA. It is 

calculated as 1.38 times the actual dust concentration found 

with a Dorr Oliver cyclone operated at 2 L min-l. The 

calculated value is an estimate of the respirable concentration 

that would be found by a horizontal elutriator.) The DPM 

concentrations provided a range around the current DPM-TC 

regulatory level of 400 I1g m-3 (8-h TWA). 

To provide an indication of the potential interference posed 

by coals from different regions, two other coals, one from Mine 

Y and one from Mine Z, also were examined. To simulate a 

certain size distribution and to obtain a variety of coal dust 

concentrations, the bulk ores were collected at the mines and 

crushed in a laboratory apparatus at the Pennsylvania State 

University (see Table 1). Mine Z is in the Eastern Province, 
while Mine Y is in the Rocky Mountain Province. Coal, DPM 

and mixed coal-DPM samples were again collected in the 

laboratory chamber as described above. 

The size distribution and carbon content of Mine Y coal dust 

were not known. The carbon content of Mine Z was also 

unknown. To determine the carbon content (TC and EC) of 

each dust, we collected respirable dust samples on quartz fiber 

filters along with respirable gravimetric samples on PVC filters 

when just coal dust was in the chamber. Carbon analyses 

(method 5040) were done on the quartz filters that collected 

the respirable dust. By knowing the respirable dust mass and 

the amount of carbon in that mass, the carbon content (TC and 

EC) of each coal dust was calculated. A Marple impactor also 

was operated in the dust chamber to determine the particle size 

distribution for Mine Y. 

U 

~ 

.l!i " 

.Q 

';/?, e 

160 

140 

120 

100 

80 

60 

40 

0 

0 200 400 600 800 1000 

Fig.2 Expected bias (%) in DPM-EC results due to submicrometer, 

coal-source EC. DPM Cassettes were used to sample respirable coal 

dust (Keystone Black) at two concentrations: 2.4 mg m- 3 and 5.4 mg m-3 The amount of EC collected on the DPM Cassette when exposed 

to each of the two dust concentrations was divided by a range ofDPMEC concentrations to estimate the error (%) that would be expected if 

thc DPM Cassette is used to monitor DPM-EC in mines with high coal 

dust levels. 

Carbon analysis 

The carbon content of the sampled aerosols was determined by 

NIOSH Method 5040,20 which employs a Sunset Laboratory 

OC-EC Carbon Analyzer (Sunset Laboratory, Inc., Forest 

Grove, OR), at the Pittsburgh Research Laboratory. The 

reported carbon concentrations are the average of the triplicate 

samples receiving simultaneous exposures. 

Gravimetric measurement 

Gravimetric samples, along with control filters, were desiccated 

and then equilibrated in a controlled environment (72 OF and 

50% relative humidity) before weighing. Balance precision was 

better than 5 !lg. 

The airborne concentration of coal dust and DPM aerosol 

was calculated by dividing the mass gained by the filter 

(representing the collected mass) by the total volume of air 

sampled by the pump. The reported gravimetric results are the 

average of triplicate measurements. 

Results and discussion 

In a previous study, the DPM Cassette was shown to be very 

efficient at DPM collection. ls In this work, we focused on the 

DPM Cassette's ability to exclude coal dust. The carbon in 

most coals is mainly organic carbon (OC), with the remainder 

being Ec. 14 (During the analysis process, some of the OC in 

coal dust forms EC, which is a process commonly referred to as 

charring. Correction for char is accomplished in the NIOSH 

Method 5040 by monitoring laser transmission through the 

filter.) Thus, coal dust interference is expected to be greater 

with TC (TC = OC + EC) determinations than with EC. 

Table 3 shows the EC and TC concentrations found by the 

DPM Cassettes at different concentrations of Mine X respirable dust. 

Table 3 EC from coal dust X on DPM cassettes 

Respirable dust 

concentration/mg m- 3 

Coal-EC collected 

on SKCDPM 3 cassette/Ilg mCoal-TC collected on SKCDPM 3 cassette!llg m0.21 4 17 

0.6 17 65 

1.27 22 115 

2.45 22 83 

5.46 34 125 

A significant amount ofTC was found on the DPM 

Cassette filters, even for dust concentrations that were half the 

regulated level. Because adsorbed OC vapor can collect on the 

sample filter and positively bias the particulate TC result, the 

DPM Cassette actually contains two filters that are stacked. In 

theory, the second filter is also exposed to the OC vapor but 

not the particulate carbon and can be used to correct the first 

filter for non-particulate OC. Thus, the TC results were 

corrected by subtracting the TC, which occurs as OC, found 

on the second quartz filter from the first filter TC value. 

In contrast to TC, only about 34 Ilg m-3 (8-h TWA) of EC 

was found at a high coal dust concentration (8-h TWA = 5.46 

mg m-3

, 3.8 times the regulated limit). These results are quite 

similar to those found with the BOM impactors in laboratory 

tests. 14 This level (34 Ilg m-3) would only be a significant 

interference when measuring very low DPM-EC concentrations. 

We can estimate the percent bias in the DPM-EC measurement at a given concentration (DPM-EC) through the following equation: 

Coal EC 

Estimated bias (% ) for DPM EC = DPM EC x 100 

where Coal EC is the coal-source EC concentration measured 

when the DPM Cassette was exposed only to coal dust at a 

certain concentration and DPM-EC is the DPM-EC concentration at which we want to estimate the percent bias. For 

example, if the amount of coal-source EC (22 Ilg m-3) found 

after exposing the DPM Cassette to 2.4 mg m-3 of Keystone 

Black respirable coal dust is divided by a range of DPM-EC 

concentrations (20-800 Ilg m- 3

) (Fig. 2), biases in the DPMEC due to coal-source EC can be calculated. 

As shown in Fig. 2, when measuring DPM-EC in the 

presence of respirable coal dust at concentrations over the 

regulatory limit for the worst case dust (Keystone Black) the 

bias does not become greater than 25% until DPM-EC concentrations are below 80 Ilg m- 3 

A similar observation was made when we looked at mixed 

coal-DPM samples. As described earlier, all DPM Cassettes 

were exposed to the same amount of DPM, but one set was 

also exposed to coal dust. The difference between the two sets 

should provide the amount of EC contributed by coal dust. 

For the mixed samples, the bias in the DPM-EC concentration 

was calculated as follows: 

B' (0;' )EC = ECDPM/coal - ECDPM x 100 JaS ° DPM ECDPM 

where ECDPM/coal is the average EC concentration for three 

DPM Cassettes exposed to both DPM and coal dust, and 

ECDPM is the average of three DPM Cassettes exposed to 

DPM only. 

As can be seen in Fig. 3, if the Keystone Black coal dust 

concentration was 5.46 mg m -3 (8-h TWA) or below, the bias 

introduced by the coal dust was less than 20% when ECDPM 
concentrations were greater than 200 ~g m-3 (8-h TWA). 

When ECDPM concentrations were above 100 ~g m- 3 (8-h 

TWA) in the presence of a respirable coal dust (Keystone 

Black) level of 2.45 mg m-3

, about 1.6 times the regulatory 

limit, the bias was less than 25%. At this dust level, the 

Keystone Black coal dust was a significant contributor to the 

ECDPM/coal concentration when the ECDPM concentration was 

about 50 ~g m-3 (Sh-TWA). Thus, in the absence of an 

independent measure of the DPM-EC, measurement of such 

a mixture (50 ~g m- 3 DPM-EC and 2.45 mg m-3 respirable 

Keystone coal dust) would falsely attribute the EC from coal 

as DPM-EC. 

90 

80 

70 

60 

u 

50 '" 

~ ~ 

40 .$'" 

,::, 

<l< 

30 

20 

10 

0 

-10 

54 

DPMEC 

0.22 0.6 1.27 5.46 I 5.46 5.46 5.46 

Respirable dust cone. (mg! m3 ) 

Fig.3 Percent bias in the DPM-EC results for mixed coal-DPM samples collected with DPM Cassettes. Data were collected over a range ofDPM 

and coal concentrations (sec text for details). Coal dust was Keystone Black. 

25~----------------------------------------------------------------~ 

Mine Y 

21 llg/nf DPM-EC 

20+--------------------------------------------------------------------4 

'" 15 ~ MineY ~ MineZ MineZ MineY 

U 71 llg/m 3 DPM-EC 

r-1 

204 ~g1nf DPM-EC 164 )J.g1nf DPM-EC ~ " 10 

5 

o 

1.52 	 1.34 2.89 2.89 2.89 

Respirable dust conc. (rug/m 3) 

Fig.4 Percent bias in the DPM-EC results for mixed eoal-DPM samples collected with the DPM Cassettes. Coal dusts were from Mines Y and Z 

(sec text for details). 
The Keystone Black coal dust is abnormally fine and contains a high percentage of carbon (MMD 4.5 ~lm and fixed 

carbon of 75%) and thus represents an industry worst case. 

Other dusts with lower carbon contents (and larger particles) 

are expected to pose less interference. When looking at two 

different dusts from Mines Y and Z, we saw only a very small 

EC contribution. Mine Y dust size was small (MMD around 

4 ).tm), but it had lower carbon content (44% TC, 1% EC) than 

Keystone Black dust. Mine Z had a MMD of about 5 ).tm and 

TC and EC contents of about 70% and 15-20%, respectively. 

Bias in the DPM-EC results for the DPM Cassettes that 

collected DPM plus Mine Y and Mine Z dusts are shown in 

Fig. 4. Neither coal dust from the two mines contributed 

significantly to the EC results at the concentrations of DPM 

and coal dust tested (at and above the regulatory limit). Dust 

from Mine Y did not contribute even at high dust concentrations (2 times the regulatory limit) and very low DPM concentrations. With a Mine Y respirable coal dust concentration 

of 2.89 mg m-~-3, the DPtv1-EC bias was less than 20% for a 

DPM-EC concentration of 21 ).tg m-3 . At the same coal dust 

level, the bias was less than 10% for DPM-EC concentrations 

of71 ).tg m- 3 and 135).tg m- 3 With an average concentration 

of coal dust from Mine Z of about lAO mg m-3 (8-h TWA) 

(close to the regulatory limit) and DPM-EC levels near 200 ).tg 

m- 3 (8-h TWA), no significant contribution of EC from Mine 

Z coal dust was observed. 

Conclusion 

When monitoring DPM in the presence of coal dust, the DPM 

Cassette behaved similarly to commercial and UMN/BOM 

prototype impactors evaluated previously. In laboratory tests, 

it prevented most of the coal dust from collecting on the filter, 

even at high dust concentrations. The EC contributed by coal 

dust was minor except when relatively low DPM concentrations were sampled in the presence of a relatively high concentration of a very fine coal dust having high EC content. This 

is not representative of most coal seams. Limited testing with 

more representative coal dusts indicates that lower concentrations of DPM could be more accurately measured at coal dust 

concentrations near the regulatory limit. More laboratory and 

field data are needed to definitively determine a minimum 

DPM-EC concentration that can be measured without significant interference of coal dust. 

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