Inaccuracy ofarea sampling for measuring 

the dust exposure ofmining machine 

operators in coal mines 

F.N. KISSELL AND H, K. SACKS 

F.N. Kissell and HK Sacks are both senior

scientists with the Pittsburgh Research,

Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA. 

Introd uction can be made to run continuously 

This study examines the accu- over long periods. rather than for 

racy of area sampling for measuring just an eight-hour shift. Area samthe dust exposure of mining ma- pling also relieves workers from the 

chine operators at coal mine work- nuisance of having to wear a saming faces. Area sampling refers to 

general air monitoring for measuring employee exposures. typically at 

some fixed location near the worker. 

The work was prompted by the 

development of a prospective new 

type of dust sampling instrument called the machinemounted continuous respirable dust monitor (MMCRDM). However. the MMCRDM is more than a new 

sampling instrument. It also changes the location where 

samples are collected. The current method of workingface compliance sampling for coal mine dust uses "personal sampling" equipment worn by workers. The 

MMCRDM. housed in a 73-kg (160-lb) hox. must be 

mounted in a fixed location. Thus. it can only be used for 

area sampling. 

Modern industrial hygiene practice has been to 

avoid area sampling and to sample airborne contamj,. 

nants using "personal sampling" eq uipment worn by 

workers. It is well known that personal samples provide 

more accurate results than area samples when the contaminant source is nearby. Near contaminant sources. the 

dilution air and the contaminants are not evenly mixed. 

Therefore. exposure measurements must be taken from 

the worker's hreathing zone to be accurate (Leidelet al.. 

1977). However. area sampling equipment can have 

more functionality because of the relaxed size and 

weight restrictions 'On the instrumentation. For example. 

an area-sampling instrument can be more sensitive and 

pling device for the entire work 

shift. Thus, depending on the level of 

inaccuracy introduced. it is possible 

to imagine circumstances where 

area sampling provides better results overall. 

Inaccuracy resulting from use of the MMCRDM has 

two sources. One is the MMCRDM itself (Kissell and 

Thimons. 2001 ).1 

'The results of MMCRDM measurements and the performance of the 

MMCRDM itself are being reported elsewhere. 

The other is the potentially more significant source of inaccuracy associated with the switch 

to area sampling. This study deals only with the inaccuracy resulting from the switch to area sampling. For this 

purpose. the authors compared breathing zone samples 

to area samples using "personal samplers" in both locations. The effort had two parts: The first part was a literature survey that extracted data from previous area 

sampling studies. The second part was the authors own 

study on area sampling at one continuous miner face and 

at one 10ngwall face. ' 

Abstract 

This study examines the accuracy of area sampling 

for measuring the dust exposure ofmining machine operators in coal mines. The specific objective of this research was to find locations where an area sampler 

might work better than earlier studies have indicated. 

The results show thatJixed-location area sampling can­ not accurately predict the dust exposure of a machine 

operator, even when the best fIXed location is sought, the 

fixed location is quite close to the operator and the bias 

due to the dust concentration gradient is corrected. Industrial hygienists have known for many years that area 

sampling is unsuitable for measuring air contaminant 

exposure . ., in the workplace. Near contaminant sources, 

the dilution air and the contaminants are not evenly 

mixed. Therefore, when workers are near contaminant 

sources, exposure measurements must be taken from the 

worker's breathing zone to be accurate. 

literature survey on area sampling 

The authors found 12 studies that are relevant to 

area sampling of dust in coal mine working faces: 

Listak et al. (1999) conducted the most recent study 

on area sampling. comparing fixed-location area sampies 
F.a._ 1 

Miner-bolter and loader showing 

operator and fixed-location sites. 

Key

® Operator site 

L3

• 

Fixed location site 

to operator breathing zone samples. Data at deep cut trations at three locations (headgate, support #50 and 

sections in five mines where remote operators controlled tailgate) along the face. 

the mining machines were gathered. Listak et a1. con- Kissell and Jankowski (1993) summarized several 

cluded that there was little predictive capability between studies conducted by Foster-Miller to reduce continuous 

the two locations. If the fixed-point dust level was 1.5 mg! miner dust. During this work. Foster-Miller measured 

m3• then the 95% confidence level predicted operator dust levels at the boom hinge point and in the operator 

breathing zone exposure could vary from zero to 2.6 mgl cab.The mean concentration ratio (hinge point: operator 

m3• cab) was 4.15, and the standard deviation was 1.85. 

In a study of nine longwalls, Sun et al. (1997) found Babbitt et al. (1990) obtained dust concentration 

no relationship between the dust concentration at the profiles around a longwall shearer during mining. The 

shearer operator and the concentration at the tailgate. profiles showed a strong gradient across the shearer. The 

However, Sun et a1. also described another study con- machine had a well-designed shearer-clearer system that 

ducted at one longwall in Australia. The Australian study functioned to hold the dust cloud against the face as it 

found a correlation between the shearer operator dust moved downwind from the shearer. When shield movers 

concentration and the average of the observed concen- worked within 15 m (50 ft) of the return side of the 

shearer. their dust levels were the 

same or less than those measured at

the shearer. 

Kelly et a1. (1990) measured

tracer gas gradients at a full-scale 

modellongwall shearer. Methane 

gas was released at the drums and 

the concentration at various locations around the shearer was measured. One location was at the 

headgate-side operator position and 

another was the zone downwind of

the headgate-side drum on the face

side of the shearer. Both the gradient and the variability in the gradient were high. The average ratio 

(face concentration/headgate operator concentration) in 32 tests was 66. 

with a standard deviation of 56. In 

addition, Kelly et a1. obtained a dust 

profile map of the headgate area

during mining. The dust level varied 

froin less than 0.5 mg/m3 to more

than 1.5 mg/m3 across the entry.

TABLE 1 

Results of tests conducted bv Divers et al. (1982). 

Shift 

Respirable dust 

concentration, mg/m3 

Cab operator 

Respirable dust 3 concentration, mg/m

Remote Ratio 

cab: remote 

1 	 4.34 0.14 31 

2 3.33 	 0.09 37 

3 1.95 	 0.08 24 

TABLE 2 

Results of the five published studies that provide enough 

information to calculate a mean concentration ratio. 

Published study No. of mines Mean 

ratio FlO RSB 

Listak et aI., 1999 5 3.07 0.59 

Kissell and Jankowski, 1993 5 4.15 0.45 

Kelly et al. 1990 Lab test 66 0.85 

Divers et al. 1982 1 30.7 0.21 

Kost and Saltsman, 1977 6 3.53 0.81 

Jayaraman et a1. (1987) studied

methods to reduce the dust level of

continuous miner operators who use 

"adio remote control to operate ma-
chines equipped with dust scrubbers. It was found that. 

while these scrubbers remove a large portion of the dust, 

correct positioning of the operator can realize additional 

gains. When the machine operator positioned himself 

directly in front of the blowing curtain, his dust exposure 

level was 0.2 mg/m3, When the operator moved away 

from the curtain and positioned himself next to the 

miner, his dust exposure level was 3.1 mg/m3. 

Peng and Chiang (1986) took dust measurements 

near the shearer during longwall mining. Depending on 

the cut direction and the operator position measured, the 

concentration over the shearer ranged from 18% to 71 % 

above the operator concentration. Peng and Chiang also 

obtained other data about the dust gradient along the 

face. In the study, the dust concentration in the walkway 

9 m(30 ft) downwind of the shearer varied from being 

similar to 12 times higher when compared to the walkway at the shearer. Changes in the shearer water sprays 

caused this large variation. 

Grayson and Peng (1984) conducted a regression 

analysis on the dust data from a longwall panel to predict 

the concentration at specific locations. It was found that 

by using "location on face" as a single independent variable (VI), dust level C in mg/m3 could be predicted as m 

C -0.2835 + 0.8106 VI, where the correlation coeffi- m 

cient r was equal to 0.8054, and ,:z equaled 0.65. The fitted model explained 65% of the total variation. 

However, to improve the predictive capabiJity, it was 

necessary to incorporate other variables such as condition of roof. method of cutting. cutting time and air quantity. 

In a study of six mines. Jankowski and Organiscak 

(1983) found that dust concentrations in the walkway 9 

in (30 ft) on the return side of the shearer varied from 

being similar to three times higher when compared to 

the walkway at the shearer. Jankowski and Babbitt 

(1986) got similar results in a laboratory study using 

tracer gas, finding that the gas concentration downwind 

of the shearer varied from being similar to ten times 

higher than the walkway concentration at the shearer. 

Divers et a1. (1982) conducted a three-shift dust 

study in a mine that used remote control to guide the 

miner.The mine also used a push-puB ventilation system 

with both blowing and exhaust curtains in the working 

place. The remote control operator positioned himself 3 

to 6 m (10 to 20 ft) behind the miner. Respirable dust 

samples were taken at the cab and at the remote control 

operator location. The results are shown in Table l. 

PIGURI2 ~:. 

: ~ . 'r-r-r-t-T..,....I'""T".,...,..,..,.....,.---y-,.............,,....,...,............,...,.......,,....,.."'P"'I'.......,...,.~.........,....,-t;'..... 

Longwall operator and 

fixed-location 

sites. 

Heodgo!e - end • hearer opero!or··:,;: I Stage loader .':/:1': 

. ~.: Fixed location site . ' .. 

TABLI3 

Federal #2 Mine results for the fixed-location sites and the personal sample in 

the cab. 

Concentration ration between: Shown as Mean ratio RSD 

FlO 

Fixed loc. cab/pers,. sample cab L 1/E1 0.865 0.27 

Fixed loc. right botter/pers. sample cab L2/E1 0.726 0.27 

Fixed .Ioc. left bolter/pers. sample cab L3/E1 1.22 0.46 

Fixed loc. right 10ader/pers. sample cab L4/E1 0.488 0.406 

Fixed loc. left loader/pars. sample cab L5/E1 0.429 0.438 

2 The concentration ratio is analogous to the bias as described by 

Kennedy et al. (1995). Bias Is the relative discrepancy between the mean of 

a distribution of measurements and the true concentration. In this paper. the 

conclusions are based on the assumption that all bias wi!! be corrected. Failure to correct bias leads to greater errors than described here. 

:} In dust sampling. the failure to correct area sampling bias invites ·stra· 

tegic sampliog"- that is, placement of dust samplers to achieve readings consistently higher or lower than the true exposure of the worker. 

4 It was assumed that the data cited in the literature. survey are normally 

distributed. In the studies at the Federal #2 and Baker mines. BatteUe concluded that the data from these mines was normally distributed. 

Rankin and Rodgers (1980) conducted dust concentration surveys at locations away from the working face 

in intake air. Dust concentration correlations between 

area sampling locations and personal sampling of the 

section boss and shuttle-car operator were found. How-
ever, Rankin and Rogers cautioned not to expect good 

correlations near working-face dust sources. because 

shift-to-shift coefficient of variation values there varied 

from 25% to 60%. 

Kost and Saltsman (1977) measured dust concentrations near continuous miner operators at eight sections 

in six different mines. It was concluded that the dust profiles showed a zone of high concentration near the face. 

separated from a zone of clean air. The boundary between these two zones was near the operator. which 

made the location of the sampler critical. Kost and 

Saltsman saw a large variation in the dust gradient from 

mine to mine. concluding that no single correction factor 

applies for all mines. Kost and Saltsman also saw a large 

variation in the concentration gradient from shift to shift 

in the same section, concluding that even a single-section 

correction factor is impractical. 

TABLE 4 

Federal #2 Mine results for the fixed-location sites and the remote operator 

location. 

Concentration ration between: Shown as Mean ratio RSD 

FlO 

Fixed loc. cab/pers. Sample L1/E5 2.49 0.285 

remote op. 

Fixed loc. right bolter/pers. sample L2/E5 2.07 0.299 

remote op. 

Fixed .Ioc. left bolter/pers. sample L3/E5 3.32 0.304 

remote op. 

Fixed loc. right loader/pers. sample L4/E5 1.33 0.243 

remote op. 

Fixed loc. left loader/pars. sample L5/E5 1.17 0.325 

remote op. 

Establishing an accuracy criterion 

The issue at hand is whether a measurement made at 

a "fixed location" (an area sample) will effectively show 

how much dust a worker. typically the operator of a mining machine. is breathing. Because the dust source is 

nearby. the average concentration at the two locations 

will obviouslv be different. In other words. there will be 

a gradient that can be represented by a ~'concentration 

ratio."lf the variability in this ratio is small. then a fixedlocation measurement can be used to show worker exposure when the concentration ratio is applied as a 

correction factor.:!_l ' 

For the literature survey results to be useful. one 

must convert those results to a common denominator 

and then compare them to wen-established measures of 

accuracy. Fortunately. five of the studies gave enough information to enable the authors to calculate a concentration ratio between the fixed location and the mining 

machine operator for each shift. These concentration ratios were then averaged over all shifts in the test. and the 

standard deviation calculated. The relative standard deviation (RSD) was calculated from the standard deviation and the mean concentration ratio. Then. from the 

RSD in this concentration ratio. the authors could determine whether the. accuracy criteria had been met. 

Two accuracy criteria. ±25% and ±50%. were used. 

These are analogous to the NIOSH instrumentation accuracy criterion (Kennedy et a1.. 1995) and the European 

Community standard for "screening measurements" 

(CEN. 1994). For normally distributed data~, 95% of the 

measurements fa1l within the range ±1.96s.where s is the 

standard deviation. For example. assuming that the mean 

is 100. for the criterion of ±25%. then 1.96s 25 or s 

12.7. Because the mean is 100. the standard deviation divided by the mean (called the relative standard deviation 

or RSD) is 0.127. Thus. the ±25% accuracy criterion is 

met at RSD =0.] 27 or less5• For the criterion6 of ±50%. 

2s = 50 or s 25. Hence, the RSD is 0.25. and the ±50% 

criterion is met at RSD 0.25 or less. 

Results from the literature survey 

The five published studies that provided enough information to calculate a mean concentration ratio between the fixed location and the mining machine 

operator (FlO) and to calculate a relative standard deviation for the concentration ratios are shown in Table 2. 

These values for RSD fail to meet the ±25% criterion. 

for which the RSD is 0.127. and. with only one exception 

(Divers et al.). they also fail to meet the 50% criterion. 

for which the RSD is 0.25. The average shift-weighted 

RSD for aU the mines studied (omitting the lab test) was 

0.58. 

Comprehensive study on area sampling in two mines 

The results from the literature survey showed that 

area sampling resulted in poor accuracy - that is. high 

RSD values. Therefore. between January and March 

1999. the authors conducted a comprehensive area sampling study in two mines to determi~e if they could find 

locations where area sampling might yield better results. 

For this two-mine study. the authors used the personal samplers normally employed for coal mine compliance sampling - that is. a to-mm Dorr-Oliver cyclone. a 

5 Assuming that all bias is corrected. 

6 25 is the value corresponding to 95.4% of the measurements. 

7A more complete statistical analysis of the area sampling studies at the 

Federal #2 and Baker mines was undertaken by Battelle Inc. under contract to 

NIOSH. Copies of this Battelle report are available from the authors. 

8 In this mine, the air on the longwall face moved from the tailgate to the 

headgate. and so the headgate-end shearer operator was the designated 

sampling position. 
37-mm filter and a Mine Safety Appliances CD. Elf-EScDrt flo.W-CDntrolled pump Dperated at 2 Llmin. 

Pump calibratiDn was checked before each sampling shift using a 

Gillibrato.r calibrato.r and a filter 

IDad. 

The fixed IDcatiDn sample (the 

surrogate fDr the MMCRDM) was a 

package Df three samplers, with the 

average cDncentratiDn Df the three 

designated as the fixed-locatiDn 

measurement. FDr the persDnal 

samples.. Dne sampler was used with 

the cyclDne attached to' the lapel Df 

the wDrker in the cDnventiDnal manner. MSHA pre- and pDstweighed 

the filters to' a precisiDn o.f 11 J,.lg in 

its autDmated sample-weighing facility (Ko.gut et a1.. 1997). One filter 

blank was established fDr each shift 

of samples. and filter pDst-weights 

were co.rrected fo.r weight changes 

in the blank. 

The data were analyzed using 

the same "cDncentratiDn ratio." approach emplo.yed in the literature 

survey. The authDrs adDpted7 this 

appro.ach to. a]]Dw a direct compariSDn to. the literature survey results. 

Federal "2 Mine. The authDrs 

cDnducted the Federal #2 study Dn a 

miner-bDlter sectiDn. Dust samples 

were co.llected for 11 shifts at five 

wDrker expDS1,lre sites and five Po.Ssible fixed-IDcatio.n sites fo.r the 

MMCRDM. Co.ncentratiDn ratio.s were calculated between each Df the five fixed-locatiDn sites. and the two. 

mDst prominent DperatDr persDnal-expDsure sites as 

shDwn in Fig. 1. These DperatDr expDsure sites were in 

the miner cab and at the IDcatio.n where the o.peratDr 

WDuid stand if the machine were being Dperated remDtelv. 

Table 3 sho.WS the results fDr the fixed-IDcatiDn sites 

and the persDnal sample in the cab. Table 4 ShDWS the 

results fDr the fixed-Io.catiDn siles and the perso.nal 

sample at the remDte Dperator IDcatiDn. All Df the RSD 

values fail to. meet the ±25% criteriDn. and all but one 

fail the ±50% criteriDn. 

Baker Mine. The authDrs conducted the Baker Mine 

study on a IDngwall sectiDn. Eleven shifts of dust samples 

were collected at six worker expDsure sites and at seven 

Po.ssible fixed-Io.catio.n sites fDr the MMCRDM. Co.ncentration ratiDs were calculated between the mo.st prominent wo.rker persDnal-exPo.sure site, the headgate-end 

shearer DperatDr'i (HGSO) and each Df the seven fixedlocation sites. as shDwn in Fig. 2. 

These fixed-Io.catio.n sites were at shield #42. shield 

#63, shield #84, Dn the shearer at the headgate end, on 

the shearer at the tailgate end and at two. places Dn the 

stagclDader. Results are shown in Table 5. All Df the 

RSD values fail to. meet bDth the ±25% and the ±50% 

criteria. The mDst surprising finding is the lack Df CDrrelatiDn between the persDnal sample o.n the headgate-side 

shearer operatDr and the fixed IDcatio.n o.n the headgate 

side Df the shearer (LlIEl). with an RSD value Df 0.503. 

The results frDm the Federal #2 and Baker mines 

were unfavorable. with an Dverall average RSD o.f 0.37. 

TABLE 5 


Baker Mine results. 

Concentration ratio between: Shown as Mean RSD

ratio FlO 

Fixed loc.shield #42/pers.sample 

HGSO L42/E1 0.798 0.380

Fixed loc.shield #63/pers.sample 

HGSO L63/E1 0.713 0.450

Fixed loc.shield #84/pers.sample 

HGSO L84/E1 0.490 0.302 

Fixed loc.shearer head gate end/pers. 

sample HGSO L 1/E1 1.138 0.503 

Fixed loc.shearer tailgate end/pers. 

sample HGSO L2/E1 1.015 0.591 

Fixed loc.#1 stage loader/pers.sample 

HGSO L3/E1 1.564 0.423 

Fixed loc.#2 stage loader/pers.sample 

HGSO L4/E1 1.574 0.378

Subset of data from samplers located 

within 760 mm (30 in.) of worker 

The literature survey and the tWD-mine study 

yielded unfavDrable results. TherefDre, the autho.rs reanalyzed all Df the data in the belief that samplers that 

were clo.ser to. each Dther might provide better co.rrelatiDn. The authDrs focused Dn a subset of two. cases in 

which the fixed 10catiDn was within 760 mm (30 in.) Df 

the machine Dperator. The literature survey and the tWDmine study each provided Dne case. 

Kost and Saltsman study Kost and Saltsman (1977) 

cDnducted a dust gradient study in six mines to assess the 

impact of mo.ving the sampler away from the continuDus 

miner o.peratDr. The dust cDncentratiDn measured at the 

operatDr's lapel and the dust cDncentratiDn measured 

elsewhere on the mining machine were co.mpared. In 

fo.ur Df the mines. KDst and Saltzman placed samplers Dn 

the rear PDSt Df the canDpy that cDvered the operatDr 

cab. These were the samplers clDsest to. the operatDr, and 

they were never mDre than 610 mm (24 in.) fro.m the 

sampler o.n the DperatDr's lapel. 

TABLES 

Kost and Saltsman study results. 

Mine Fixed No. of Mean ratio RSD 

location (F) shifts (FlO)

c right rear post 12 0.666 0.39

c left rear post 12 0.806 0.33 

o rear cab 10 0.690 0.38

E left rear post 8 0.934 0.60

E right rear post 8 1.07 0.58

F rear cab 8 0.956 0.28
Table 6 gives the mean concentration ratio for each 

mine and the std.dev.lmean or relative standard deviation (RSD). Every value fails both the ±25% and the 

±50% criteria. This is a surprising result considering that 

the samplers were within a few feet of each other. 

Federal #2 study. In the Federal #2 study, three data 

pairs represented cases where the operator sampler and 

the fixed-location samplers were very close to each 

other. These were as follows: the fixed-location measurement at the miner cab vs. a personal sample on the continuous miner operator who sits in the cab (LI/EI): the 

fixed location at the right bolter vs. the personal sample 

at the right bolter (L2/E2); and the fixed location at the 

left bolter vs. the personal sample at the left bolter (L3/ 

E3). The mean ratios and RSDs for these cases are 

shown in Table 7. These values also fail both the ±25% 

and the ±50% criteria. 

TABLE 7 

Federal #2 study results. 

Concentration ration between: Shown 

as 

Fixed loc. cab/pers, 

sample cab L1/E1 

Fixed loc. right botter/pers. 

sample right bolter L2/E2 

Fixed loc. left bolter/pers. 

sample left bolter L3/E3 

Mean RSD Distance 

ratio FlO 

0.865 0.27 18 in. 

0.859 0.32 30 in. 

1.00 0.39 30 in. 

The Federal #2 samplers were very close to each 

other. The fixed-location samplers at the cab (L1) were 

only 460 mm (18 in.) from the personal sampler worn by 

the operator in the cab (El). In addition, the fixed-location samplers at each bolter (L2 and L3) were only 760 

mm (30 in.) from the corresponding personal sampler 

(E2 and E3). Yet the RSDs for ratios LIIE1. L2/E2 and 

L3/E3 were 0.27.0.32 and 0.39, respectively. The average 

RSD for this two-mine subset was 0.39. These results 

show a surprisingly wide variation in dust levels between 

samplers located within a few feet of each other. 

Variance from the samplers 

The high RSD values for the fixed-location/personal 

sampler ratios warranted an analysis of the samplers 

themselves. In the two-mine study. each fixed location 

value was the average of a three- sampler package in 

which the cyclone inlets were only 75 to 125 mm (3 to 5 

in.) apart. In the Federal #2 Mine study, 11 shifts of dust. 

samples were taken at five fixed-location sites, for 55 values. 

In the Baker Mine study. 11 shifts of dust samples 

were taken at seven fixed-location sites, for 77 values. 

The mean sampler-to-sampler RSD for the 132 fixed-location values in both mines was 0.12. An RSD value of 

0.12 for samplers only 75- to 125-mm (3- to 5-in.) apart 

accounts for part of the poor correlation observed for 

samplers separated by greater distances. 

Conclusions 

The results of this work show that fixed-location 

area samples cannot predict the shift dust exposure of a 

machine operator. even if the best fixed location is 

sought. the fixed location is quite close to the operator 

and the bias due to the concentration gradient is corrected. 

The average RSD from the literature survey was 

0.58. From the two-mine comprehensive study. the value 

was 0.37, and. from the subset of area samplers within 

760 mm (30 in.) of the worker, the value was 0.39. A part 

of this variability is due to the samplers themselves. 

which had an RSD of 0.12.• 

Acknowledgments 

The authors gratefully acknowledge the important 

contribution of those who .collected the data for this 

analysis: NIOSH employees Bruce Cantrell. Calvin 

Garbowsky, Gregory Green, Thomas Mal. Shawn 

Vanderslice and Jeanne Zimmer~ MSHA employees 

Karl Ahlen. David Atchison. Ken Fields. Everett Gerbec 

and Brian Murphy. 

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