Determining the Spatial Variability of Personal Sampler 

Inlet Locations 

Robert Vinson,1 Jon Volkwein,1 and Linda McWilliams2 

1NIOSH, Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania 

2NIOSH, Surveillance and Research Support Branch, Pittsburgh, Pennsylvania 

This article examines the spatial variability of dust concentrations within a coal miner’s breathing zone and the 

impact of sampling location at the cap lamp, nose, and lapel. 

Tests were conducted in the National Institute for Safety and 

Health Pittsburgh Research Laboratory full-scale, continuous 

miner gallery using three prototype personal dust monitors 

(PDM). The dust masses detected by the PDMs were used to 

calculate the percentage difference of dust mass between the 

cap lamp and the nose and between the lapel and the nose. 

The calculated percentage differences of the masses ranged 

from plus 12% to minus 25%. Breathing zone tests were also 

conducted in four underground coal mines using the torso 

of a mannequin to simulate a miner. Coal mine dust was 

sampled with multi-cyclone sampling cans mounted directly 

in front of the mannequin near the cap lamp, nose, and lapel. 

These four coal mine tests found that the spatial variability of 

dust levels and imprecision of the current personal sampler 

is a greater influence than the sampler location within the 

breathing zone. However, a one-sample t-test of this data did 

find that the overall mean value of the cap lamp/nose ratio was 

not significantly different than 1 (p-value = 0.21). However, 

when applied to the overall mean value of the lapel/nose ratio 

there was a significant difference from 1 (p-value < .0001). 

This finding is important because the lapel has always been 

the sampling location for coal mine dust samples. But these 

results suggest that the cap location is slightly more indicative 

of what is breathed through the nose area. 

Keywords dust samplers, inlet location, respirable dust 

BACKGROUND 

Monitoring of personal respirable dust exposure is an 

important step in eliminating many dust-related occupational illness and diseases. The Federal Coal Mine Health 

and Safety Act of 1969, the predecessor for the Federal Mine 

Safety and Health Act of 1977, mandates that coal mine dust 

levels be monitored and controlled to below 2 mg/m3 for a 

working shift. To date, this monitoring process has relied on a 

coal mine dust personal sampler unit to collect a filter sample 

in the mine environment. The filter is then sent to a laboratory 

for analysis and the results returned to the miners days or 

weeks after the actual sample was taken. Current methods do 

not provide timely feedback to detect or correct excessively 

dusty conditions. In the Report of the Secretary of Labor’s 

Advisory Committee on the Elimination of Pneumoconiosis 

Among Coal Mine Workers,

(1) several recommendations call 

for the development of continuous respirable dust monitors to 

help protect workers’ health. 

To address these recommendations, the National Institute of 

Occupational Safety and Health (NIOSH) Pittsburgh Research 

Laboratory along with the Mine Safety and Health Administration (MSHA) issued a development contract to Rupprecht & 

Patashnick Co., Inc. (R&P, now the Thermo Fisher Scientific) 

to develop a one-piece, person wearable, respirable dust 

monitor. This unit is referred to as the Tapered Element 

Oscillating Microbalance (TEOM) Series 3600 Personal Dust 

Monitor, or PDM. The PDM is essentially a miniaturized 

TEOM that is based on proven technology of the TEOM Series 

1400a Ambient Particulate Monitor used worldwide by air 

monitoring organizations. 

The complete unit incorporates both the dust monitor and 

a miner’s cap lamp. Coal dust is drawn into the PDM through 

a brass inlet attached to the cap lamp (which mounts to the 

bill of the hard hat) and through a section of conductive 

tubing that is connected to the dust monitoring section of the 

PDM. The PDM then monitors respirable dust concentrations 

in near real-time with TEOM technology. Locating the PDM 

sampler inlet at the cap lamp raised questions about the loss of 

dust through the tubing and whether the cap lamp location is 

equally representative of a miners’ respirable dust exposure in 

comparison with the traditional lapel location. Previous work 

demonstrated that less than 2% of the respirable dust will be 

lost in the tube from the cap lamp to the PDM.(2) This work 

investigates the impact of inlet location. 

Industrial hygiene practice requires that personal exposure 

to atmospheric contaminants be measured in a worker’s 

personal breathing zone. The breathing zone has been variously 

identified as close to a person’s nose and mouth, within 30 cm 
(1 ft) of a person’s nose and mouth, or simply representative 

of the person’s exposure. MSHA currently defines acceptable 

sampling locations to be within 1 m (36 in) of the miner.(3) It 

is possible that the cap lamp location may be less indicative of 

exposure if dust generation occurs more in front of and below 

the worker’s shoulders. In fact, Guffey et. al.(4) suggest that 

when a gas emission source is at waist level, the lapel samples 

are greater than those at the nose or ear locations. Alternatively, 

in mining, it is also possible that bumping the roof with the 

cap lamp or liberating dust at the roof during roof supporting 

operations may cause more dust to be sampled at the cap lamp 

than at the lapel location. 

Cohen et al.(5) studied inlet location on a mannequin in a 

laboratory dust room exposed to three test aerosols: a radon 

aerosol (activity mean diameter of 0.15 μm), a magnetite 

aerosol (mass median aerodynamic diameter [MMAD] = 1.6 

μm), and an Arizona Road Dust aerosol (MMAD = 7.5 μm). 

Analysis of the average mean ratio of concentration at the lapel 

compared with that at the nose for all three aerosols was 0.98 ± 

0.01 and for the forehead the nose ratio was 1.01 ± 0.02. These 

findings showed that for uniformly dispersed aerosols, the bias 

is essentially equivalent for any of these sampling locations. 

Cohen et al.(5) further developed a field method to analyze 

for sampling location bias using three continuous reading, 

light-scattering aerosol monitors. Dust levels were recorded 

from positions at the forehead, nose, and lapel in a beryllium 

casting operation. Results were highly variable, and additional 

data were needed to apportion the source of the variability. 

This high variability is consistent with large spatial variability 

of dust concentrations found in mining and summarized by 

Kissell and Sacks.(6) 

The current study was conducted to better define the 

influence of sample inlet location in the underground coal mine 

environment. 

METHODS 

T wo methods were used to examine dust concentrations 

within the miner’s breathing zone. The first method 

used NIOSH’s PRL full-scale continuous miner gallery(7) 

and three prototype PDMs,(8) which were mounted onto a 

modified backpack frame, and the inlets were then positioned 

at the miner’s cap lamp, nose, and lapel. The test gallery 

was filled with coal dust at typical mine levels, and airflows 

were generated to simulate a continuous miner section. The 

researcher carried the PDMs and continuously walked in the 

mine gallery where miners would typically work. 

The second method used to examine the dust concentrations 

within the miner’s breathing zone, involved collecting gravimetric dust samples at the cap lamp, nose, and lapel in the 

working sections of four underground coal mines. The head 

and torso of a full-scale mannequin was used in place of a 

coal miner. The mannequin was mounted on a metal base that 

had a vertical shaft the same height as the mannequin mounted 

directly in front of the mannequin (Figure 1). 

FIGURE 1. Mannequin and multicyclone can setup used in coal 

mine tests 

Three multi-cyclone sampling cans were mounted on the 

vertical shaft at the cap lamp, nose, and lapel of the mannequin. 

Each multi-cyclone sampler is made up of a metal can 10.8 cm 

in diameter and 12.7 cm high with a lid having a 1.9 cm 

diameter hole in its center. Each can contains cyclones and 

filter cassettes of three coal mine dust personal sampling units 

(CMDPSU) (Figure 2). A CMDPSU is made up of a 10-mm 

FIGURE 2. Cutaway view of multicyclone sampling can 
nylon cyclone with a preweighed filter cassette connected to 

an MSA Elf Escort pump with a section of 0.64-cm flexible 

tubing. The flexible tubing passes through three holes in the 

bottom of the multi-cyclone sampler connecting the cyclones 

to MSA Elf Escort pumps. The MSA Elf Escort pumps were 

calibrated with a Gilibrator (Sensidyne Inc., Clearwater, Fla.) 

primary standard flow meter to 2.0 ± 1% L/min. Dust samples 

were collected on preweighed 37 mm diameter MSA filter 

cassettes, which are similar to the cassettes used for the federal 

dust sampling program. 

A single gravimetric personal sampler is not a precise 

sampling device when it is necessary to sample small differences between multiple samplers. To ensure that multiple 

samplers measure the same dust environment requires the 

canister approach to minimize the spatial variability between 

sampling inlets. The purpose of this work was to examine the 

spatial variability of dust concentrations within the miner’s 

breathing zone. The actual respirable dust concentration in the 

mine air is not of primary importance here. What is essential is 

that the three cyclones in each canister are sampling the same 

dust concentration. Each sampling canister acts as a miniature 

aerosol chamber. 

Placing three cyclones in the canister significantly reduced 

spatial sampling variability between the cyclones. Although 

the location and size of the canisters may have influenced 

the dust concentrations, the primary concern was the relative 

concentrations at each canister inlet. 

TEST PROCEDURE 

Full-Scale Gallery 

The full-scale surface test gallery simulates a continuous 

miner working section with the face 12.2 m deep, 5.5 m wide, 

and roughly 2.0 m high. The gallery contains a full-scale 

mockup of a continuous mining machine with a 0.9 m diameter 

cutting drum rotating at 50 rpm. The machine was positioned 

at the face. Exhaust curtain (set back 9.1 m from the face) 

ventilation was created in the laboratory by drawing air via a 

gallery fan through a curtain positioned on the left side of the 

test gallery. The machine used a water spray system consisting 

of 24 hollow cone sprays positioned above, below, and along 

the sides of the cutting head. 

Coal dust was introduced into the gallery at the miner’s 

cutting head via a compressed air/eductor system. The eductors 

mixed the coal dust with the compressed air to deliver the dust 

to the cutting head. One eductor discharged along the left front 

side of the cutting drum, while the other discharged along the 

right front of the drum. The rotating cutting drum mixed the 

dust with the ventilation airflow. 

Two series of six tests each were run in the mine gallery. 

The three PDMs were programmed to run throughout each 

test series and store dust mass and concentration sampling data 

every minute. Each test ran 30 min. Between the tests, air filters 

were inserted into the PDM sampling inlets for at least 5 min. 

With the filters on the inlets, the PDMs recorded zero increase 

in dust mass that, in addition to the time, gave a marker to the 

file that separated each set of stored PDM test data. 

Between each test, the PDM inlet locations were switched 

so that each of the three PDMs sampled at the three sampling 

locations. This was done to minimize any effect caused by 

difference in response between the PDMs. Two series of 6 

tests were conducted for a total of 12 tests. This meant that 

each PDM sampled at each sampling location for four of the 

tests. After each test series, the PDM files were downloaded 

into an Excel 2003 workbook for analysis. 

In both series of the gallery tests, PDM #3 gave a high 

response to the dust mass even though its inlet location was 

changed for each test. To improve the inter-sample precision of 

the PDM to detect smaller differences, data were normalized 

with a correction factor. The correction was applied to the 

PDM #3 data to match the response of the other two PDMs. 

Theoretically, by rotating each PDM equally to each sample 

location, the ratio between any pair of PDMs should be 1. This 

was not true for PDM #3 and, consequently, a correction factor 

was derived. This correction is the ratio of the PDM #3 slope 

to the average of the slopes of PDMs #1 and #2. For Series 

One tests the ratio was 0.881; and for Series Two tests the 

ratio was 0.781. The responses of PDM #3 were multiplied 

by these ratios for their respective test series. The corrected 

response curves of PDM #3 and the responses of PDMs #1 

and #2 were then used to find the dust masses that each PDM 

recorded for each test in both series. This was done for each 

test by simply taking the total mass reading at the beginning of 

a test and subtracting it from the total mass reading at the end 

of the same test. The resulting dust masses for each PDM at 

each location for every test are listed in Table I. This data was 

then used to calculate the differences in collected dust masses 

between the cap lamp and the nose and also between the lapel 

and nose. 

Four Coal Mines 

The mannequin/multi-cyclone sampling system was used 

to collect samples in two longwall sections on the downwind 

side of the machine called the tailgate return, on the back right 

side of a continuous mining machine, and on the front of a twin 

boom roof bolting machine. 

The NIOSH researchers together with mine personnel 

determined the actual placement of the sampling system to 

protect the equipment and to give a representative indication 

of a miner’s location. After the mannequin/multi-cyclone 

sampling system was secured to the mining equipment, the 

sampling pumps were turned on and the sampling time noted. 

The test time to collect an adequate dust sample was estimated 

by the researchers based on previous experience or, in some 

cases, use of a light scattering dust monitor to obtain sufficient 

mass of dust for analysis. At the end of the test, the time 

was noted and the sampling pumps turned off. The 37-mm 

filters with the collected dust masses were removed from their 

cyclones and stored. If time allowed, a new set of 37-mm filters 

was installed along with clean 10-mm cyclones, and another 

test conducted. 
TABLE I. PDM Responses for Each Gallery Test 

Cap Nose Lapel Ratio Ratio 

Test # (mg) (mg) (mg) Cap/Nose Lapel/Nose 

Test Series One 

1 0.429 0.469 0.444 0.916 0.948 

2 0.329 0.404 0.392 0.816 0.971 

3 0.198 0.263 0.274 0.754 1.043 

4 0.346 0.431 0.421 0.802 0.976 

5 0.334 0.423 0.457 0.789 1.079 

6 0.296 0.383 0.429 0.772 1.120 

avg 0.808 1.023 

std 0.057 0.068 

rsd 0.071 0.067 

Test Series Two 

1 0.360 0.386 0.406 0.933 1.053 

2 0.328 0.318 0.355 1.031 1.116 

3 0.390 0.421 0.398 0.926 0.944 

4 0.319 0.293 0.301 1.089 1.030 

5 0.302 0.314 0.293 0.962 0.933 

6 0.202 0.197 0.189 1.029 0.963 

avg 0.995 1.007 

std 0.065 0.072 

rsd 0.065 0.072 

The 37-mm filters were pre- and postweighed (along with 

three control filters) in the NIOSH PRL filter weighing room. 

The pre- and post filter weights, along with the sampling 

times, sampling pump flow rates, and control filter weights 

were input into an Excel 2003 workbook that calculated 

the average concentration, standard deviation, and relative 

standard deviation of the triplicate dust samples in the multicyclone sampling cans. The average concentrations from each 

multi-cyclone sampling can was then used to calculate the 

concentration differences and ratios between the cap lamp and 

the nose and also between the lapel and nose. 

RESULTS AND DISCUSSION 

Full-Scale Gallery 

In the NIOSH PRL test gallery, two series of six tests each 

were run. Differences between the two series include the use of 

different researchers wearing the apparatus and, in the second 

series, a longer length of time elapse between the initial dust 

generation and the test start time. 

The respirable masses in milligrams detected by each PDM 

at the cap, nose, and lapel are listed in Table I. The table also 

lists the mass ratios between the cap and nose and between the 

lapel and nose. The average standard deviation and the relative 

standard deviation (RSD) are listed at the bottom of each test 

series. 

We subjected the RSDs to the methodology developed 

by Kissell and Sacks.(6) They examined dust concentration 

ratios developed by dividing dust concentrations taken at fixed 

locations by dust concentrations taken on mining machine operators. They were looking for a possible correlation between 

the two sampling locations. This was similar to our objective. 

They used NIOSH’s accuracy criterion, which requires “that 

a method will give a result that is within ±25% of the true 

concentration with a probability of 0.95 for an individual 

observation.”(6,p.35) 

Ideally, if there is no difference in the nose or lapel sampling 

locations, all of the ratios in Table I have a numerical value 

of one. Applying NIOSH’s criteria, the ratios must be within 

1.25 and 0.75. For the measurements to fall within the range 

95% of the time, the measurements must fall within 1.96s 

where s is the standard deviation. When the mean value of 

the ratios is 1 and the standard deviation is 0.25, the maximum 

RSD is 0.217. All of the RSDs in Table I are less than 0.217, 

which means that both the cap and lapel sampling locations 

meet NIOSH’s sampling method criteria. Therefore, either the 

cap or lapel are valid sampling locations with respect to the 

nose area. 

Four Coal Mines 

The mannequin dust sampling system was used to collect 

297 dust samples from four coal mines. Twelve of the 297 

samples were rejected leaving 285 dust samples for analysis. Of 

the 12 samples that were rejected, the breakdown is as follows: 

The tubing was pulled off the sampling pump of two samples, 

the sampler pump battery was dead for three of the samples, a 

pre-weigh error was recorded on one of the samples, and six 

of the samples were deemed outliers by Grubb’s test,(9) which 

is a statistical procedure for detecting outlying observations. 

The valid dust samples were used to produce 33 breathing 

zone test comparisons. The mean gravimetric dust concentrations at each location and summary statistics by mine and 

overall are shown in Table II. In all cases of in-mine testing, 

the mean lapel concentration was lowest. The highest mean 

concentration varied between the nose and cap lamp location. 

Included in this table are the upper and lower 95% confidence 

limits of the mean concentrations and ratios. A histogram in 

Figure 3 shows the mean concentrations along with standard 

error bars. For the overall ratio data set with 33 samples, the cap 

lamp location was about 1.1% higher than the nose location, 

and the lapel location was about 5% less than the nose. 

A paired t-test was used to determine whether a difference 

existed between the cap/nose and lapel/nose ratios. One of the 

assumptions of this test is that the difference values (cap/nose 

minus lapel/nose) are normally distributed. The entire data 

set of 33 observations was evaluated for normality with the 

Shapiro-Wilk test. This test did not reject the null hypothesis 

that the distribution was normally distributed (p-value = 

0.1120); thus, the parametric t-test for related samples was 

used. Because multiple t-tests were employed, the level that 

was used to determine statistical significance was adjusted 

from .05 to .01 using a Bonferroni correction(10) (.05 divided 

by the number of comparisons). This correction was applied 

to control the probability of finding a significant difference 

when one does not really exist (Type I error). The results 
TABLE II. Summary Statistics of Four Coal Mine Studies 

95%Cl 

Mine Variable Mean Std. Error For Mean 

Longwall (A) Cap LampA 1.804 0.208 [1.312, 2.296] 

(n=8) NoseA 1.852 0.214 [1.346, 2.357] 

LapelA 1.705 0.185 [1.266, 2.143] 

Cap lamp/Nose 0.972 0.010 [0.948, 0.996] 

Lapel/Nose 0.930 0.020 [0.883, 0.977] 

Ratio Diff 0.042 0.019 [−0.004, 0.088] 

Continuous miner Cap LampA 1.142 0.109 [0.876, 1.408] 

(n=7) NoseA 1.169 0.113 [0.893, 1.445] 

LapelA 1.109 0.116 [0.825, 1.393] 

Cap Lamp/Nose 0.979 0.015 [0.943, 1.016] 

Lapel/Nose 0.942 0.014 [0.907, 0.977] 

Ratio Diff 0.038 0.024 [−0.022, 0.097] 

Bolter Cap LampA 1.573 0.369 [0.722, 2.425] 

(n=9) NoseA 1.483 0.318 [0.750, 2.217] 

LapelA 1.411 0.267 [0.796, 2.027] 

Cap Lamp/Nose 1.047 0.019 [1.003, 1.092] 

Lapel/Nose 0.976 0.019 [0.932, 0.020] 

Ratio Diff 0.071 0.030 [0.001, 0.141] 

Longwall (B) Cap LampA 7.475 0.861 [5.491, 9.460] 

(n=9) NoseA 7.219 0.835 [5.294, 9.143] 

LapelA 6.834 0.794 [5.002, 8.665] 

Cap Lamp/Nose 1.035 0.005 [1.023, 1.048] 

Lapel/Nose 0.947 0.012 [0.919, 0.975] 

Ratio Diff 0.088 0.012 [0.060, 0.117] 

Overall Cap LampA 3.147 0.533 [2.063, 4.232] 

n=33 NoseA 3.070 0.511 [2.030, 4.110] 

LapelA 2.897 0.483 [1.914, 3.881] 

Cap Lamp/Nose 1.011 0.009 [0.993, 1.029] 

Lapel/Nose 0.950 0.009 [0.932, 0.967] 

Ratio Diff 0.062 0.011 [0.038, 0.085] 

Amg/m3. 

of the paired t-tests are presented in Table III. The longwall dust concentrations measured at the lapel location. This finding 

(B) had a mean difference value (.088) that was significantly was supported by a one-sample t-test that was compared with 

different from zero. the overall mean ratios to 1 (see overall values at the bottom 

The cap lamp location will measure dust concentrations of Table II). This test showed that the overall mean value of 

that are closer to what the nose position measures than the the cap lamp/nose ratio (1.011) was not significantly different 

TABLE III. Results of Hypothesis Tests of Sampling Locations (tests conducted at four coal mines) 

Hypothesis: Mean Difference of Ratios = 0 

Mean Difference t-Value 95% CI 

Mine Count (Cap/Nose-Lapel/Nose) (Paired t-test) For Mean p-Value 

Longwall (A) 8 0.042 2.17 [−0.004, 0.0876] 0.07 

Continuous miner 7 0.038 1.53 [−0.022, 0.0972] 0.18 

Bolter 9 0.071 2.34 [0.0011, 0.1414] 0.047 

Longwall (B) 9 0.088 7.17 [0.0599, 0.1168] <.0001 

Total 33 0.062 5.39 [0.0383, 0.0849] <.0001 

Note: Tests conducted at four coal mines. 
FIGURE 3. Histograms of mean value dust concentrations with standard error bars for fourcoal mine tests 

than 1 (p-value = 0.21). However, for overall mean value of 

the lapel/nose ratio (0.950), there was a significant difference 

from 1 (p-value <.0001). Thus, in this study, the lapel sampled 

less dust than the nose position. 

Individual mine section results (Table II) showed that 

the roof bolter and longwall (B) sections had higher dust 

concentrations at the cap lamp location compared with the 

lapel location. The other two sections showed a somewhat 

smaller difference, but when all of the data were examined 

together (see Overall values in Table II), the cap lamp sampling 

location was about 2.5% higher than the nose sample location, 

and the lapel sample was about 5.6% low compared with the 

nose location. These differences are in the range of individual 

sampler precision of 5.1% determined by Kogut.(11) Because 

individual sampler precision and the differences measured in 

these mine studies are within the same range, historic dust 

exposure data taken at the lapel should be comparable to dust 

exposure data sampled at the cap lamp location. 

CONCLUSIONS 

The gallery testing of dust mass loadings at the cap lamp, 

nose, and lapel indicated that little difference in inlet 

locations could be determined beyond the spatial variability 

of the gallery. The calculated percentage differences of the 

masses differed at each location, but the percentage difference 

was less than plus or minus 25%. In field sampling situations, 

the spatial variability of dust levels and imprecision of the 

current personal sampler in taking an individual measurement 

is, in general, a greater influence than the change in breathing 

zone sampler location from lapel to cap lamp. The four coal 

mine studies and the mannequin/multi-cyclone system indicate 

that sampling at the cap lamp is a slightly better indicator of the 

dust concentration at the nose level than at the lapel. A sample 

inlet adjacent to the cap lamp is within the accepted definitions 

of the breathing zone. The gallery data did not clearly indicate 

a difference between positions. The mine data shows that there 

is no difference between the nose and cap lamp location, but 

that there is a difference between the cap lamp location and 

the lapel. The effect is small and within the precision of the 

CMPDSU. 

REFERENCES 

1.	 U.S. Department of Labor (DOL): Report of the Secretary of Labor’s 

Advisory Committee on the Elimination of Pneumoconiosis Among 

coal Mine Workers. Arlington, Va.: DOL, Mine Safety and Health 

Administration, 1996. 

2.	 Peters, T.M., and J.C. Volkwein: Analysis of sampling line bias on 

respirable mass measurement. Appl. Occup. Environ. Hyg. 18:458–465 

(2003). 

3. “Mandatory	 Health Standards–Underground Coal Mines,” Code of 

Federal Regulations Title 30, Part 70.207e. 2006. 

4.	 Guffey, S.E., M.E. Flanagan, and G. VanBelle: Air sampling at the 

chest and ear as representative of the breathing zone. AIHAJ 62:416–427 

(2001). 

5.	 Cohen, B.S., H.H. Harley, C.A. Martinelli, and M. Lippmann: 

Sampling artifacts in the breathing zone. In Aerosols in the Mining and 
Industrial Work Environments, Vol. 1. V.A. Marple and B.Y.H. Liu (eds.). 

Ann Arbor, Mich.: Ann Arbor Science, 1983. pp. 347–360. 

6.	 Kissell, F.N., and H.K. Sacks: Inaccuracy of area sampling for measuring 

the dust exposure of mining machine operators in coal mines. Mining 

Engineering 54(2):33–38 (2002). 

7.	 Goodman, G.V.R., and D.E. Pollock: Use of a directional spray system 

design to control respirable dust and face gas concentrations around a 

continuous mining machine. J. Occup. Environ. Hyg. 1:806–815 (2004). 

8.	 National Institute of Occupational Safety and Health (NIOSH): 

Performance of a New Personal Respirable Dust Monitor for Mine Use, 

by J.C. Volkwein, R.P. Vinson, L.J. McWilliams, D.P. Tuchman, and S.E. 

Mischler (Pub. No. 2004-151). Pittsburgh, Pa.: Department of Health and 

Human Services, Centers for Disease Control and Prevention, NIOSH, 

2004. 

9.	 Grubbs, F.E.: Procedures for detecting outlying observations in samples. 

Technometrics 11(1):1–21 (1969). 

10.	 Maxwell, S.E., and H.D. Delaney: Designing Experiments and Analyzing 

Data: A Model Comparison Perspective. Belmont, Calif.: Wadsworth, 

2000. pp. 171–174. 

11.	 Kogut, J., T.F. Tomb, P.S. Parobeck, A.J. Gero, and K.L. Suppers: 

Measurement precision with coal mine dust personal sampler. Appl. 

Occup. Environ. Hyg. 12:999–1006 (1997).