.

Steven J. Page1

Crystalline Silica Analysis: A Comparison of Calibration 

Materials and Recent Coal Mine Dust Size Distributions

ABSTRACT: Since 1982 standard calibration materials recommended for respirable crystalline silica 

analysis by the Mine Safety and Health Administration (MSHA) P7 Infrared Method and the National 

Institute for Occupational Safety and Health (NIOSH) X-ray Diffraction (XRD) Analytical Method 7500 

have undergone minor changes in size distribution. However, a critical assumption has been made that the 

crystalline silica in ambient mine atmosphere respirable dust samples has also remained essentially 

unchanged in particle size distribution. The objective of this work, therefore, is to compare recent 

particle size distributions of underground coal mine dust and the silica component of these dusts with 

estimated aerodynamic particle size distributions of calibration standard materials MIN-U-SIL 5, 

Berkeley 5, and SRM 1878 used by two crystalline silica analysis techniques. This work provides 

resolution to a previously reported discrepancy involving the proper sample dilution for the particle sizing 

method used. 

Dust impactor sampling data for various locations in 13 underground coal mines were collected 

between 1991 and 1998 and analyzed for the respirable mass median aerodynamic diameters. The data 

suggest that the MSHA P7 Method will underestimate the silica content of the sample by at most 8.5 % in 

the median size range of 0.9–3.6 μm and that it is unlikely one would obtain any significant error in the 

MSHA P7 Method analysis when the method uses Berkeley 5, MIN-U-SIL 5, or SRM 1878 as a

calibration standard material. 

The results suggest that the NIOSH Analytical Method 7500 would be more appropriate for a dust 

sample that is representative of the total (no cyclone classifier) rather than the respirable airborne dust, 

particularly since the mass fraction in the size range below 4 μm is usually a small percentage of the total 

airborne dust mass. However, the NIOSH Analytical Method 7500 is likely to underestimate the silica 

content of an airborne respirable dust sample by only 5–10 %. The results of this study also suggest that 

any changes that may have occurred in the median respirable size of airborne coal mine dust are not 

significant enough to cause any appreciable error in the current methods used for respirable crystalline 

silica analysis.

KEYWORDS:silica, coal, analysis, XRD, infrared, calibration materials

Introduction 

Crystalline silica (referred to hereafter as silica) dust has long been recognized as an

occupational health hazard to the respiratory system responsible for the insidious disease known 

as silicosis [1–6]. In underground mining this silica dust is created from several sources which 

generally include, but are not limited to, cutting, drilling, or milling of rock material. For coal 

mining this rock material may be found as an inherent constituent of the coal, although this is 

usually not a significant source of silica. The majority of the silica is found in the immediate 

overlaying or underlaying rock strata which are frequently removed during the coal mining 

process. Also, significant rock inclusions known as partings occur within the coal seam itself 

 

Manuscript received Mar. 11, 2004; accepted for publication Aug. 3, 2005; published Jan. 2006. Presented at ASTM 

Symposium on Silica: Sampling and Analysis on 22–23 April 2004 in Salt Lake City, UT.

1

 Research Physicist, National Institute for Occupational Safety & Health, Pittsburgh, PA.
and must be removed with the coal. Surface mining operations can also produce significant 

quantities of airborne respirable silica due to the amount of overburden which must be removed.

Monitoring the respirable coal mine dust exposure for compliance with Federal regulations is 

mandatory to protect the health of miners [7]. For all coal mines, if the silica content of the dust 

sample is measured to be 5 % or less, the respirable dust standard is 2.0 mg/m3. If the silica 

content exceeds 5 %, the respirable dust standard is reduced according to the formula 10 ÷ % 

silica [8]. For metal and nonmetal mines, defined as all non-coal producing mines, the respirable 

dust standard for respirable dust that contains silica (> 1 %) is calculated according to the 

formula 10 ÷ (% silica + 2). However, there is no technique available for the direct measurement 

of the respirable silica dust concentration or size distribution in the mining environment. The 

only means to estimate the respirable silica dust concentration is to perform an analysis of the 

respirable dust sample collected, which may contain both coal and silica dust as well as other 

minerals. 

Currently, the two fundamental techniques used for routine estimation of the silica content 

of a respirable dust sample are X-ray diffraction (XRD) and infrared (IR) absorption analysis. 

The techniques recommended by the National Institute for Occupational Safety and Health 

(NIOSH) are Method 7603 (crystalline silica in coal mine dust by IR) and Method 7500 

(crystalline silica by XRD) [9]. For determination of the silica content of respirable coal mine 

dust samples, the Mine Safety and Health Administration (MSHA) employs the MSHA P7 

Infrared Method, hereafter referred to as P7 [10,11]. The MSHA and NIOSH IR methods are 

similar but with some differences noted in the sample preparation. Airborne dust samples from 

metal and nonmetal mines are analyzed by MSHA using MSHA Method P2, which is nearly 

identical to NIOSH Method 7500.

The results of both XRD and IR techniques have long been recognized to be dependent on 

particle size [12–15]. However, these techniques are considered reasonably accurate if the silica 

particle size distribution of the sample approximates the size distribution of the calibration 

material for the methods and the XRD intensity peak area is used instead of intensity peak height 

[9]. The criteria under which Methods 7603, 7500, P2, and P7 were developed and implemented 

were closely scrutinized and impose strict limits on sample requirements and analysis protocol. 

Methods 7500 and P7 have evolved from the procedures recommended for silica determination 

of coal mine dust samples in 1982. At that time the product MIN-U-SIL 52 was recommended as 

the standard calibration material, based on this material’s particle size distribution closely

matching the silica particles found in respirable dust samples from ambient underground mine 

atmospheres [9,16]. 

Since 1982, the recommended standard calibration materials have undergone relatively minor 

changes in size distribution. However, a critical assumption has been made that the silica in 

ambient mine atmosphere respirable dust samples has also remained essentially unchanged in 

particle size distribution. This may not be true in some situations since the mining industry has 

undergone many changes in mining technology, production, and dust control methods. For 

example, at the time of P7 development and implementation only a small percentage of

continuous mining machines in coal mines were equipped with flooded bed scrubbers to collect 

the dust generated during mining. It is well documented that scrubbers are very efficient in 

capturing ambient dust particles down to 2 μm aerodynamic diameter [17,18]. Also, among the 

changes in mining technology is a significant increase in the power delivered by mining

machinery. Since mining frequently involves cutting the much harder rock material, which is 

 

2

 U.S. Silica Company, Berkeley Springs, WV. 
usually the major source of silica, it is possible that there has been a change in the amount of 

milling or grinding and an increase in the actual breakage of this rock material due to these more 

powerful machines. It would appear reasonable to suspect that some change could occur in the 

median size of the airborne respirable silica compared to the early 1980s when the MIN-U-SIL 5 

material was chosen as the best representation of airborne respirable coal mine silica dust [16].

The objective of this work, therefore, is to compare particle size distributions of respirable 

underground coal mine dust with estimated aerodynamic particle size distributions of calibration 

standard materials MIN-U-SIL 5, Berkeley 5, and SRM 1878 by two silica analysis techniques. 

This was accomplished by evaluating the particle size dependency of the MSHA P7 method and 

the NIOSH XRD method on size fractions of MIN-U-SIL 10. MIN-U-SIL 10 was chosen to 

provide a broad size range so that four size fractions could be obtained. This work provides 

resolution to a previously reported discrepancy involving the proper sample dilution for the 

particle sizing method used [19]. The discrepancy resulted in an incorrect choice for the sample 

dilution and the subsequent size dependency relationships for the analytical techniques used.

Materials, Methods, and Analysis

Materials

The work was carried out on size fractions of MIN-U-SIL 10 silica (reported by the 

manufacturer to be 99.5 % pure), as described and reported previously [19]. The calibration 

material used for P7 was Berkeley 5 silica, which has the same manufacturer specifications as 

MIN-U-SIL 5 but which is produced at a different plant, according to the manufacturer. It is 

noted that the P7 method employed by MSHA at the time of this work used MIN-U-SIL 5 for the 

calibration material. The P7 procedure used in this work was routinely validated by comparison 

with analyses performed by MSHA and no discrepancies were ever found. The calibration 

material for XRD was SRM 1878 produced from MIN-U-SIL 5 by the former National Bureau 

of Standards and distinct from the currently available and recommended SRM 1878a, prepared 

by the National Institute for Standards and Technology (NIST).

 

Particle Sizing

Size distribution based on spherical equivalent diameter (dSED) was performed by an

independent laboratory. The size-classified bulk powder silica samples were prepared for size 

distribution analysis using two different dilutions. The first dilution dispersed the powder in IPA 

to achieve a suspension concentration of approximately 700 mg/L and stirring for 5 min, 

sonicating in a 200 Watt ultrasonic bath for 10 min, followed by re-stirring. The second dilution 

used the same low concentration suspensions that were produced for P7 analysis of the size 

fractions, which were in the range of 31–38 mg/L, followed by the sonication and stirring

process. To examine possible sizing variations due to suspension concentration, the silica 

sample material in the largest size fraction was analyzed for size over a suspension range of 45–

714 mg/L. This was achieved by subsequent dilutions of the 714 mg/L suspension. Analysis of 

all samples was performed on a Horiba3 LA-920 particle size analyzer which classifies the 

particles into 68 size intervals below dSED = 200 μm. For each sample the median dSED was 

calculated. 

 

3

 Horiba Instruments, Irvine, CA.
Two groups of gravimetric size distribution data were also reported previously [19]. Group 1 

consists of 83 underground samples from locations on 13 continuous mining and longwall 

mining operations. Group 2 size distribution data consists of 29 samples from underground 

continuous mining sections as well as from rock material fed to a double roll crusher in a 

laboratory wind tunnel.

 

Analysis

Two measures of individual particle size have been used in this study. These are the 

spherical equivalent diameter (dSED), reported by the optical particle size analyzer, and the mass 

equivalent aerodynamic diameter (dAED), measured by an aerosol impactor. Since dAED is chosen 

as the relevant measure of particle size for aerosols sampled by aerodynamic properties, it is 

necessary to establish a reasonably accurate means to convert values of dSED measured for the 

size fractions obtained from MIN-U-SIL 10. According to Baron and Willeke [20], it is possible 

to estimate dAED from the dSED . This conversion has been previously described [19]. In the 

present work, gravimetric measurement of the median dAED using an inertial impactor for the 

individual size fractions obtained from MIN-U-SIL 10 was not possible since insufficient 

material was available. However, gravimetric measurement of median dAED by an impactor and 

conversion to median dSED for the whole product of MIN-U-SIL 10 was performed. The 

assumption was made that this procedure applied to the median size of the distribution and was 

applicable to particles of that size and the individual size fraction distributions of MIN-U-SIL 10 

having median sizes (dAED)i and (dSED)i .

Prepared laboratory samples of size-classified bulk powder were analyzed for silica content 

by (1) P7 at the NIOSH Pittsburgh Research Laboratory using a PerkinElmer4 Spectum GX 

FTIR spectrometer and (2) the NIOSH XRD method at an independent laboratory participating 

in the Proficiency Analytical Testing (PAT) program and accredited by the American Industrial 

Hygiene Association. The P7 and XRD methods used were performed according to the method 

protocol as specified by MSHA and NIOSH, respectively, with the exception that samples 

prepared for P7 analysis were from size-classified bulk powder, as described earlier, instead of 

from an airborne respirable dust sample. XRD samples were prepared from the same sample 

deposit on which P7 analysis had been performed by low-temperature ashing of the deposit and 

substrate, followed by redeposition on a silver membrane filter. Silica reported by XRD was 

based on intensity peak area, as required by method 7500.

Group 2 of impactor samples were analyzed for silica by removing the dust collected on the 

individual stages and compositing into three groups. The se groups consisted of impactor stages 

1, 2, and 3 (21.3–9.8 μm); stages 4, 5, and 6 (6.0–1.55 μm); and stages 7, 8, and final (0.93–0.1 

μm). Each composite was analyzed for silica according to the NIOSH XRD method at an 

independent accredited PAT laboratory.

Results and DiscussionBParticle Sizing

The median value (dAED)10 for MIN-U-SIL 10 was determined to be 3.83 μm from an average 

of four measurements using Sierra Model 298 9-stage personal sampling impactors. The

appropriate median value (dSED)10 was determined to be 2.97 μm on the Horiba LA-920 particle 

 

4

 PerkinElmer, Wellesley, MA.
size analyzer. Using (dSED)10 = 2.97 μm yielded (dAED)10 = 3.89 μm, good agreement with the 

value obtained from the gravimetric determination.

Table 1 summarizes median size data for three materials used for calibration of the P7 and 

the XRD methods. The median dSED value of 1.70 μm for MIN-U-SIL 5 is that reported by the 

manufacturer. The median dSED values for Berkeley 5 (1.61 μm) and SRM 1878 (1.70 μm) were 

measured by the same technique as used for the analysis of the MIN-U-SIL 10 size fractions. 

These values are reported to show the relatively small differences in median size between the 

original calibration material MIN-U-SIL 5 and two other standard materials used for instrument 

calibration during these tests. Calculated values of median dAED for the calibration materials 

listed in Table 1 are reported.

TABLE 1—Calibration material median size data.

Calibration materials dSED

a

μm

dAED

b

μm

MIN-U-SIL 5c 1.70f2.23 

Berkeley-5

d

 1.61 2.12 

SRM 1878e 1.70 2.23 

 a Spherical equivalent diameter.

 b Aerodynamic equivalent diameter.

 c 1982 recommended calibration standard.

 d Calibration material used for P7 analysis in this work.

 e Calibration material used for XRD Method 7500 analysis in this work.

 f Value reported by manufacturer using sedimentation method.

Table 2 summarizes the fractiona ted MIN-U-SIL 10 particle size analysis results. Sample 

weight is the amount of size-classified material dispersed in isopropyl alcohol (IPA) to obtain the 

particle sizing dilution. The size fractions are categorized by median dSED . Size values have also

been expressed in terms of the median dAED as previously described. Each row represents an 

average particle size based on three measurements of that sample. Repetition for three different 

samples was performed to obtain the average values in the last two columns of Table 2. The 

coefficient of variation (CV) for repeatability for all median dSED reported in Table 2 was less 

than 0.5 %, based on three replicate measurements. 

The effect of particle sizing suspension concentration was also investigated. A series of 

suspension concentrations using the largest size fraction were generated. Beginning with an 

initial silica/isopropyl alcohol (IPA) concentration of 714 mg/L, four successive factor-of-two 

dilutions were prepared. Figure 1 shows the results which indicate a significant bias in the 

particle sizing due to sample concentration. It has been previously reported by this author that it 

was not known if this bias was due to an inherent suspension concentration dependency in the 

LA 920 sizing instrument or sample preparation for the dilution tests [19]. The previous work 

reasoned that, assuming no bias in sample preparation such as differential particle settling, the 

LA 920 would produce a positive size versus suspension concentration relationship on a purely 

statistical basis. This would occur due to too few of the larger particles being present in the dilute 

suspension for proper measurement, resulting in an underestimation of median particle size at the 

lower dilution concentrations. Therefore, the demonstrated bias would likely have been in

sample preparation.
TABLE 2—Summary particle sizing replication data for MIN-U-SIL 10 size fractions.

Sample

weight,

mg

Sizing

dilution,

mg/L

dSED 

a

average,

μm

Std.dev.

N=3,

μm

Pooled

Average,

μm

dAED

 b

average,

μm

0.333 33 6.90 0.019 ... ...

0.333 33 7.06 0.036 ... ...

0.333 33 7.02 0.026 6.99 9.12

0.333 33 6.82 0.004 ... ...

0.333 33 6.90 0.046 ... ...

0.333 33 6.90 0.022 6.87 8.96

0.328 33 4.16 0.006 ... ...

0.328 33 4.17 0.012 ... ...

0.328 33 4.05 0.010 4.13 5.40

0.328 33 4.04 0.004 ... ...

0.328 33 4.05 0.020 ... ...

0.328 33 4.07 0.009 4.05 5.29

0.383 38 2.12 0.002 ... ...

0.383 38 2.12 0.004 ... ...

0.383 38 2.12 0.003 2.12 2.78

0.383 38 2.02 0.003 ... ...

0.383 38 2.02 0.001 ... ...

0.383 38 2.07 0.006 2.04 2.68

0.307 31 1.26 0.001 ... ...

0.307 31 1.26 0.003 ... ...

0.307 31 1.24 0.001 1.25 1.65

0.307 31 1.24 0.004 ... ...

0.307 31 1.24 0.002 ... ...

0.307 31 1.23 0.003 1.24 1.64

 a Spherical equivalent diameter.

 b Aerodynamic equivalent diameter estimate based on average of particle shape factors 1.82 and 1.36.

However, re-examination of the data suggests that the bias is due to suspension concentration 

effects. Figure 1 also shows the relationship between suspension concentration and the measured 

laser transmission of the LA 920. The close parallel between the aerodynamic diameter and laser 

transmission data strongly suggests that increasing the suspension concentration produces

secondary scattering interpreted by the instrument as scattering from smaller particles which do 

not actually exist. This effect has previously been documented [21]. Therefore, only the data 

based on dilute silica/IPA samples will be presented as well as the result of these changes.
y = -0.0018x + 10.1760

R

2

 = 0.9948

8.6

8.8

9.0

9.2

9.4

9.6

9.8

10.0

10.2

0 200 400 600 800

Suspension Concentration, mg/L

Aerodynamic dia., 

mm

50

60

70

80

90

100

Laser transmission, %

Aerodynamic diameter

Laser transmission

FIG. 1—Suspension concentration effect on the reported median aerodynamic sizes.

Results and Discussion--P7 

Table 3 summarizes the MIN-U-SIL 10 size fraction analysis results. The average reported 

mass and standard deviations were determined from triplicate P7 measurements of each sample. 

Figure 2 shows the silica reported by P7 for three independent samples of each size fraction at 

the dilute silica suspensions used for particle sizing in the size range of 1.5–10 μm. The error 

bars for reported silica represent the standard deviation of nine measurements, comprised of 

triplicate measurements of three samples.

A cubic equation was chosen in this size range, based on previously published data by Foster 

and Walker [14], to fit the data with an adjusted R-squared value of 0.99. The present analysis 

closely mimics the results of Foster and Walker’s infrared technique as a function of particle size 

at 799 cm–1 wavenumber. It was reported by Dempster and Ritchie [22] that the absorbance 

decrease below approximately 1.5 μm is due to an amorphous quartz layer on the silica particles 

which becomes an appreciable volume percentage as the surface area-to-volume ratio increases 

at smaller particle sizes. Figure 2 also includes a function that represents a particle crystallinity 

percentage, based on a 0.03 μm amorphous layer on a 100 % crystalline silica particle. The 

combination of the curves in Fig. 2 provide the reader with some estimate of P7 performance at 

very small particle sizes.
TABLE 3—Summary data for P7 infrared and NIOSH 7500 XRD methods for silica on 

MIN-U-SIL 10 size fractions.

SILICA P7 REPLICATION X-RAY: PEAK AREA ANALYSIS ONLY 

Reported

mass 

average,

mg

Std. 

dev.

N=3,

mg

Silica

average,

percent

Silica

std. dev.,

percent

Reported

mass

average, 

mg

Reported

mass 

average,

mg

Silica

average,

percent

Silica

std. dev.,

percent

dAED

a

average,

μm

0.100 0.002 ... ... 0.301 ... ... ... ...

0.097 0.004 ... 0.357 ... ... ... ...

0.096 0.004 29.4 1.1 0.327 0.328 98.6 8.4 9.12

0.070 0.004 ... ... 0.333 ... ... ... ...

0.107 0.001 ... ... 0.345 ... ... ... ...

0.107 0.004 28.5 5.6 0.342 0.340 102.1 1.9 8.96

0.206 0.006 ... ... 0.285 ... ... ... ...

0.219 0.003 ... ... 0.291 ... ... ... ...

0.219 0.018 65.5 3.5 0.301 0.292 89.1 2.5 5.40

0.227 0.004 ... ... 0.334 ... ... ... ...

0.215 0.011 ... ... 0.313 ... ... ... ...

0.225 0.012 67.8 3.1 0.291 0.313 95.3 6.6 5.29

0.383 0.016 ... ... 0.375 ... ... ... ...

0.383 0.009 ... ... 0.351 ... ... ... ...

0.371 0.019 98.9 3.8 0.334 0.353 92.3 5.4 2.78

0.375 0.008 ... ... 0.360 ... ... ... ...

0.390 0.016 ... ... 0.371 ... ... ... ...

0.386 0.015 100.1 3.6 0.378 0.370 96.5 2.4 2.68

0.317 0.008 ... ... 0.261 ... ... ... ...

0.324 0.007 ... ... 0.283 ... ... ... ...

0.318 0.009 104.1 2.5 0.250 0.265 86.2 5.5 1.65

0.291 0.006 ... ... 0.260 ... ... ... ...

0.294 0.017 ... ... 0.273 ... ... ... ...

0.336 0.004 100.0 7.7 0.254 0.262 85.5 3.2 1.64

a

 Aerodynamic equivalent diameter estimate based on average of particle shape factors 1.82 and 1.36.

Table 3 shows that if the median dAED of the silica particles in dust samples is between 1.6 

and 2.8 μm, then P7 provides results which are reasonably accurate. Comparison of the median 

dAED for the calibration materials shows that they are in close agreement with the P7 size range of 

accuracy. However, it is seen from Table 3 and Fig. 2 that there is significant underestimation of 

the reported silica percentage for particle sizes greater than 4 μm. Previous research has shown 

that MIN-U-SIL 5, MIN-U-SIL 15, and MIN-U-SIL 30 have median dAED values of 1.16, 1.2, 

and 1.2 μm, respectively, after passing through a 10-mm nylon cyclone [9]. This result occurs 

because these materials all contain a significant amount of very small material below 5 μm and 
the cyclone is very effective at uniformly removing the larger, nonrespirable silica particles. 

This suggests that it is unlikely one would obtain any significant error in P7 analysis of an 

airborne respirable dust sample which passes a 10-mm cyclone.

y = 0.431x3 - 7.000x2 + 22.063x + 82.760

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8 9 10 11 12

Median Aerodynamic Diameter, mm

Reported Silica, %

0

20

40

60

80

100

120

Particle Crystallinity, %

Dilute Silica/IPA particle

sizing suspension; Y-error

bars = standard deviation

Calculated percent

crystallinity for a particle

with a 0.03 micrometer

amorphous surface layer

FIG. 2—Reported silica as determined by the MSHA P7 Method.

Results and Discussion BXRD Method

Table 3 and Fig. 3 summarize the XRD analysis on the same MIN-U-SIL 10 size fractions 

which were prepared and analyzed by P7. When compared to Fig. 2, it is immediately obvious 

in Fig. 3 that XRD yields much less error than P7 in sizes above 4 μm median dAED . However, it 

is seen from Table 3 and Fig. 3 that there is some underestimation of the reported silica 

percentage for particle sizes smaller than approximately 2 μm. Although XRD is affected less by 

particle size than P7 over the size range less than 10 μm, it would appear that this method may 

underestimate the silica content of an airborne respirable dust sample which passes a 10-mm 

cyclone. Comparison of the median dAED for the calibration materials shows that they appear to 

fall in the region where XRD shows some depressed response. The observed decreasing

response seen in Fig. 3 is known to occur with XRD analysis [12,23,24]. The work of Edmonds 

et al. [23] showed the XRD intensity falling to nearly zero in the size range of 1–2 μm for MINU-SIL 15 material, possibly indicating a significant fraction of noncrystalline material in this size 

range. However, Fig. 3 suggests that the MIN-U-SIL 10 material fractionated in the present 

study does not have a significant fraction of noncrystalline silica in the 1–2 μm size range. XRD 

analysis would appear to be most appropriate for a dust sample which is representative of the 
total airborne dust, particularly since the mass fraction in the size range below 4 μm is a small 

percentage of the total airborne dust mass.

y = 1.544x + 85.959

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8 9 10 11 12

MEDIAN SIZE, aerodynamic diam, mm

REPORTED SILICA, pct

Dilute silica/IPA particle sizing suspension

FIG. 3—NIOSH XRD Analytical Method 7500 results on same samples analyzed by MSHA 

P7 Method.

Results and DiscussionBImpactor Sampling Data

To investigate a potential change in median size of underground coal mine dust, 83 impactor 

samples (Group 1) were analyzed for the median dAED of the respirable fraction according to the 

former ACGIH definition and the individual median dAED values ranged from 0.9 to 3.6 μm 

[19,25–27]. Using the polynomial regression equation from Fig. 2, P 7 will underestimate the 

silica content of the sample by at most 8.5 % in the MMAD size range of 0.9–3.6 μm. For the 

underground respirable size distributions, XRD could underestimate the silica content by 12.7 %. 

Although it is not known what the median dAED of the silica component for these distributions is, 

the fact that these are aerodynamic estimates of the respirable fraction suggests that the median 

dAED of the silica component should be in approximately the same size range. 

Group 2 impactor samples (Table 4) were analyzed for total dust and silica content in three 

size ranges: R1 = 21.3–9.8 μm, R2 = 6.0–1.55 μm, and R3 = 0.93–0.1 μm. Eighteen of twentynine samples have a larger percentage of silica in the combined ranges of R2 and R3 compared to 

the total dust in these ranges. However, allowing for an accepted 8 % relative standard deviation 

error propagation for the silica determination in the combined ranges of R2 and R3, 23 of 29

samples have a larger percentage of silica compared to the total dust in these ranges. It is 

apparent that the largest contributor to this result is the percentage of silica in the size range of 

6.0–1.55 μm. Although the values of the median dAED of the dust sample and silica content are 

not known, it is possible to do a student’s t-test for paired two-sample means analysis on the 

values of (R2 + R3) in Table 4. The rationale for this choice is that the median dAED of a 
respirable dust sample is likely to be in this range. A two-tailed analysis yields tdata = 1.98 < 

tcritical = 2.05 with 28° of freedom at the 5 % significance level. These results suggest that the 

median dAED of the dust sample silica content is not statistically different from the median dAED

of the total dust sample.

 TABLE 4—Total dust and silica dust weight distribution for composited impactor stages.

Mine Sample Percent of total dust weight Percent of silica dust weight 

location in impactor μm size ranges in impactor μm size ranges

R1 R2 R3 (R2+R3) R1 R2 R3

a

(R2+R3)

21.3- 9.8 6.0-1.55 0.93- 0.1 6.0-0.1 21.3- 9.8 6.0-1.55 0.93- 0.1 6.0-0.1

N intake 71.8 17.0 11.2 28 79.8 18.0 2.3 20

return 89.1 7.1 3.7 11 83.6 14.7 1.7 16

return 64.3 25.9 9.8 36 33.8 64.4 1.8 66

lab crusher 76.0 22.3 1.7 24 66.3 32.7 1.1 34

lab crusher 65.1 30.8 4.1 35 55.3 43.7 1.0 45

lab crusher 43.0 45.6 11.4 57 31.8 59.9 8.3 68

O intake 34.2 20.4 45.3 66 24.8 33.1 42.1 75

return 50.3 38.5 11.2 50 50.7 41.9 7.5 49

return 49.8 45.7 4.5 50 52.1 44.0 3.9 48

return 60.2 34.6 5.2 40 55.6 37.7 6.7 44

lab crusher 61.2 38.8 0.0 39 60.5 39.5 0.0 40

lab crusher 63.6 36.4 0.0 36 61.1 38.9 0.0 39

P lab crusher 67.4 32.6 0.0 33 66.8 33.2 0.0 33

lab crusher 57.2 42.8 0.0 43 54.8 45.2 0.0 45

lab crusher 57.6 42.4 0.0 42 63.8 36.2 0.0 36

lab crusher 50.4 49.6 0.0 50 53.8 46.2 0.0 46

lab crusher 58.1 41.9 0.0 42 69.3 30.7 0.0 31

lab crusher 57.8 42.2 0.0 42 54.4 45.6 0.0 46

lab crusher 60.1 39.9 0.0 40 55.0 45.0 0.0 45

Q intake 39.3 46.4 14.3 61 28.1 56.1 15.8 72

return 38.2 30.6 31.2 62 19.5 58.5 22.0 81

return 38.3 38.3 23.3 62 39.2 51.2 9.6 61

return 40.6 43.0 16.4 59 31.8 59.9 8.3 68

lab crusher 57.2 38.3 4.5 43 56.1 42.1 1.8 44

lab crusher 64.6 31.6 3.8 35 68.8 29.6 1.6 31

lab crusher 61.3 35.6 3.1 39 61.2 36.7 2.1 39

R lab crusher 73.9 23.9 2.2 26 75.3 20.4 4.3 25

lab crusher 71.1 24.1 4.8 29 74.3 21.6 4.1 26

lab crusher 69.8 26.9 3.3 30 76.4 19.7 3.8 24

Note: 23/29 samples have (R2+R3)silica/(R2+R3)total > 1 after adjustment for an assumed 8 % relative standard 

deviation error propagation in the silica determination R2+R3.

a Values of R3 reported in Table 3 are 13 % higher than reported by laboratory analysis to correct for the 

underestimation at 0.5 μm midpoint.

A significant error could be introduced into P7 analysis of a respirable coal sample if the 

silica dust being sampled contains relatively few particles in the 1–2 μm size range. The result 

of this would be an increase in the median size of the cycloned material and it is reasonable to 

ask under what circumstances might this occur. Among the changes in coal mining technology 
is a significant increase in the power delivered by mining machinery to mine the coal. Since coal 

mining frequently involves cutting the much harder rock material, which is usually the source of 

silica, it is possible that there has been a decrease in the amount of grinding and an increase in 

the actual breakage of this rock material due to these more powerful machines. On this basis, it 

would appear reasonable to suspect some change in the median size of the airborne respirable 

silica compared to the early 1980s when the MIN-U-SIL 5 material was chosen as the best 

representation of airborne respirable silica dust in underground coal mines. However, the results 

of this work suggest that the median dAED of the respirable silica component of coal mine dust 

still remains within the size range in which P7 is calibrated. It must be pointed out that the P7 

results describe performance only for dust samples that have been classified according to a

respirable definition similar to the penetration of a 10 mm cyclone or, at a minimum, samples 

which have a median dAED in the size range mentioned. Therefore, application of P7 to impactor 

stage samples that possess 50 % cut points outside of the approximate size range of 0.9–3.6 μm 

is likely to produce significant underestimation errors due to the particle size sensitivity of P7.

Conclusions

The effect of particle size on the MSHA P7 Infrared Method and NIOSH XRD Analytical 

Method 7500 has been measured using size fractions of MIN-U-SIL 10 silica. Although the 

effect of particle size on these measurement techniques has been studied and presented

previously, the objective of this work was to compare particle size distributions of underground 

coal mine dust with the particle size dependency of these two silica analysis techniques using 

Berkeley 5 and SRM 1878 calibration standard materials. 

Impactor sampling data for various locations on continuous mining and longwall mining 

operations from 13 underground mines were analyzed for the respirable fraction and the resulting 

median dAED. The individual median dAED ranged from 0.9 to 3.6 μm. It appears that P7 will 

underestimate the silica content of the sample by at most 8.5 % in the median dAED size range of 

0.9–3.6 μm and that it is unlikely one would obtain any significant error in P7 analysis of an 

airborne respirable dust sample that passes a 10-mm cyclone when the method uses Berkeley 5, 

MIN-U-SIL 5, or SRM 1878 as a calibration standard material. 

It appears that XRD is more appropriate for a dust sample that is representative of the total 

rather than the respirable airborne dust, particularly since the mass fraction in the size range 

below 4 μm is a small percentage of the total airborne dust mass. However, the results of this 

study indicate that XRD may not be as suitable as P7 for respirable dust samples. Comparison of 

underground respirable size distributions indicates that XRD could underestimate the silica 

content by as much as 12.7 % if the respirable aerodynamic equivalent diameter is 0.9 μm and 

8.5 % if the respirable aerodynamic equivalent diameter is 3.6 μm. Since it is not common to 

find mine environments with respirable mass median aerodynamic diameters as low as 0.9 μm, 

the NIOSH Analytical Method 7500 is likely to underestimate the silica content of an airborne 

respirable dust sample by only 5–10 %. 

The results of this study suggest that any changes that may have occurred in the median 

respirable size of airborne coal mine dust are not enough to cause any appreciable error in the 

current methods used for respirable crystalline silica analysis. Although the crystalline purity of 

the standard calibration materials used is not known, the differences reported in this work cannot 

be attributed to a significant noncrystalline component.
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