Coal Dust Particle Size Survey of U.S. Mines 

by Michael J. Sapko, Kenneth L. Cashdollar, and Gregory M. Green 

Pittsburgh Research Laboratory, 

National Institute for Occupational Safety and Health, 

Pittsburgh, PA, U.S.A. 

Harry C. Verakis 

Mine Safety and Health Administration 

Triadelphia, WV, U.S.A. 

Abstract 

The National Institute for Occupational Safety and Health (NIOSH) and the Mine Safety and 

Health Administration (MSHA) conducted a joint survey to determine the range of coal particle 

sizes found in dust samples collected from intake airways of U.S. coal mines. The last 

comprehensive survey of this type was performed in the 1920’s. The size of the coal dust is 

relevant to the amount of rock dust required to inert the coal dust, with more rock dust needed to 

inert finer sizes of coal dust. 

Dust samples were collected by MSHA inspectors from several mines in each of MSHA’s 

ten bituminous Coal Mine Health and Safety Districts. Samples were normally collected in 

several intakes at each mine. The laboratory analysis procedures included acid leaching of the 

sample to remove the limestone rock dust, sonic sieving to determine the dust size, and low 

temperature ashing of the sieved fractions to correct for any remaining incombustible matter. 

The results indicate that particle sizes of mine coal dust in intake airways are finer than those 

measured in the 1920’s. The significance of these finer coal dust sizes is discussed relevant to 

the amount of rock dust required to prevent coal dust explosions. 

Disclaimer: The findings and conclusions in this report are those of the authors and do not necessarily 

represent the views of the National Institute for Occupational Safety and Health or the Mine Safety and 

Health Administration. 

1. Introduction 

Despite the worldwide research on coal mine safety, coal mine explosions involving 

fatalities and injuries still occur. Experimental studies by the Pittsburgh Research Laboratory1 

1

 The Pittsburgh Research Laboratory was part of the U.S. Bureau of Mines until 1996, when it was transferred to 

the National Institute for Occupational Safety and Health (NIOSH). 

(PRL) and similar agencies in other countries have shown that mixing a sufficient quantity of 

inert rock dust with coal dust will prevent coal dust explosions by acting as a heat sink. The U.S. 

requirement specified in Title 30, Code of Federal Regulations, Section 75.403 mandates that the 

nation's coal mines maintain an incombustible content of at least 65 percent in the non-return 

(intake) airways and at least 80 percent in the return airways where the potential for 

accumulation of finer float coal dust is greater. The U.S. regulation law also requires an 

additional 1.0% incombustible by weight for each 0.1 % methane in the ventilating air in intakes 

and 0.4% additional incombustible for each 0.1% methane in returns. 

 
The 65% total incombustible content required for intakes was based on the measured size 

of coal dust found in mines during the 1920’s and the amount of rock dust required to inert that 

size of coal in full-scale experimental mine tests (Nagy, 1981). The term “mine size coal” was 

adopted in about 1925 and refers to coal dust, all of which passes a U.S. Standard No.20 sieve 

(850 μm) and contains 20% minus 200-mesh (75 μm). The justification for adopting it is given 

in Bureau of Mines Technical Paper (TP) 464 (Rice and Greenwald, 1929). In the 1920’s, 

representative dust samples were collected from mine passageways that were not rock-dusted, 

and then they were sized using sieves. TP 464 states that these coal dust samples collected from 

the mine floors had 5% to 40% of the material less than 200 mesh. TP 464 further indicates says 

that the values were weighted as far as possible, and for 80% of the mines, the final values 

ranged from 15% to 25% through 200 mesh. Therefore, dust having 20% through 200 mesh was 

used in the experimental mine tests that determined the current 65% total incombustible 

requirement for intake roadways (Rice and Greenwald, 1929 and Nagy, 1981). TP 464 states 

that dust collected from ribs, roof, and timbers was finer in size, with 40% to 75% finer than 200 

mesh. TP 464 does not give any additional details on the total number of mines surveyed or the 

total number of samples analyzed for coal particle size. The 80% incombustible content for 

return airways is based on the finer float coal dust that may be deposited there. Nagy (1981) 

defines float coal as dust that is finer than 200 mesh. 

To comply with regulations, mine personnel periodically dust the mine intake and return 

airways with an inert material, such as pulverized limestone (rock) dust. The term “inert” in this 

sense means that the material does not support combustion. In determining compliance with the 

regulation, inspectors for the Mine Safety and Health Administration (MSHA) periodically 

collect samples of deposited dust from different areas in a mine. When samples are collected in 

any given mine, they are usually collected at 500-ft intervals along an entry. Generally, a band 

sample is collected, which includes dust from the floor, ribs (walls), and roof at each location in 

the mine. The inspector then screens the sample through a 10-mesh sieve while in the mine, 

bags the sample and sends it to MSHA's laboratory at Mt. Hope, West Virginia for determination 

of the incombustible content. The fineness of the coal dust component is not measured and 

therefore not specifically considered in assessing the level of dust explosion protection afforded 

by the 65% inert requirement for intake airways. 

The results are presented of a recent coal dust particle size survey to determine the range 

of coal particles sizes found in dust samples collected from intake airways from 47 U.S. coal 

mines in MSHA’s ten bituminous coal mine health and safety districts (see figure 1). (MSHA 

District 1 covers anthracite mines in Pennsylvania, which do not require rock dusting.) This 

research is relevant to the amount of rock dust needed to prevent coal dust explosions under 

current and changing mining operations. 

2. Coal particle size effects 

Since the amount of rock dust required to inert a mixture varies with coal particle size 

(Rice et al., 1922; Rice and Greenwald, 1929; Nagy, 1981; Cashdollar and Hertzberg, 1989; 

Weiss, Greninger, and Sapko, 1989), the measurement of the incombustible concentration 

without considering the effect of coal particle size is not, by itself, sufficient to determine the 

possible explosion hazard. The effect of coal particle size on the explosibility is best illustrated 

in figure 2, which shows the incombustible required to prevent explosion propagation in largescale dust explosion experiments conducted in the NIOSH-PRL Bruceton and Lake Lynn 

Experimental Mines. The graph shows the amount of incombustible required to prevent 

 
propagation for Pittsburgh high volatile bituminous coal dust containing 10 to 75 % of particles 

passing a 200 mesh sieve (<75 μm). The dashed line shows the amount to inert, based on the 

BEM data (Rice et al., 1922; Rice and Greenwald, 1929). These are the data used to support the 

current 65% incombustible requirement for intake airways. The LLEM data points (Weiss, 

Greninger, and Sapko 1989; Sapko et al. 2000) show close agreement with the BEM data for the 

fine size coal at 80% minus 200 mesh. For the mine size coal at 20% minus 200 mesh, the data 

show that somewhat more rock dust is required to prevent explosions in the LLEM than in the 

BEM. 

3. Experimental procedures 

To assess current variations in coal particle size distribution from various underground 

coal mining operations, MSHA coordinated the acquisition of mine dust samples from ten Coal 

Mine Safety and Health Districts. The dust samples were those collected routinely by mine 

inspectors for compliance with 30 CFR 75.403. The samples were sent to MSHA’s Mt. Hope 

Laboratory in WV and analyzed for their total incombustible content (TIC). The TIC includes 

the moisture in the samples, the ash in the coal, and the rock dust. The incombustible analysis 

procedure (Montgomery, 2005) begins with passing the sample through a 20-mesh sieve (850 

μm) and then oven drying the minus 20 mesh material for one hour at 105°C and recording the 

weight change. The weight loss constitutes the moisture in the sample. Next, the dried sample is 

heated in an oven that is ramped up over 1.5 hours and held at 515°C for about 2.5 hours to burn 

off the combustible coal fraction, thereby leaving the incombustible ash. This low temperature 

ashing (LTA) burns off the coal, but does not decompose the limestone rock dust. The amount 

of the remaining ash plus the as-received-moisture is reported as %TIC. After the MSHA 

laboratory completed their incombustible analyses, the remaining material of the dust samples 

was sent to NIOSH-PRL for the coal particle sizing analyses. 

At PRL, the limestone dust was leached from the sample using hydrochloric acid, and a 

sieve size analysis was conducted on the remaining non-leachable residue. In the laboratory acid 

leaching method, dilute hydrochloric acid was added to the dust sample in a beaker and heated 

on a hotplate. The acid reacted with the rock dust producing foam while releasing carbon dioxide 

(CO2). Additional acid was added until the foaming stopped. The hotplate kept the slurry near 

its boiling point for about one hour. After the slurry cooled, the acid-insoluble residue was 

filtered from the acid. The solid residue was rinsed with isopropanol and then transferred to a 

large evaporating dish. The residue was dried at 110°C for 3 hours. Agglomerates were broken 

with a spatula. The residue consisted of coal plus other insoluble mineral matter (such as silica 

from the rock dust and shale from roof or floor rock in the mine). 

The dried residue was classified into the different size fractions using a sonic siever, 

which combined two motions to provide particle separation: a vertical oscillating column of air 

and a repetitive mechanical pulse. In addition, the tops of the sieves were brushed to break up 

any remaining agglomerates. The sieves are 8-cm in diameter and include the following sizes: 

20 mesh (850 μm), 30 mesh (600 μm), 40 mesh (425 μm), 50 mesh (300 μm), 70 mesh (212 

μm), 100 mesh (150 μm), 140 mesh (106 μm), 200 mesh (75 μm), 270 mesh (53 μm), and 400 

mesh (38 μm). After the sieving was completed, the amount of sample on each sieve was 

recorded. 

Since the residue from the leaching process contained other inert mineral matter (such as 

clay and silica dust) that did not react with the acid, a correction to the size analysis must be 

made. First, the residue was grouped into three size fractions: minus 200 mesh, 200 by 70 mesh, 

 
and plus 70 mesh. Then these three fractions were heated at 515°C to determine the 

incombustible or non-coal content, using a LTA method similar to that of the MSHA laboratory 

at Mt. Hope. The sieve size analyses were then corrected for the non-coal content in the three 

size groupings. For most of the samples analyzed, the mineral matter was finer in size than the 

coal, but the reason for this is not obvious. 

The total size analysis procedure (acid leaching, sieving, and correction for incombustible 

matter) was verified by using prepared mixtures of coal and rock dust. First, the size of the coal 

sample was determined by sieving. Then, samples of coal and rock dust (usually 35% coal and 

65% rock dust) were mixed together, and then the rock dust was leached from the mixture. The 

residue was then sieved and corrected via LTA for any remaining incombustible matter in the 

size fractions. This verification test was done for samples of Pittsburgh seam high volatile coal, 

Blue Creek seam medium volatile coal, and Pocahontas seam low volatile coal. In general, the 

size analyses after leaching were close to the original size analyses, with agreement to within 1 

to 3% for the amount of minus 200 mesh. 

4. Results 

For this study, samples of mine dust from 47 coal mines in the ten MSHA bituminous 

districts were size analyzed. For each mine, samples were usually collected from two or more 

entries. For most analyses, multiple samples from a mine entry were combined to give an 

average size for that entry. Most of the samples were band samples, but some were floor and rib 

samples or floor and roof samples. Table 1 lists the intake coal dust size data by MSHA Coal 

Mine Health and Safety District. Columns two and three of the table list the number of mines 

and total number of combined samples per district. The third and fourth columns list the average 

percent minus 200 mesh (75 μm) and minus 70 mesh (212 μm) with the associated standard 

deviations, respectively. The last column lists the average mass median particle diameter and 

standard deviation. The averages for all MSHA districts are 32% minus 200 mesh, 64% minus 

70 mesh, and a mass median of 147 μm. This is significantly finer than the size measured in the 

1920’s. 

Table 2 lists the average sizes for various coal seams or groups of coal seams. The 

eastern bituminous coal seams are those from the Appalachian Mountains from Pennsylvania to 

Alabama. Only the seams that included samples from two or more mines are listed. The mideastern seams are those in Illinois, Indiana, and western Kentucky. These seams are known by 

different names in different states, as listed in the table. The western coal seams include various 

high volatile C bituminous (hvCb) coals in Colorado or Utah. The coal samples from the 

Hazard #4 seam in Kentucky and the Blue Creek seam in Alabama are the finest in size, with 

40% or more less than 200 mesh. 

 
Table 1. Average coal size from mines in ten MSHA Health and Safety Districts 

District No. of Mines No. of Samples –200 M, % –70 M, % Dmed, μm 

2 5 12 28 ± 4 60 ± 5 161 ± 23 

3 7 15 33 ± 11 63 ± 10 149 ± 50 

4 3 8 33 ± 5 64 ± 10 148 ± 33 

5 4 7 36 ± 10 70 ± 14 134 ± 42 

6 4 21 32 ± 7 60 ± 7 155 ± 35 

7 4 13 37 ±11 65 ± 9 126 ± 51 

8 3 8 26 ± 6 59 ± 8 169 ± 42 

9 4 7 28 ± 4 61 ± 5 160 ± 25 

10 4 26 29 ± 4 61 ± 5 151 ± 25 

11 6 19 39 ± 9 77 ± 14 116 ± 40 

Table 2. Average coal particle size from various coal seams. 

Coal Seams No. of 

Mines 

No. of 

Samples –200 M, % –70 M, % Dmed, μm 

Eastern Bituminous Coal Seams 

Pittsburgh, hvb 9 21 32 ± 9 64 ± 8 144 ± 35 

Upper or Lower Kittanning, hvb 2 5 25 ± 5 53 ± 6 197 ± 37 

Alma, Upper Elkhorn #1 or #3, hvb 3 18 33 ± 7 61 ± 6 149 ± 34 

Hazard #4, hvb 2 8 40 ± 13 69 ± 7 104 ± 45 

Pocahontas #3, lvb 2 5 35 ± 7 64 ± 8 140 ± 39 

Pratt coal seam, hvb 2 6 31 ± 6 63 ± 11 155 ± 34 

Blue Creek coal seam, mvb 4 13 43 ± 14 83 ± 25 98 ± 37 

Mid-Eastern Bituminous Coal Seams 

Springfield, Illinois #5, or W. Kentucky #9 3 16 29 ± 5 62 ± 7 150 ± 31 

Herrin, Illinois #6, or W. Kentucky #11 3 12 26 ± 3 59 ± 4 164 ± 24 

Western Bituminous Coal Seams 

various hvCb seams in Colorado 2 4 27 ± 5 58 ± 5 170 ± 31 

various hvCb seams in Utah 3 4 26 ± 5 61 ± 7 164 ± 29 

5. Summary 

Dust explosibility is strongly dependent on the fineness of the coal particles in a coal and 

rock dust mixture. Underground coal mining technology has changed significantly since the 

1920’s when data were collected on the particle size of coal dust distributed in mine 

passageways. Coal mining has become highly mechanized, and this has resulted in increased 

coal productions rates. Particle size can vary both within and between mines, since size is 

 
dependent on several factors such as mine type (i.e., longwa1l or continuous miner, along with 

cutting speed and depth) and coal seam type. In addition to total incombustible content and 

methane concentration, the coal dust particle size should be considered as an essential part of the 

explosibility assessment strategy in underground coal mines. The present coal size study 

indicates shows that the coal dust in intake airways of U.S. mines is finer than that measured in 

the 1920’s (Rice and Greenwald, 1929). Based on the inerting data from the Bruceton and Lake 

Lynn Experimental Mines, the present size of coal would require more incombustible to be 

effectively inerted than the 65% incombustible specified in current regulations. 

Further study is being undertaken to collect and analyze dust samples in a selection of 

mines to assess the feasibility of a hand-held instrument to evaluate the explosibility hazard. 

NIOSH in collaboration with MSHA (Sapko and Verakis, 2006) has devised a prototype handheld instrument, based on optical reflectivity, which can provide a direct assessment of the 

potential explosibility of a coal and rock dust mixture which is independent of coal dust particle 

size. The field studies will be used to evaluate the practicality for in-situ assessment of dust 

mixture explosibility with the newly developed hand-held instrument. 

References 

Cashdollar, K. L., and Hertzberg, M. (1989). Laboratory Study of Rock Dust Inerting 

Requirements: Effects of Coal Volatility, Particle Size, and Methane Addition. 

Proceedings of the 23rd International Conference of Safety in Mines Research Institutes, 

Washington, DC, September 11-15, 1989, pp. 965-977. 

Nagy, J. (1981). The Explosion Hazard in Mining. U.S. Department of Labor, Mine Safety and 

Health Administration, Information Report 1119, 69 pp. 

Montgomery, T. (2005). Personal communication, MSHA, Mt. Hope WV. 

Rice, G.S., H.P.Greenwald (1929). Coal-Dust Explosibility Factors Indicated by Experimental 

Mine Investigations 1911 to 1929. U.S. Bureau of Mines Technical Paper 464, 45 pp. 

Rice, G. S., Jones, L. M., Egy, W. L., and Greenwald, H. P. (1922). Coal-Dust Explosion Tests 

in the Experimental Mine 1913 to 1918, Inclusive. U.S. Bureau of Mines Bulletin 167, 

639 pp. 

Title 30, U. S. Code of Federal Regulations (30 CFR), Section 75.403, July 1, 2005. 

Sapko, M.J. and H.Verakis (2006). Technical Evaluation of Coal Dust Explosibility Meter. 

SME 2006 Annual Meeting, St. Louis, MO. March 26-29, 2006, pp.7 

Sapko, M. J. Weiss, E. S., Cashdollar, K. L., and Zlochower, I. A. (2000). Experimental Mine 

and Laboratory Dust Explosion Research at NIOSH. Journal of Loss Prevention in the 

Process Industries, 13, 229-242. 

Weiss, E.S., Greninger, N.B., and Sapko, M.J. (1989). Recent Results of Dust Explosion Studies 

at the Lake Lynn Experimental Mine. Proceedings of the 23rd International Conference 

of Safety in Mines Research Institutes, Washington, DC, September 11-15, 1989, pp. 843-

854. 

 
District 1 

District 2 

District 3 

District 5 

District 6 

District 7 

Hawaii Puerto Rico 

District 4 District 8 

District 9 

District 10 

District 11 

Alaska 

Figure 1. – MSHA Coal Mine Safety and Health Districts 

100 

90 

Non-Propagation 

80 

70 

60 

50 Propagation 

40 

30 BEM, non-propagation 

LLEM, non-propagation 20 

BEM, propagation 

10 LLEM, propagation 

0 

0 10 20 30 40 50 60 70 80 90 100

Minus 200 mesh content of coal dust, % 

Total Incombustible Content, %

Figure 2. Effect of particle size of coal dust on the explosibility.