Rock dusting considerations in underground coal mines 

M.L. Harris, E.S. Weiss, C-K. Man, S.P. Harteis & G.V. Goodman 

National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, Pennsylvania, USA 

M.J. Sapko 

Sapko Consulting Services, LLC., Finleyville, Pennsylvania, USA 

ABSTRACT: Rock dusting to prevent coal dust explosions has been in widespread use in U.S. underground coal 

mines since the early 1900s. Underground coal mining technology has changed significantly over this same time 

period, becoming highly mechanized and produces finer coal dust particles which are more explosive. Despite the 

advances in mining technology, mine-wide dust sampling practices have remained essentially unchanged. There 

are many factors associated with the practice of rock dust sampling that, if not properly considered, can adversely 

impact the effectiveness of the rock dust and the potential explosibility of the coal dust. Dust on elevated surfaces 

is dispersed and entrained by the developing explosion much more readily than dust on the floor. The increased 

use of meshing to control roof and rib spall provides elevated surfaces for coal dust to collect which significantly 

increases the potential for dust explosion propagation if not adequately inerted. In addition to holding more coal 

dust, the meshing makes the collection of representative dust samples by using current band sampling equipment 

inadequate. This paper discusses these and other related factors that could result in a potential undetected dust 

explosion hazard, when using current dust sampling procedures, in an area that otherwise appears to be adequately 

protected with rock dust. Recommendations are made for further investigation into how these and other factors 

affect explosion propagation and the need for sampling procedure changes. 

1 Introduction 

A coal dust explosion can generate sufficient air pressure 

and associated turbulence to disperse dust from entry 

surfaces and draw it into the expanding combustion zone. 

Heat transfer to the coal dust particles results in the 

production of volatiles and tars. These products react with 

the oxygen in the air at high temperatures, and the heat 

released from this exothermic reaction is converted into 

work of expansion of the semi-confined air. Similarly 

entrained rock dust acts as a heat sink, drawing energy out 

of the system. As the explosion progresses, surrounding 

dust is fluidized and dispersed into the propagating 

explosion. The magnitude of the explosion is related to the 

relative amounts of coal, rock, and other dust entrained at 

the flame front. The amount of dust entrained depends on 

the size of the explosion-produced aerodynamic 

disturbance. 

In underground coal mining, coal dust is produced at 

the face, at conveyors, at transfer points, and by the normal 

movement of machines. Coarse coal dust settles rapidly 

while finer coal particles remain airborne. Fine dust can be 

moved relatively long distances by ventilating air before 

settling. Although water sprays are effective for removal of 

dust particles at the face and along the conveyor, rock dust 

distribution is still required to render the residual coal dust 

accumulations inert in order to protect against a 

propagating explosion. 

2 Rock Dust Sampling Procedure 

The Mine Safety and Health Administration (MSHA) 

inspectors conduct rock dust surveys in each advancing 

working section to determine compliance with the 

regulations (30 CFR 75.403). If the working section has 

advanced and the loading point has moved 150 m (500 ft) 

or more since the last survey, a new survey is conducted. 

Samples are gathered every 150 m (500 ft) (MSHA, 2008). 

MSHA inspectors collect dust samples according to 

established procedures (MSHA, 2008). These procedures 

dictate that the band or perimeter method be used to collect 

dust samples from the roof, ribs, and floor creating one 

“band” sample. This band sample includes 25 mm (1") 

deep material from the floor. Once collected, the sample is 

thoroughly mixed, coned, and quartered to take a portion 

for analysis. This sampling essentially assumes a 

homogeneous mixture of coal, rock, and other dust on all 

surfaces. Underlying this assumption is that in the event of 

an explosion, the aerodynamic disturbance ahead of the 

flame front will scour dust from all surfaces and will tend 

to either enhance or inhibit propagation depending on the 

incombustible content (% IC) (Owings et al, 1940). 

Once the sample is gathered, the collected dust is 

sieved through a 10-mesh screen (1.7 mm) and a portion of 

the sample is bagged and labeled. The survey samples are 

sent to the MSHA Mt. Hope laboratory for moisture and 

low temperature ash analysis to determine % IC of the 

minus 20 mesh fraction. 
 

If a sample location is too wet to take a dust sample, 

the location is tracked and inspected for one year. If during 

the year, the sample location is dry enough to take a dust 

sample, then a survey is conducted in that location. 

3 Issues Associated with Rock Dusting 

Current mining technologies have created a need to review 

and revise sampling procedures and hence corresponding 

rock dusting practices. 

3.1 Current Rock Dust Requirements 

Currently, generalized rock dusting is the primary means 

of defence against coal dust explosions in U.S. mines. 

Code of Federal Regulations, 30 CFR 75, Subpart E 

(Combustible Materials and Rock Dusting) requires the 

use of rock dust in bituminous coal mines (30 CFR 75.402) 

to abate the hazard of accumulated coal dust. Regulations 

state rock dust shall be distributed upon the top, floor, and 

sides of all underground areas of a coal mine in such 

quantities that the percent of incombustible content (% IC) 

of the combined coal dust, rock dust, and other dust shall 

be not less than 65%. In the return aircourses where the 

dust is expected to be finer, the % IC shall be no less than 

80% (30 CFR 75.403). 

The current requirements are based on a 1920s coal 

dust particle size survey. As a result of the 1920s research, 

“mine size” coal dust was defined as coal dust that passes 

through a U.S. Standard No. 20 sieve (850 μm) with 20% 

passing through a 200-mesh sieve (75 μm) (Rice & 

Greenwald, 1929). Float coal dust was defined as minus 

200-mesh coal dust particles that may be deposited on the 

roof, ribs, and timbers in a mine (Nagy, 1981). Current 

rock dust regulations mandating a 65% IC dust mixture 

provide no margin of safety since the NIOSH Lake Lynn 

Experimental Mine (LLEM) tests have shown that even a 

~68% IC dust mixture with the “mine size” Pittsburgh 

seam coal dust will propagate dust explosions (Sapko etal., 

1989; Weiss et al., 1989; Greninger et al., 1990). The 

LLEM entry geometries are typical of current U.S. coal 

mines (Triebsch & Sapko, 1990). 

The mining methods that produced the 1920s survey 

data have changed dramatically throughout the last 

century. A particle size survey of US coal mines recently 

conducted by NIOSH concluded the old definition of 

“mine size” dust is no longer applicable or representative 

(Sapko et al., 2007). 

MSHA divides U.S. coal mines into 11 districts 

(Figure 1). The intake airway dust samples were examined 

and compared according to these MSHA districts. The 

fractions of minus 200-mesh (75 μm) ranged from 27% in 

District 9 to 37% in District 11. Minus 70-mesh (212 μm) 

fractions ranged from 59% in Districts 2 and 6 to 73% in 

District 11. The median diameter of the samples ranged 

from 128 μm in District 11 to 172 μm in District 9. The 

overall averages are 31% minus 200-mesh, 61% minus 70-

mesh, with a median dust particle diameter of 156 μm. 

These samples are significantly finer than those of the 

1920s survey (Cashdollar et al., 2009; Sapko et al., 2007). 

The same information was analyzed for the various 

coal seams sampled. The Blue Creek (District 11, 

Figure 1) and Hazard #4 (District 6, Figure 1) coal seams 

have the finest sized particles with a median diameter of 

98 μm and 104 μm respectively.

Figure 1	 MSHA districts (Sapko et al., 2007). 

 For these seams, the finer 

particle sizes were found in the Blue Creek coal seam 

containing about 40% dust particles minus 200-mesh (75 

μm) and 76% minus 70-mesh (212 μm) and in the Hazard 

#4 seam containing 40% minus 200 mesh and 69% minus 

70-mesh (Cashdollar et al., 2009; Sapko et al., 2007). 

Since the average particle size is significantly different 

than that on which current regulations are based, current 

%IC requirements are also different. Based on recent 

large-scale inerting experiments conducted within the 

LLEM (Figure 2), at least 76.4% IC is required to prevent 

explosion propagation for medium-size coal dust (37% < 

75 μm); i.e., an average of the finer coal dust found in 

current intake areas.

Figure 2 	 Effect of particle size of coal dust on the 

explosibility of Pittsburgh seam bituminous 

coal. (Cashdollar et al., 2009). 

 NIOSH recommends 80% IC in both 

intake and return airways in order to prevent flame 

propagation (Cashdollar et al, 2009). 
 

3.2 Sample Depth 

As a part of the recent large-scale dust explosion testing at 

the LLEM in 2008, 13 sets of dust scouring measurements 

were collected. The dust scoured is indicative of the depth 

of dust participating in an explosion. Two parallel rails 

were filled with a coal dust mixture and positioned in the 

entry with the dust leveled between the rails. Using a 

displacement gauge mounted on a portable aluminum bar, 

measurements were taken of the dust levels before and 

after the explosions (Figure 3).

Figure 3 	 Measuring the amount of dust scoured during an 

explosion. 

 The dust scoured during an 

explosion ranged from 0.7 mm (0.03") to 2.6 mm (0.1 in) 

with an average of 1.7 mm (0.06") and a standard 

deviation of 0.5 mm (0.02"). This is much less than the 25 

mm (1 in) that is specified in the MSHA band sampling 

procedures. Therefore, the current 25 mm (1") sampling 

depth of dust does not represent the dust that actually 

contributes to initial explosion propagation. If there is a 

layer of coal dust, the band sample can be diluted with 

rock dust by sampling to a full depth of 25 mm (1"), 

thereby giving a false sense of safety. A sample depth of 3 

mm (1/8") appears to better represent the level to detect 

potential deficiencies in rock dust (Sapko et al., 1987; 

Harris et al., 2009). 

3.3 Use of Mesh 

The position of the coal dust along the perimeter of an 

entry is a more important factor affecting explosion 

propagation than is commonly recognized. Dust on the 

ribs, roof, or elevated surfaces is dispersed by the 

explosion wind much more readily than dust on the floor. 

If the overhead dust is disproportionally coal dust, the 

explosion hazard is intensified. If the dust is primarily rock 

dust, the explosion hazard is reduced (Hartman et al. 

1956)). It is obvious that roof and rib sampling procedures 

are critical. 

Wire mesh is commonly used in mines to prevent skin 

failure from the ribs and roof. When material spalls from 

the ribs or roof, it can fall on miners or create slipping and 

tripping hazards. The installed meshing contains the 

spalled material to minimize the risk of injuries to miners. 

For control of roof skin failure, wood planks, steel straps 

or channel, and various meshes such as welded wire, chain 

link fencing, or synthetic grid materials are being used 

(Bauer & Dolinar, 2000). As seen in Figure 4, the mesh is 

placed as close to the roof and ribs as possible using bolts 

but is not completely flush which creates areas for coal 

dust to accumulate. 

Figure 4 Synthetic mesh supporting roof and rib. (Photo 

courtesy of Tensar Earth), (Bauer and Dolinar, 

2000). 

The use of mesh prevents adequate band sampling of 

the ribs and roof. With the mesh on the ribs, it is difficult 

for inspectors to brush the dust from the ribs into a pan for 

sample collection. The space between the mesh and strata 

allows the dust to fall behind the mesh or to be blown 

away by the ventilation airflow rather than to be collected. 

Also, the dust on the roof is sampled only in mines with a 

low roof that can be accessed without the aid of a ladder or 

extended sampling equipment. In mines where the roof is 

beyond reach and cannot be practically or safely sampled, 

a sample from the roof is not collected. It is currently 

acceptable to take a sample of the floor and ribs to the 

maximum height that it can be done safely and practically 

(MSHA, 2008). Therefore if coal dust were present on the 

mesh support, in these instances, it would not be collected 

for analysis and a potential explosion hazard may go 

undetected. Also, when combining a limited quantity roof 

and rib sample with a 25 mm (1 in) deep floor dust sample, 

the potential coal dust explosion hazard can easily go 

undetected. 

Diligently applying rock dust to these elevated surfaces 

with roof and rib mesh will reduce the dust explosion 

hazard. Nevertheless, improved sampling methods and 

procedures are needed to better assess the dust explosion 

hazards in view of these new technologies that prevent 

skin failure. 

3.4 Distance Between Samples 

Current sampling practices for determining compliance 

requires samples be collected at least every 150 m (500 ft) 

of mine entry (MSHA, 2008). Limited experiments were 

conducted in a single entry at the Bruceton Experimental 

Mine (BEM) to determine the effect on explosion 

propagation of alternate zones deficient in incombustible 

content. Alternate zones contained 9% less than and 9% 
 

more than the limiting incombustible content (Nagy 1981). 

When alternate zones were 18 and 27 m (60 and 90 ft) in 

length, flame propagation was arrested. When the alternate 

zones were 36 m (120 ft) in length, explosion propagation 

was obtained. These limited trials indicate with 

approximately 30 m (100-ft) length of entry, 9% deficient 

in incombustible has a significant effect on explosion 

propagation even though the rock dust deficient zones are 

compensated by excess incombustible in adjacent zones. It 

can be presumed that if the deficiency in incombustible 

were greater than 9%, a zone of less than 30 m (100 ft) 

would affect explosion development. 

Results from one recent large-scale explosion 

experiment conducted within the LLEM (Test #511) 

indicates that a propagating explosion can significantly 

increase in intensity while propagating through a 64 m 

(210 ft) zone of nearly 68% IC and jump over (through) 

and well beyond an adjacent 100 m (330 ft) zone 

containing over 81% IC. This preliminary result indicates 

that the minimum requirement of 150 m interval between 

band sampling locations within an entry needs to be 

reassessed. 

3.5 Particle Size of Rock Dust 

MSHA specifies rock dust as pulverized limestone, 

dolomite, gypsum, anhydrite, shale, adobe, or other inert 

material, preferably light colored, 100% of which will pass 

through a sieve having 20 meshes per linear inch (840 μm) 

and 70% or more of which will pass through a sieve 

having 200 meshes (75 μm) per linear inch (30 CFR 

§75.2). The research supporting the rock dust particle size 

effects on coal dust explosion propagation was performed 

in 1933 and reported in Bureau of Mines Bulletin 369. 

Preliminary sieve analysis of field samples of rock dust 

indicated that the rock dust meets the (30 CFR) 

specification of 100% less than 20-mesh (840 μm) and 

70% less than 200-mesh (75 μm). However, the integrated 

surface mean particle size showed considerable variation. 

In small-scale laboratory tests, the larger the rock dust 

particle size, the more rock dust is required to inert and 

prevent an explosion from propagating. It has been shown 

in various small chamber tests that by reducing the size of 

the rock dust particles, the surface area of the rock dust 

increases and promotes greater radiant heat absorption 

(Dastidar et al., 1997). 

In view of current results from the NIOSH coal dust 

particle size survey in US mines and preliminary size 

analyses of rock dusts, the effect of rock dust particle size 

in preventing coal flame propagation should be 

re-examined through large-scale explosion tests. 

3.6 Effectiveness of Different Rock Dust Types 

The effectiveness of rock dust in arresting explosion 

propagation has been proved by experiment and practice. 

The precise mechanism by which rock dust (generally 

limestone dust) quenches flame has not been fully 

explained, but is believed to be absorption of thermal 

energy from the heated gases and absorption of radiant 

energy which reduces the preheating of unburned coal 

particles ahead of the flame front. Even though limestone 

is not considered a chemical inhibitor some decomposition 

does take place which absorbs additional energy. 

Man & Teacoach (2009) have shown that the limestone 

acts more than a simple heat sink. Calcium carbonate does 

not only act as a physical heat sink in the prevention of a 

coal dust explosion, but also absorbs some energy in a 

calcination process to convert the calcium carbonate to 

calcium oxide or calcium hydroxide. 

Laboratory explosion inerting experiments conducted 

in the 20 liter chamber (Cashdollar & Hertzberg, 1989) 

also indicate that not all commercial rock dusts are equally 

effective in inerting Pittsburgh pulverized coal dust (80% 

< 75 μm). For example, marble dust was less effective than 

limestone even though both dusts fulfill the 30 CFR size 

specifications. The marble dust has a higher percentage of 

mass below 75 μm but less below 200 μm than other rock 

dusts. The smaller amount of very fine particles reduces 

the effectiveness of the rock dust as a heat sink and any 

rapid energy absorption in the flame front through the 

decomposition process. 

Currently limestone, dolomite, gypsum, anhydrite, 

shale, adobe, or other inert material are permitted by 30 

CFR for use in intake and return airways for preventing 

coal dust explosion propagation. The only size requirement 

is that 70% by mass be less than 200 mesh (75 μm) and 

100% be less than 20 mesh (840 μm). 

In view of these results, there is a need to re-examine 

the particle size specifications for rock dusts and determine 

the material specific requirements relevant to the 

incombustible content needed to prevent explosion 

propagation. 

3.7 Moisture Fluctuation in a Mine 

Mitchell et al. (1962) studied the effectiveness of water as 

an inert for neutralizing the coal dust explosion hazard. 

The study emphasized that surface water evaporates 

readily from dusts. Thus, in a passageway where the dust 

is wet, changes in weather or ventilation system could dry 

the dust and make it unsafe in a relatively short period of 

time. Where adequate rock dust has been applied, this 

drying effect is not a factor. According to 30 CFR § 

75.403-1 “moisture contained in the combined coal dust, 

rock dust and other dusts shall be considered as a part of 

the incombustible content of such mixture.” However, dust 

surface moisture within a mine can fluctuate making 

moisture content an ineffective measure of safety. 

It has long been recognized that the trend shows mine 

explosions occur primarily during the winter season when 

the humidity is low for long periods of time (Mannakee, 

1910: Scholz;, 1908; Kissell, 2006). Due to the potential 

variability of the moisture content of the dust, it may be 

prudent to not include surface moisture in the total 

incombustible content of the sample. 
 

4 Summary 

Dust sampling studies were conducted by NIOSH 

researchers to evaluate the amount of mine floor dust 

involved in an explosion, to evaluate the explosion hazard 

of collected samples in terms of coal dust particle size, and 

examined the current rock dust inerting levels to prevent 

explosion propagation hazards. The sampling procedures 

and techniques essentially described by Owings et al. 

(1940) are still being used. 

Dust explosion assessment procedures developed many 

years ago have not adequately kept pace with advances in 

mining methods and the implementation of new ground 

control technologies. Results from preliminary 

experimental mine explosion dust scouring experiments 

and dust particle size surveys indicate that the current dust 

sampling procedures are not fully adequate for identifying 

potential dust explosion hazards and should be reassessed 

in view of recent research findings and current mining 

practices. 

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