Respirable quartz hazard associated with coal mine roof bolter 

dust 

G.J. Joy, T.W. Beck & J. M. Listak 

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

ABSTRACT: This study examined the potential for coal mine roof bolter dust to be a source of a worker’s 

exposure to respirable quartz. For the models of roof bolter studied, the dust from bolt hole drilling is collected by 

a dry vacuum exhaust system. The collected dust-laden air passes through a cyclone size-separator (pre-cleaner) 

and into a collector box containing two additional cyclones and a filter cartridge. The filtered air is then 

discharged through a muffler into the ambient mine air. The pre-cleaner is designed to remove oversize dust, with 

the pre-cleaner undersize dust passing to a collector box. The pre-cleaner periodically dumps collected dust onto 

the mine floor. To assess the hazard presented by the roof bolter dust, bulk samples of the pre-cleaner dust and 

collector box dust were obtained from roof bolters operating in Mine Safety and Health Administration Coal 

Districts 4, 5, 6, and 7. Forty-six samples of each dust type were collected from twenty-six mines. Dust size 

distributions were determined for each sample. The quartz content was also determined for forty samples. The 

fraction of pre-cleaner dust smaller than 10 micrometers ranged from 5.3% to 35.4%. Quartz content, by weight, 

of the pre-cleaner dust ranged from 9.8% to 53%. The collector box dust was significantly smaller in size with the 

fraction of dust below 10 micrometers ranging from 13% to 86.7%, its quartz content ranged from 1.0% to 79%. 

Airborne dust samples along with detailed observations of the bolter operation were collected at two of these 

mines to further evaluate potential for worker exposure. This study quantifies the high content of respirable quartz 

in the roof bolter pre-cleaner cyclone dump dust and the collector box dust. 

1 Introduction 

Miners are exposed to several occupationally related 

agents that have the potential to degrade health. These 

agents include physical forms such as noise, vibration, 

and temperature extremes; and chemical forms such as 

respirable dusts, diesel particulates, and asphyxiants. 

Underground coal miners can be subject to high 

concentrations of respirable dust from coal extraction and 

transportation activities, and this dust may contain quartz. 

Extended exposure to elevated levels of respirable coal 

mine dust may result in coal workers pneumoconiosis 

(CWP), and if the respirable dust contains quartz, the 

miner may develop silicosis. Both of these lung diseases 

are incurable, and silicosis may continue to worsen after 

exposure has ceased. Consequently, the best means to 

control these diseases is to prevent their occurrence. 

Pneumoconiosis in coal miners, including cases of 

rapidly progressive disease, has been reported to be 

increasing among underground coal workers in the 

Southern Appalachian Region (SAR) (CDC, 2006; 2007). 

The SAR area is not strictly defined; it is generally 

described as southern West Virginia, the eastern third of 

Kentucky, and western Virginia. The area is largely 

encompassed by Mine Safety and Health Administration 

(MSHA) Coal Districts 4, 5, 6 and 7. The fraction of coal 

miners with rapidly progressive CWP in these MSHA 

districts based on recent surveillance is given in Table 1 

(Antao, et al., 2005). Counties with fewer than five 

miners evaluated, or less than 40% with rapid progression 

are not included. 

As part of an on-going research effort into potential 

explanations for this increase, the National Institute for 

Occupational Safety and Health (NIOSH) examined the 

particle size distribution and quartz content of dust 

generated by the installation of roof bolts in a group of 

mines in these four MSHA Coal Districts. Forty-six bulk 

samples of roof bolting machine pre-cleaner cyclone 

dump dust and collector box dust were collected from 

twenty-six underground coal mines. Real-time and 

integrated airborne respirable dust concentrations were 

measured on three mining sections in two mines in 

District 6. The real-time airborne dust concentrations 

profiles were examined to identify any concentration 

changes that might be associated with pre-cleaner cyclone 

dust discharge events. 

Roof bolting, as a ground control approach, was 

introduced to underground coal mining in the 1940s, 

although some applications were attempted earlier. 

Today, roof bolting is the principal method of roof 

support in underground mines. In 2005, an estimated 68 

million roof bolts were installed, including 1 million cable 

bolts (Tadolini & Mazzoni, 2006). 

Several different types of bolt-anchoring systems 

exist, but regardless of the type of bolt used, all require a 

hole to be drilled to accept the bolt. The diameter of the 

hole created to accept the bolt is larger than the bolt 

diameter. As an example, one roof bolt manufacturer 

recommends hole diameters ranging from 1.1 to 1.6 times 

the bolt diameter (Jennmar Corporation, 2009). The 

volume of drill cuttings – the dust – will be slightly 
 

greater than the hole volume due to the loss of 

consolidation, but ignoring that small difference, the 

amount of dust generated on a ‘typical’ underground coal 

entry 5.5 m wide x 6.1 m deep (18 ft x 20 ft) using a 1.2 

m long, 16 mm diameter bolts on a 1.2 m center pattern (4 

ft long x 5/8” diameter bolts on 4-ft center pattern) would 

be about 0.05 m3

 (1.7 ft3). This represents a considerable 

amount of dust that could potentially be released into a 

small, confined volume subject to variable ventilation 

rates. 

Table 1 Proportion of evaluated miners with rapidly 

progressive CWP by state and county.

 Miners Miners Miners 

Examined Evaluated Evaluated with 

(n) 1996- (n)# Rapid 

2002 Progression (%) 

Kentucky 

Floyd 68 6 50.0% 

Harlan 400 24 41.7% 

Knott 289 9 66.7% 

Leslie 123 5 60.0% 

Letcher 504 23 43.5% 

Martin 141 10 60.0% 

Perry 223 9 44.4% 

Pike 748 73 53.4% 

Virginia 

Buchanan 736 122 49.2% 

Dickenson 292 31 51.6% 

Lee 142 5 80.0% 

Russell 99 13 46.2% 

Tazewell 121 26 61.5% 

Wise 1,095 63 58.7% 

West Virginia 

Grant 42 7 57.1% 

Nicholas 140 11 45.5% 

Preston 199 16 50.0% 

Raleigh 302 51 43.1% 

Randolph 17 8 62.5% 

Upshur 170 13 46.2%
 

# A minimum of two medical examinations, including
 

pre-1996, was required for a miner to be evaluated for 

progressive disease.
 

The dust released from drilling bolt holes has long 

been recognized as a potential health hazard (Mark, 

2002). MSHA regulates miner exposure to respirable dust 

generated from drilling under the requirements of 30 CFR 

Part 70.100 (respirable dust) and 30 CFR Part 70.101 

(respirable dust when silica is present). MSHA also 

regulates how the control of dust generated from drilling 

is to be achieved in 30 CFR Part 72.630. The dust 

controls specified include permissible dust collectors, the 

application of water or water containing a wetting agent 

through the drill steel, by ventilation, or by any other 

equally effective, MSHA-approved method. 

In the United States, dry vacuum dust collectors are 

commonly used as the control method on roof bolting 

machines in coal mines. The arrangement and 

specifications of the dust control systems differ depending 

on the equipment manufacturer. The mines included in 

this study were all utilizing roof bolters manufactured by 

J.H. Fletcher & Co. There was no pre-selection decision 

to limit the study to a single manufacturer; the selected 

mines comprised a convenience sample. 

In general terms, the dust collector currently used on 

Fletcher bolting machines is composed of an initial 

cyclone called the pre-cleaner, followed by a collector 

box where undersized dust from the pre-cleaner either 

impacts on a plate, or enters a dust collection bag. The 

airstream exits the collector box after passing through two 

additional cyclones and a final cartridge filter. The precleaner cyclone is intended to remove larger size dust 

particles, and dump this fraction onto the mine floor. The 

removal of the larger size dust reduces the volume of 

material that must be removed from the airstream in the 

collector box. The air blower is located downstream from 

the cartridge filter, so the entire system is designed to 

handle dust-laden air under negative pressure. 

The dust accumulated in the collector box must be 

removed occasionally to maintain system performance. 

Various approaches to reduce the dust exposure of miners 

from this activity utilizing engineering changes and/or 

work practices have been developed. These include 

providing a handled tool to scrape the dust from the box, 

and manufacturer-developed procedures incorporating the 

use of existing mine ventilation and respiratory protective 

equipment. On machines equipped with dust collection 

bags, the bag containing dust is removed from the 

collector box and placed against a rib or other location 

away from traffic. Miner exposure to airborne dust from 

cleaning the collector box has been shown to be reduced 

when bags are utilized as compared to machines without 

the dust bag configuration (Listak & Beck, 2008). 

Roof bolter operator exposure to respirable coal mine 

dust (RCMD) is generally low. Much of the RCMD 

exposure of bolter operators results from operating the 

bolting machine downwind in the return airflow from the 

continuous mining machine. Mine ventilation plans, 

which specify mandatory dust control methods, generally 

limit the time the bolting machine may operate downwind 

from the continuous miner, but it can be very difficult to 

eliminate this undesirable situation completely. The 

concern regarding roof bolter activities focuses on the 

respirable quartz content of the dust created when drilling 

into the rock roof. This exposure, as demonstrated by 

MSHA inspector sampling data, can be problematic 

(Table 2) (MSHA). 

The original equipment manufacturer (OEM) roof 

bolter dust collector system is very effective in controlling 

dust from drilling, but proper operation, maintenance and 

cleaning are very important to preserve this level of 

performance (Thaxton, 1984). Questions have been raised 
about the potential for exposure from the pre-cleaner 

cyclone dust discharge and from the disposal of collector 

box dust. The concern about the pre-cleaner cyclone 

discharge centers on the potential for this dust to become 

airborne as a result of the dust falling approximately 0.3 

m (1 ft) from the pre-cleaner to the mine floor, and/or 

becoming re-suspended after being deposited onto the 

mine floor. The collector box dust presents a potential 

exposure hazard when the accumulated dust is removed 

from the box and dust collection bags have not been 

utilized. A further potential exists once the collector box 

dust has been transferred onto the mine floor. 

Mechanisms to make dust from the mine floor become 

airborne may include foot and mobile equipment traffic, 

supplemented by ventilation air currents. The likelihood 

of re-entrainment of coal dust from a mine floor was 

examined in wind tunnel tests and in an experimental 

mine (Shankar & Ramani, 1996a, 1996b). At the lowest 

air velocity reported in these tests – 2.13 m/s (419 ft/min) 

– re-entrainment of dust was observed with only airflow 

as the source of energy. The amount of re-entrainment 

increased further when persons walked through the dust. 

The measured increase in re-suspension varied with 

height above the floor and downwind distance from the 

dust source. 

Table 2 Roof Bolter Occupation Respirable Dust and 

Quartz Exposures. 

MSHA Inspector Roof Bolter Occupations Samples 

1999 – 2008 

Respirable Coal 

Mine Dust 

Respirable Quartz 

Sample N % > Dust 

Standard* 

Sample N % > 100 

μg/m3 

50,072 6.3% 9,624 21.5% 

* The dust standard associated with a sample is 2.0 

mg/m3

 unless the quartz content of the airborne 

respirable dust exceeds 5%. 

In these in-mine tests, coal dust was spread onto a flat, 

dry and smooth floor surface. The testing also utilized 

high ventilation air velocities. These conditions do not 

closely represent typical underground coal mine 

production sections where the floor is normally uneven 

and wet, and the ventilating air velocities are rarely as 

high as those in the test conditions. These differences 

limit the application of the test results to producing mines. 

The hazard from re-suspension of floor-deposited dust 

after a production section evolves into a transportation 

heading and bears frequent traffic from mobile equipment 

(e.g., shuttle cars) and personnel has not been 

investigated. 

In a test conducted under contract to the United States 

Bureau of Mines to examine the question of reentrainment of roof bolter dust from the floor of two 

active coal mines, an object weighing approximately 2.25 

kg (5 lbs) was dropped onto dust piles while a Simslin 

real-time aerosol monitor was operated 1 m (3 ft) 

downwind and 30.5 cm (1 ft) above the floor (Colinet et 

al., 1985). In five tests at one mine, no indication of 

increased dust was observed on the aerosol monitor. In 

four tests at a second mine, the average dust concentration 

on the aerosol monitor was 0.2 mg/m3

. The report 

concluded that vehicular traffic was not a significant 

source of quartz. No information about the air velocity or 

dryness of the mine floor was reported in association with 

these tests. 

The study reported here developed additional 

information about the size distribution and quartz content 

of the dust discharged to the mine floor from the precleaner cyclone (the oversize fraction), and from the dust 

contained in the collector box. 

2 Methods 

The mines included in this study comprise a convenience 

sample, and although a sizeable number were sampled, a 

representative sample of mines in MSHA Coal Districts 4, 

5, 6, and 7 is not implied. The majority of the bulk dust 

samples were collected by district-based MSHA 

inspectors as they performed their scheduled duties. Bulk 

dust samples were collected from five mines in District 4, 

ten mines in District 5, five mines in District 6 and six 

mines in District 7. The mines sampled by NIOSH 

personnel were in District 6 and volunteered to participate 

in the research; they were also a convenience sample and 

were not selected on a statistical basis. 

Bulk samples were collected from the collector box 

and from the pre-cleaner cyclone dump discharge into 

clean, unused plastic containers. Particle size distributions 

(volume basis) were obtained for each of the bulk samples 

by sieving, followed by analysis using a Saturn 5200 

DigiSizer low-angle laser scattering instrument. This 

analysis measures light scattered from particles in a liquid 

suspension according to Mie theory. The weight-basis 

quartz content of the bulk dust was determined by x-ray 

diffraction analysis (CDC, 2003). 

On a subset of mines, NIOSH collected air samples 

during the roof bolting process. Six air-sampling racks, 

each consisting of two coal mine dust personal sampling 

units (CMDPSU) and one Thermo Fisher pDR-1000AN 

real time aerosol monitor were used on each roof bolting 

machine. The CMDPSU is the standard sampling system 

used by MSHA to measure respirable coal mine dust. The 

pDR monitors were programmed to log concentrations on 

2-second intervals. The air samplers were operated 

continuously while the roof bolting machine was 

operating. Air samples were not collected if the roof 

bolter was in the return airflow from the continuous 

mining machine. 

The sampling racks were located at the rear corner, 

above the pre-cleaner cyclone and at the operator’s 

controls on the left and right sides of the roof bolting 

machine. The racks were suspended from the body of the 

roof bolting machine as permitted by material storage and 

operational access requirements, and were approximately 

0.7 m (28”) above the mine floor. 
Figure 1 Real-time respirable dust concentration output from a pDR aerosol monitor. The circled area is an example of a 

low baseline dust interval. 

Figure 2 Pre-cleaner cyclone dump events and respirable dust concentration recorded by a pDR real-time aerosol monitor 

above the pre-cleaner cyclone (pre-dump), and at the downwind operator’s position. Vertical lines indicate precleaner cyclone dump events. 
PASW Statistics 17.0 was used to generate descriptive 

statistics and analyze size distributions and quartz content. 

3 Results 

Particle size distributions were determined for forty-six 

bulk dust samples from the pre-cleaner cyclone dump 

material and the collector box. Quartz content was 

determined for forty of the bulk dust samples. Table 3 

presents summary statistics for the analytical results. The 

size distributions and quartz content of the pre-cleaner 

cyclone dump dust samples and collector box dust samples 

and were compared to identify any significant differences. 

Table 3 Summary statistics for size distribution and quartz 

content analytical results. 

Pre-cleaner Bulk Dust 

Mean

% < % < % < 

100 μm 10 μm 5 μm 

Quartz % 

(w/w) 

 43.6 17.9 11.3 27.3

Std Dev 12.3 5.9 3.8 12.4 

Median 43.7 18.0 11.2 24.5

Min 16.4 5.3 3.3 9.8

Max 81.9 35.4 22.2 53.0

Collector Box Bulk Dust 

Mean

% < 

100 μm 

% < 

10 μm 

% < 

5 μm 

Quartz % 

(w/w) 

 75.6 38.1 12.8 26.2

Std Dev 17.1 17.8 5.8 13.3 

Median 76.8 34.2 11.8 23.0

Min 34.4 13.0 3.3 1.0

Max 99.3 86.7 28.2 79.0

The size distributions, as represented by the percent of 

dust less than 10 μm, and the quartz content of the bulk 

dust samples were tested for normality by evaluation of 

normal probability plots and by the Kolmogorov-Smirnov 

test. Both distributions were found to be significantly nonnormal; size distribution result: D = 0.168, p < 0.001, df = 

92; quartz content result: D = 0.118, p < 0.001, df = 80. 

Median values of the percent less than 10 μm size of 

the pre-cleaner dust and the collector box dust were 

compared using the Wilcoxon Signed Ranks test. The 

collector box dust is significantly smaller in size than the 

pre-cleaner dump dust, (Z = -5.70, p < 0.001). There was 

no significant difference between the median

concentrations of quartz in the collector box dust and the 

pre-cleaner dump dust according to the same statistical 

test, (Z = -0.582, p = 0.56). 

The real-time airborne respirable dust concentration 

recorded by the pDR monitor varied substantially during 

the operating cycle. It was affected by air velocity over the 

bolting machine, the position of ventilation curtains, travel 

of the bolting machine, and the actions of the miners 

installing the bolts, in particular from the stocking and rearrangement of supplies and materials on the machine 

deck. Occasionally the hollow drill steel would become 

plugged by dust, and this would cause transient very high 

dust levels at the operator’s position due to a loss of 

vacuum at the drill bit. Any contribution to airborne dust 

from pre-cleaner cyclone dump events was expected to be 

brief in duration and possibly small in magnitude. To 

improve chances of if identifying this increase, bolting 

periods exhibiting a consistently low baseline level of 

airborne dust were examined, as depicted in Figure 1. 

No definitive association between increases in airborne 

respirable dust and pre-cleaner cyclone dump events was 

observed during this study (Figure 2). 

Eighteen of the CMDPSU filter samples co-located 

with the pDR real-time aerosol monitors were analyzed for 

respirable quartz. All of the samples analyzed were below 

the limit of quantification for quartz. 

4 Summary 

Roof bolter dust, both from the pre-cleaner cyclone dump 

and accumulated in the collector box is a potential 

inhalation hazard due to the fraction of dust less than 

10 μm in size, and the quartz content of the dust. The precleaner cyclone dust was significantly larger, as 

determined from the fraction smaller than 10 μm, when 

compared to the collector box dust. This demonstrates that 

the pre-cleaner functioned properly in removing the larger 

dust size fraction from the airstream. However the precleaner dust does still contain a substantial amount of 

respirable dust. 

To maintain the effectiveness of the OEM’s roof bolter 

dust collector requires the periodic removal of dust 

deposited in the collector box. The task of cleaning the 

accumulated dust potentially exposes miners due to the 

proximity of the miner’s head and face to the dust, and the 

possibility of dust adhering to the miner’s clothing. 

Appropriate work procedures and equipment are necessary 

to minimize exposure during this task. 

Utilization of dust collection bags has been shown to 

reduce operator exposure to respirable dust when 

performing this task. 

In limited field-testing, no noticeable increases in 

airborne dust concentration could be attributed to precleaner cyclone dump events using a pDR real time aerosol 

monitor. However, additional laboratory and in-mine study 

of this issue is warranted due to the difficulty of 

controlling all potential confounding variables in an 

operating mine section. 

A miner’s task training should address the hazards of 

roof bolter dust, including the importance of proper 

operation and maintenance of the roof bolter machine dust 

collector, means to minimize exposure when cleaning out 

the collector box, and avoiding or minimizing disturbance 

of roof bolter dust deposited to the mine floor from either 

the pre-cleaner cyclone or collector box. 
Acknowledgement 

NIOSH acknowledges and thanks the MSHA inspectors in 

Districts 4, 5, 6, and 7 for their assistance with this 

research. 

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