Introduction

The United States Federal Coal 

Mine Health and Safety Act of 1969 

requires that the concentration of 

respirable dust in the coal mine atmosphere not exceed 2.0 mg/m3

 for 

a working shift (Code of Federal 

Regulations, 2007). This limit is measured gravimetrically as an 8-hour 

time-weighted average concentration 

of the respirable dust. If the quartz 

content on the dust fi lters exceeds 5 percent (by weight), 

the dust standard is reduced according to the following 

expression: 10/(percent quartz). For example, if the quartz 

content is 20 percent (by weight), the limit is reduced 

to a maximum of 0.5 mg/m3 total respirable dust. Mine 

Safety and Health Administration (MSHA) inspectors 

collected 5,000 samples from roof bolting occupations 

during the years 2000 through 2004. More than 1,000 of 

these samples showed excessive exposure to respirable 

silica dust (MSHA, 2004). 

Past research has shown that most roof bolter operator dust exposure comes from upwind sources such as the 

continuous miner (Colinet et al., 1985). However, with 

inadequate ventilation, dust generated by roof drilling 

operations will contribute to elevated occupational exposures. Further research is required to develop and evaluate improvements in roof drilling dust-control methods 

to minimize these exposures. This paper compares respirable dust levels measured around roof bolting machines 

equipped with either a vacuum drilling or mist-drilling 

system. Tests were conducted in both underground and 

Evaluation of dust exposures associated with 

mist drilling technology for roof bolters

laboratory settings. 

Roof bolting machines are typically equipped with an MSHA-accepted (Code of Federal Regulations, 

2007) vacuum dust collection system 

to capture dust while drilling. A vacuum pump on the bolter draws the 

dust through the hollow bit and drill 

steel into a precleaner. The precleaner removes the large particles from 

the air stream and deposits them on 

the mine fl oor when discharged. The precleaned dustladen air is then routed to an enclosed dust box. The box 

has multiple chambers and functions as a rough size classifi er, removing the dust from the air stream according 

to its size. The typical box consists of four compartments 

with a single canister fi lter in the fi nal chamber. Previous 

research has documented the effectiveness of this system 

if properly maintained (Thaxton, 1984). 

On roof bolter machines using mist drilling technology, the mist head bolter injects a combination of water 

and compressed air through the drill steel and drill bit to 

control drill dust. Air is supplied by an onboard compressor, while the water is supplied by an onboard water tank 

or by a water hose attached to the machine. As a positive 

displacement method of fl ushing the drill hole, the wet 

drill cuttings are expelled from the hole around the outside of the drill steel. 

Laboratory test design and methods

The laboratory portion of the research was conducted in a surface test facility at the Pittsburgh Research 

Laboratory of the National Institute for Occupational 

Safety and Health (NIOSH). All tests were performed 

in the Acoustical Testing Laboratory, typically used for 

full-scale sound power testing of mining equipment. The 

room has a volume of approximately 1,300 m3

 (46,000 cu 

ft). During testing, the doors to the room were closed to 

prevent dust from escaping and to prevent possible contamination from outside sources. 

The test platform was a J.H. Fletcher & Co. (Huntington, WV) roof-bolting machine. This walk-through 

dual head bolter was outfi tted with a standard vacuum 

collection system and a mist drilling system. The drill was 

outfi tted with a round 25.4-mm- (1.0-in.-) diameter hollow drill steel for all tests. For vacuum tests, Kennametal 

(Latrobe, PA) KCV4-1-RR bits were used, while Kennametal KWH-1-400 bits were used for mist tests (Fig. 

1). These are both carbide-tipped bits with a drilling diameter of 25.4 mm (1 in.). A new bit was used for each 

new hole.

Granite with a compressive strength of 165 MPa 

Abstract

Overexposure to respirable dust remains a widespread 

health hazard to roof bolter occupations in underground 

coal mines. Recently implemented mist drilling technology 

was evaluated for its effectiveness in controlling respirable 

dust exposures for roof bolting personnel. This work 

presents the fi ndings from NIOSH fi eld and laboratory 

studies comparing respirable dust levels on mist drilling 

roof bolter machines to dust levels measured on machines 

using a conventional vacuum drilling system. Gravimetric 

and instantaneous respirable dust samples were collected 

for multiple drilling cycles. These dust measurements were 

used to assess the potential dust control capabilities relative 

to conventional drilling. Controlled laboratory tests found 

0.3 mg/m3

 more respirable dust with mist drilling than with 

the conventional vacuum method. A separate fi eld investigation of the two techniques observed higher dust levels 

around the mist bolting machine.

T.W BECK AND G.V.R. GOODMAN

T.W. Beck and G.V.R. Goodman, members SME, are research 

engineer in dust control and section chief of ventilation and 

explosion prevention, respectively, with the National Institute 

for Occupational Safety and Health (NIOSH), Pittsburgh, PA. 
(24,000 psi) was used as the drilling test media. The high 

compressive strength of granite allowed for a longer drilling and sampling duration. A 0.8-m- (30-in.-) thick block 

of this material was secured to the steel test stand (Fig. 2). 

Holes were drilled to a depth of 0.5 m (20 in.) to prevent 

drilling through the granite block and releasing a large 

amount of dust and mist into the laboratory atmosphere. 

The thrust and drill rotation speed were held constant 

for all tests at 22.0 kN (4,900 lbs) and 250 rpm, respectively (Fig. 1). The bolter operator controlled the feed 

rate manually.

During laboratory mist drill testing, the onboard mist 

system supplied 2.3 L/min (0.6 gpm) of water at 700 kPa 

(100 psi). Compressed air was supplied at 400 L/min (13 

cu ft per min) at 700 kPa (100 psi). These values correspond to drilling tests conducted by NIOSH during previous fi eld studies (Goodman et al., 2006).

The vacuum portion of testing used a typical MSHAaccepted (Code of Federal Regulations, 2007) four-compartment dust collection system with a single canister 

fi lter (Donaldson and Co., Minneapolis, MN). The dust 

collection box was cleaned prior to the series of vacuum 

tests. The precleaner, which separates oversized material from the system before it reaches the dust box, was 

isolated from the atmosphere to prevent contamination 

of the samples. A sealed container was fi tted to the precleaner outlet to contain any possible releases. 

Sampling was accomplished with a combination of 

gravimetric and instantaneous light-scattering instrumentation. Samplers were attached to the test stand in four 

locations (Fig. 3) to eliminate any bias due to air movements. The sampling racks were located at the midpoint 

of each side of the test stand, at an average distance of 

1.2 m (4 ft) from the drill steel and 0.3 m (1 ft) below the 

base of the rock. Each sampling location consisted of 

two gravimetric sampling units and one instantaneous 

dust monitor. 

For gravimetric sampling of dust, Escort ELF constant flow pumps (Mine Safety Appliances Co., Pittsburgh, PA) pulled dust-laden air through 10-mm nylon 

cyclone separators at a rate of 2 L/min (Code of Federal 

Regulations, 2007). The respirable mass was deposited 

onto preweighed 37 mm (1.5 in.) PVC fi lters that were 

subsequently desiccated and postweighed to determine 

dust levels. The fi lter weights were blank-adjusted to account for any variability in weighing conditions. The gravimetric samplers were operated continuously during 

vacuum and mist system drill tests due to safety considerations. Consequently, fi lter weights included dust from 

both drilling and non-drilling activities. For this reason, 

instantaneous dust sampling was used in conjunction with 

the gravimetric data to determine the dust level for each 

individual hole drilled. 

Instantaneous dust sampling was performed using 

personal DataRAM (pDR) dust monitors (Thermo 

Electron, Franklin, MA). The pDR recorded dust concentrations every fi ve seconds during testing. The pDR 

measurements are affected by aerosol size distribution 

and content. Because of this, the pDR values must be 

corrected by the gravimetric measurements. This is accomplished by calculating the ratio of the gravimetric 

concentration to the average pDR concentration for the 

same time period. In this study, the ratio can be calculated 

for each sampling location for both testing conditions. 

Individual pDR data points for the desired time period 

were multiplied by the previously determined correction 

ratio and averaged together to obtain a gravimetrically 

corrected pDR concentration. All pDR concentrations 

presented in the paper have been adjusted using this correction procedure. 

The following test procedure was used to assess the 

differences in dust control effectiveness using the mist 

system and the conventional vacuum system. The background dust concentrations were measured in the test 

chamber prior to each series of drilling tests. A drilling 

test was conducted using either the mist or vacuum system for dust control. The duration of each drilling test 

was the time needed to drill 0.5 m (1.6 ft) into granite. 

This time varied from 100 to 300 seconds, depending on 

drilling technique. A two-minute period elapsed between 

each hole to stabilize and measure idle dust levels. A total 

of 60 tests were conducted, 30 using the mist system and 

30 utilizing the vacuum system.

Analysis of laboratory bolter performance 

There was very little respirable dust in the test chamber prior to any drilling. The corrected pDR background 

dust concentration before the series of 30 vacuum tests 

was 0.03 mg/m3, while the dust concentration before the 

FIGURE 1

Vacuum drilling bit (left) and mist drilling bit (right). 

FIGURE 2

Laboratory test confi guration. 
mist drilling tests was 0.01 mg/m3

. This indicates that 

nearly all dust generated during the drilling tests can be 

attributed to the corresponding technique.

The laboratory tests showed that mist drilling resulted 

in higher dust concentrations than conventional vacuum 

drilling. The gravimetrically corrected pDR measurements showed an average respirable dust concentration 

of 0.04 mg/m3 during vacuum drilling. Sampling during 

mist drilling resulted in concentrations of 0.39 mg/m3. 

Standard deviations between individual sampling locations were found to be 0.03 mg/m3

 for vacuum tests and 

0.17 mg/m3 for mist tests. Corrected pDR data for each 

sampling location are displayed in Table 1. Figure 4 shows 

the corrected pDR dust concentration for each hole 

drilled as an average of all four sampling locations. 

For both test conditions, the average idle dust levels 

were found to be nearly the same as dust levels during 

drilling. Though there was no dust generated by drilling 

between tests, the dust generated during previous drilling 

tests remained in the test chamber. 

During this study, elevated dust emissions were observed during the collaring of the holes. This was experienced regardless of dust control technique. Once a hole 

was drilled to a depth of about 50 mm (2 in.), the visible 

dust emissions diminished. Throughout the mist drilling 

tests, an appreciable stream of aerosol was emitted from 

the holes. In addition to water mist, this aerosol contained 

a respirable fraction of dust that was deposited on the 

gravimetric fi lters. On completion of each vacuum hole, a 

small puff of dust was observed. However, this did not appear to have a large impact on the dust levels attributed 

to this technique.

The visible dust emissions appeared to migrate towards sampling Locations #3 and #4 during all mist 

drilling tests, as was confi rmed by elevated readings by 

the samplers in these locations. This was possibly due 

to small, but consistent, air currents present in the test 

chamber, which contributed to the variability in readings 

between sampling locations. The same condition was not 

observed for vacuum drilling. This may be due to the low 

dust levels documented for this technique. 

Vacuum drilling was observed to have a greater penetration rate during these laboratory tests. The average 

time to drill a 0.5-m (20-in.) vacuum hole was 100 seconds, while it took 300 seconds to drill 

the same depth using the mist system. 

This translated into a penetration rate 

of 5 mm/s (0.2 ips) for vacuum drilling 

and 2.5 mm/s (0.1 ips) for mist drilling. 

These numbers were lower than those 

reported for previous laboratory bolter work under similar test conditions 

(Matetic, 2006; Peterson et al., 2006). 

Field evaluation

The fi eld portion of this study was 

conducted at an underground coal 

mine in Virginia. In this study, two separate twin-boom J.H. Fletcher & Co. 

roof bolting machines were sampled 

during production drilling and bolting. 

One machine was outfi tted with a conventional vacuum system to control 

dust, while the other machine used 

a mist drilling system. The vacuum-equipped machine 

used a precleaner on each side to remove the oversized 

dust fraction. Each bolting arm of the vacuum bolter 

used a four-chamber permissible dust collection system 

with a single canister fi lter. The mist drilling system supplied water to the left and right bolting arms via a 420-L 

(110-gal) onboard water tank at rates of 2.6 L/min (0.7 

gpm) and 1.1 L/min (0.3 gpm), respectively. Air was supplied to the left and right arms at a rate of 0.01 m3

/s (20 

cfm) to each side. Both machines used round drill steels. 

The vacuum machine used “dust hog”-type drill bits and 

the mist machine used “wet” bits. These bits were both 

similar to those used in laboratory testing. Entry airfl ows 

were measured for each location drilled. The entry airfl ows averaged 5.2 m3

/sec (11,000 cfm) for vacuum drilling and 3.8 m3

/sec (8,000 cfm) for mist drilling. Because 

there was only one bolting crew, these machines did not 

operate at the same time.

Three shifts of data were collected for each bolting 

machine. Sampling was performed by gravimetric samplers using a combination of permissible sampling pumps 

operating at a rate of 2.0 L/min (0.53 gpm), with 10-mm 

(0.4 in.) nylon cyclones and preweighed 37-mm (1.5 in.) 

PVC filters. The filters were subsequently desiccated, 

postweighed and blank adjusted to account for any variFIGURE 3

Locations of samplers around bolter for laboratory testing (not to scale). 

FIGURE 4

Gravimetrically corrected drilling dust concentrations for each hole drilled.
ability in weighing conditions. Samplers were attached 

near the left and right operator work locations, approximately 0.3 to 0.6 m (1 to 2 ft) outby their controls (Fig. 

5). These are the preferred sampling locations when a 

personal sample cannot be collected on the roof bolter 

operator (Ondrey et al.). Separate samplers were used 

for the vacuum and mist drilling machines, and each set 

was sampled only when that specifi c machine operated. 

Thus, any dust collected on the fi lters could be assigned 

to a specifi c dust-control system. Gravimetric samplers 

were also positioned in the intake air for each bolting 

machine. 

During the underground study, the mist bolter 

worked downwind of the continuous mining machine, 

while the vacuum-equipped bolter worked upwind. Such 

downwind operation of roof bolters has been shown to 

significantly increase operator dust exposure (Goodman and Organiscak, 2002; Kissell, 2003). This resulted in 

higher intake dust levels for the mist bolter than for the 

vacuum bolter. Also, elevated dusts levels were recorded 

at the left and right side operator area sampling locations 

for the mist bolter. With the contribution of intake dust 

removed from all machine samples, dust levels around 

the mist bolting machine still exceeded those around the 

vacuum machine. The overall average gravimetric dust 

concentration for each dust control method was 0.30 

mg/m3

 for vacuum and 1.44 mg/m3 for mist. The average 

dust concentration for each drilling method is listed in 

Table 2. 

During this study, NIOSH researchers noted that 

dust was generated every time a hole was collared, using 

either the vacuum or mist system. Once the bit penetrated a depth of 20 to 30 mm (0.8 to 1.2 in.), the problem 

disappeared. This observation was similar to the occurrences noted during laboratory testing. During collaring, 

a risk exists in that the respirable fraction of this dust can 

be a health hazard to the operators if insuffi cient airfl ow 

is present.

NIOSH researchers documented a large difference 

in water fl ow rates to each bolting arm during this study. 

However, when comparing the dust concentrations measured between the right and left side of the mist bolting 

machine, this difference was not apparent. This indicated 

that both water fl ow rates resulted in similar dust generation characteristics.

Summary and conclusions

NIOSH researchers compared the dust control capabilities of a mist drilling system and a vacuum drilling 

system in both laboratory and underground settings. In 

both portions of the study, the mist drilling system was 

found to generate more respirable dust than vacuum 

drilling. In the laboratory phase of testing, sampling 

during vacuum drilling measured dust levels of 0.04 mg/

m3. Mist drilling tests performed under the same conditions yielded nearly ten times more dust, with area dust 

concentrations of 0.39 mg/m3. Several factors, including 

bit selection, air/water pressures and quantities and rock 

composition, may have detrimentally impacted the dust 

capture effectiveness of the mist drilling technique. 

Because the mist bolter worked downwind of the 

continuous mining machine for the duration of the fi eld 

study, higher intake dust levels were observed for this 

machine. After removing the intake dust contribution, 

underground tests measured dust concentrations of 

0.30 and 1.44 mg/m3 for vacuum and mist drilling, respectively. Although the gravimetric dust concentrations of the fi eld study were signifi cantly higher than 

the concentrations encountered in the laboratory study, 

these results confi rmed the laboratory results, as the dust concentrations generated during 

mist drilling in the fi eld tended 

to be approximately fi ve times 

the concentrations observed for 

vacuum drilling. 

Conventional vacuum drilling is a negative displacement 

drilling technique. The system 

is designed to draw the drill 

cuttings through the drill steel 

and capture them in a dust 

collection box. The lower dust 

levels measured during laboratory and field testing indicate 

the potential effectiveness of 

this system at minimizing dust 

 Bolter drilling system

 Vacuum Mist

Sampling Idle, Drilling, Idle, Drilling,

location mg/m3

 mg/m3 mg/m3 mg/m3

1 0.05 0.06 0.22 0.23

2 0.03 0.03 0.15 0.23

3 0.03 0.03 0.49 0.48

4 0.03 0.04 0.52 0.61

Overall1

 0.03 (0.02) 0.04 (0.03) 0.34 (0.16) 0.39 (0.17)

1

Standard deviations are shown in parentheses.

Table 1

Gravimetrically corrected dust concentrations for each sampling location.

FIGURE 5

Locations of samplers around bolters for fi eld testing (not 

to scale).
emissions when operated under 

these conditions.

As a positive displacement 

drilling method, the mist drilling 

system is designed to wet the drill 

cuttings and expel them from the 

hole around the drill steel. If the 

drill cuttings are not completely 

wet, the risk exists for the respirable portion to be released into 

the atmosphere. Considering the 

elevated dust concentrations 

measured here, it is likely that 

this may be occurring during both 

fi eld and laboratory mist drilling 

testing, indicating that additional 

water or a change of drilling parameters might be necessary to 

reduce the dust emissions for this 

drilling technique. Further investigation should be undertaken to better identify opportunities for improvement of 

this technology’s performance.

Acknowledgments

The authors recognize L. Alcorn, P. Kovalchik, R. Matetic, P. McElhinney, J. Peterson and A. Prokop for their 

contributions in completing portions of the study. 

Disclaimers

The fi ndings 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. Mention of any company name or product does 

not constitute endorsement by the National Institute for 

Occupational Safety and Health.

References

“Code of Federal Regulations, Title 30, Mineral Resources,” 2007, 

Offi ce of the Federal Register, National Archives and Records Service, 

General Services Administration, Washington, D.C., Parts 33, 70, and 74.

Colinet, J.F., Shirey, G.A., and Kost, J.A., 1985, “Control of Respirable Quartz on Continuous Mining Sections,” Contract Report Number 

J0338033, U.S. Bureau of Mines, Pittsburgh, PA.

Goodman, G.V.R., Beck, T.W., Pollock, D.E., Colinet, J.F., and Organiscak, J.A., 2006, “Emerging technologies control respirable dust 

exposures for continuous mining and roof bolting personnel,” in Proceedings of the 11th U.S./North American Mine Ventilation Symposium, 

Mutmansky and Ramani, eds., London, pp. 211-216.

Goodman, G.V.R., and Organiscak, J.A., 2002, “Evaluation of methods for controlling silica dust exposures on roof bolters,” Transactions 

of Society for Mining, Metallurgy, and Exploration, Littleton, CO, Vol. 

312, pp. 133-138.

Kissell, F.N., 2003, Handbook for Dust Control in Mining, Department of Health and Human Services (NIOSH) Publication #2003-147, 

Information Circular No. 9465, Washington, D.C.

Matetic, R.J., 2006, “Assessment and Evaluation of Noise Controls 

on Roof Bolting Equipment and A Method For Predicting Sound Pressure Levels in Underground Coal Mining,” Ph.D. Dissertation, West 

Virginia University, College of Engineering and Mineral Sciences, Department of Mining Engineering, Morgantown, WV.

Mine Safety and Health Administration Standardized Information 

System, 2004, “Respirable Coal Mine Quartz Dust Data,” Available at 

U.S. Department of Labor, MSHA, Arlington, VA.

Ondrey, R., Stoltz, R., Atchison, D., and Gerbec, E., “Dust Sampling 

Roof Bolting Operations,” Mine Safety and Health Administration, 

Available at MSHA Website, http://www.msha.gov/s&hinfo/TECHRPT/

dust/dustsamp.pdf

Peterson, J.S., Kovalchik, P.G., Matetic, R.J., 2006, “Sound power 

level study of a roof bolter,” Transactions of Society for Mining, Metallurgy, and Exploration, Littleton, CO, Vol. 320, pp. 171-177.

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a means to control quartz,” Proceedings of the Coal Mine Dust Conference, Morgantown, WV, pp. 137-143.

 Bolter drilling system

 Vacuum Mist

Shift Right side Left side Right side Left side

sampled mg/m3

 mg/m3 mg/m3 mg/m3

Shift 1 0.61 0.30 1.57 1.49

Shift 2 0.36 0.36 0.95 1.43

Shift 3 0.11 0.07 1.63 1.54

Overall1

 0.36 (0.20) 0.24 (0.12) 1.38 (0.31) 1.49 (0.04)

1

Standard deviations are shown in parentheses.

Table 2

Gravimetric dust concentrations measured during fi eld bolter testing. Values are 

with intake dust contribution removed.