Emerging technologies control respirable dust exposures for continuous 

mining and roof bolting personnel 

G.V.R. Goodman, T.W. Beck, D.E. Pollock, J.F. Colinet, & J.A. Organiscak 

Pittsburgh Research Center, National Institute for Occupational Safety and Health, Pittsburgh, PA 

ABSTRACT: This work presents the findings from a number of NIOSH studies evaluating the impacts of 

emerging technologies that may reduce dust exposures for continuous mining and roof bolting personnel. 

These technologies include use of a wet head cutting drum on a continuous mining machine that places water 

sprays on the cutting drum instead of placing them on a manifold outby the drum. Evaluations at two separate 

operations showed that the use of the wet head sprays reduced dust levels at the miner operator in the return 

air, although these reductions were quite variable. NIOSH also assessed the effectiveness of a canopy air curtain for protecting roof bolting personnel. The data showed that dust levels were lower beneath the air curtain 

than outside the air curtain. This study also suggested several future modifications to the air curtain to increase its effectiveness. Finally, NIOSH evaluated mist drilling technology for its effectiveness in controlling 

respirable dust exposures of bolting personnel. A recent study showed that higher dust levels existed around a 

machine using the mist system compared to a machine using a conventional vacuum system for dust control. 

Unfortunately, the true impact of mist drilling was confounded by this machine operating downwind of the 

continuous miner for much of the study. 

1 INTRODUCTION 

The Federal Coal Mine Health and Safety Act requires that the concentration of respirable dust in the 

coal mine atmosphere not exceed 2.0 mg/m3 for a 

working shift. If quartz content on the dust filters 

exceeds 5% by weight, the dust standard is reduced 

according to the following expression: 10/(% 

quartz). Compliance with the standard or a reduced 

standard maintains quartz dust exposures at or below 

100 μg/m3. 

Over 17,000 samples collected by Mine Safety 

and Health Administration (MSHA) inspectors during the period 2000-2004 (MSHA, 2004) were analyzed for both respirable coal and quartz dusts. This 

included over 12,000 samples at the miner operator 

occupation and 5,000 samples at roof bolter occupations. Nearly 2,000 samples of the miner and bolter 

showed excessive occupational exposure to respirable coal dust while 3,000 samples at the same occupations showed quartz exposures in excess of 100 

μg/m3

. 

Such levels present unacceptable health risks to 

the U.S. coal miner. The high incidences of coal 

and quartz overexposures suggest that existing control systems do not consistently limit respirable 

quartz dust exposures at roof bolter and miner operator occupations. Research must, therefore, focus on 

evaluation and improvement of these needed controls. 

Continuous mining machines typically are 

equipped with external water spray systems and a 

fan-powered dust collector (scrubber), while roof 

bolting machines are equipped with vacuum (dry) 

dust collection systems. Novel technologies have 

been developed that offer the potential to further reduce dust exposures. A “wet head” continuous mining machine (water sprays situated in the cutting 

drum), a filtered air supply (canopy air curtain) for a 

roof bolting machine, and mist drilling technology 

are available for implementation on continuous mining operations. Each of these technologies offers the 

potential for reducing the dust exposures of underground coal miners. 

This paper documents NIOSH evaluations of 

these emerging technologies for controlling worker 

exposures to respirable dust. Recommendations are 

provided for effective use of these methods. 
2 CONTINUOUS MINER DUST CONTROL 

2.1 Wet Head Cutter Drum 

Water sprays are the most widely used technique for 

limiting worker exposure to respirable dust in underground coal mining. Water sprays wet surfaces 

to prevent airborne dust generation, which is most 

effective when the water sprays are placed as close 

as possible to these surfaces. Typical mining machines place water sprays approximately 30-40 cm 

away from cutting bits to protect the spray manifolds. Wet head cutting drum designs, on the other 

hand, introduce water via sprays located directly behind each cutting bit on the cutting drum. This 

places water at each cutting bit and in the region 

where the dust cloud forms, a significant advantage 

for limiting formation and growth of this cloud. 

Early studies of a prototype wet head machine 

showed 25-40% reductions in respirable dust levels 

at the mining machine operator (Strebig, 1975; Kost 

et al, 1976). Return dust levels showed similar reductions, an important consideration for miners 

working downwind of the continuous miner. However, comprehensive studies of wet head machines 

were not conducted to evaluate the impacts of external spray and scrubber use on dust reduction potentials. 

Poor mechanical reliability and high maintenance 

requirements of the wet head drum delayed its initial 

acceptance by the underground coal mining industry 

(Belle et al, 2002). A major manufacturer of underground mining equipment has recently introduced a 

redesigned version of the wet head cutting drum that 

improves both reliability and maintenance. 

NIOSH conducted two multi-shift studies of this 

new design to evaluate the effectiveness of the wet 

head cutting drum for limiting respirable dust exposures for the miner operator and downwind personnel. Pairs of gravimetric samplers were hung in the 

intake and return air of the continuous mining machine while the miner operator wore a single sampler. These samples were not collected for a full 

shift; hence the results could not be correlated to 

compliance sampling. 

2.2 Wet head study at Mine A 

This study compared dust levels around a single machine that could be configured with either a wet 

head spray system or a regular spray system. The 

machine ran with three configurations: (1) with the 

regular drum sprays and with the flooded-bed dust 

scrubber, (2) with the wet head sprays and with the 

scrubber, and (3) with the wet head sprays but without the scrubber. NIOSH evaluated the first and 

second configurations for five shifts each and the 

third configuration for two shifts. 

Mining height averaged 2.0 m for all test configurations with rock partings accounting for half of the 

height in some entries. A combination of a floodedbed dust scrubber rated at 3.3–3.5 m3

/s and an exhaust line curtain hung along the right rib ventilated 

the face. Exhaust curtain flow averaged 7.1 m3/s for 

configuration 1, 6.2 m3

/s for configuration 2, and 5.6 

m3

/s for configuration 3. Shift production averaged 

500 tons (range 290-575 tons) for configuration 

1,620 tons (range 360-810) for configuration 2, and 

420 tons (range 330-500 tons) for configuration 3. 

The regular spray system consisted of twentyseven hollow-cone water sprays for dust control 

mounted externally on the frame of the continuous 

miner, with an equivalent number located on the cutting boom. Water spray pressure on the external 

sprays varied from 1,034 to 1,276 kPa measured 

with a dial gauge on a right-hand end-ring spray. 

The wet head drum contained 73 small orifice solid 

stream sprays (each delivering 1.1 liters/min at 689 

kPa) oriented to spray directly on the 73 cutter bits. 

These sprays could be turned on or off depending on 

required test conditions. Wet head spray pressures 

varied from 448 to 655 kPa when measured by company personnel prior to testing. Water flow rates on 

the continuous miner were 174.3 liters/min with the 

wet head sprays operating and 181.9 liters/min with 

the regular sprays. In addition to the wet head 

sprays, several sprays on the cutting boom remained 

open to provide cooling water to the cutting motors 

and contributed to the total flow measured for the 

wet head spray system. 

Average gravimetric dust levels measured at the 

miner operator and in the return airway were slightly 

lower using the wet head cutting drum and the dust 

scrubber (figure 1). Removing the contribution of 

intake dust from miner operator and return dust levels showed that operation of the wet head sprays reduced dust levels by 0.5 and 0.3 mg/m3

, respectively 

at these two locations. However, the true impact of 

these sprays on respirable dust levels was likely hidden by the variability of the gravimetric data collected at these locations. For instance, miner operator dust levels ranged from 2.20 to 4.52 mg/m3

, 0.94 

to 4.26 mg/m3, and 1.94 to 6.15 mg/m3 for configurations 1, 2, and 3, respectively. Gravimetric dust 

levels in the return exhibited similar variations. 

Relatively high quartz dust levels in the intake airway were observed for configurations 1 and 2 (table 

1). These levels tended to be higher when rock was 

being crushed in the feeder breaker and its dust entered the intake air. This and the cutting of a sandstone rider above the coal seam elevated miner op-
erator quartz dust levels on several shifts. The highest quartz exposures for configuration 1 were recorded at the miner operator and in the return while 

cleaning up a roof fall from the previous shift. 

Table 1. Quartz dust levels for three configurations 

during wet head study, Mine A. 

Respirable quartz levels, 

Average (range), ug/m3 

Configuration Intake Miner Operator Return 

 1 103 (0-262) 356 (140-613) 413 (230-739) 

 2 82 (0-192) 311 (89-753) 359 (161-579) 

 3 20 (0-39) 372 (191-553) 621 (436-806) 

The data showed much higher dust levels for configuration 3, indicating that the wet head sprays 

could not control dust levels without the dust scrubber. The large amount of dust around the continuous 

miner made it difficult for the operator to see the 

cutter head. Based upon the sampling results, 

NIOSH recommended that the wet head machine not 

operate without the scrubber. 

0

2

4

6

8

Regular machine +

scrubber

Wet head + scrubber Wet head w ithout

scrubber

Sampling location

Respirable dust, mg/m 3

Intake Miner operator Return

Figure 1. Gravimetric dust levels for Mine A, with and 

without wet head sprays. 

2.3 Wet head study at Mine B 

This study compared dust levels around two continuous mining machines: one equipped with wet 

head sprays and the other equipped with a conventional spray system. Cutting height varied with face 

location but averaged 1.8 m for the regular machine 

with an average rock thickness of 0.5 m. Cutting 

height and rock thickness averaged 1.7 m and 0.3 m, 

respectively, for the wet head machine. Evaluation 

of the regular and wet head machines took place 

over four shifts. 

Section ventilation was single split with the wet 

head continuous miner operating upwind of the 

regular mining machine during the study (operation 

was not simultaneous). Blowing face ventilation for 

both machines was provided by an intake line curtain hung along the right rib and by a flooded-bed 

dust scrubber rated at 3.2 m3

/s on the wet head machine and 3.0 m3

/s on the regular mining machine. 

Airflow quantities behind the intake curtain averaged 4.6 m3

/s for the wet head mining machine and 

4.4 m3/s for the regular machine. Production for the 

wet head averaged 610 tons (range 430-740 tons) 

and 680 tons (range 580-910 tons) for the regular 

miner. 

The wet head continuous miner contained a total 

of 89 sprays, 63 of which were small orifice sprays 

located on the cutter head. The wet head sprays 

(each delivering 1.5 liters/min at 689 kPa) were oriented to spray directly on the 63 cutting bits. Wet 

head spray pressure was approximately 621 kPa as 

measured by company personnel prior to testing. 

With the scrubber operating, water flow on the wet 

head machine varied from 144 to 159 liters/min 

measured by a flow meter on the right side of the 

machine frame. External spray pressure was 1,034 

kPa measured with a dial gauge on this machine. 

Six sprays were open on the cutting boom to provide 

cooling water to the cutting motors. 

The regular machine contained a total of 41 dust 

control sprays, each delivering 2.6 liters/min at 689 

kPa. Water pressure on the sprays was 1,034 kPa 

when measured by a dial gauge on the mining machine. A flow rate measurement was not possible on 

this machine because of a broken flow meter. 

The intake and return samplers assigned to each 

machine ran only when that mining machine operated. The results of this study indicate lower dust 

levels at the three sampling locations when using the 

wet head (figure 2). After removing the intake dust 

contribution, the reduction at the miner operator location was only 0.2 mg/m3

 when operating the wet 

head sprays. Interestingly, the reduction in the return was nearly 2 mg/m3

 when using the wet head 

sprays. 

0

1

2

3

4

5

Wet head machine Regular Machine

Sampling location

Respirable dust, mg/m3

Intake Miner operator Return

Figure 2. Gravimetric dust levels for Mine B, with and 

without wet head sprays. 

Respirable quartz dust levels varied between the 

wet head and regular mining machines. Measurable 

levels were not detected on the intake airway or 

miner operator samples of the wet head machine. 
The return airway showed a respirable quartz level 

of 81 ug/m3 on the first shift, while return airway 

samples from other shifts showed no detectable 

quartz mass. Measurable levels of respirable quartz 

were not found in the intake airway samples of the 

regular mining machine. Quartz dust levels for the 

miner operator of the regular machine averaged 70 

ug/m3 (range 42 to 97 ug/m3) while return airway 

levels averaged 143 ug/m3 (range 108-193 ug/m3

). 

The true impacts of the wet head sprays on respirable coal and quartz dust levels were likely confounded by excessive variability in production, ventilation, and geologic factors between the two 

machines. It is the contention of the authors that 

dust reductions shown in figure 2 and quartz reductions described earlier may not have been due solely 

to the wet head sprays, but also to these variations. 

3 ROOF BOLTER DUST CONTROL 

3.1 Canopy Air Curtain 

Previous work by NIOSH researchers showed that 

dust levels for roof bolter operators could increase 

when working downwind of the continuous mining 

machine (Goodman et al, 2002). A canopy air curtain system was developed by NIOSH researchers 

and tested as a means for reducing these exposures. 

This device consists of a hollow metal plenum with 

a perforated surface on one side. The plenum is 

connected by flexible tubing to the output of a small 

centrifugal fan. A filter is placed on the intake side 

of the fan. Placed on the underside of the operator’s 

canopy, filtered air flows from the perforated plate 

and over the breathing zone of the person working 

beneath the air curtain (figure 3). 

Centrifugal fan

Inlet filter

Flex tubing Air curtain

flow

Bolter operator's

canopy

Figure 3. Schematic of canopy air curtain. 

Laboratory testing of this device showed that its 

effectiveness in limiting dust levels beneath the canopy was mostly a function of the velocity of the air 

moving across the face of the air curtain. Higher velocity air disrupted the air flowing from the air curtain, thus reducing its effectiveness. Testing of the 

air curtain showed dust reductions of 40% to 60% at 

a low air velocity of 0.3 m/s, a typical value in the 

bolter headings of many underground coal mines 

(Goodman et al, 2001). 

An underground study by NIOSH evaluated the 

canopy air curtain for its effectiveness in controlling 

respirable dust exposures for the bolter operators. 

Respirable dust levels were measured around a twin 

boom roof bolting machine equipped with a canopy 

air curtain mounted on the underside of the canopy 

of the downwind operator (figure 4). The canopy air 

curtain was constructed of 14-gauge steel and measured 61 cm long, 25 cm wide, and 5 cm thick. The 

underside of the air curtain was a section of perforated plate steel (2.4-mm diameter holes, 4.8-mm 

staggered spacing). The inside of the air curtain 

contained a 3.2-mm cell polycarbonate honeycomb 

material to straighten and baffle the air flowing into 

the air curtain. A small centrifugal fan (American 

fan, model AF-10-R12327-6) supplied 0.11 m3

/s to 

the air curtain and was mounted at the rear of the 

roof bolting machine adjacent to the tramming cab. 

A 6.1-m length of 10.2-cm diameter spiral tubing 

connected the air curtain to the centrifugal fan. Airflow to the air curtain was cleaned by an EPG model 

G110120 filter (Donaldson, Inc., Minneapolis, MN) 

mounted on the intake side of the fan. The canopy 

air curtain system provided a steady stream of filtered air over the operator’s breathing zone. 

Figure 4. Canopy air curtain installed on roof bolter. 

Dust sampling was conducted for three shifts beneath the air curtain and on the left and right sides of 

the bolting machine on the tool trays. Coal and 

quartz dust levels beneath the air curtain were 1.2 

mg/m3

 and 40 ug/m3 lower than dust levels on the 

tool trays (figure 5). However, these samples were 

collected close to the perforated plate of the air curtain because of headroom constraints and, as such, 

represent an “optimum” scenario for dust protection 

effectiveness. Data collected during this study suggested several ways to improve the efficiency of the 
0

1

2

3

Coal dust levels, mg/m 3

0

20

40

60

80

100

120

140

Quartz dust levels, ug/m 3

Coal dust Silica dust

Beneath air 

curtain

Tool trayleft sideTool trayright side

Sampling locations

Figure 5. Respirable coal and quartz dust levels beneath 

air curtain and around roof bolter. 

canopy air curtain, such as increasing air curtain size 

to increase coverage area and reducing tubing 

lengths from the fan to the air curtain (requiring positioning of the fan closer to the drill head). 

3.2 Mist drilling 

A mist head bolter injects a combination of water 

and compressed air through the drill steel to the drill 

bit to control drill dust, instead of using a conventional vacuum system with canister or bag filters. 

Air is supplied by an on-board compressor while the 

water is supplied by an on-board water tank or by a 

water hose dragged behind the machine. 

An underground study compared dust levels 

around two roof bolting machines, one using a mist 

system and the other using a conventional vacuum 

system to control drill dust. The mist bolting machine injected water and compressed air to the left 

bolting arm at rates of 2.6 liters/min and 75.8 liters/min, respectively. Water and air were supplied 

to the right arm at rates of 1.1 liters/min and 75.8 liters/min, respectively. The vacuum bolter used a 

four-compartment permissible vacuum dust collection system with a single canister filter to collect 

drill dust. These machines did not operate simultaneously. 

Two gravimetric samplers were attached near the 

left and right operator’s work location on each machine approximately 30-60 cm outby their controls. 

The bolter area dust level for each machine was the 

average of the dust concentrations from the left and 

right samples. Gravimetric samplers were also hung 

in the intake air of each machine. The sampling 

pumps on each machine operated only while that 

machine ran. Three shifts of data were collected for 

each bolting machine. 

For much of this study, the mist bolter worked 

downwind of the continuous mining machine, resulting in higher intake dust levels for this bolter than 

the vacuum bolter that typically worked in the clean 

air upwind of the continuous miner. As a result, 

gravimetric dust levels measured around the mist 

bolter were significantly elevated compared to dust 

levels around the vacuum machine (figure 6). Even 

after removing the contribution of intake dust, dust 

levels around the mist bolting machine still exceeded those around the vacuum machine. 

Quartz contents on the intake filters averaged 

10.6% (range 10.3 to 10.9%) at the vacuum machine 

and 9.5% (range 7.7 to 11.1%) at the mist machine 

for a three-shift study. Quartz contents averaged 

12.3% (range 9.6 to 16.3%) around the mist machine 

and 9.0% (range 7.6 to 10.4%) around the vacuum 

bolter. The higher quartz content around the mist 

bolter was likely a result of this machine working 

downwind of the continuous miner as it cut rock 

(rock thickness varied with face location, but ranged 

from 0.6 to over 1.0 m out of a mining height of 1.7 

m). This high quartz dust content coupled with the 

high gravimetric dust levels led to higher quartz dust 

concentrations around the mist bolter. 

0

1

2

3

4

5

6

Mist Vacuum

Bolter type

Respirable dust, mg/m3

Intake Bolter area

Figure 6. Impact of mist drilling on gravimetric dust levels. 

4 CONCLUSIONS 

NIOSH evaluated emerging technologies that limit 

respirable dust exposures for operators of continuous 

mining and roof bolting equipment. These technologies include use of a wet head cutting drum on a 

continuous mining machine that places water sprays 

on the drum and closer to the dust cloud. An evaluation at one operation showed that use of the wet 

head sprays reduced dust levels 0.5 mg/m3

 at the 

miner operator and 0.3 mg/m3

 in the return when adjusted for intake dust levels. Dust reductions at a 

second operation were about 0.2 and 2.0 mg/m3

 at 

the miner operator and in the return airway, respectively. In both studies, the true impacts of the wet 

head sprays were likely confounded by variations in 

gravimetric data, production, ventilation, and geologic factors. NIOSH also assessed the protection 

afforded roof bolter personnel by use of a canopy air 
curtain, a device that blows filtered air over the operator’s breathing zone. The data showed a reduction in dust levels of over 1.2 mg/m3

 beneath the air 

curtain compared to levels around the bolter. This 

study also revealed several ways to improve the efficiency of the air curtain, including increasing air 

curtain size to increase coverage area and reducing 

tubing lengths from the fan to the curtain. Mist 

drilling controls dust by injecting a combination of 

water and compressed air through the drill steels to 

the drill bit, instead of using a conventional vacuum 

system to draw dust back through the steel. A recent 

NIOSH study compared two bolting machines, one 

using a mist system and the other using a conventional vacuum system. The mist bolter worked 

downwind of the continuous miner for much of the 

study causing higher intake dust levels for this 

bolter. As a result, dust levels averaged 1.1 mg/m3

higher around the mist machine than around the 

vacuum machine, even after removing the intake 

dust contribution. 

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Controlling Occupational Exposures of Roof 

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International Mine Ventilation Congress, Krakow, Poland, 17-22 June, 2001. Krakow: 

EMAG. 

Goodman, G.V.R. and Organiscak, J.A. 2002. 

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Technique to Water-Flush the Bits on a Fixed 

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the MSHA Technology Center, Dust Division, 

Pittsburgh, PA 15236. 

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