1 

AN EVALUATION OF METHODS FOR CONTROLLING SILICA DUST EXPOSURES ON ROOF BOLTERS 

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

Natl Inst for Occuptnl Sfty and Health 

Pittsburgh, PA 

ABSTRACT 

This paper examines methods for limiting occupational 

silica exposures for roof bolting personnel in underground coal 

mining. A canopy air curtain and an air tube were evaluated as 

means to provide filtered air over the breathing zone of these 

personnel. Laboratory testing showed that dust reductions 

beneath the air curtain and air tube approached 60 percent and 

40 percent, respectively, with low interference air velocities. The 

data also showed that dust reductions were highest for positions 

closest to the device and that these levels decreased with 

distance. 

This paper also evaluates a metal rake, a flexible insert, 

and a rigid insert for cleaning the roof bolter dust collector box. 

Less dust was produced when using the rigid insert. Surprisingly, 

the flexible insert was sometimes no better than scraping or 

raking the dust onto the mine floor. However, little dust got on the 

coveralls when using either of the inserts. 

INTRODUCTION 

The Federal Coal Mine Health and Safety Act limits 

respirable dust exposures of mine workers to a time weighted 

average of 2.0 mg/m3

 for a working shift (U.S. Government 

Printing Office, 1999). If the respirable dust sample contains 

more than 5 percent silica by weight, the dust standard is reduced 

according to the formula 10 ) (percent silica). Compliance with 

the 2.0 mg/m3

 standard or a reduced standard maintains silica 

dust levels at or below 100 Φg/m3

. 

Data from the Mine Safety and Health Administration 

(MSHA) revealed that more than 3400 respirable dust samples 

were collected at underground coal roof bolter occupations in 

1999. Of these, nearly 70% exceeded 5 percent silica content on 

the dust samples, while nearly one-quarter of the samples 

exceeded 100 Φg/m3

 (MSHA, 1999). Clearly, roof bolter 

occupations exhibit a continued risk for overexposure to 

respirable silica dust. These high levels of noncompliance could 

result from operation of the roof bolter downwind of the 

continuous mining machine or from inadequate maintenance of 

the vacuum dust collection system. 

Data collected by NIOSH researchers showed that silica 

dust levels in the roof bolter intake increased when working 

downwind of the continuous mining machine. These dust levels 

were measured by placing four gravimetric samplers in the bolter 

intake. Two samplerswere used when bolting upwind of the 

mining machine and the other two used when bolting downwind. 

Comparing dust levels from each sampling set showed the effects 

of bolting downwind of the mining machine. 

The samplers consisted of constant-flow sampling pumps 

pulling dust-laden air through a 10-mm nylon cyclone at 2 lpm 

(liters/minute) to deposit the respirable fraction onto preweighed 

37-mm PVC filters. All filters were subsequently weighed and 

dust levels calculated. The filters were sent to an independent 

laboratory for analyses of crystalline silica using the infrared P-7 

Method (MSHA, 1973). 

The data showed that silica dust levels in the intake airway 

of the roof bolter increased by nearly 70 Φg/m3

 when operating 

downwind of the mining machine. At another operation, silica 

dust levels measured in the bolter intake increased by nearly 40 

Φg/m3

 when operating downwind of the mining machine. Similar 

results were found in Colinet et al (1985), Kok et al (1985), and 

Taylor et al (1986). To limit these silica exposures, many MSHA 

dust control plans limit the time that the roof bolter works 

downwind of the mining machine. 

Improper cleaning of the dust collector box and dust filters 

is another dust source for roof bolter personnel. Although the 

cleaning process has the potential to produce high silica dust 

levels, durations for these exposures are typically low (Colinet et 

al, 1985). However, bolter headings can be poorly ventilated so 

that any dust generated by dust collector box cleaning may linger 

near the bolter operators, increasing the duration of their 

exposure (Kok et al, 1985). Miners also can get dust on their 

clothing when cleaning the collector boxes and filters. Future 

movement by these workers may generate additional dust until 

the clothing is cleaned. Furthermore, dust from the collector 

boxes is generally dumped into tramways where it can be 

entrained into the mine atmosphere by movement of men and 

equipment. 

NIOSH studies also revealed that dust boxes and filters 

were not frequently cleaned during the shift causing dust to 

accumulate in the collector boxes and to eventually bypass the 

filters. Dust flowed from the mufflers of the bolter and formed a 

white haze around the machines, suggesting that immediate 

corrective action (cleaning collector box, changing filters) should 

be taken. 

Eventual cleaning of the dust collector boxes and filter 

reduced dust levels by 10 to nearly 80 percent at two surveyed 

operations (figure 1). The values in this figure are averages for 

bolting an entire cut before andthen after cleaning the collector 

boxes or filters. The mini-RAM units used in this figure are 

outputs from this light scattering device and are representative of 

a dust concentration. Little differences were evident between 

cleaning the collector boxes or cleaning just the filters. Although 

cleaning the collectors and filters reduced instantaneous dust 

readings, some dust continued to flow from the mufflers 

suggesting that fine dust particles were present throughout much 

of the vacuum system downstream of the filters. According to a 

bolter manufacturer, only a thorough cleaning and purging of the 

dust collection system may clear this dust. 

Infrequent cleaning and subsequent contamination of the 

vacuum dust collection system led to high silica dust levels at the 

roof bolter operator’s sampling location. During the two studies 

mentioned previously, dust levels were measured by placing 

gravimetric samplers on the frame of the roof bolter approximately 

1-1.2 m outby the controls of the return side bolter operator. 
2 

0

2

4

6

8

10

12

14

123456

Dust level, mini-RAM units

Before cleaning After cleaning

# = cleaned collector boxes/filters

* = cleaned filters only

*

#

* # *

#

Figure 1. Dust levels before and after cleaning dust boxes 

and/or filters. 

Gallery exhaust fan

*

 Dust 

Feeder

Test rig for filtered

air suppliers Fan

FilterTubing

x

x

x

Intake 

airflow

Return 

airflow

Return curtain

Regulator

Gravimetric dust

sampler

Figure 2. Test setup for filtered air suppliers. 

Silica dust concentrations and percentages at these 

surveyed operations ranged from 103 to 327 Φg/m3

 and 9.1 to 

13.7 percent. Levels were measured when operating upwind of 

the continuous mining machine and in clean intake air. Therefore, 

silica exposures arose from dust generated by the bolter and not 

by the mining machine. 

This paper describes methods for controlling occupational 

silica exposures for roof bolting personnel in underground coal 

mines. A canopy air curtain and an air tube system are tested as 

a means to limit exposures by supplying filtered air over the 

breathing zone of these personnel. Finally, dust levels are given 

for various methods of cleaning the roof bolter dust box. 

TESTING OF FILTERED AIR SUPPLIERS 

Two filtered air suppliers, the canopy air curtain and the air 

tube, were tested separately in a full-scale facility at the 

Pittsburgh Research Laboratory. This facility simulated a bolter 

heading 1.9 meters high, 5.5 meters wide, and 12.2 meters deep. 

This heading was ventilated by an exhausting line curtain hung 

0.9 m from the left side of the simulated heading. Each device 

was mounted 1.6 m above the floor on a wooden test rig placed 

1.0 m from the return curtain and 0.9 m from the curtain mouth 

(figure 2). 

Three gravimetric samplers each were placed 30.5 cm and 

61.0 cm beneath the filtered air supplier while three samplers 

were placed around the device to measure environmental dust 

levels. One was placed 1.5 meters in front of the test rig and one 

sampler 0.9 m to the left and right sides of the test rig. These 

three samplers were positioned 1.1 m above the floor. All 

sampling equipment was sized and calibrated similarly to that 

used in the underground studies. 

A 13.0 ∀ 0.9 percent mixture of silica dust in coal dust was 

used for this testing. This dust was introduced into the gallery 

12.2 m upwind of the air curtain or air tube to simulate dust flow 

from a mining machine working upwind of a roof bolter. 

Reductions in dust levels were calculated by comparing dust 

levels beneath the filtered air supplier to dust levels surrounding 

the device. Selected filters were analyzed for silica content using 

the MSHA P-7 Method. 

Canopy Air Curtain Testing

The canopy air curtain is constructed of a square steel 

frame 43.2 cm wide, 43.2 cm long, and 7.6 cm thick. Air enters 

this device at one side of the frame and passes along an intake 

plenum and through a series of aluminum honeycomb flow 

straighteners (figure 3). The airflow then enters another 

honeycomb flow straightener before exiting through a perforated 

steel plate. The perforations are 2.38-mm diameter holes drilled 

on 6.35-mm centers. The perforated plate is roughly 40.6 cm by 

40.6 cm. 

The tested system consisted of the canopy air curtain 

connected to a 5.6-kW (7.5-hp) centrifugal fan (American Fan 

Co., OH) by 20.3-cm (8 in) diameter metal tubing. A Donaldson 

Company (Minneapolis, MN) ECG11-2501 Konepac filter was 

mounted on the inlet side of the fan to filter dust from the air 

supplied to the air curtain (Goodman and Organiscak, 2001). 

Tests were run with air quantities of 0.094 (200 cfm) and 

0.283 m3

/s (600 cfm) supplied to the air curtain and with 

interference air velocities of 0.32 m/s (60 fpm) and 0.64 m/s (120 

fpm). Interference airflow is the ventilation airflow moving in the 

bolter heading and, at higher velocities, may distort air patterns 

emanating from the canopy air curtain. The lower value of 0.32 

m/s represents the lowest velocity permitted in bolter headings. 

Five tests were made at each set of canopy flow and interference 

air velocity to assess errors in dust reduction potentials. Test 

durations were 50 minutes. 

Figure 3. Canopy air curtain construction and testing. 

Figure 4 gives mean dust reductions and 95% confidence 

levels for distances of 30.5 and 61.0 cm beneath the canopy. 

Positive reductions signify protection from environmental dust 

concentrations, whereas negative reductions show increases in 

dust concentrations. 

-40

-20

0

20

40

60

80

Dust reductions, %

30.5 cm from canopy

61.0 cm from canopy

Low curtain flow,

low interference 

velocity

Low curtain flow,

high interference 

velocity

high curtain flow,

low interference 

velocity

high curtain flow,

high interference 

velocity

Figure 4. Canopy air curtain performance. 

The data showed that efficiencies always were highest 

when sampling close to the canopy and that these efficiencies 

decreased with distance. Also, reductions were highest when 

interference air velocity was lowest. This suggests that dust 
3 

reductions approaching 60 percent were possible in headings with 

low interference air velocities, a likely occurrence in many bolter 

headings. Increases in interference air velocity adversely affected 

the performance of the canopy air curtain. Detailed statistical 

analyses of test parameters and theirinteractions are given in 

Goodman and Organiscak (2001). 

Silica contents averaged 6.10 ∀ 0.40 percent for samples 

30.5 cm beneath the air curtain, 5.95 ∀ 0.56 percent for samples 

61.0 cm beneath the canopy, and 6.13 ∀ 0.35 percent for 

samples outside the canopy. Statistical analyses showed that 

respirable silica contents beneath the canopy were not 

significantly affected by changes in either air curtain quantity or 

interference air velocity. Subsequent pair wise comparisons 

revealed no significant differences between silica contents 

measured under the canopy and those contents outside the 

canopy suggesting that the canopy air curtain was equally 

effective on both coal and silica particles. 

Air Tubing Testing

Although the canopy air curtain provided adequate dust 

reductions, its 7.6-cm thickness could detract from its usefulness 

in lower seam operations. To provide more headroom for the roof 

bolter operator, an air tube was placed on the edge of the canopy 

and tested for its efficiency in reducing concentrations of 

respirable coal and silica dusts. The air tube was a 45.7-cm (18-

in) length of 63.5 mm (2.5 in) internal diameter PVC tubing. A 90-

degree section of the tubing was removed over a 40.6-cm (16-in) 

length and covered with a perforated steel plate. The perforations 

were 2.4-mm diameter holes drilled on 6.4-mm centers. 

Polycarbonate hexagonal flow straighteners with 3.2-mm (1/8-in) 

openings redirected the tubing airflow to exit from the ninetydegree section through the perforated plate (figure 5). Airflow 

was distributed over the tube length by means of an internal 

baffle. 

The air tube was connected to an American Fan Company 

0.6-kW (0.75-hp) centrifugal fan using 12.7-cm (5-in) diameter 

rigid tubing and 10.2-cm (4-in) diameter flexible tubing. A 

Donaldson Company G11-0120 Radial Seal dust filter was 

mounted on the intake side of the fan. This device was mounted 

on theunderside of a surface representing the bolter operator 

canopy. Initial testing revealed that placing the tube on the 

upwind side of the canopy and angling it 45 degrees downward 

delivered more air over the operator’s breathing zone. 

Air quantities of 0.057 and 0.103 m3

/s (120 and 220 cfm) 

and interference air velocities of 0.32 m/s and 0.64 m/s were 

evaluated in this testing. Again, five tests of 50 minute duration 

were run with each combination. 

Figure 5. Air tube construction and testing. 

At a distance of 30.5 cm from the air tube, dust reductions 

approached 40 percent with low interference velocity in the bolter 

heading (figure 6). Increasing interference air velocity adversely 

affected air tube performance, especially at the low tubing flow 

where negative dust reductions occurred at both 30.5 and 61.0 

cm from the tubing. Like the canopy air curtain, dust reductions 

decreased as distance from the air tube increased. 

Silica dust content averaged 6.75 ∀ 0.51 percent for those 

filters 30.5 cm beneath the air tube and 7.00 ∀ 0.37 percent for 

samples taken from the environment surrounding the air tube. 

Statistical analyses showed that at 95% confidence, no significant 

differences in silica contents existed between samples 

surrounding the air tube and samples beneath the air tube. 

Furthermore, changes in silica contents were not significantly 

affected by changes in tubing air quantity or interference air 

velocity. As with the canopy air curtain, the air tube was equally 

effective on both coal and silica particles. 

The air tube produced dust reductions approaching 40 

percent in tests with low airflow. Although the air tube was more 

adversely affected by higher interference airflows than the canopy 

air curtain, the air tube offerscertain advantages. This device is 

less obtrusive than the air curtain because it is mounted at the 

edge of the operator’s canopy. Air tube operation also may 

require a smaller fan which will produce less heat and noise. 

CLEANING THE ROOF BOLTER DUST BOX 

Roof bolter personnel can reduce their occupational 

exposure to respirable silica dust by maintaining the vacuum dust 

collection system on the machine. This includes proper cleaning 

of the dust collector box by removing drill cuttings and placing 

them in the return or against the rib. 

This study examined three methods of cleaning the dust 

collector box. These include an unacceptable, but very common, 

method of using a metal rake to scrape dust from the collector 

box onto the mine floor. Dust likely falls onto the worker’s 

clothing. A second method inserts a rigid box with open top into 

the largest compartment of the bolter’s dust collector box. This 

rigid box measures approximately 38 cm wide by 28 cm high by 

32 cm deep and is constructed of 16-gauge steel with recessed 

handles welded on the front and top sides. When full of dust, the 

rigid box is carried to the rib and carefully dumped to control the 

dust cloud. A final method inserts a lightweight, flexible bag in the 

largest dust collector box compartment. This bag is sized 

similarly to the rigid box and is constructed of lightweight bratticetype material with handles sewn on the top. This bag also is 

carried to the rib and dumped when full of dust. 

The different methods were evaluated on an EIMCO 300-10 

(EIMCO, LLC; Bluefield, WV) single boom roof bolter. This 

machine uses a permissible four-compartment Donaldson 

vacuum dust collector with a single cannister-type filter. The 

bolting machine was placed in a heading ventilated by an 

exhausting line curtain hung 0.9 m (3 ft) from the right rib. 

Ventilation problems limited face airflow to 0.5 -1.0 m3

/s (1000 -

2000 cfm) behind the curtain. The dust collector box was located 

on the right side of the bolter chassis. 

Figure 6. Air tube performance. 

To avoid potential health risks of working with silica dust, 

limestone rock dust instead was used. Sieve analyses of rock 

dust and drill dust from this drill showed that the rock dust had a 

finer composition (table I). Two-22.7 kg (50 lb) bags of rock dust, 
4 

were opened and set onthe mine bottom near the drill chuck. The 

vacuum hose running to the drill chuck was disconnected and, 

after energizing the bolter, was used to vacuum-up the rock dust 

pile. After de-energizing the machine, the dust collector box was 

cleaned using one of the methods described earlier. New rock 

dust was used every five tests to avoid altering the size fraction of 

the dust. The filter was cleaned every three tests. 

 Table I. Sieve analyses of drill dust and rock dust 

 Cumulative Percent Finer Than Size (%) 

Sieve size #4 #7 #16 #30 #40 #60 #100 #200 Fines

Opening size (Φm) 4750 2800 1180 600 425 250 150 75 <75

Drill dust 100 98.4 85.0 64.4 53.6 38.2 26.9 16.8 0 

Rock dust 100 100 100 100 100 99.9 61.0 25.8 0 

Due to the short test durations, gravimetric sampling at 2 

lpm did not collect sufficient mass on the filters to permit accurate 

analyses. Instead, all samples were collected at 4.2 lpm. Use of 

this higher flow rate required a change in the cyclone 

preseparator from a 10-mm nylon cyclone to a Gk-2.69 metal 

cyclone (BGI, Inc., Waltham, MA). Constant flow pumps pulled 

dust-laden air through BGI cyclones at 4.2 lpm. The respirable 

fraction was then deposited onto preweighed 37-mm PVC filters. 

These filters were weighed following each test and dust levels 

calculated. 

Accurate measurement of the respirable size fraction was 

not compromised by changing sampling rate and cyclone design. 

Work conducted by Kinney and Gussman (1997) showed that a 

d50=4.2 Φm was obtained with the BGI cyclone sampling at 4.2 

lpm. This was similar to the value obtained when sampling at 2 

lpm with the 10-mm nylon cyclone (NIOSH, 1995). 

For each test, only the largest compartment of the dust 

collector box was cleaned and this kept test durations at one 

minute. Five replicates of each cleaning method were made and 

were split between Researcher A and Researcher B. The five 

tests were averaged to establish an average dust concentration 

for each cleaning method. Dust levels were measured with two 

BGI gravimetric samplers in the breathing zone of the researcher 

cleaning the dust collector box. Separate samplers were used for 

each cleaning method to note any differences in dust levels. 

The test results in figure 7 show that cleaning thedust 

collector box using the rigid box insert produced minimum dust 

levels compared to those measured when using the brattice bag 

and rake. Not only were dust levels smaller, but we observed 

little dust on the coveralls of the researchers. 

Surprisingly, the brattice bag insert was not very effective 

and sometimes was less effective than raking the dust onto the 

mine floor. This was attributed to dust “puffing” off the bag as it 

flexed and bent while being pulled out of the dust collector box 

and while being carried to the rib. It is possible that the “puffing” 

problem will be smaller when used with coarser drill dust. Like 

the rigid box, cleaning the dust collector box with the brattice bag 

insert did not get much dust on the researcher’s coveralls. 

The rake produced much dust as the dust mass fellfrom the 

collector box onto the mine floor. In the process, dust got on the 

coveralls of the researchers. Furthermore, there was no way to 

get the dust to the rib without rehandling the dust. When cleaning 

the dust collector box using either the rigid box or the flexible bag, 

carefully rolling the box or bag at the rib successfully limited the 

size of the dust cloud. 

SUMMARY 

Recent field work by NIOSH researchers showed that high 

silica dust levels around a roof bolting machine arose when 

working downwind of the continuous mining machine. One 

means to limit exposure is the use a canopy air curtain or an air 

tube to provide a constant flow of filtered air over the breathing 

zone of roof bolter personnel. This laboratory work evaluated the 

effectiveness of each device for limiting exposures to respirable 

coal and silica dust. 

The results showed that dust reductions were highest when 

sampling close to each device and that efficiencies decreased 

rapidly with distance. Furthermore, reductions with both devices 

tended to decrease rapidly when interference air velocities 

increased, although the air tube was most affected by such 

increases. Increases in air flow supplied to the canopy air curtain 

and air tube generally led to increased dust reductions. The 

canopy air curtain provided dust reductions approaching 60 

percent in headings with a low interference air velocity, a likely 

condition in many bolter headings. Reductions approached 40 

percent when using the air tube under similar conditions. 

0

2

4

6

8

10

12

14

16

1234

Test Number 

Dust levels, mg/m3

Metal rake

Flexible bag

Rigid box

Figure 7. Results of dust box cleanings. 

Silica dust contents were not affected by changes in flow 

quantity or interference air velocity when using the canopy air 

curtain or the air tube. Generally, silica contents measured under 

the air curtain and air tube were not significantly different from 

those measured outside. 

Roof bolter personnel can control their exposures to 

respirable silica dust through proper cleaning of the dust collector 

box by removing drill cuttings and placing them against the rib. 

Three different methods were examined for cleaning this box. 

The first used a metal rake to scrape the cuttings onto the mine 

floor. The other two methods used a rigid metal box or a flexible 

bag inserted in the largest compartment of the dust collector box 

to hold the drill cuttings. When full of dust, the rigid box or flexible 

bag was carried to the rib and carefully dumped. 

Less dust was produced when using the rigid boxinsert. 

Not only was less dust generated when dumping the rigid box, but 

we observed less dust on the clothing. The flexible bag did not do 

as well as the rigid box and sometimes did no better than 

scraping the dust onto the mine floor. This was attributed to 

“puffing” of dust as the bag flexed and bent while being carried to 

the rib. As with the rigid box, little dust got on the clothing. Much 

dust was present on the clothing when scraping the dust onto the 

mine floor. Also, there was no way to get this dust against the rib 

without rehandling it. 

REFERENCES 

Colinet, J.F., Shirey, G.A., and Kost, J.A., 1985, “Control of 

Respirable Quartz on Continuous Mining Sections,” 

Contract Report No. J0338033, U.S. Bureau of Mines, 

Pittsburgh, PA. 

Goodman, G.V.R. and Organiscak, J.A., 2001, “Laboratory 

Evaluation of a Canopy Air Curtain for Controlling 
5 

Occupational Exposures of Roof Bolters,” Proceedings, 7th

International Mine Vent. Congress, Krakow, Poland, June, 

pp. 299-305. 

Kok, E.G., Adam, R.F.J., and Pimentel, R.A., 1985, “Control of 

Respirable Quartz on Continuous Mining Sections,” 

Contract Report No. J0338078, U.S. Bureau of Mines, 

Pittsburgh, PA. 

Kinney, L.C. and Gussman, R.A., 1997, “Characterisation and 

Modelling of a Family of Cyclone Aerosol Preseparators”, J. 

of Aerosol Science, Vol. 28, No. 4, April, pp. 677-688. 

Mine Safety and Health Administration (MSHA), 1973, “Standard 

Method No P-7, Infrared Determination of Respirable 

Quartz in Respirable Coal Mine Dust,” Internal Document, 

US Dept. of Labor, Pittsburgh, PA. 

Mine Safety and Health Administration (MSHA), 1999, 

“Respirable Coal Mine Quartz Dust Data,” available from 

the MSHA Technology Center, Dust Division, Pittsburgh, 

PA 15236. 

National Institute for Occupational Safety and Health (NIOSH), 

1995, “Occupational Exposure to Respirable Coal Mine 

Dust - Criteria for a Recommended Standard,” U.S. Dept. of 

Health and Human Services, Centers for Disease Control 

and Prevention, 336 pp. 

Taylor, L.D., Thakur, P.C., and Riester, J.B., 1986, “Control of 

Respirable Quartz on Continuous Mining Section,” Contract 

Report No. J0338077, U.S. Bureau of Mines, Pittsburgh, 

PA. 

U.S. Government Printing Office, 1999, Mineral Resources Code 

of Federal Regulations Title 30, Parts 70 and 75.