LABORATORY EVALUATION OF A CANOPY AIR CURTAIN 

FOR CONTROLLING OCCUPATIONAL EXPOSURES OF ROOF BOLTERS 

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

Pittsburgh Research Laboratory 

National Institute for Occupational Safety 

and Health 

Pittsburgh, PA 15236 USA 

Pittsburgh Research Laboratory 

National Institute for Occupational Safety 

and Health 

Pittsburgh, PA 15236 USA 

ABSTRACT 

In the United States, respirable coal mine dust exposures are limited to a 2 mg/m3

 time weighted average for a 

working shift. If the silica content of the sample exceeds 5 percent, the 2 mg/m3

 standard is reduced according to 10 )

(% silica) to limit silica dust exposures to 100 μg/m3

. Examinations of dust sampling data collected by the federal 

coal mine inspectorate showed that roof bolter occupations continue to be at risk for overexposure to respirable silica 

dust. Testing at the Pittsburgh Research Laboratory of the National Institute for Occupational Safety and Health 

assessed the effectiveness of a canopy air curtain for controlling occupational exposures to respirable silica and coal 

dusts. Initial testing found that zones of higher velocity airflow shrank as distance beneath the canopy increased. This 

suggested that protection from occupational dust exposures would likely decrease as distance beneath the canopy 

increased. Further testing revealed that coal dust reduction efficiencies were most affected by changes in interference 

air velocity and, to a lesser extent, changes in canopy air quantity. However, respirable silica dust percentages under 

the canopy were not affected by such changes and, in fact, were not significantly different from silica percentages 

measured outside the canopy. This implied that the air curtain was equally effective on both respirable coal and 

respirable silica dust particles.

KEYWORDS 

Coal mine dust, engineering controls, silica dust, underground mining, coal mining 

INTRODUCTION 

The Federal Coal Mine Health and Safety Act (the 

Act) limits the respirable dust exposure 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) (% silica). This maintains silica dust 

levels at or below 100 μg/m3

. 

In the United States, coal mine dust samples are 

collected using constant flow pumps to pull dust-laden 

air through a 10-mm nylon cyclone preseparator at a 

rate of 2.0 liters/minute. This deposits the respirable 

mass onto pre-weighed 37-mm filters. The Act 

stipulates that the calculated dust concentration be 

multiplied by 1.38 to convert it to an equivalent 

concentration measured with a British Mining Research 

Establishment sampling device. 

Data from the Mine Safety and Health Administration 

revealed that more than 3200 respirable dust samples 

were collected at roof bolting occupations in 1998. Of 

these, more than 12 percent of the samples exceeded the 

2 mg/m3

 standard for respirable dust while 25% of the 

samples exceeded the 100 μg/m3

 level for respirable 

silica dust (MSHA, 1999). Clearly, roof bolters 

occupations continue to be at risk for overexposure to 

respirable silica dust. 

Previous work showed that several operational 

factors contributed to this increased risk. Higher levels 

of silica dust were measured at the roof bolter when 

this machine operated in the return air of the 

continuous mining machine. Besides exposures from 

silica dust generated during cutting and loading cycles, 

exposures often increased when the mining machine 

cut rock (Organiscak et al., 1990; Kok et al., 1985). 

Exposures also increased when dumping the dust 

collector box, although this source was very 

controllable with proper care and attention. Respirable 

silica levels generated during bolting could vary 

considerably depending upon the condition of the dry 

collection system. Both studies showed, however, that 
these collection systems were very effective when 

properly maintained. 

One means to limit downwind exposures is use of a 

filtered air supply. Placed on the underside of the 

bolter canopy, the filtered air supply or canopy air 

curtain (CAC) can blow a constant stream of fresh, 

filtered air over the breathing zone of the bolter 

operator. Such devices were previously developed 

under contract with the U.S. Bureau of Mines and were 

investigated for dust reduction performance 

(Donaldson Company, Inc., 1975). When placed under 

the canopy of a continuous miner, dust reductions 

varied from 30 to 75 percent, depending upon the 

length of time the operator spent beneath the air 

curtain. However, this air curtain design did not allow 

adequate headroom beneath the operator=s canopy. A 

second-generation CAC was developed later that 

improved both headroom and distribution of airflow 

around the operator’s head (Donaldson Company, Inc., 

1985). Laboratory testing of this improved device 

showed an 89% reduction in respirable dust. Work 

conducted by the Mining Research and Development 

Establishment (MRDE) in Great Britain showed that 

reductions in operator exposure suffered when the 

external or interference air velocity exceeded 0.51 

m/sec (MRDE, 1981). High interference air velocities 

could overwhelm the airflow of the CAC and push 

dust under the canopy. Subsequent work in lower 

airflow headings showed dust reductions of 73% in 

underground trials (MRDE, 1983). 

Due to the lower velocities common in bolter work 

places, the air curtain may be a viable means of 

controlling occupational exposures at roof bolter 

positions. This work evaluates the effectiveness of a 

canopy air curtain system for controlling exposures to 

both respirable coal and silica dusts. Relationships are 

also defined between the protection provided by the air 

curtain and variables such as interference air velocity 

and canopy airflow. The effects of velocities emanating 

from the canopy and their impacts on the protection of a 

bolter operator situated beneath this canopy are 

evaluated. 

EXPERIMENTAL DESIGN 

The canopy air curtain was tested in a full-scale 

facility at the Pittsburgh Research Laboratory. This 

facility simulated a bolter heading 1.93 meters high, 

5.49 meters wide, and 12.19 meters deep (Figure 1). 

This heading was ventilated by a brattice curtain hung 

91.4 cm from the left side of the simulated heading. The 

main gallery fans drew from 2.83 to 5.66 cubic meters 

per second (m3

/s) of ventilation airflow through this 

curtain. A continuous recording anemometer was placed 

behind the ventilation curtain 7.01 m from the curtain 

mouth and 1.14 m above the floor to measure this 

airflow. 

The canopy air curtain is constructed of a square steel 

frame 43.2 cm wide by 43.2 cm long. Air enters this 

device at one side of the frame and passes along an 

intake plenum and through a honeycomb flow 

straightener to disperse the air along the intake side of 

the curtain equally. The airflow then enters another 

honeycomb flow straightener before exiting through a 

perforated steel plate. The perforations were 2.38-mm 

diameter holes drilled on 6.35-mm centers. The 

perforated plate was roughly 40.6 cm by 40.6 cm. 

Gallery exhaust fan

*

 Dust 

Feeder

*

Anemometer

Test 

rig

Fan

FilterTubing

x x x

x

x

x

xxx

xxx

Top view

Side view

# = RAM monitor plus gravimetric sampler

x = gravimetric sampler

x

#

x

Intake 

airflow

Return 

airflow

Return curtain

Test rig views

Canopy air

curtain

Regulator

30.5 cm

30.5 cm

30.5 cm

20.3 cm

20.3 cm

Figure 1. Experimental setup

The air curtain was mounted 1.63 m above the floor 

on a wooden test rig placed 1.02 m from the return 

curtain and 91.4 cm from the curtain mouth. A 5.6 kW 

(7.5 hp) centrifugal fan was connected to the air curtain 

with 20.3-cm diameter metal ducting. This fan 

generated airflow quantities of 0.094, 0.188, and 0.283 

m3

/s that were checked with 10-point pitot tube 

traverses in a straight run of the ducting. Because this 

fan provided more air than required for this study (0.944 

m3

/s at 3.72 kPa static pressure), air was bled as needed 

behind the return curtain. Static pressures across the 

canopy were 0.22 kPa, 0.45 kPa, and 1.04 kPa at flows 

of 0.094, 0.188, and 0.283 m3

/s, respectively. 

A Donaldson Company (Minneapolis, MN, USA) 

ECG11-2501 Konepac filter was attached on the inlet 

side of the fan to filter dust from the air being supplied 

to the air curtain. This filter was selected due to its high 

efficiency (99.9% for particles less than 200 μm) and 

low initial pressure restriction (0.44 kPa at 0.283 m3

/s) 

(Donaldson Company, Inc., 1987). 
Initial evaluations of the canopy air curtain defined 

velocity profiles at distances of 30.5, 61.0, and 91.5 cm 

beneath the perforated plate. Air curtain quantities of 

0.094, 0.188, and 0.283 m3

/s were used. Velometer 

measurements were made on a preset grid pattern of 

7.6-cm increments (Figure 2). These evaluations were 

made without the presence of gallery airflow 

(interference airflow) to more easily define practical 

limits of protection with the air curtain. 

3(7.6) 9(22.9) 15(38.1) 21(53.3)

3(7.6)

9(22.9)

15(38.1)

21(53.3)

27(68.6)

x = velocity measurement location

9 (22.9) = distance along measurement grid, inches (cm)

Canopy perimeter

Airflow to 

canopy

Figure 2. Velocity measurement grid 

Subsequent testing evaluated the ability of the air 

curtain to reduce dust exposures for personnel working 

beneath this device. A mixture of silica dust in coal dust 

was used for this testing. Samples of this silica-coal 

mixture sent to an independent laboratory for bulk 

analysis showed that the feed dust averaged 13.0 ± 0.9 

percent silica. This dust was introduced into the test 

gallery 12.19 m upwind of the canopy air curtain to 

simulate dust flow from a continuous miner working 

upwind of a roof bolter. Respirable coal dust levels 

surrounding the air curtain averaged 4.01 ± 0.14 mg/m3

for all tests while respirable silica dust levels averaged 

6.19 ± 0.36 percent. 

Dust samples were taken by constant flow sampling 

pumps pulling dust-laden air through 10-mm nylon 

cyclone separators at a rate of 2.0 liters/min to deposit 

the respirable mass onto preweighed 37-mm filters. All 

filters were subsequently weighed and dust levels 

calculated. Selected filters were sent to an independent 

laboratory for P-7 silica analysis (MSHA, 1973). 

For this testing, three gravimetric dust samplers each 

were placed 30.5 cm, 61.0 cm, and 91.5 cm beneath the 

canopy air curtain. In each set of three, one sampler was 

placed directly beneath the center of the perforated plate 

with the remaining two samplers placed 20.3 cm away 

along a diagonal line drawn between two opposite 

corners (Figure 1). Three samplers were also placed 

away from the air curtain to measure surrounding or 

environmental dust levels. One was placed 1.52 meters 

in front of the air curtain, one was placed 91.4 cm to the 

left side of the test rig, and the final sampler placed 91.4 

cm to the right of the test rig. These three samplers were 

situated 1.07 m above the floor. Finally, one RAM 

(Realtime Aerosol Monitor, GCA Corp., Bedford, MA) 

was placed 1.52 m in front of the air curtain and 1.07 m 

above the floor to monitor dust levels during testing. 

To provide robust assessments of the protection 

provided by the canopy air curtain, wide ranges were 

selected for both interference air velocities and canopy 

air flows. Gallery air quantities of 2.83 m3

/s and 5.66 

m3

/s were selected corresponding to interference 

velocities of 0.32 m/s and 0.64 meters per second (m/s), 

respectively, around the air curtain test rig. Air 

quantities supplied to the canopy air curtain ranged from 

0.094 to 0.283 m3

/s. Tests were conducted for each 

combination of interference air velocity and canopy air 

flow. Five replicates were made to assess the potential 

error in each combination. Two interference air 

velocities combined with two canopy air quantities 

defined the four corners of the experimental design 

square. The presence of curvature or non-linearity was 

defined between the dependent variable of dust 

reduction and the independent variables of interference 

air velocity and canopy air flow. To evaluate potential 

curvature, five replicates were run at the middle point of 

each independent variable. The middle point was 

defined by a gallery flow of 4.25 m3

/s, corresponding to 

an interference velocity of 0.48 m/s, and an air curtain 

air flow of 0.189 m3

/s. Tests were run for 50 minutes to 

provide measurable dust quantities for all samples. 

The dust concentration at each point beneath the 

canopy was the average of all replicates for that set of 

conditions. Dust levels outside the canopy were 

calculated by averaging the three exterior dust 

measurements for all replicates of a specific set of 

conditions. The average protection efficiency was then 

calculated as one minus the ratio of dust concentration 

beneath the canopy to concentration outside the canopy, 

multiplied by 100. 

ANALYSIS OF TEST DATA 

Air Curtain Velocities

Tests of velocities were made at 30.5, 61.0, and 91.5 

cm beneath the perforated plate with the canopy air 

curtain operating at 0.094, 0.188, and 0.283 m3

/s. With 

0.094 m3

/s supplied to the air curtain, the velocity 

profile 30.5 cm beneath the curtain is shown in figure 3. 

Nearly one-half of the area beneath the air curtain 

received between 1.0 and 1.5 m/s airflow. Air flow from 

the centrifugal fan, entering at a corner of the air 

curtain, led to the higher flow velocities in this area. The 

opposite corner of the air curtain, representing the 

furthest distance from the inlet, was a region of low air 
velocity. As distance beneath the canopy increased, the 

zone of higher velocity air shrank noticeably, although 

it remained beneath the center of the canopy. Zones of 

lower velocity air then covered most the area beneath 

the air curtain. This suggested that, although protection 

from occupational dust exposures likely decreased as 

distance beneath the canopy increased, zones of 

increasing protection were found beneath the center of 

the canopy. 

3(7.6) 9(22.9) 15(38.1) 21(53.3)

3(7.6)

9(22.9)

15(38.1)

21(53.3)

27(68.6)

9 (22.9) = distance along measurement grid, inches (cm)

Airflow to 

canopy

= 0 - 0.5 meters/sec.

= 0.5 - 1.0 meters/sec.

= 1.0 - 1.5 meters/sec.

Canopy perimeter

Figure 3. Velocity contours for canopy flow = 0.094 

m3

/s, 30.5 cm beneath canopy 

3(7.6) 9(22.9) 15(38.1) 21(53.3)

3(7.6)

9(22.9)

15(38.1)

21(53.3)

27(68.6)

9 (22.9) = distance along measurement grid, inches (cm)

= 2.0 - 3.0 meters/sec.

= 1.0 - 2.0 meters/sec.

= 0 - 1.0 meters/sec.

Canopy perimeter

Airflow to 

canopy

Figure 4. Velocity contour for canopy flow = 0.283 

m3

/s, 30.5 cm beneath canopy 

As air curtain flow increased to 0.283 m3

/s, zones of 

higher air velocity appeared near the inlet. At a distance 

of 30.5 cm beneath the canopy, velocities exceeding 1.0 

m/s were present beneath a large portion of the area 

under the canopy (Figure 4). However, velocities 

diminished with distance from the inlet and with 

distance beneath the canopy implying that, despite 

higher canopy air curtain flows, zones of decreased 

protection could remain. 

Dust Reductions With Air Curtain

Testing initially measured dust reduction efficiencies 

at the extreme points of the test matrix; that is, air 

curtain flows of 0.094 and 0.283 m3

/s and interference 

air velocities of 0.32 and 0.64 m/s. Subsequent testing 

then determined the presence of any curvature or nonlinearity between canopy air flow, interference air 

velocity, and dust reduction potentials. This involved 

repeated testing at the center point of the matrix; that is, 

an air curtain flow of 0.188 m3

/s and an interference air 

velocity of 0.48 m/s. Table 1 gives mean dust reduction 

efficiencies and 95% confidence levels for distances of 

30.5, 61.0, and 91.5 cm below the air curtain. Higher 

efficiencies signify increased protection from exterior 

dust concentrations. 

Not surprisingly, the data showed that, for each 

combination of canopy flow and interference velocity, 

efficiencies were highest when sampling close to the 

canopy and that these efficiencies decreased rapidly 

with distance. This was expected given the high 

velocities found near the canopy and their rapid 

decrease at greater distances. 

Table 1. Mean reduction efficiencies and 95% 

confidence limits. 

Air Reduction efficiencies (%) 

curtain 

flow 

(m3

/s) 

Interference air 

velocity

(m/s) 

30.5 cm 61.0 cm 91.5 cm 

0.094 0.32 56.6 ± 2.9 42.7 ± 6.7 29.4 ± 16.1 

0.094 0.64 28.5 ± 20.7 -19.9 ± 12.8 -22.2 ± 7.6 

0.283 0.32 62.1 ± 5.5 52.5 ± 5.0 47.1 ± 14.8 

0.283 0.64 25.2 ± 4.5 -2.6 ± 13.8 -9.9 ± 9.5 

0.188 0.48 40.5 ± 7.9 28.5 ± 16.2 6.9 ± 11.1 

Notes: 

1. samples for each combination of air curtain 

flowand air velocity. 

2. Negative efficiencies are increases in dust 

concentrations. 

ANOVA analyses defined variabilities in dust 

reduction efficiencies through the contributions of 

canopy air flow, interference air velocity, and the 
 

interaction of these two variables. Comparing the 

average reduction at the extreme points of the test 

matrix to the average reduction at the center point gave 

the error associated with curvature or non-linearity 

(Myers and Montgomery, 1995). The ANOVA tables 

are given in table 2 for average dust reduction 

efficiencies at 30.5, 61.0, and 91.5 cm beneath the air 

curtain. 

These analyses show that changes in interference air 

velocity significantly affected dust reduction 

efficiencies for all distances beneath the canopy. 

Changes in air curtain quantity were significant at 61.0 

and 91.5 cm beneath the canopy, yet insignificant at 

30.5 cm beneath the canopy. Perhaps, canopy efficiency 

was maximized at this distance. The AB interaction of 

the two independent variables was not significant at any 

distance showing that the effect of each variable was 

independent of the magnitude of the other. Finally, these 

ANOVA analyses showed that for distances of 30.5 and 

61.0 cm beneath the canopy, dust reduction efficiencies 

varied linearly with air curtain quantity and interference 

air velocity. Curvature in the relationship was detected 

only at the extreme distance of 91.5 cm beneath the 

canopy. However, very few instances are envisioned 

where a roof bolter operator would work this far 

beneath the air curtain. Most work time would be spent 

within 61.0 cm of this device and the effects of 

curvature between the test variables were insignificant 

at this distance. 

The impacts of increasing curtain airflow and 

interference air velocity were evaluated by comparing 

average dust levels with each variable at the low level to 

dust levels with that variable at its high level. 

Hypothesis testing then assessed the significance of the 

difference between these mean dust levels. With the null 

hypothesis being no difference in mean dust levels, ttests were used to accept or reject this assumption. Two 

tails were specified to account for any increase or 

decrease in dust level due to change in variable level. 

Subsequent analyses constructed 95 percent confidence 

intervals for the difference in mean dust levels. An 

interval containing zero means that the difference in 

mean dust levels was not statistically significant at this 

level of confidence. 

Increasing air curtain quantity from 0.094 m3

/s to 0.283 

m3

/s generally resulted in statistically significant 

improvements in efficiency and, consequently, improved 

protection from exterior dust levels. The ranges of 

improvements in figure 5 show that for a low interference 

air velocity of 0.32 m/s, significant improvements in 

efficiency were seen at 30.5 and 61.0 cm below the 

canopy (p<0.05). The improvement at 91.5 cm was not 

significant (p>0.05). Efficiency improved at the high 

interference air velocity (0.64 m/s) only for 61.0 and 91.5 

cm below the canopy. No significant change in efficiency 

was seen at 30.5 cm (p>0.10). 

Figure 5 shows that increasing air curtain quantity 

led to small or even negative changes in efficiency at 

30.5 cm beneath the canopy. This is consistent with 

the ANOVA analysis in table 2 showing that air 

curtain quantity was not a significant influence on 

canopy efficiency at 30.5 cm. For other distances, 

however, increases in canopy air curtain quantity 

generally led to statistically significant increases in 

efficiency. 

Table 2. Summary of ANOVA results for air curtain 

sampling. 

Source of 

variation Sum of Squares Degrees of 

freedom F-ratio 

30.5 cm beneath air curtain 

A (air curtain 

quantity) 

6.16 1 0.08 

B (interference 

air velocity) 

5276.05 1 64.22#

AB (interaction) 95.66 1 1.16 

Curvature 27.25 1 0.33 

Error 1232.34 15 

Total 6637.46 19 

61.0 cm beneath air curtain

A (air curtain 

quantity) 

913.546 1 12.966#

B (interference 

air velocity) 

17344.283 1 246.161#

AB (interaction) 69.006 1 0.979 

Curvature 42.172 1 0.599 

Error 1056.886 15 

Total 19425.893 19 

91.5 cm beneath air curtain

A (air curtain 

quantity) 

1127.551 1 15.125#

B (interference 

air velocity) 

14758.201 1 197.969#

AB (interaction) 34.716 1 0.466 

Curvature 518.746 1 6.959#

Error 1118.216 15 

Total 17557.43 19 

Note: 

Critical F-value: F0.05,1,15 = 4.54 #

 = significant at 95% level of confidence 
Increasing interference air velocity, however, from 

0.32 to 0.64 m/s led to statistically significant reductions 

in efficiency at all sampling locations (Figure 6). This 

was most evident at locations 61.0 and 91.5 cm below 

the air curtain where mean reductions in efficiencies 

ranged from -52% to -63%. The effects were much less 

at a distance of 30.5 cm from the canopy where mean 

reductions in efficiencies ranged from -28% to -36%. 

Comparing absolute magnitudes of changes in figure 5 

to those in figure 6 suggests that changes in interference 

air velocity, rather than changes in air curtain quantity, 

had a much greater impact on air curtain efficiency. 

This is supported by the ANOVA results of Table 2. 

-30 -10 10 30 50

30.5

61.0

91.5

Distance beneath canopy (cm)

Change in efficiency

0.64 m/s

0.32 m/s

Figure 5. Effects of changing air curtain quantity (mean 

changes and 95% confidence limits shown) 

These analyses imply that use of a canopy air curtain 

on a roof bolter would be most effective in areas of low 

air velocity such as bolting faces. In areas of higher 

interference air velocity, the protection offered by the 

device likely would be compromised. Furthermore, 

higher canopy airflow, such as that used in this work, 

led to little improvement in efficiency. Perhaps smaller 

capacity and, therefore, quieter fans may be as effective 

as larger fans. 

-100 -80 -60 -40 -20 0

30.5

61.0

91.5

Distance beneath canopy (cm)

Change in efficiency

0.283 m3/s

0.094 m3/s

Figure 6. Effects of changing interference air velocity 

(mean changes and 95% confidence limits shown)

Average dust concentrations and efficiencies were 

calculated using the average of three samplers situated 

beneath the air curtain. The previous assessment showed 

that the highest velocity air streams existed below the 

center of the air curtain and that these velocities 

diminished toward its periphery. Center efficiencies 

were calculated in table 3 and found much higher than 

the efficiencies across the entire air curtain. 

ANOVA analyses of centerline efficiencies showed 

trends similar to those found with the average 

efficiencies. Interference air velocity significantly 

affected average efficiency at all distances from the air 

curtain. Air curtain quantity, again, was significant only 

at distances of 61.0 and 91.5 cm beneath the canopy. No 

evidence was found of interaction between the variables 

of air curtain quantity and interference air velocity, nor 

any evidence of non-linearity between these variables. 

Table 3. Mean reduction efficiencies and 95% 

confidence limits beneath center of air curtain. 

Air Reduction efficiencies (%) 

curtai

n flow 

(m3

/s) 

Interfer

ence air 

velocity 

(m/s) 30.5 cm 61.0 cm 91.5 cm 

0.094 0.32 89.5 ± 1.9 55.6 ± 11.5 33.6 ± 18.2 

0.094 0.64 64.6 ± 41.8 -19.9 ± 15.3 -21.8± 10.5 

0.283 0.32 90.2 ± 8.0 77.2 ± 5.1 57.4 ± 22.4 

0.283 0.64 73.7 ± 12.2 12.7 ± 36.8 -4.4 ± 7.4 

0.188 0.48 88.3 ± 6.6 28.5 ± 16.2 6.9 ± 11.1 

1. 5 samples for each combination of air curtain flow 

and air velocity. 

2. Negative efficiencies are increases in dust 

concentration.

Efficiencies calculated using three samplers beneath 

the air curtain were much less than center efficiencies. 

However, these average efficiencies presented a more 

realistic picture of the actual dust reduction potentials. 

During roof bolting, the bolter operator likely will be 

moving beneath the canopy, rather than staying beneath 

the center. An efficiency calculated from three samplers 

establishes a lower limit on protection from an external 

dust source. Improved protection is, of course, available 

by staying beneath the center of the air curtain. 

Out of five total tests run with each combination of 

canopy air curtain flow and interference air velocity, 

filters from three of these tests were selected for 

subsequent silica analyses. ANOVA analyses showed 

that respirable silica percentages at distances of 30.5, 

61.0, and 91.5 cm beneath the CAC were not 

significantly affected by changes in either air curtain 

quantity or interference air velocity. Subsequent 

pairwise comparisons revealed no significant difference 
between silica contents under the canopy and silica 

contents outside the canopy (Figure 7). 

This implies that the canopy air curtain was equally 

effective on both respirable coal and respirable silica 

dust particles. If the canopy air curtain was selective in 

its control of either dust particle, then silica percentages 

under the canopy would have been significantly 

different from percentages outside the canopy. 

5,0

5,5

6,0

6,5

7,0

30.5 cm 61.0 cm 91.5 cm Outside

Sample location

Silica content (%)

Figure 7. Average silica dust percentages with 95% 

confidence limits 

Although silica percentages did not change under the 

canopy, coal dust concentrations generally decreased. 

Therefore, silica dust concentrations also decreased for 

samples under the canopy. The canopy air curtain 

provided similar control of both respirable coal and 

respirable silica dusts. 

SUMMARY 

Data collected from the Mine Safety and Health 

Administration show that roof bolting occupations 

continue to be at risk for overexposure to respirable 

silica dust. One means proposed to limit exposures is 

the use of a canopy air curtain to provide a constant 

flow of filtered air over the breathing zone of the bolter 

operator. This laboratory work evaluated the 

effectiveness of such a system for limiting respirable 

coal and silica dust exposures. 

Initial evaluations of velocity profiles beneath the 

canopy showed that higher velocity air streams existed 

near the inlet to the air curtain and that these velocities 

diminished with distance from the inlet. With increasing 

distance beneath the canopy, the zone of high velocity 

air shrank noticeably, although it was still present 

beneath the center of the canopy. 

Subsequent testing showed that dust reduction 

efficiencies were highest for those locations close to the 

canopy and that these values decreased rapidly with 

distance beneath the canopy. ANOVA analyses revealed 

that changes in interference air velocity significantly 

affected dust reduction efficiencies while changes in 

canopy air flow did not always lead to significant 

changes. No interactive effects were found between 

interference airflow and canopy air quantity. Similar 

results were found when considering only the higher, 

centerline dust reduction efficiencies. A final analysis 

showed that respirable silica dust percentages were not 

significantly affected by changes in either canopy air 

flow or interference air velocity. In fact, silica contents 

of samples under the canopy were not significantly 

different from those outside the canopy. Due to 

reductions in respirable coal dust levels under the 

canopy, however, silica dust concentrations decreased 

under the canopy. 

REFERENCES 

Donaldson Company, Inc., 1975, “Develop and Test 

Canopy Air Curtain Devices,” Contract Report No. 

H0232067, U.S. Bureau of Mines, Pittsburgh, PA 

Donaldson Company, Inc., 1987, “Improved Canopy 

Air Curtain Systems,” Contract Report No. J0318014, 

U.S. Bureau of Mines, Pittsburgh, PA 

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 

Mine Safety and Health Administration (MSHA), 1973, 

“Standard Method No. P-7, Infrared Determination of 

Respirable Quartz in Respirable Coal Mine Dust,” 

Internal Document, U.S. Department 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, USA 15236 

Mining Research and Development Establishment 

(MRDE), 1981, “Studies of Air Curtain Technology 

with Particular Application to the Provision of Clean 

Air for Underground Mines,” Final Report, ECSC 

Project 7251-17/8/085 

Mining Research and Development Establishment 

(MRDE), 1983, “Application of Air Curtains and 

Other Methods for Controlling Ventilation and 

Providing Clean Air at the Workplace,” Final Report, 

ECSC Project 7256-13/039/08 

Myers, R.H. and Montgomery, D.C., 1995, Response 

Surface Methodology. John Wiley and Sons, Inc., 

New York.Organiscak, J.A., Page, S.J., and 

Jankowski, R.A., 1990, “Sources and Characteristics 

of Quartz Dust in Coal Mines,” Information Circular 

9271, US Bureau of Mines, Pittsburgh, PA 

U.S. Government Printing Office, 1999, Mineral 

Resources. Code of Federal Regulations Title 30, 

Parts 70 and 75