CONTINUOUS MINER SPRAY CONSIDERATIONS FOR OPTIMIZING 

SCRUBBER PERFORMANCE IN EXHAUST VENTILATION SYSTEMS 

John Organiscak, National Institute for Occupational Safety and Health, Pittsburgh, PA 

Timothy Beck, National Institute for Occupational Safety and Health, Pittsburgh, PA 

Abstract 

A majority of continuous mining machines employ a 

water spray system and a machine mounted flooded-bed 

scrubber to suppress and capture dust during coal mining. 

These machine mounted dust control systems must be 

designed to function within the localized face ventilation 

system at the mining section to control both dust and 

methane. Spray systems can impede or improve the 

scrubber effectiveness in controlling dust or methane at 

the mining face. Laboratory experiments were conducted 

to examine the effect of spray type, spray pressure, 

machine body blocking sprays, and scrubber airflow on 

dust and gas levels while using a 12.2 m (40 ft) exhaust 

ventilation curtain setback from the face. These 

experiments were conducted with the mining machine 

positioned at the end of a simulated 6.1 m (20 ft) sump 

and slab cut. Results indicate that the hollow cone 

nozzles with blocking sprays best complemented the 

flooded-bed scrubber performance in an exhaust 

ventilation system. This external spray system notably 

reduced dust and gas levels on the off-curtain side of the 

mining machine for both the sump and slab cut as 

compared to the flat spray nozzles. Higher scrubber 

airflows reduced dust and gas levels on the curtain side 

and in the return of the continuous mining machine. The 

remote operator position, located on the off curtain side 

and parallel to the inlet end of the exhaust curtain, 

sustained the most stable and lowest dust levels around 

the mining machine. 

Introduction 

Coal miner overexposure to respirable coal and 

crystalline silica (or quartz) dust can cause 

pneumoconiosis and silicosis, respectively, which are 

debilitating and potentially fatal respiratory lung diseases. 

Although significant progress has been made in the 

United States (US) with the reduction of coal workers’ 

pneumoconiosis, severe cases continue to occur among 

coal miners, especially within several geographic clusters 

of the Appalachian coal region (Antao et al. 2004). 

Mining also has some of the highest incidences of 

worker-related silicosis, with mining machine operators 

being the occupation that is most commonly associated 

with the disease (NIOSH 2008). 

The US Mine Safety and Health Administration 

(MSHA) enacts and enforces mine worker safety and 

health standards to mitigate mine worker injuries and 

occupational diseases. MSHA’s permissible coal mine 

dust exposure limit is 2.0 mg/m3 during an 8 hr shift for 

coal mine workers as defined by the Mining Research 

Establishment (MRE) Criteria (30 CFR 70-72, 74 2009). 

If more than 5% quartz mass is determined to be in the 

coal mine worker dust sample using MSHA’s P7 infrared 

method (Parobeck and Tomb 2000), the applicable 

respirable dust standard is reduced to the quotient of 10 

divided by the percentage of quartz in the dust sample. 

This reduced dust standard based on the percentage of 

quartz content is intended to limit worker respirable 

crystalline silica (quartz) exposure to 0.1 mg/m3

 or less 

for the shift. 

Coal mine worker overexposure to coal and quartz 

dust continues to be a problem at underground coal 

mining operations in the US. Over 90 % of mechanized 

mining units operating in US underground coal mines are 

continuous mining machines (MSHA 2009). The 

percentage of valid MSHA inspector dust samples for 

continuous mining machine operators from 2004 to 2008 

that exceeded the respirable coal dust standard and 

reduced dust standard was 7.2 % and 19.5 %, respectively 

(US Department of Labor 2009). Therefore, many 

continuous mining machine operators continue to be 

overexposed to coal and quartz dust. 

The primary dust controls used on most continuous 

mining machines are water sprays and flooded-bed 

scrubbers. Initially flooded-bed scrubbers were used with 

blowing face ventilation systems in gassy coal seams to 

help remove dust being blown over face workers at the 

 1 
mining face while providing satisfactory face methane 

removal for curtain setback distances up to 15.2-m (50-ft) 

(Volkwein et al. 1985, Jayaraman et al. 1990). With the 

development of remote control technology for continuous 

mining machines, flooded-bed scrubbers were also being 

adopted on exhaust face ventilation systems for use in 

extended-cut mining applications (beyond 6.1 m or 20 ft 

of entry advance). Research has shown that remote 

positioning away from the mining machine during 

extended-cut mining was a significant factor in lowering 

operator dust exposures on both blowing and exhaust 

ventilation systems (Fields et al. 1990). The best 

continuous miner operator position for blowing 

ventilation is in front of the discharge end of the intake 

curtain (Jayaraman et al. 1987, Goodman and Listak 

1999). The best operator position for exhaust ventilation 

is parallel to or outby the inlet end of the return curtain on 

the opposite side of the entry (Colinet and Jankowski 

1996, Goodman and Listak 1999). 

Since continuous miner operators don’t or can’t 

always stay at these optimum positions during mining, 

their dust exposure can notably increase at other positions 

around the rear of the mining machine (Goodman and 

Listak 1999). Previous research on machine mounted 

scrubbers in blowing face ventilation systems have shown 

the lowest dust levels at the rear corners and return of the 

mining machine were achieved when the face ventilation 

to scrubber airflow ratio is at or slightly above 1 

(Jayaraman et al. 1992). Another scrubber study with 

blowing ventilation showed that dust rollback at the rear 

of the mining machine was reduced when the face 

ventilation curtain setback distance was increased from 

6.1 m (20 ft) to 12.2 m (40 ft) and/or when blocking 

sprays are used on both sides of the mining machine 

outby the scrubber inlets (Goodman 2000). Machine 

mounted scrubber research with exhausting face 

ventilation systems showed dust levels increased at the 

remote operator position outby the mining machine when 

using a larger curtain set back distance 12.2 m (40 ft) 

versus 9.1 m (30 ft), external directional sprays, and/or 

under boom sprays (Goodman et al. 2006). Although the 

external directional sprays redirected dust past the 

scrubber inlets and increased operator dust levels, these 

sprays noticeably reduced gas levels on the off-curtain 

side of the face. On the other hand the under boom sprays 

showed increases to both operator dust levels and gas 

levels at the face. 

In order to improve dust and gas control around a 

continuous mining machine using a scrubber and external 

sprays with exhaust ventilation, the National Institute for 

Occupational Safety and Health (NIOSH) conducted 

additional experiments in its full scale continuous miner 

gallery at the Pittsburgh Research Center (PRL). The 

objective of these experiments was to examine external 

 

water spray configurations that are complementary to 

scrubber performance for exhaust ventilation systems. 

The experimental factors studied were spray nozzle type 

(hollow cone vs. flat), water spray pressure (550 kPa 

vs.1100 kPa or 80 psig vs.160 psig), blocking sprays (off 

vs. on) and scrubber airflow (reduced vs. maximum). 

This paper describes the experiments conducted and the 

dust and gas level results measured around the mining 

machine. 

Experimental Design 

 Laboratory experiments were conducted within a 

full-scale continuous miner gallery as shown in Figure 1. 

The gallery entry dimensions were 5.5 m (18-ft) wide by 

2.0 m (6.5 high) with a full-scale plywood mockup of a 

Joy CM141

1

 Mention of any company name or product does not 

constitute endorsement by the National Institute for 

Occupational Safety and Health. 

 continuous mining machine positioned at a 

simulated mining face. This mining machine was 

equipped with a flooded-bed scrubber, several banks of 

external spray nozzles, and a 0.91 m (36 in) diameter 

cutting drum that rotates at 50 rpm. The flooded-bed 

scrubber utilized a 30-layer pleated stainless steel filter 

wetted by 3 spraying system full cone QPH-6.5 nozzles 

(Spraying Systems, Wheaton, IL) at 340 kPa (50 psig) 

and was powered by a variable frequency ac drive speed 

controlled fan. Scrubber inlets were located under each 

side and center of the cutter boom near the hinge point. 

External sprays consisted of 15 top mounted boom sprays 

directed at the top of the rotating drum, 3 under boom 

throat sprays directed at the loading pan, and 3 sprays on 

each side of the cutter boom directed at the drum’s end 

rings. Two blocking sprays were vertically mounted 3 

inches apart on each side of the mining machine body 

located two feet outby the scrubber inlets and two feet 

above ground level. These blocking sprays were oriented 

at a 15° angle away from the machine body towards the 

rib and were operated at the same pressure as the other 

external sprays. 

Coal dust and sulfur hexafluoride (SF6) gas were 

introduced in front of and along the length of the rotating 

cutting drum. Pulverized coal dust (Keystone mineral 

black 325BA, Keystone Filler & Manufacturing Co., 

Muncy, PA) was fed into the gallery at 25 grams/minute 

with a screw feeder (Vibra Screw, Inc., Totowa, NJ) and 

two LH-1/2 brass eductors (Penberthy, Prophetstown, IL) 

operated with 30 kPa (4 psig) of compressed air. One 

eductor discharged dust through a hose along the left front 

side of the drum and the other eductor discharged dust 

through a hose along the right front side of the drum. SF6

gas was also released from tubing at each end of the dust 

 2
 3 

discharge hoses to mix in the gas with the dust. A model 

1303 multipoint gas doser (California Analytical 

Instruments, Orange, CA) released the SF6 gas at a flow 

rate of 6 milliliters/sec. The rotating drum ensured their 

mixing and simulated dust and gas emissions from the 

face during mining. 

Respirable dust and SF6 gas concentrations were 

measured at several locations around the mining machine 

as shown in Figure 1. Respirable dust concentrations 

were measured with coal mine dust personal sampling 

units (CMDPSU), comprised of an ESCORT-Elf constant 

flow air sampling pump pulling dust laden air through a 

10-mm nylon cyclone (respirable dust classifier) and 

depositing the respirable fraction onto a pre-weighed 37-

mm filter cassette (MSA, Pittsburgh, PA). A pair of these 

samplers (CMDPSU) were placed and operated at the 

remote operator (Oper) position, the right rear corner 

(RRC) of the mining machine, the left rear corner (LRC) 

of the mining machine and the return (Return) air course. 

The pairs of dust concentrations measured were averaged 

to determine the dust concentration at each sampling 

location. SF6 gas measurements were made using a 

California Analytical Instruments model 1312 

photoacoustic gas monitor which sequentially drew gas 

samples through tubing from the off-curtain side (OCS) of 

the cutting boom, the curtain side (CS) of the cutting 

boom, and the return (Return) air course. This data was 

collected with a computer based data acquisition system 

and the gas concentrations at each location were averaged 

for the test. 

Figure 1. Plan view of continuous miner gallery test setup. 

Scrubber 

Plan View 

CS

OCS

Return 

RRC 

Oper 

LRC 

Blocking Sprays 

Blocking Sprays 

5.5 m 

Key: -Dust Sampling Location 

 - Tracer Gas Sampling Location

1.2 m 

6.1 m

Slab Section 

12.2 m 

Other operating parameters measured and recorded 

were water spray pressure, machine water flow, face 

return airflow and scrubber airflow. Water pressure and 

flow were measured with electronic instruments and 

recorded with a computer data acquisition system. Face 

return airflow was measured at the inlet end of the 

ventilation curtain with a handheld vane anemometer 

(moving traverse) with the scrubber off at the beginning 

and end of each test. Scrubber airflow was also measured 

with a handheld vane anemometer at the discharge end of 

the scrubber (moving traverse) at the beginning and end 

of each test. A moving traverse airflow measurement at 

the scrubber exhaust was found to be very comparable to 

a fixed point traverse measurement within the scrubber 

duct. 

For these scrubber/spray experiments the continuous 

miner gallery was configured for exhaust curtain 

ventilation with a 12.2-m (40-ft) setback from the face as 

shown in Figure 1. A 2-level, 4-factor experimental 

design was conducted and is shown in Table 1. Return 

airflow for these experiments was set to approximately 

1.25 times the maximum scrubber airflow rate. The 

maximum scrubber airflow averaged 2.27 m3

/s (4810 

ft3

/min) and the return airflow averaged 2.90 m3/s (6150 

ft3

/min) for these experiments. The water sprays tested 

were Spraying Systems 3/8-BD-3 hollow cone nozzles 

(77° spray angle @ 550 kPa or 80 psig) and Spraying 

Systems 3/8-TT-5006 flat nozzles (56°spray angle @ 550 

kPa or 80 psig). These nozzles were chosen because their 

specifications showed comparable water flow rates at 

similar water pressures. All the external sprays, including 

the blocking sprays, used the same nozzle type and were 

operated at the same water pressure during their 

experimental comparisons. The flat spray pattern 

orientation was parallel to the roof for the top boom 

sprays and parallel to the ribs for the side boom and 

blocking sprays. The low and high operational water 

spray pressures averaged 560 kPa (81 psig) and 1110 kPa 

(161 psig), respectively. Approximately a 20 % reduction 

in scrubber airflow was also used to simulate a realistic 

decrease from material buildup on the filter screen during 

the shift. The reduced scrubber airflow was controlled by 

decreasing the fan speed with the variable frequency 

drive, yielding a 1.78 m3

/s (3780 ft3/min) average for 

these experiments. These experimental factors were 

examined for both a simulated 9.1-m (20-ft) sump and 

slab cut. All testing is limited to examining airborne dust 

captured around a continuous mining machine and do not 

represent dust suppression from coal wetting. 

Each experimental factor combination in Table 1 was 

replicated for at least 3 tests. Experimental tests were 

blocked or separately conducted for the sump and slab 

cuts for experimental practicality. Experimental tests 

were also blocked by nozzle type. One test of spray 

pressure, blocking sprays, and scrubber airflow 

combinations was randomly conducted for each nozzle 

type before they were changed. The nozzle types were 

alternated to complete the 3 test replicates. All 

experimentally controlled test factors were precisely 

maintained and had relative standard deviations (RSD = 

(standard deviation /average) x 100%) less than 3% of 

their measured average. 
Table 1. – Experimental Design 

 

Experimental Sump Cut, 9.1 m (20 ft) Deep Slab Cut , 9.1 m (20 ft) Deep 

Test Factors Low Level (-1) High Level (+1) Low Level (-1) High Level (+1) 

Nozzle Type Hollow Cone Flat Hollow Cone Flat 

Spray Pressure 550 kPa 1100 kPa 550 kPa 1100 kPa 

Blocking Sprays Off On Off On

Scrubber Airflow Reduced ~ 20% Maximum Reduced ~ 20% Maximum 

Experimental Results 

Test replicate averages and standard errors were 

determined for respirable dust and SF6 gas concentrations 

at the multiple locations around the mining machine 

during these experiments. The averages and standard 

errors for the sump cut are illustrated in Figures 2-4 and 

for the slab cut illustrated in Figures 5-7. The tests 

without blocking sprays are presented on the left side of 

these figures and the blocking spray tests are presented on 

the right side of these figures. Their x-axes are labeled by 

water pressures in descending order and scrubber airflow 

in ascending order. 

Stepwise regression analysis was also conducted on 

the experimental data to examine the significant test 

factor relationships (at the 95 % confidence level) with 

dust and gas concentrations. The low and high 

experimental test factor levels were represented as -1 and 

+1, respectively, in the regression model. Regression 

analyses were separately conducted at each dust and gas 

sampling location, during the sump and slab cut. Since 

the dust concentrations measured at the RRC and LRC 

locations exhibited an extensive data range, nonnormality and unequal variances, natural logarithms of 

these concentrations were used to stabilize their 

regression model variance (Myers and Montgomery, 

1995). The most significant experimental test factors are 

shown in Table 2 with a + symbol illustrating a direct 

relationship and a – symbol illustrating a negative 

relationship in the regression models. Since the operator 

position had minimal concentration changes in these 

experiments and there were very few regression factor 

interactions at the other sampling locations, these 

regression results were not shown in Table 2 for 

simplicity. 

Sump Cut 

Figures 2 and 3 show the dust concentrations measured 

for the sump cut on the off-curtain side and curtain side of 

the entry, respectively. The SF6 gas concentrations 

measured for the sump cut are shown in Figure 4. Table 2 

shows the significant dust and gas relationships in these 

figures. 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

1100kPa-Low 

550kPa-Low

1100kPa-Max

550kPa-Max

1100kPa-Low 

550kPa-Low

1100kPa-Max

550kPa-Max

Spray Pressure-Scrubber Airflow

Dust Conc., mg/m3

Hollow-RRC Flat-RRC Hollow-Oper Flat-Oper

No Blocking Sprays Blocking Sprays

Figure 2 – Sump cut dust concentrations on the offcurtain side of the entry. 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

1100kPa-Low

550kPa-Low

1100kPa-Max

550kPa-Max

1100kPa-Low

550kPa-Low

1100kPa-Max

550kPa-Max

Spray Pressure-Scrubber Airflow

Dust Conc., mg/m3

Hollow-LRC Flat-LRC Hollow-Return Flat-Return

No Blocking Sprays Blocking Sprays

Figure 3 – Sump cut dust concentrations on the curtain 

side of the entry. 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

1100kPa-Low

550kPa-Low

1100kPa-Max

550kPa-Max

1100kPa-Low

550kPa-Low

1100kPa-Max

550kPa-Max

Spray Pressure-Scrubber Airflow

SF6 Gas Conc., ppm

Hollow-OCS Flat-OCS Hollow-CS

Flat-CS Hollow-Return Flat-Return

Blocking Sprays

No Blocking Sprays

Figure 4 – Sump cut gas concentrations. 

 4 
Table 2 – Significant Dust and Gas Concentration Relationships 

Sample-Location 

Sump Cut Slab Cut 

Nozzle Spray Blocking Scrubber Nozzle Spray Blocking Scrubber 

Type Pressure Sprays Airflow Type Pressure Sprays Airflow 

Dust-RRC + + + - -

Dust-LRC - + -

Dust-Return - - -

Gas-OCS + - + -

Gas-CS - + + -

Gas-Return + - + -

Key: + and – symbols refer to direct and indirect relationships, respectively, at the 95% confidence level. 

The most significant factors affecting dust

concentrations in the sump cut were the nozzle type and 

scrubber airflow. The hollow-cone nozzle appeared to 

exhibit very little dust rollback to the RRC of the 

machine, whereas the flat nozzle created significant dust 

rollback to the RRC of the machine. Blocking sprays 

didn’t seem to have a significant effect on controlling dust 

in the sump cut with either of the spray nozzle types. The 

dust concentrations on the curtain side of the entry were 

most affected by scrubber airflow. Dust levels at the LRC 

and in the Return were significantly reduced with higher 

scrubber airflows. The remote operator position had the 

lowest and most stable dust concentrations observed for 

these sump cut tests. 

The most significant factors affecting SF6 gas 

concentrations in the sump cut were nozzle type and 

blocking sprays. Figure 4 shows that the highest gas 

concentrations are at the OCS of the continuous miner 

boom with no blocking sprays operating. Hollow cone 

sprays achieved lower gas concentrations on both sides of 

the miner boom with no blocking sprays as compared to 

the flat sprays. The blocking sprays significantly reduced 

the OCS gas concentrations, especially for the hollow 

cone spray nozzles. The decrease in OCS gas levels were 

somewhat offset by an increase in gas levels on the CS of 

the continuous miner boom. The increased gas 

concentrations at the CS location with the blocking sprays 

were still lower than the concentrations at OCS without 

the blocking sprays. 

Slab Cut 

 Figures 5 and 6 show the dust concentrations 

measured for the slab cut on the off-curtain side and 

curtain side of the entry, respectively. The SF6 gas 

concentrations measured for the sump cut are shown in 

Figure 7. Table 2 shows the significant dust and gas 

relationships in these figures. 

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

1100kPa-Low

550kPa-Low

1100kPa-Max

550kPa-Max

1100kPa-Low

550kPa-Low

1100kPa-Max

550kPa-Max

Spray Pressure-Scrubber Airflow

Dust Conc., mg/m3

Hollow-RRC Flat-RRC Hollow-Oper Flat-Oper

No Blocking Sprays Blocking Sprays

 Figure 5. Slab cut dust concentrations on the off-curtain 

side of the entry. 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

1100kPa-Low

550kPa-Low

1100kPa-Max

550kPa-Max

1100kPa-Low

550kPa-Low

1100kPa-Max

550kPa-Max

Spray Pressure-Scrubber Airflow

Dust Conc., mg/m3

Hollow-LRC Flat-LRC Hollow-Return Flat-Return

No Blocking Sprays Blocking Sprays

 Figure 6. Slab cut dust concentrations on the curtain side 

of the entry. 

All the experimental factors significantly affected 

dust concentrations at the RRC of mining machine in the 

slab cut. Both spray nozzle types showed prominent dust 

rollback to the RRC of the machine with no blocking 

sprays operating. The flat spray nozzles exhibited 

significantly higher dust concentrations than the hollow 

cone sprays at this RRC location. Lower water pressures 

and higher scrubber airflows significantly reduced this 

rollback effect. Application of the blocking sprays 

appeared to eliminate all dust rollback to the RRC 

location, reducing these dust concentrations to nearly 

Oper position levels. 

The dust concentrations on the curtain side of the 

entry were most affected by water pressure and scrubber 

airflow. Dust levels at the LRC were significantly 

increased by higher spray pressures, while higher 

scrubber airflows reduced dust concentrations at the LRC 

 5 
and Return sampling locations. Similar to the sump cut, 

the remote operator position again had the lowest and 

most stable dust concentrations for these slab cut tests. 

The most significant factors affecting SF6 gas 

concentrations in the slab cut were again nozzle type and 

blocking sprays. Figure 7 shows that the highest gas 

concentrations are at the OCS of the continuous miner 

boom with no blocking sprays operating. The hollow 

cone sprays achieved lower gas concentrations on both 

sides of the miner boom with no blocking sprays as 

compared to the flat sprays. Blocking sprays application 

again significantly reduced the OCS gas concentrations, 

especially for the hollow cone spray nozzles. The 

decrease in OCS gas levels were somewhat offset by an 

increase in gas levels on the CS of the continuous miner 

boom. The increased gas concentrations at the CS 

location with the blocking sprays were still lower than the 

concentrations on the OCS location without the blocking 

sprays. 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

1100kPa-Low

550kPa-Low

1100kPa-Max

550kPa-Max

1100kPa-Low

550kPa-Low

1100kPa-Max

550kPa-Max

Spray Pressure-Scubber Airflow

SF6 Gas Conc., ppm

Hollow-OCS Flat-OCS Hollow-CS

Flat-CS Hollow-Return Flat-Return

Blocking Sprays

No Blocking Sprays

Figure 7. Slab cut gas concentrations. 

Conclusions 

 Laboratory experiments were conducted to examine 

the effect of spray type, spray pressure, machine body 

blocking sprays, and scrubber airflow on dust and gas 

levels while using a 12.2 m (40 ft) exhaust ventilation 

curtain setback from the face. From these experiments 

the key observations were made. 

• The remote operator position had the lowest and 

most stable dust concentrations as compared to 

the rear corners and return of the continuous 

mining machine. 

• Hollow cone nozzles exhibited less dust rollback 

than flat sprays on the off-curtain side of the 

mining machine for both the sump and slab cuts. 

• Blocking sprays notably reduced dust 

concentrations on the off-curtain side of the 

mining machine for the slab cut with negligible 

dust changes for the sump cut. 

• Higher water spray pressure was more 

detrimental in increasing dust concentrations at 

the back corners of the mining machine for the 

slab cut as compared to the sump cut. 

• Hollow cone nozzles and blocking sprays both 

noticeably reduced gas concentrations at the offcurtain side of the continuous miner boom. 

• Higher scrubber airflows reduced dust levels on 

the curtain side of the continuous mining 

machine, and reduced both dust and gas levels in 

the return. 

Given these experimental observations, it appears 

that the hollow cone nozzles with blocking sprays best 

complemented the flooded-bed scrubber performance in 

an exhaust ventilation system. This external spray system 

notably reduced dust and gas levels on the off-curtain side 

of the mining machine for both the sump and slab cut as 

compared to the flat spray nozzles. Using lower water 

spray pressures noticeably reduced dust rollback to the 

rear corners of the mining machine (RRC and LRC) 

primarily during the slab cut. Higher scrubber airflows 

reduced dust and gas levels on the curtain side and in the 

return of the continuous mining machine. Finally, the 

remote operator position, located on the off curtain side 

and parallel to the inlet end of the exhaust curtain, 

sustained the most stable and lowest dust levels around 

the mining machine. 

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