Controlling respirable dust in underground coal mines in the 

United States 

J.F. Colinet, J.P. Rider & E.D. Thimons 

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania, United States 

ABSTRACT: As mining operations in the United States (US) have become more productive, controlling the dust exposure of mine workers has become more challenging. In response, US mining 

operations are applying basic controls at elevated levels and are looking to emerging control technologies in an effort to better control airborne respirable dust levels. 

The Pittsburgh Research Laboratory (PRL) of the National Institute for Occupational Safety and 

Health (NIOSH) conducts research to develop and/or improve control technologies that reduce the 

respirable dust exposure of workers in underground coal mines. The goals of this research involve 

optimizing the use of water sprays and ventilating air, as well as, evaluating emerging control technologies. An overview of dust controls typically utilized in underground US coal mines will be 

provided. An update on ongoing PRL research efforts that are evaluating new control technologies 

will also be presented. 

1 INTRODUCTION 

Since 1973, a respirable dust standard of 2.0 mg/m3

 over an 8-hour shift has been enforced in 

underground coal mining by the Mine Safety and Health Administration (MSHA). However, if silica 

in the collected sample exceeds 5%, a reduced dust standard is established by dividing 10 by the 

percent silica. For example, if the sample contains 10% silica, a reduced dust standard of 1 mg/m3 

(10/10% silica) is enforced. Periodic sampling is conducted by MSHA and the mine operators to 

measure compliance with the applicable dust standard. Currently, MSHA samples each mechanized 

mining unit (MMU) four times per year, while the operator is required to collect samples on a bimonthly basis. 

Underground coal mining accounts for 33% of the coal production in the United States. Of this, 

51% of the production is produced by longwall mines (EIA 2006). Approximately 45 longwalls 

are in operation at a given time. Average longwall production as reported by mine operators during 

compliance dust sampling in 2006 was 4,800 tons (5,300 short tons) per shift (Niewiadomski 2007). 

This production has been achieved through improved machine reliability and improved operating 

practices. Average longwall panel size in 2002 was 287 m (940 ft) wide and nearly 3050 m (10,000 

ft) long (Rider and Colinet 2007). These improvements in longwalls challenge dust control efforts. 

In 2006, 16% and 14% of compliance samples exceeded the applicable dust standard for the tailgate 

shearer operators and jacksetters, respectively. 

In addition to improvements and changes being realized on US longwalls, continuous mining 

operations have also seen dramatic changes. Average production on the approximately 780 continuous miner sections during compliance sampling reached 690 tons (760 short tons) per shift in 2006 
(Niewiadomski 2007). The vast majority of continuous miners are now operating with flooded bed 

scrubbers and taking extended cuts that are greater than 6 m (20 ft). An increase in the number of 

mines utilizing supersections (sections with two continuous miners) has also occurred. However, 

utilization of super sections has increased the potential for roof bolter operators to work downwind 

of a continuous miner, resulting in increased dust exposure. A negative change that has occurred has 

been an increase in the quantity of rock that is being cut as seam conditions deteriorate. Cutting of 

this rock has the potential to add significant quantities of silica dust to the mine environment, resulting in 51% of mines operating on reduced dust standards. In 2005, 11% of continuous miner operator and 12% of roof bolter operator samples exceeded their applicable dust standard. 

Since the initiation of the x-ray surveillance program in the US coal mining industry in 1970, the 

prevalence of Coal Workers’ Pneumoconiosis (CWP) in the workforce has been declining. For mine 

workers with 25 or more years of experience, the prevalence rate of CWP has dropped from 28.2 % 

in 1973 to 3.3 % in 1999 (NIOSH 2003). However, recent x-ray surveillance data have uncovered 

cases of rapidly progressing CWP and also revealed an upturn in the prevalence rate (CDC 2006). 

As a result of changes that have been occurring in the underground coal industry in the US, the 

continued overexposure of workers to respirable dust, and recent x-ray findings, the need to improve 

existing dust control technology and develop new control methods remains a major focus for NIOSH 

and the US mining industry. 

2 LONGWALL MINING DUST CONTROL 

Ventilating air and water sprays are the primary controls used for protecting workers from overexposure to respirable dust. Ventilation provides reduced dust levels through dilution of generated 

dust and through transporting dust away before it can migrate to the breathing zones of miners. As 

a result, efforts must be made to maximize the quantity and quality of ventilating air that reaches 

the face. This is achieved by ensuring that dust levels in the intake air steam are low, that stoppings 

and curtains are tight, and that as much of the intake air is directed down the face as possible. Outby 

dust sources can be controlled by wetting roadways, minimizing outby dust-producing activities (unloading supplies, removing stoppings), and ensuring that sufficient coal wetting occurs before being 

transported out the belt entry. In recent surveys of US longwalls, the average face airflow was found 

to be 3.4 m/sec (665 fpm) with an estimated average air quantity of 31.6 m3

/sec (67,000 cfm). This 

air quantity represents a 65% increase in airflow from 10 years ago (Rider and Colinet 2007). 

As clean intake air is brought onto the face, the potential exists for this air to be contaminated by 

dust generated at the crusher/stageloader. In response, water sprays should be placed on both sides of 

the crusher and at the stageloader-to-section belt transfer (Jayaraman et al., 1992). The objective of 

these sprays is to wet the coal product and prevent respirable dust from becoming airborne. Consequently, water quantity is more critical than water pressure, so larger orifice, full-cone sprays operating at water pressures below 414 kPa (60 psi) are recommended. Also, the entire crusher/stageloader 

unit should be completely enclosed to prevent dust that is generated within the unit from escaping 

into the ventilating air. Mines have successfully utilized steel plating, conveyor belting, expanding 

foam, and line brattice to seal these units. Belting can also be hung at the inlet to the crusher and 

brattice can be used to enclose the transfer point from the stageloader-to-section belt. Figure 1 shows 

two enclosed stageloaders and conveyor belting hung at the crusher inlet. 

In order to maximize the quantity of intake air that reaches the face, a gob curtain can be hung 

in the headgate entry (Jankowski et al. 1993). This is simply a section of line brattice that is 

hung between the rib and first shield in the headgate entry to turn intake air down the face. Several longwalls have expanded on this technique by extending brattice behind the shield legs on 
the first 5-10 shields to further improve the seal around the headgate and turn as much air down the 

face as possible. 

Water spray application is the other primary control being used to substantially reduce the dust 

exposure of longwall face workers. The first attempt to control dust generation from the shearer 

occurs as the shearer bits cut the coal. All shearers in the US are equipped with water sprays in the 

cutting drums. In general, water spray pressure to the shearer drums should be limited to a maximum 

of 690 kPa (100 psi) to prevent dust from being blown into the walkway by the sprays. Also, past 

research has shown that full-cone or jet sprays are the most effective type of spray patterns to use in 

the shearer drums. These sprays increase wetting without inducing substantial air movement around 

the drum. 

Figure 1. Enclosed stageloaders and crusher. 

In recent surveys of ten US longwalls, all operations were using directional spray systems on 

their shearers (Rider and Colinet 2007). The exact type, number, and location of these sprays varied 

significantly between mines, but all were operating on the principle of splitting the ventilating air as 

it reaches the headgate side of the shearer and holding the dust-laden air near the face. This air split 

is facilitated through the use of a “splitter arm,” which is a steel arm that extends from the headgate 

side of the shearer body parallel to the headgate ranging arm. Sprays are mounted on this splitter 

arm and oriented at an angle to the face in the direction of the airflow coming from the headgate. 

Additional spray manifolds are located along the body of the shearer to further enhance the movement of the ventilating air along the face. Typically, a section of conveyor belting is also hung from 

the splitter arm as a physical barrier between the face conveyor and walkway. Figure 2 illustrates 

a directional spray system operating on a shearer. The physical structure of the splitter arm and 

water sprays on the arm force the dust-laden portion of the ventilating air toward the face, while the 

“clean” split of air travels down the walkway. Spray manifolds on the back side of the shearer help 

to maintain this dust near the face and move the dust cloud past the shearer. Since the sprays in these 

directional spray systems are attempting to move air, the operating pressure is critical and pressures 

of at least 1035 kPa (150 psi) should be utilized. Hollow cone sprays and/or venturi sprays are effective in this application. 

To assist in protecting the tailgate side shearer operator, companies have started using spray manifolds mounted at the tailgate end of the shearer. Typically, these sprays are located directly above 

the panline and oriented parallel to the tailgate ranging arm. The water spray pattern and induced 

airflow help move dust-laden air past the tailgate side operator before the dust reaches the walkway. 

Figure 3 illustrates a manifold of these sprays mounted on the shearer body. Hollow cone sprays operated at pressures at or above 1035 kPa (150 psi) would be effective air movers in this application. 
Measurement of water usage on US longwall operations shows that an average of 495 lpm 

(130 gpm) is being supplied to the shearer, with an additional 285 lpm (75 gpm) being added with 

stageloader/crusher sprays (Rider and Colinet 2007). One operator reported using 850 lpm (225 

gpm) on their shearer (Dezeuuw 2007). 

Figure 2. Headgate side splitter arm and directional sprays on shearer. 

Figure 3. Spray manifold on tailgate end of shearer. 
3 CONTINUOUS MINER DUST CONTROL 

Ventilation for continuous mining operations in the US is supplied either through exhaust or 

blowing ventilation systems. For the exhaust system, fresh air is brought to the face in the working 

entry and line brattice or tubing is installed within the entry to create an air separation. Dust-laden 

air is then drawn from the face through the tubing or behind the curtain. This method of ventilation 

is typically the most effective from a dust control viewpoint since it keeps the main working entry 

and the continuous miner and shuttle car operators exposed to fresh air. 

For blowing ventilation, the clean air is brought to the face through tubing or behind brattice and 

discharged toward the face. Dust-laden air is then carried out of the face through the entry. This type 

of ventilation typically penetrates deeper toward the face and is more effective for methane control. 

Ideally, the miner operator can position himself at the discharge of the brattice/tubing and work in 

fresh air. However, the shuttle car operators are positioned in the dust-laden return air. 

Most operations in the US are taking extended cuts of more than 6m (20 ft) in depth and use 

continuous miners equipped with fan-powered, flooded-bed scrubbers. These scrubbers assist in 

moving air toward the face and in capturing airborne dust generated by the miner cutter heads. They 

typically pull dust-laden air through one to three inlets on the miner boom and pass the air through 

a filter panel being wetted with water sprays. The dust mixes with the water droplets and is removed 

from the airstream when the droplets are captured by a demister located behind the filter panel. Over 

90% of respirable dust can be removed by a scrubber. The cleaned air is then discharged from the 

back of the miner and directed toward the return. It is critical that sufficient water is applied to the 

entire filter panel area to extend the effectiveness and performance of the scrubber. Typically, fullcone sprays operated at 415 kPa (60 psi) are used to wet the scrubber filter. Periodic removal and 

cleaning of the filter is also necessary to maintain optimum performance of these dust collectors. 

Research has shown that the selection of the scrubber filter can have a significant impact on dust 

collection efficiency. If dust control is a concern, the more efficient filters (30-layer stainless steel 

or bottle brush filters) can be utilized to maximize dust capture (Colinet and Jankowski 2000). 

In addition to the scrubber, all miners are equipped with an external spray system. These sprays 

are typically located on top of the miner boom behind the cutting drum and oriented toward the bits 

on the miner cutting drums. These sprays wet the coal as it is being cut and prevent dust from becoming airborne. Research has shown that flat fan sprays that are operated at less than 690 kPa (100 

psi) are most effective at reducing dust while preventing rollback (Jayaraman et al. 1984). Rollback 

occurs when cone type sprays are used at higher pressures. The air-moving action of the cone sprays 

can force dust back toward the operator, particularly as spray pressures are increased. 

In addition to scrubbers and boom sprays, it is important to utilize low-pressure, high-volume 

sprays under the boom of the miner and in the conveyor throat. These sprays are designed to wet the 

coal under the boom and prevent dust liberation as the coal is loaded and transported to the shuttle 

cars. These sprays are typically limited to 415 kPa (60 psi) or less. 

NIOSH research has also shown that “blocking sprays” can be mounted on the sides of the miner 

outby the scrubber inlets to help contain dust near the face. These blocking sprays give the scrubber 

and external sprays an improved opportunity of capturing this dust. The blocking sprays are also fan 

sprays that are mounted with a vertical orientation to develop a water barrier along the side of the 

miner boom. The operating pressure of these sprays can be higher than 690 kPa (100 psi) to increase 

the impact zone of the sprays. 

Within the last few years, the leading continuous miner manufacturer in the US has been offering 

a miner that is equipped with water sprays in the cutting drums. These “wet head” miners place the 

spray directly behind the cutting bit and offer improved control of frictional ignition with the potential to reduce dust levels. Figure 4 illustrates the wet head miner and spray nozzle location. NIOSH 
has evaluated two wet head miners and measured dust reductions at the mine operator ranging from 

0.2 to 0.5 mg/m3 (Goodman et al. 2006). 

Each miner in these surveys was equipped with approximately 70 sprays on the cutting heads. 

As designed, the cooling water for these miners was not utilized through the wet head sprays. Consequently, the manufacturer and mine operator wanted to limit the water supplied to the cutting head. 

Each spray was limited to 1.5 lpm (0.4 gpm) in order to maintain overall water flow to the miner at 

an acceptable level. The restricted flow to the wet head sprays may have limited the dust reductions 

to the range observed in these surveys. Additional work with the manufacturer is planned to further 

explore the dust reduction potential of the wet head design. 

Figure 4. Wet head miner and spray location behind bit. 

4 ROOF BOLTER DUST CONTROL 

Roof bolter operators are often exposed to respirable silica dust during drilling. This dust can 

come from drilling into the roof strata or from operating downwind of the continuous miners. Most 

roof bolters in the US are utilizing a dry vacuum dust collection system that pulls dust through the 

drill steel back to a dust collector box. The dust is removed from the airstream and deposited in 

chambers of the collector box or captured by a filter cartridge. When operated and maintained properly, this system can be very effective in capturing and removing dust generated by drilling. 

As the collector box fills, it is necessary for the bolter operator to empty the box and clean 

the filter cartridge. Both of these tasks can expose the bolter operators to elevated levels of silica. 

One method of minimizing the dust exposure of workers when cleaning the box is through the use 

of collector bags that contain the dust and reduce loading on the filter cartridge. These bags fill with 

dust and can be removed without significant dust exposure to the workers. The bag is then discarded 

without the potential for further contamination. Since the dust loading on the filter cartridge is reduced, the filter does not need to be cleaned or replaced as often. This also reduces the potential for 

dust getting beyond the seal of the cartridge, where it can be discharged through the muffler back 

into the mine atmosphere. It should be noted that the dust collector must be fitted with an automated 

pre-dump that separates larger particles prior to reaching the collector. This minimizes the loading 

of the collector bag and extends its usable time. 

Laboratory and field testing of the collector bags has shown that they reduce dust loading on the 

filter cartridge by approximately 80 % (Listak and Beck 2007). Figure 5 illustrates the difference in 

dust deposition within the collector when the bag is utilized. As shown, the collector bag contains 

the dust and allows the collector to be serviced with less dust exposure. 
Figure 5. Dust deposited in the collector box (left) and the collector box with bag utilized (right). 

Another recent control that has shown promise is a canopy air curtain. The canopy air curtain 

is mounted on the underside of the bolter canopy and blows air down over the bolter operator. This 

air is drawn from the ventilating air in the entry and passes through a filter prior to being blown over 

the operator. Consequently, a stream of filtered air is passing over the operator, which prevents dust 

from drilling or dust generated upwind by the miner from reaching the bolter operator. 

Laboratory testing has shown a 50% reduction of dust under the air curtain. An underground 

survey confirms that the air curtain has the potential to reduce operator dust and was well received 

by the operator (Goodman et al. 2006). Figure 6 illustrates the principle of operation for the canopy 

air curtain and also shows the air curtain being tested underground. This testing also identified 

modifications that could be made to the air curtain to improve its effectiveness. NIOSH is currently 

evaluating a second generation air curtain in the laboratory. 

Figure 6. Canopy air curtain being developed for roof bolting machines. 

5 DISCUSSION 

A number of basic control technologies for longwall and continuous miner operations have been 

discussed in this paper. Information on several emerging control technologies has also been presented. However, this paper only touched upon a limited number of controls. A more comprehensive 

summary of dust control principles and technologies is summarized in a NIOSH handbook (Kissell 
2003). This handbook provides a concise discussion of multiple control technologies and includes 

references that can be accessed for greater detail on individual controls. 

In order for any and all of these dust controls to realize their maximum potential, it is critical 

that maintenance of the controls becomes a routine part of operating practices. Management must 

encourage mine workers to regularly examine the installed dust controls and provide the opportunity 

for control technologies to be maintained. An effective program for reducing the respirable dust 

exposure of mine workers must also contain an education and training component along with appropriate control technologies. Workers must be made aware of the potential health risks associated 

with breathing excess respirable dust. Also, the importance of proper utilization of the dust control 

technologies must be stressed and supported by mine management. In the US, mine management 

must post the approved ventilation plan at the mine. This plan includes the types of controls to be 

used and approved operating levels for these controls. 

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