12

 

 

United States 

Jay F. Colinet, Andrew B. Cecala, John A. Organiscak, Douglas E. Pollock, Gregory J. Chekan 

and Ed D. Thimons 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Abstract 

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

Health (NIOSH) work to reduce respirable dust exposures in mining operations. One focus of this research is silica 

dust control for surface and underground operations in the metal/nonmetal mining industry. PRL researchers 

analyze dust sampling results from the Mine Safety and Health Administration (MSHA) to identify high-risk 

occupations to guide research efforts. Research is then conducted in an effort to improve existing technologies 

and/or develop new controls for reducing worker exposure to airborne silica dust. During the last few years, PRL 

researchers have conducted research related to the following goals: improving protection provided by enclosed cabs 

on mobile surface mining equipment, reducing dust generation from surface drills, reducing dust levels in iron ore 

operations, and developing a new method to clean dust from workers’ soiled clothes. A description of each of these 

controls and subsequent dust reductions quantified through site surveys will be provided. 

Background 

Overexposure to respirable silica dust can lead to the development of silicosis, a debilitating and 

potentially fatal lung disease. Consequently, exposure to silica dust is limited by regulations 

enforced by MSHA. When MSHA suspects that silica dust may be present in a metal/nonmetal 

mining operation, a gravimetric sample of the respirable fraction of airborne dust is collected. 

This sample is then analyzed for silica content using X-ray diffraction. If the percent of silica in 

the sample exceeds 1%, a respirable dust standard is calculated by dividing 10 by the (% silica + 

2). For example, if a sample contains 8% silica, the respirable dust standard would be 1.0 mg/m3 

[10 ÷ (8+2)] = 1.0]. If the dust concentration of the collected sample exceeds this limit, the 

sample is out of compliance. 

NIOSH routinely examines MSHA compliance sampling data in order to identify occupations 

that are at elevated risk of overexposure. Research is then directed toward identifying and 

implementing controls that improve the dust protection for these high-risk workers. In addition, 

NIOSH interacts with stakeholders representing industry, labor, academia, and government and 

receives input into areas of concern. This input is also utilized in determining the direction of 

research. As a result, NIOSH researchers conducted research to address silica dust control for 

enclosed cabs on mobile equipment, rotary surface drills, an iron ore processing plant, and 

cleaning workers clothes in a silica sand processing plant. Details from this research follow. 

Filtration Systems for Enclosed Cabs 

Potential worker exposure to respirable silica dust is common in surface metal/nonmetal mining 

operations, since a significant portion of the overburden contains high silica-bearing strata. 

Many of the workers at these mines operate mobile equipment that is equipped with enclosed 
13

deteriorate to a point where they no longer provide adequate sealing. Also, older cabs may not 

have filtration and pressurization systems, or they have poorly designed systems that are 

incapable of maintaining acceptable air quality within the cab. In an effort to improve the 

protection provided by enclosed cabs, NIOSH retrofitted filtration and pressurization systems to 

a haul truck at an underground stone mine and a surface drill at a silica sand operation (Chekan 

et al., 2003). 

At the underground limestone mine, a haul truck that had been manufactured in 1975 was 

selected for evaluation. This truck was used to load fines from the processing plant, transport 

these fines to different locations, dump the fines, and then return to the plant for the next load. 

This truck was fitted with a heating and air-conditioning unit that had minimal dust filtering 

capabilities. This unit was mounted inside the operator’s cab on the roof. 

For this study, the original air-conditioning system on the haul truck was replaced with an 

external unit mounted on the top of the cab. The new system included heating, cooling, and 

pressurization, with high-efficiency filter media for both the pressurizer filter and the final filter 

in the recirculation unit. These filters were rated at 99% efficiency for particles greater than 0.5 

microns. In addition, the integrity of the enclosed cab was improved by replacing the door seals 

with foam weather stripping and sealing gaps in the cab with caulking. 

Gravimetric dust samplers were operated within the cab and outside of the cab directly below the 

operator’s front windshield. Sampling was conducted for three shifts with the original 

equipment and then three shifts after the new filtration system was installed. Silica content of 

the samples from each shift was determined with X-ray diffraction. The data was then averaged 

for each of the test conditions and is summarized in Table 1. As shown, installation of the new 

filtration system made a significant improvement to the protection afforded by the enclosed cab. 

Table 1. Average dust concentrations at the haul truck with each filtration system. 

Test Condition 

Average Respirable Dust Concentrations Average Respirable Silica Concentrations 

Outside Cab 3 mg/m

Inside Cab 

 mg/m3 

Reduction, 

% 

Outside Cab 3 mg/m

Inside Cab 3 mg/mReduction,% 

Baseline 0.425 0.279 34 0.035 0.023 34 

Retrofit 1.010 0.317 69 0.071 0.017 76 

It should be noted that the dust conditions outside of the cab for all three days of testing during 

retrofit sampling were as least twice as high as the baseline testing. These higher concentrations 

influence the amount of dust actually penetrating the cab and resulted in slightly higher dust 

levels inside the cab with the retrofit system. However, if the interior dust levels from the 

retrofit testing were normalized for the outside concentrations from the baseline levels, a 52% 

reduction in respirable dust and a 63% reduction in respirable quartz dust in the cab are achieved 

with the new system. This is a measure of the actual improvement in the cab working 

environment that could have been expected if outside dust levels had been equal. 
14

original filtration system. Gravimetric sampling was conducted inside and outside of the cab to 

compare the protection provided by the original filtration system to that of the retrofitted system. 

Sampling results indicated that the dust levels outside of the cab again differed significantly 

between the testing of the original and new systems. After normalizing the dust concentrations 

measured inside the cab for the differences observed outside, the new system reduced dust levels 

in the cab by 67%. 

The original filtration system was tested during the winter months. With this system, heat to the 

cab was supplied by a heater located on the floor of the cab, as shown in Figure 1. During 

testing, it appeared that the floor heater was creating higher dust levels in the cab by blowing air 

across the dirt tracked into the cab on the operator’s boots. 

Figure 1. Floor heater in surface drill cab. 

The drill was taken into the shop and particle counting instrumentation used to evaluate the 

filtration system and floor heater. The optical particle counting results indicated that the floor 

heater raised respirable dust levels from 0.03 to 0.26 mg/m3 in the shop tests. The new 

filtration/pressurization had heat supplied at the roof of the cab, allowing for elimination of this 

floor heater and the internal dust source. This testing also illustrated the importance of having a 

filtered recirculation circuit that will remove dust generated inside of the cab. 

Surface Drill Dust Collector 

MSHA compliance sampling data has historically shown that drill operators at surface mines are 

typically at the highest risk of being overexposed to respirable silica dust. As a result, NIOSH 

has examined methods to lower the dust levels liberated by surface drills. The information 

already discussed for enclosed cabs also has direct application. 

The majority of the surface drills being operated in the United States are equipped with dry dust 

collectors. Typically, these drills have the inlet to the dust collector located at the rear corner of 

the drill table and are designed to collect airborne dust as it is blown out of the drill hole by the 

bailing airflow. NIOSH has conducted laboratory research to evaluate the relationship between 

the bailing airflow and the collector airflow for these systems. In addition, NIOSH designed a 

modified collector inlet that moves the inlet around the drill stem. Preliminary tests were 
15

plate was designed with a triangular top section that traversed the circumference of the plate 

inside the hood surrounding the drill stem. This was intended to be self-cleaning so that any 

larger sized material making it inside the hood could slide back out through the perimeter of the 

hood inlet gap. A rubber seal (made out of conveyor belting) was utilized to seal the deflection 

plate gap around the drill stem. Figure 2 shows a standard and the modified collector inlet. 

 

Original collector inlet 

Modified collector inlet 

Figure 2. Original and modified collector inlet locations. 

Laboratory experiments were conducted to evaluate three factors that impact dust liberation: 1) 

the amount of open area or bottom gap between the ground and drill table shroud, 2) the dust 

collector airflow to bailing airflow ratio, and 3) the collector inlet location. Since down-the-hole 

bailing airflow is usually set for maximum drill penetration rate and bit cutter life, varying this 

parameter over a broad operating range for dust control is usually not a functional consideration. 

In the lab experiments, the drill hole bailing airflow was set at a constant flow rate and the 

collector airflow was changed to achieve the desired collector to bailing airflow ratio. The drill 

hole bailing airflow varied between 170 and 179 L/s to yield a bailing air velocity between 23.4 

and 24.9 m/s, representing typical drilling conditions found on medium sized drills in the US. 

These tests were conducted in a large, 3.66-m wide by 3.05-m deep by 2.44-m high dust 

chamber. The simulated drill shroud was located in the center of the dust chamber with the 

dimensions of 1.52-m wide by 1.22-m deep by 1.22-m high. This shroud size is within the range 

found on medium-sized, rubber tire or track-mounted rock drills, typically drilling holes 12.7 to 

20.3 cm in diameter with about 178 kN to 222 kN of drill pull down pressure. 

Laboratory tests were conducted to evaluate a range of operating parameters (Organiscak and 

Page, 2005). The ground-to-shroud gap was varied from a minimum of 5.1 cm to a maximum of 

35.6 cm. The dust-collector-to-bailing-airflow ratio was varied from a minimum of 2:1 

(collector volume of about 349 L/s) to a maximum of 4:1 (collector volume of about 698 L/s). 

The baseline tests were conducted with the collector inlet located toward the back corner of the 

drill table. Tests were also conducted with the inlet relocated above the drill hole. Figure 3 

illustrates the dust levels measured for the different operating conditions. 

Laboratory testing showed that the inlet hood concept notably improved drill dust collector 

system capture over the conventional inlet location, particularly with large leakage areas at the 
16

m16.0 g/ m

14.0 

ons, 12.0 ita

10.0 

rnt

8.0 

eonc

6.0 

 Ct

4.0 

us

2.0 

D

0.0 

5.1 cm 20.3 cm 35.6 cm 5.1 cm 20.3 cm 35.6 cm 

Shroud Gap Height, cm 

2:1 ratio 

3:1 ratio 

4:1 ratio 

Figure 3. Laboratory dust concentrations escaping the shroud for different test configurations. 

bottom of the shroud enclosure. The most significant improvements were between 63 and 91% 

dust reductions measured at collector-to-bailing airflow ratios of 3:1 and 4:1 with shroud gap 

heights of 20.3 and 35.6 cm. The laboratory tests also showed that the two key influential factors 

on collector inlet capture were the collector-to-bailing airflow ratio and the shroud gap height for 

both inlet configurations. Nearly an order of magnitude increase in dust leakage was observed 

when the shroud gap height was increased from 5.1 to 20.3 cm with the conventional inlet 

configuration. The higher collector-to-bailing airflow ratios and inlet hood configuration 

mitigates this dust leakage at the higher shroud gap heights (20.3 and 35.6 cm, respectively). 

Given the laboratory results measured with the modified inlet hood, field testing was conducted 

to examine its operational viability and performance. The field study plan was to conduct area 

dust sampling around the drill shroud in its existing state for several shifts and resample after the 

inlet hood was installed. The inlet hood had to be significantly redesigned from the laboratory 

version to custom fit this particular drill deck underside 

The shroud enclosures used on all the drills at this mine were an angle curtain design that could 

be hydraulically raised and lowered to seal with uneven ground surfaces. During the initial visit 

to the mine, the shroud on the test drill had signs of wear that allowed dust to leak out. However, 

a day before the baseline study began the drill was serviced, including dust filter replacement and 

repair of the shroud holes. After the shroud was repaired, it maintained a good seal with the 

ground, with average dust levels outside of the shroud equal to 0.18 mg/m3

 during baseline 

testing. These findings support the data from the laboratory tests that illustrated the benefits of a 

sealed shroud. 

In spite of the low baseline dust levels, the inlet hood was installed to examine its operational 

functionality, since the inlet hood had been custom designed to fit this particular drill. The 

modified inlet hood appeared to have a slight but negligible dust reduction to 0.11 mg/m3. 

During installation, the drill’s bailing airflow was measured to be 380 L/s with collector airflow 

of 1008 L/s. Immediately after the inlet hood installation, the collector airflow dropped to 963 

L/s. From an operational perspective, over 1,500 m of drilling was conducted with the inlet hood 

installed. Follow-up phone discussions with the mine about the inlet hood’s operating status 

indicated that the inlet hood ductwork eventually filled with cuttings after a week of operation. 
17

 

Dust Control at an Iron Ore Processing Plant 

NIOSH was contacted by stakeholders that expressed interest in lowering dust levels in the 

grinding and concentrator mill portion of an iron ore processing plant. This multi-level plant had 

a generally open structure with steel grating separating the different levels of the plant. During 

the winter months, the plant processed hematite ore with the doors and other air intakes into the 

plant closed. The hematite typically contained silica in the range of 10–15% and airborne 

respirable silica was a concern. Therefore, the plant maintained a mandatory respirator zone in 

the region of the 12 primary grinding mills located at the north end the plant. Figure 4 illustrates 

the general layout of the grinding and concentrator building. 

Figure 4. Location of major processes in grinding and concentrator building. 

The ventilation provided to this building was through a network of over 70 roof fans that could 

be operated in either an intake or exhaust mode. In addition to the roof-mounted fans, each of 

two air heaters supplied 4,250 m3

/min of heated intake air into the basement of the structure, one 

on the eastern and the other on the western side of the structure. The ventilation setup at this 

facility changed somewhat over the course of the year based on outside air temperatures. 

Typically, the air exhausted from the structure was reduced during the winter months, with the 

goal of providing enough ventilation to remove contaminants while trying to maintain an 

adequate temperature within the structure. 

It appeared to NIOSH researchers that the ventilation fans were being operated in a random 

fashion, with significant imbalance between intake and exhaust air (Cecala et al., 2006). 

Gravimetric and instantaneous dust samplers were used at numerous locations throughout the 

plant during a baseline dust survey to quantify airborne respirable dust levels under typical 

operating conditions. NIOSH then requested that the number and pattern of operation for the 

fans be changed to induce a more balanced, directional flow of air through the plant. 

The original ventilation setup included 12 exhaust fans operating in the primary grinding area, 

nine exhaust fans and three intake fans operating in the pebble mill area, and 19 exhaust fans 

operating in the filter and flotation area. The only other mechanical ventilation provided was 
18

the structure was 54,670 m3

/min, as compared to 17,000 m3/min being brought into the structure. 

These ventilation air volumes were calculated based upon the rated fan capacities and the 

assumption that all the fans were operating at their rated output. 

Figure 5. Roof fans at iron ore plant. 

Since the baseline sampling data showed that the highest dust levels were observed around the 

primary grinding mills, fan operation was reconfigured to direct airflow from the southern region 

of the plant toward these grinding mills. Two different fan configurations were tested with this 

goal in mind. The final and most effective ventilation setup was composed of 24 exhaust fans 

and 12 intake fans, along with the two heater intake fans. The volume of air exhausted from the 

structure was approximately 32,000 m3/min, with 29,600 m3/min being brought into the structure 

through the roof intake fans and two heater fans. This operating pattern resulted in a 31% dust 

reduction (1.01 mg/m3

 to 0.70 mg/m3) at the six sampling stations located along the primary 

grinding mills. Dust levels near the pebble mills and flotation cells were within 0.02 mg/m3

 of 

levels observed during baseline sampling. This improvement is especially significant because it 

was achieved simply through better utilization of an existing control technology. 

Clothes Cleaning Booth 

Past research (Cecala and Thimons, 1986) has shown that dust-soiled work clothes can be a 

substantial source of personal dust exposure for workers. Currently, the only approved MSHA 

method for cleaning soiled clothes is to use a HEPA filter vacuuming system. To perform this 

technique, a worker uses the vacuum hose and manually moves the nozzle over his/her soiled 

clothing in an attempt to remove the contaminant. Because this task is difficult and timeconsuming, some workers choose to use a compressed air hose to blow dust from their work 

clothing, even though this is not an approved method of cleaning. For either method, the most 

difficult task is to clean the worker’s back and legs effectively. 

NIOSH worked with a leading silica sand producer to develop the clothes cleaning booth as an 

improved method to cleaned soiled work clothes (Pollock et al., 2006). An enclosed booth was 

purchased and installed at the silica sand plant to provide a worker sufficient space to effectively 

perform the cleaning operation. Above the door was an open grate that served as an intake for the 

ventilating air. A return air plenum located on the bottom-back wall of the booth was ducted to 
19

37.3 Pa, which prevented dust leakage from the booth into the plant. 

An air nozzle spray manifold was installed in the booth and used compressed air to remove the 

dust from workers’ clothes. The air spray manifold was fabricated from 63.5mm, schedule-40 

steel pipe that was capped at the base. Twenty-six flat fan air nozzles were mounted along the 

manifold, spaced on 50.8 mm centers. The bottom nozzle was a circular design located 152.4 

mm from the floor. This nozzle was used in coordination with a ball-type adjustable fitting that 

was directed downwards to clean the individual’s work shoes or boots. At a pressure of 206.8 

kPa, the air spray manifold system expels 4.7 m3

 of air for the typical cleaning period. The air 

spray manifold was actuated by the worker operating a timer-set pneumatic valve located on the 

top of the manifold. The pneumatic valve had a safety interlock that would automatically shut the 

air supply to the manifold if the exhaust ventilation system failed to keep the booth under 

sufficient negative pressure. In order to supply this compressed air volume to the air nozzles for 

effective cleaning, a 0.45 m3

 reservoir tank was used and typically pressurized to 1,034.2 kPa. 

The air reservoir was located directly behind the cleaning booth and hard-piped to the air spray 

manifold located inside the booth. Supply air to the manifold was regulated down to 206.8 kPa. 

The air regulator was located in a lock-box enclosure to prohibit anyone from tampering with the 

air pressure. The worker performing the cleaning process is required to wear a half-mask fittested respirator with an N100 filter, hearing protection, and full-seal goggles. 

Performance of the booth was compared to that of the approved HEPA vacuuming method and 

using a compressed air line. Each test was timed by a stopwatch to determine the actual cleaning 

time. Results of testing indicated that the manifold cleaned the clothes at least 10 times faster 

and removed 50% more dust than the single air nozzle or vacuuming methods. The average 

cleaning time with vacuuming, the air hose and the cleaning booth was 372, 178, and 18 seconds, 

respectively. These values represent averages calculated for two NIOSH test personnel, two 

types of coveralls, and a total of 96 tests. Figure 6 shows the relative effectiveness of the 

cleaning techniques tested. 

Conclusions 

These NIOSH research efforts have identified a number of control technologies that can be 

implemented to reduce the dust exposure of mine workers in the metal/nonmetal industry. All of 

the controls were demonstrated at mining operations to provide guidance for their application at 

mining sites. However, a key factor in the success of applying any of these controls will be 

continued attention to operating details and maintenance of the controls. Too often, NIOSH has 

observed less than optimal performance from proven control technologies as a result of 

ineffective maintenance procedures. Likewise, mine workers must be made aware of the 

importance of properly utilizing dust controls in order to protect themselves on a daily basis from 

exposure to respirable dust. 
20

 

Pre-cleaning Single Air Hose Vacuuming Air Manifold 

Figure 6. Effectiveness of various cleaning methods. 

 

 

 

References 

CECALA, A.B., ZIMMER, J.A., COLINET, J.F., and TIMKO, R.J., 2006. Lowering Respirable Dust at an Iron 

Ore Concentrator Plant through Improved Ventilation Practices. 11th U.S./North American Mine Ventilation 

Symposium – Mutmansky & Ramani (eds), Taylor & Francis Group, London, p.189-196. 

CECALA, A.B., and THIMONS, E.D., 1986. Impact of Background Dust Sources on Dust Exposures of Bag 

Machine Operators. U.S. Bureau of Mines, Information Circular 9089. 

CHEKAN, G.J., CECALA, A.B., and COLINET, J.F., 2003. An Evaluation of Cab Filtration and Pressurization 

Systems: Two Case Studies. Proceedings of the 2003 Environment, Safety & Health Forum, National Stone, Sand 

& Gravel Association, Phoenix, AZ, Sept. 22-24, p. 114-129. 

ORGANISCAK, J.A., and PAGE, S.J., 2005. Development of a Dust Collector Inlet Hood for Enhanced Surface 

Mine Drill Dust Capture. International Journal of Surface Mining, Reclamation & Environment, 19(1), p. 12-28. 

POLLOCK, D.E., CECALA, A.B., ZIMMER, J.A., O’BRIEN, A.D., and HOWELL, J.L., 2006. A New Method to 

Clean Dust from Soiled Work Clothes. 11th U.S./North American Mine Ventilation Symposium – Mutmansky & 

Ramani (eds), Taylor & Francis Group, London, p.197-201.