Improving Silica Dust Control Through Targeted Research 

Jay F. Colinet and Edward D. Thimons 

National Institute for Occupational Safety and Health 


Pittsburgh, PA 

1 


ABSTRACT 

NIOSH conducted a strategic planning effort to identify 

those areas in mining with the greatest research needs. 

Data analysis and stakeholder input identified the need to 

reduce worker exposure to respirable crystalline silica 

dust as a high priority area. MSHA dust sampling data 

were analyzed to target the mining commodities and 

occupations with the greatest percentage of samples 

containing silica above compliance levels. In response, 

the Pittsburgh Research Laboratory (PRL) has 

implemented a multi-faceted research program to improve 

silica dust control for these problematic commodities and 

occupations. A review of silica compliance trends, PRL 

silica research programs, and recent control technology 

developments are provided here. 

BACKGROUND 

The Federal Coal Mine Health and Safety Act of 1969 

and the Federal Mine Safety and Health Act of 1977 

established exposure limits and sampling requirements to 

assess cpmpliance with the respirable dust standards in 

mining. The Mine Safety and Health Administration 

(MSHA) is responsible for the collection of personal 

respirable dust samples, which are measured 

gravimetrically as an 8-hour time-weighted-average 

concentration. For coal mining, a permissible exposure 

limit (PEL) of2 mg/m3 for respirable coal mine dust has 

been established. The PEL is reduced if the respirable 

crystalline silica content ofthe samples exceeds 5%. 

Crystalline silica analysis (quartz) for coal mine dust 

samples is conducted using infrared spectrometry 

(Parobeck and Tomb 2000). The reduced PEL is 

calculated by dividing the percent silica into 10 (e.g., 

10/10% silica, would reduce the PEL to 1.0 mglm3). 

Noncompliance occurs when the measured dust 

concentration exceeds 2 mg/m3 or the reduced PEL, when 

applicable. 

For metal/nonmetal mining, personal respirable dust 

samples are collected by MSHA inspectors and analyzed 

for crystalline silica dust (quartz, cristobalite, tridymite) 

using x-ray diffraction (Parobeck and Tomb 2000). All 

samples with a mass of at least 0.1 mg are analyzed for 

silica content. Samples with a silica content greater than 

or equal to 1% silica are considered respirable quartz 

(RQ) samples and are used to calculate a PEL. The PEL 

is determined by dividing 10 by the (% respirable quartz + 

2). Therefore, a sample containing 10% silica in a 

metal/nonmetal mine would result in a PEL of 0.833 

mglm3(e.g., 10/12 = 0.833). Noncompliance occurs when 

the measured dust concentration is greater than 1.2 times 

the PEL. 

When the health and safety functions ofthe U.S. Bureau 

of Mines were transferred into NIOSH, a strategic 

planning effort was initiated to utilize available data and 

stakeholder input to identify the areas with the greatest 

research needs. Part of this effort involved analyzing 

available data to assess the severity and likelihood of 

overexposure to respirable dust in the mining industry. 

STRATEGIC PLANNING EFFORTS 

In 1998, NIOSH personnel were charged with the task 

of analyzing available data to identify the greatest threats 

to the health and safety of miners. A risk analysis was 

completed, where the severity and likelihood of individual 

risks to miners were determined and ranked on a scale 

from low to high. For these purposes, severity was 

defmed as the effect of an event when it occurs (e.g., 

death, disability, injury), while likelihood indicates the 

frequency at which an event occurs. High severity was 

assigned for those illnesses resulting in death, total 

disability, or significant decrease in the quality of life, 

while low severity represented illnesses causing little to no 

permanent disability or impairment. However, the 

development of this risk analysis for health-related threats 

to mine workers was complicated by the disease itself and 

the type of data available for analysis. 

The development of health-related illnesses typically 

does not occur lll1til extended exposure to the health 

hazard has been realized. The onset of the illness may not 

manifest itselfuntil after the mine worker has left the 

mining industry or even retired. Consequently, the direct 

causal relationship associated with accident-injury 

reporting often is not available with health-related 

illnesses. 

In underground coal mining, the Coal Workers' X-ray 

Surveillance Program was created in the Federal Coal 

Mine Health and Safety Act of 1969. Under this program, 

underground coal mine workers can volunteer to 

periodically have a chest radiograph taken to evaluate for 

the development of Coal Workers' Pneumoconiosis 

(CWP). However, surface coal miners and miners 

working in the metal/nonmetal mines did not have a 

similar federally-sponsored, x-ray program available. 

Consequently, the prevalence rate of CWP for active 

underground coal miners has been documented, but the 

prevalence of silicosis in the mining workforce has not 

routinely been monitored. Another confounding factor in 

determining contraction of these diseases is that 

radiographs can be used to identify lung disease, but an 

associated work history or autopsy is needed to 

distinguish between silicosis and CWP. Consequently, 

reported cases of CWP may actually be cases of silicosis. 

The best available determination of silicosis within mine 

workers is obtained through the review of death 

certificates where silicosis is listed as the cause of death. 

Unfortunately, only 24 states identify the industry ofwork 

for the deceased on their death certificate and report this 

information to the National Center for Health Statistics 
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[NIOSH 1999]. Therefore, insufficient data is available 

to detennine the actual number of cases of silicosis within 

the mining industry. 

Severity ofRisk to Miners 

Extended exposure to airborne respirable dust can lead 

to the development ofdebilitating and potentially deadly 

lung diseases for mine workers. The principal threats to 

coal mine workers are the development of CWP and 

silicosis, with silicosis being the primary lung disease 

contracted by workers in the other mining commodities. 

In its early stages, CWP is classified as simple 

pneumoconiosis and the disease usually does not prevent 

the worker from working or carrying on with most nonnal 

activities. As the disease progresses to complicated 

pneumoconiosis or progressive massive fibrosis (PMF), 

more severe lung impairment results and the worker is 

considered pennanently disabled. Likewise, varying types 

ofsilicosis exist and range from the most severe (acute . 

silicosis) to the most common (chronic silicosis). In acute 

silicosis, the worker is exposed to extremely high silica 

concentrations over a relatively short period of 6 months 

to 2 years [NIOSH 1995]. The disease demonstrates a 

rapid progression to death from respiratory failure or 

tuber4?ulosis, typically within one year ofcontracting the 

disease. Chronic silicosis develops over an extended 

exposure period (1 0+ years) at relatively low silica 

concentrations. Chronic silicosis is similar to CWP in that 

the disease initiates as simple silicosis but can progress to 

PMF and death. The toxicity of silica dust within the 

lungs results in silicosis continuing to progress as a 

disease even after the afflicted worker is removed from 

further dust exposure. 

As indicated, limited data is available to accurately 

detennine the extent ofsilicosis among mine workers. 

From data available for 1987 to 1996, a total of232 

deaths were attributed to silicosis in the coal, metal, and 

nonmetal mining industries [NIOSH 1999]. Despite the 

lack of data available for "sand and gravel" and stone 

operations, these mining-related silicosis deaths accounted 

for approximately 22% ofthe silicosis deaths reported by 

the 24 states that identified worker industry on the death 

certificates during this period. Several significant mining 

states, including Pennsylvania (490 silicosis deaths), 

Illinois (92 silicosis deaths), and California (144 silicosis 

deaths), were among the states that did not provide 

industry-specific data for the deceased workers. 

A special x-ray program was offered to surface mine 


workers in,Pennsylvania in 1994. Over 5% of the 150 


miners eXl:).l11ined were diagnosed as having silicosis 


[MSHA 1995]. The level of silicosis found in this initial 


survey prompted two additional rounds of radiographs 


being offered on a voluntary basis to surface miners in 


Pennsylvania. Over 1200 additional miners were 


examined and results indicate that nearly 7% ofthese 


miners had contracted silicosis [Anon. 2000]. 

The potential for silicosis to cause pennanent disability 

and death, mining deaths continuing to occur from 

silicosis, and the continued prevalence ofthese diseases 

being identified in miners indicate that exposure to silica 

dust in underground and surface mining operations 

remains a very real and serious threat to miners' health. 

Consequently, a severity rating of "high" was assigned to 

silica dust exposure in NIOSH's strategic planning efforts. 

Likelihood of Risk to Miners 

For NIOSH's strategic planning, the lack ofinfonnation 

on silica-dust-induced illness made it necessary to utilize 

exposure data as a proxy to detennine the likelihood of 

contracting silicosis. This reasoning suggests that if 

overexposure to respirable silica dust continues to occur, 

then the future development of silicosis will occur in the 

mining workforce. For purposes ofestablishing a ranking 

ofthe likelihood of overexposure to dust, NIOSH adopted 

an arbitrary scale ofoverexposure that was similar to the 

ranking scale developed for the safety data in the strategic 

planning effort. If the percentage of samples exceeding 

the PEL was less that 5%, between 5 and 10%, or greater 

than 1 Q%, risk ratings of low (L), medium (M), or high 

(H) were assigned, respectively. 

As previously discussed, personal exposure data were 

used as an indicator for detennining the likelihood of 

developing silicosis. MSHA dust sampling data were 

utilized as the measure ofthe likelihood ofoverexposure 

to silica dust for mine workers. Sampling data for the 

five-year period from 1988 to 1992 were available for the 

strategic planning effort for all mining commodities of 

interest and are provided in Table I. These data represent 

a summary of all samples collected by MSHA and include 

a vast array of both underground and surface mining 

occupations that were sampled. It should' be noted that the 

sampling data for coal mining were obtained from the 

NIOSH document, "Criteria for a Recommended 

Standard: Occupational Exposure to Respirable Coal 

Mine Dust" [NIOSH 1995]. Sampling data for all other 

mining industries were obtained from the Mine Inspection 

Data Analysis System (MIDAS) database [Watts and 

Parker 1995]. These sampling results show that a 

substantial percentage ofsilica dust samples were 

exceeding the PEL in all ofthe mining industries. 

Further analysis ofthe sampling data from 1988 to 1992 

was completed to provide a more specific detennination 

of overexposure to silica dust by underground and surface 

mining location for coal mming. For all other mining 

industries, overexposure data for underground, surface, 

and mill/plant locations were available. Table II provides 

the over-exposure rates that were observed and the 

corresponding likelihood rankings. 
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Table I. Silica dust sampling results by mining industry 

Mining 

Industry 

1988 - 1992 Samples 

Number of 

samples 

%>PEL 

All mining 69,345 15 

Coal 19,783 24 

Metal 4,686 21 

Nonmetal 4,796 13 

Stone 22,428 12 

Sand & Gravel 17,652 9 

Table II. Likelihood ranking by mining location . 

Mining 

Industry Location 

Number 

of 

Samples 

%of 

Samples 

> PEL 

Rank 

Coal 

Under 

ground 

16,240 24 H 

Surface 3,543 24 H 

Metal 

Under 

ground 

1,178 

1,700 

27 H 

Surface 15 H 

Mill or 

Plant 

1,808 23 H 

Non 

metal 

Under 

ground 

424 

1,767 

16 H 

Surface 5 M 

Mill or 

Plant 

2,605 18 H 

Stone 

Under 

ground 

736 15 H 

Surface 12,486 9 M 

Mill or 

Plant 

9,206 15 H 

Sand & 

Gravel 

Under 

ground 

45 

10,027 

7 M 

Surface 7 M

Mill or 

Plant 

7,580 12 H 

High Exposure Occupations 

In addition to the above assessments, sampling data 

from the MIDAS database for the period from 1986 to 

1995 were also examined and contained information on 

dust exposures for individual occupations in the various 

mining industries. Figure 1 illustrates a number of 

occupations that were frequently sampled and had high 

over-exposures for each mining industry. This 

information identifies problematic occupations that 

warrant future research in order to lower silica exposures. 

Stakeholder Input 

Qualitative data from 22 stakeholder sources (data 

gathered from letters, questionnaires, and meetings) were 

examined to assess the magnitude of concern expressed by 

the stakeholders for the many health and safety issues 

facing the mining industry. A ranking scale was 

developed that resulted in a high, medium, or low interest 

ranking based upon the number ofstakeholders that 

expressed interest in each issue. Silica dust control 

received a "high interest" ranking from the stakeholder 

input examined. 

As a result ofthis strategic planning effort, recent dust 

control research at PRL has focused upon surface drills 

and roof bolters in coal mining, plant operations at 

noncoal mines, and underground operations at metal and 

stone mines. An overview ofthe research addressing each 

ofthese areas will be provided. 

ONGOING RESEARCH PROGRAMS 

The Pittsburgh Research Laboratory is currently 

conducting dust control research for coal and 

metal/nonmetal mining for both surface and underground 

mining occupations at high risk for overexposure to 

respirable silica dust. The focus, methodology, and recent 

outcomes for each research program will be discussed. 

Dust Control in Surface Coal Mining: As indicated by the 

compliance sampling results, the surface drill operator 

experiences the highest frequency of overexposures. 

Therefore, a multi-faceted research effort is being 

conducted to: reduce the amount of dust generated during 

drilling, improve the capture of dust liberated from the 

drill hole, and improve the protection offered by enclosed 

cabs on surface equipment. 

A number ofdrilling parameters (e.g., drill rpm, thrust, 

bit type) along with the physical parameters ofthe rock 

being drilled (e.g., type, thickness, silica content) have the 

potential to impact the amount ofsilica dust liberated 

during drilling. Research has been initiated to document 

the impact of1hese parameters on dust generation. In 

order to evaluate differences resulting from changes in 

these parameters, a unique sampling system has been 
4 


developed. A multi-stage sampler will be used to sample 

dust levels in the hose leading from the collector to the 

drill table. This sampling will quantify dust generation 

from the major strata segments and examine the impact on 

dust when drilling parameters are changed. Preliminary 

evaluation ofthis sampling technique has been completed 

in the lab and at a cooperative mine site. 

Dry dust collection systems on surface drills, which 

attempt to collect dust through a capture duct instalIed 

near the comer of the drill table, can be very effective. 

However, the distance separating the capture duct of the 

dry collector from the drill hole allows dust to escape the 

collector, particularly ifthe confmement ofthe drill 

shroud is compromised. Thus, research is being 

conducted at a full-scale test stand to position the 

collection point directly above the drill hole. A special 

hood had to be designed that would allow for dust capture 

without large drill cuttings causing the collector hood to 

plug. Full-scale laboratory tests indicate that the shroud 

can operate without clogging and improvements in capture 

efficiency are being evaluated. 

Enclosed cabs on surface mining equipment can provide 

protection from respirable dust generated on the mining 

site. However, as the equipment ages, the integrity of the 

enclosed cabs and air supply systems to these cabs 

degrade and thus reduces the protection offered by the 

cab. PRL has conducted several research efforts to 

improve cab protection through the installation of new 

filtration/pressurization units and by restoring the integrity 

ofthe seals around the cab. Significant dust reductions 

were measured when the cabs were able to obtain some 

degree of pressurization. At a coal mine site, a positive 

pressure of 0.1 inches of water gage was achieved on a 

front end loader and resulted in a 90% dust reduction 

inside the cab [Organiscak et al. 2000]. However, at this 

same site, a surface drill could not achieve positive 

pressure in the cab and no improvements in dust 

reductions were observed. During the course of the 

research, it was found that mud brought into the cab on 

the operator's boots can be entrained into the cab 

environment, particularly if a floor-mounted heater is 

used, and add to the operator's dust exposure [Cecala et 

al. 2001]. A sweeping compound was tested that was 

shown to reduce dust from this source [Organiscak et aL 

2001]. 

0~--------------------------------------------------------~ 

Metal Mlrlmgl 

I::::::::::;:::::) Coal Mining 

60 ::::l-----------------~ .. Stone Mining 

IDghwall drill Roof bolter Continnous LW return- Dimension Crusher Crusher LHD 

operator operator miner oper. side worker stone cutter operator operator operator 

Occupation 

Figure 1. Silica exposures for select occupations from different mining industries. 

Control of Silica Dust Exposures in Underground Coal 

Mining 

Roof bolter operators were identified as the highest 

exposure occupations in underground coal mines. 

Research was initiated to determine the source of these 

high exposures and then develop controls to address these 

sources. MSHA compliance sampling data were analyzed 
5 


to identify a number of mining sections that were 

operating on reduced dust standards as a result of elevated 

silica levels. Two groups ofmining sections were 

examined. The frrst group consisted of sections that were 

on reduced dust standards and were relatively consistent 

in meeting the reduced standard, while the second group 

was having difficulty maintaining compliance with the 

reduced standard. MSHA inspector data from these 

sections were evaluated to determine if differences in dust 

control technologies existed between the two groups. The 

factor that appeared to have the greatest impact was 

having the the roof bolter work downwind ofthe 

continuous miner. In this position, silica dust generated 

by the miner could be carried to theroofbolter operators. 

Therefore, research was initiated to develop air curtain 

technology for installation on roof bolters working 

downwind ofthe miner. 

A canopy air curtain was developed that would filter a 

small quantity ofdust-laden air and then discharge 

cleaned air through a plenum installed over the breathing 

zone ofa bolter operator. Laboratory tests were 

conducted and indicated that the air curtain did reduce 

dust levels in the operator's breathing zone. However, the 

height added by installation ofthe air curtain may 

preclude its use in mines with lower seam heights. 

Subsequent research was conducted in an effort to achieve 

the same type ofdust reduction while using an air tube, 

which would be less restrictive than the air plenum; 

The longwall mining method is highly productive but 

also has the potential to create dust control problems on 

the longwall face. One likely source ofsilica dust 

exposure for longwall workers is dust liberated during the 

advancement ofshield supports. With higher quantities of 

air being supplied to longwall faces, the impact ofhigher 

air velocities on this dust source has not been quantified. 

In response, a wind tunnel was fabricated to simulate dust 

dropping from the shield canopies to quantify the dust 

entrainment that occurs in high velocity airstreams. To 

date, tests have been conducted with coal dust that 

indicate the entrainment ofrespirable dust is greater than 

dilution effects when higher air velocities are introduced 

into the wind tunnel [Listak,et al. 2001J. Future tests are 

planned to evaluate the entrainment characteristics ofrock 

in these high velocity airstreams. 

Silica Dust Control in MetallNonmetal Mining 

As indicated by the MSHA compliance sampling data, 

occupations at both surface and underground 

metaVnonmetal mines are being exposed to unacceptable 

levels ofrespirable quartz. Research for this project is 

addressing both areas ofconcern. For surface 

occupations, studies are being conducted in a silica sand 

plant, a silica sand surface mine, and a dimension stone 

shop. Underground dust surveys have been conducted in 

limestone and gold mining operations. 

At the silica sand plant, the clothing ofworkers can get 

contaminated with silica dust and should be cleaned to 

avoid secondary exposure to the worker. Currently, 

MSHA requires that this dust be vacuumed from the 

clothing and does not permit the use of compressed air to 

blow the dust off. Compressed air disperses the dust into 

the air, creating the opportunity to be inhaled by the 

worker. NIOSH, in cooperation with plant personnel, has 

been investigating the use ofa clothes cleaning booth that 

utilizes compressed air but provides respiratory protection 

for the worker during the clothes cleaning process. Initial 

testing has utilized a manifold of air sprays to clean the 

workers clothes while the worker is wearing an airpurifying helmet in an enclosed booth. Preliminary results 

indicate that the air manifold removed a larger quantity of 

dust from worker coveralls in a shorter time than the 

normal vacuuming procedure. It appears that slight 

modifications to the manifold sprays w,ill offer additional 

improvements in cleaning and additional testing is 

planned. 

A rotary drill rig at the silica sand mine is being 

retrofitted with an air filtration/pressurization system to 

improve the protection offered by the enclosed cab. A 

baseline dust survey has been completed and the 

filtration/pressurization unit installed. Post-installation 

sampling is currently being conducted to evaluate the 

impact ofthe new system on dust levels inside the 

enclosed cab. 

The dimension stone shop receives large slabs ofstone 

which are then cut into smaller pieces using several 

different saws. Company officials were concerned about 

an accumulation ofdust in the building, particularly 

during winter months when shop doors are closed. In an 

effort to reduce ambient dust levels, fan-powered air 

cleaners were installed near the roof and dust sampling 

was completed. Results indicate that the filters in the air 

cleaners became loaded in a relatively short time and 

caused a reduction in air flow through the cleaners. 

Additional research is being conducted to utilize the 

expertise of a filter manufacturer to improve the 

performance characteristics of the cleaners. In addition, 

the potential to use wetted filters near specific saws is also 

being investigated. 

The number ofunderground limestone mines in the US 

is increasing and a common problem in many ofthese 

operations is the lack ofwell-defmed ventilation systems. 

A limestone mine in Pennsylvania was utilizing an 

auxiliary fan to supply blowing ventilation to transport 

dust away from an underground crusher and down a belt 

entry to the main mine return. Dust sampling was 

conducted to evaluate the effectiveness ofthis installation 

and showed that the fan did prevent dust rollback into the 

intake entries but did not efficiently move dust from the 

crusher to the return. Two alternatives, a second auxiliary 

fan or a dust collector, are being considered for 
6 


installation by mine personnel in order to reduce levels 

around the crusher. 

In metal/nonmetal mines, haul truck drivers that are 

operating without enclosed cabs or cabs that have 

deteriorated with use have the potential for high dust 

exposure. A survey was conducted at an underground 

gold mine to evaluate the dust exposure ofhaul truck 

drivers working in trucks with open cabs. Instantaneous 

samplers were utilized in conjunction with time-study 

information to quantify the dust exposure occurring during 

the loading, hauling, and dumping cycle. At this mine, it 

was found that even though the dumping operation had the 

shortest duration, dumping accounted for the highest 

percentage of dust exposure. This information could then 

be used to implement additional controls at the dumping 

station to reduce dust levels. 

SUMMARY 

NIOSH conducted a strategic planning effort to identify 

areas where limited research funds should be directed to 

be of greatest benefit to the mining industry. 

Overexposure to silica dust and development of silicosis 

continue to occur at surface and underground mining 

operations. The need for development of improved 

control technologies was identified as an area of high 

priority by the NIOSH strategic planning effort. MSHA 

sampling data were utilized to identify mining 

commodities and occupations that have high percentages 

of samples exceeding the PEL. In response, research is 

being conducted to address dust controls for these 

occupations at both surface and underground mining 

operations in the coal and metal/nomiletal industries. 

REFERENCES 

Anonymous, 2000, Silicosis Screening in Surface Coal 

Mines - Pennsylvania, 1996-1997, Centers for Disease 

Control and Prevention, MMWR Weekly, July 14,2000/ 

49(27);612-5. 

Cecala, A.B., Organiscak, J.A., and Heitbrink, W.A., 

2001, "Floor Heaters Can Increase Operator's Dust 

Exposures in Enclosed Cabs," NIOSH Technology New 

No.486, March, 2001. 

Listak, I.M., Chekan, GJ., and Colinet, I.F., 2001, 

"Laboratory Evaluation of Shield Dust Entrainment in 

High Velocity Airstreams," SME Preprint 01-144, SME 

Annual Meeting, Denver, Colorado, February 26-28. 

MSHA, 1995, MSHA News Release No. 95-014, Mine 

Safety and Health Administration, April, 1995. 

NIOSH, 1995, Criteria for a Recommended Standard: 


Occupational Exposure to Respirable Coal Mine Dust, 


National Institute for Occupational Safety and Health, 


DHHS (NIOSH) No. 95-106, 336 pp. 

NIOSH, 1999, Work-Related Lung Disease 

Surveillance Report - 1999, National Institute for 

Occupational Safety and Health, DHHS (NIOSH) No. 

2000-105, 202 pp. 

Organiscak, J.A., Page, S.1., and Cecala, A.B:, 2001, 

"Sweeping Compound Application Reduces Dust From 

Soiled Floors Within Enclosed Operator Cabs," NIOSH 

Technology News No. 487, March, 2001. 

Organiscak, J.A., Cecala, A.B., Heitbrink, W.A., 

Thimons, E.D., Schmitz, M., and Ahrenholtz, E., 2000, 

"Field Assessment of Retrofitting Surface·Coal Mine 

Equipment Cabs with Air Filtration Systems," 

Proceedings ofthe Thirty-first Annual Institute on Mining 

Health, Safety, and Research, Roanoke, Virginia, August 

27-30, pp. 57-68. 

Parobeck, P.S., and Tomb, T.F., 2000, "MSHA's 

Programs to Quantify the Crystalline Silica Content of 

Respirable Dust Samples," SME Preprint 00-159, SME 

Annual Meeting, Salt Lake City, Utah, February 28 

March 1. 

Watts, Jr., W.F. and D.R. Parker, 1995, The Mine 

Inspection Data Analysis System (MIDAS), Appl. Occup. 

Environ. Hyg. 10(4), pp. 323-330. 

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