SILICA DUST SOURCES IN UNDERGROUND LIMESTONE MINES

Gregory J. Chekan and Jay F. Colinet 

Pittsburgh Research Laboratory 

National Institute for Occupational Safety and Health 

ABSTRACT 

 NIOSH’s Pittsburgh Research Laboratory is 

currently involved in research to identify silica 

dust sources and generation in underground 

metal/nonmetal mines. The ultimate goal of this 

research is to develop control technologies to 

reduce worker exposure to respirable silica dust. 

Baseline dust surveys were conducted in 

underground limestone mines in Pennsylvania to 

investigate primary silica dust sources, 

generation levels, and controls being used. 

Three primary sources currently under 

investigation include dust generated by crushing 

facilities, face shots, and haul trucks. A summary 

of sampling procedures, resulting data, and 

methods being investigated to reduce silica dust 

will be discussed. 

INTRODUCTION

 Chronic overexposure to respirable crystalline 

silica may lead to silicosis, which creates 

irreversible and progressive deterioration once 

the dust has been deposited in the lung tissue. 

Historically, mining has been a one of the 

highest risk industries for worker exposure to 

crystalline silica dust. Through the 1980’s and 

into the 1990’s, United States Bureau of Mines 

(USBM) research program addressing 

silica dust sources and worker exposure had 

mainly focused on surface and underground coal 

mining and surface processing operations for the 

nonmetal mining industry. Numerous studies 

were conducted which have lead to the 

development of improved control technologies 

for reducing silica exposure in high-risk 

occupations in these operations. However, 

studies addressing silica dust occurrence and 

exposure in underground metal/nonmetal mines 

had not been a high priority in the USBM dust 

control research program. When the health and 

safety research functions of the USBM were 

transferred into the National Institute for 

Occupational Safety and Health (NIOSH), a 

strategic planning effort was conducted to 

identify areas of need that warranted new or 

continued research efforts. To identify high risk 

occupations in underground metal/nonmetal 

mines, the MSHA compliance sampling results 

were evaluated for contaminant code 523, which 

is defined as a full shift sample with a total mass 

gain on the filter equal to or greater than 0.1 mg 

and a crystalline quartz respirable fraction 

greater than or equal to 1% determined by X-ray 

diffraction (XRD) following the NIOSH 7500 

Analytical Method (NIOSH, 1994b). For the 

period from 1993 to 1998, the data shows that 

the average percent of these samples 
exceeding the Threshold Limit Value (TLV) 

was 15%, with considerably higher 

overexposures for high-risk occupations 

(MSHA, 1999). Consequently, a research 

project was initiated to address worker exposure 

and silica dust control for the more than 10,000 

miners currently employed in over 300 

underground metal/nonmetal mines (MSHA, 

2000). 

 MSHA classifies the metal/nonmetal division 

in four main categories; sand and gravel, metallic 

minerals, nonmetallic minerals, and stone. A 

major component of the underground 

metal/nonmetal research program is the 

investigation of silica dust in the underground 

crushed and broken limestone industry, which is 

one of the main commodities in the stone 

category. Figure 1 is the frequency of MSHA 

dust sampling (MSHA, 2001), as a percent of 

total samples taken, for the major commodities 

in the underground stone industry for the years 

1996 to 2000. The sampling data shows that 

43% of the sampling occurs in the crushed and 

broken limestone commodity. 

Sand st o ne 

( C rushed & 

B ro ken)

13 %

M arb le 

( C rushed & 

B ro ken)

6 %

Granit e 

( C rushed & 

B ro ken)

4 %

Limest o ne 

( C rushed & 

B ro ken)

43%

M arb le

( D imensio n)

5 %

Limest o ne 

( D imesio n)

5 %

Lime

24%

Figure 1. MSHA dust sampling as 

percentage of total samples taken. 

Currently, there are approximately 110 

underground crushed and broken limestone 

mines in the US, representing the largest 

segment of the underground stone industry. 

Depending on the geology, some limestones may 

have a higher sand component, while others have 

a higher calcite component. Studies on the 

sources of silica dust (Ramani, et. al., 1987) have 

shown a strong correlation between the airborne 

concentration and percent silica in the host rock. 

Historically, higher levels of silica have been 

observed in mines located in the Northeastern 

and South Central MSHA districts. The 

Northeastern and South Central Districts have 19 

and 18 mines respectively, and account for 35% 

and 25% of the total samples taken (MSHA, 

2001). 

 Due to demand, the number of underground 

crushed and broken limestone mines are 

increasing on a yearly basis as quarry operators 

exhaust their surface reserves and begin 

underground operations. Room-and-pillar 

mining methods are utilized, typically using 

pillars with square dimensions ranging between 

10.6 to18.3 m (35 to 60 ft). The entries are 

considered large mine openings with entries 

widths ranging from 9.1 to 18.3 (30 to 60 ft) and 

entry heights on development ranging from 4.9 

to 12.1 m (16 to 40 ft). After benching, entries 

can be over 18.3 m (60 ft) high. Many of the 

dust sources, which are not problematic in a 

surface operation, have now become a issue in 

the underground environment. Many of these 

sources may increase the level of respirable silica 

dust at generation points as well as in the overall 

mine atmosphere since these large openings can 

be difficult to ventilate. Occupations with the 

highest risk of overexposure include truck 

drivers, crusher operators, front-end loader 

operators, and rotary drill operators. On average, 

20 to 25% of the samples from these occupations 

exceed the TLV (MSHA, 2001).

 The NIOSH research program is addressing 

these silica issues by quantifying dust levels at 

major sources in the underground crushed and 

broken limestone industry. Baseline sampling 

surveys were conducted for three different 

sources, 1) dust generated by an underground 

dump/crusher facility; 2) dust generated by face 
shots; and 3) dust within a truck drivers cab 

generated by loading, dumping and tramming 

activities. The objective of these studies was to 

determine silica dust levels generated by these 

operations and assess the controls in use. The 

ultimate goal of this research is to develop or 

modify dust control technologies to reduce 

worker exposure to respirable silica dust. 

SAMPLING INSTRUMENTS USED IN 

SURVEY 

 Two types of dust sampling instruments were 

used in these studies. The first type and primary 

dust measuring instrument was the gravimetric 

sampler operated at 1.7 L/min with the 10 mm 

Dorr-Oliver cyclone and a 37 mm PVC filter. 

The pumps featured automatic compensation for 

changes in temperature and altitude, but 

calibration was adjusted at the mine site using a 

primary standard to within plus or minus 2.5%. 

The filters were weighed before and after 

sampling to calculate overall respirable dust 

concentrations (which includes all dust types and 

particulate) based on the sampling rate and time. 

The filters were then analyzed using XRD 

following the NIOSH Analytical Method 7500 

(NIOSH, 1994b), to determine the silica weight, 

so that the silica concentrations could be 

calculated. 

 The second type of sampling instrument was 

the MIE, Inc. personal DataRAM (pDR). The 

pDR is a real-time aerosol monitor. The 

instrument was operated in the active mode to 

monitor respirable dust. Before entering the 

unit, dust is classified using a 10 mm DorrOliver cyclone and a pump operated at flow rate 

of 1.7 L/min. The pDR measures and records the 

concentration of respirable airborne dust (which 

again includes all dust types and particulate) 

using a light scattering technique. Lightscattering instruments offer only a relative 

measure of concentrations but provide a 

continuous record of dust levels so that 

concentrations can be evaluated over any time 

interval during the sampling period. 

SAMPLING AT A DUMP/CRUSHER 

FACILITY

Sampling Strategy 

 

 Approximately 50% of all underground 

limestone mines have their crushers located 

underground (NIOSH, 1999) which can be a 

major source of silica as well as nuisance dust. 

In this particular case study, the mine is 

considering different methods of controlling dust 

at their underground crusher using either a pushpull ventilation system or a fan-powered dust 

collector. NIOSH and mine personnel agreed to 

complete a dust study to quantify dust levels 

being generated by the current operation. This 

would be accomplished by area sampling at key 

locations around the crusher to determine the 

dust levels generated from the dumping and 

crushing operations and to identify potential 

zones of high dust concentration. 

 Current dust controls for this study consisted of 

a 37.1 kw (50 hp) axial vane fan positioned inby 

the crusher as shown in figure 2. The fan was 

positioned in this area in an effort to prevent 

dust-laden air from rolling back into the intake 

air as the trucks dumped. The fans function was 

to blow dust away from the dump/crusher 

location and down the belt entry into the return 

airway. The crusher is a 222.6 kw (300 hp) jaw 

type rated at 907 t/h (1000 stph). The belt entry 

is isolated from the main developments using 

both permanent and curtain stoppings in 

crosscuts along its entire length of approximately 

152 m (500 ft). The crusher operator was 

located in an enclosed booth that was equipped 

with a pressurization and filtration system. A 

spray bar system was used at the dump location 

to control dust during the truck dumping 

operation. Any personnel entering or working in 

the vicinity of the crusher were required to wear 

personal protective equipment. 

 

 Table 1 identifies the types of dust samplers 

that were positioned at each sampling location, 

while figure 2 illustrates the relative location of 

these sampling stations around the crusher. 

 
 

Table 1. Dust sampler location and description for crusher survey. 

Site Location

Sampling Instruments

Description

Gravimetric pDR

1 Intake 4 1 2 grav samplers on each rib, pDR on one rib

2 Dump 3 1 all samplers on rib upwind of dump site

3 Crusher 3 1 samplers at control booth above crusher

4 Belt 3 1 inby open mandoor in stopping at return

5 Return 4 1 2 grav samplers on each rib, pDR on one rib

6 Entry A 2 0 on rib in entry parallel to belt entry

Figure 2. Location of sampling stations, 

blowing fan, and stoppings. 

Samples were collected for three consecutive 

days with an average sampling time of about 

five hours per shift. During this time, the 

number of trucks that dumped and the tonnage 

processed through the crusher were recorded. In 

addition, anemometer readings were taken at a 

1.2m by 2.1 m (4ft by 7 ft) doorway at the end 

of the belt entry leading to the return to monitor 

airflow from the crusher to the return airway. 

This information is given in table 2 and shows 

consistent values for all three sampling days. 

Table 2. Production and air velocity 

measured during sampling. 

Shift Number 1 2 3

Number of trucks 129 128 107

Measured tonnage, 

metric tons (short 

tons)

4624

(5098)

4711

(5194)

4214

(4647)

Average air velocity 

at doorway, 

m/s (fpm)

2.8

(565)

2.6

(506)

2.3

(460)

Results 

 Figure 3 summarizes the average 

concentrations for the 3 sampling days for the 

respirable dust and silica dust. The following is 
notable for each of the six locations: 

Site 1- Intake: This station had respirable and 

3

silica concentrations of 0.42 and 0.06 mg/m , 

respectively. These dust levels were the lowest 

observed from all locations and indicates that 

very little if any dust is migrating from the 

crusher back into the main developments on the 

intake side. 

Site 2 - Dump: When compared to the crusher 

and belt, this station has lower respirable and 

silica concentrations. This suggests that the 

37.1 kw (50 hp) fan is preventing dust rollback 

from the crusher as the trucks dump. 

Site 3 - Crusher: This location had the highest 

concentrations of both respirable and silica dust. 

Of interest, is the fact that respirable 

concentrations increase threefold from the dump 

to the crusher location, a distance of roughly 

18.2 m (60 ft). This indicates that the current 

fan is preventing dust migration back from the 

crusher, but lacks the ability to effectively move 

it away from the crusher. Observation from 

inside the operator’s booth showed that during 

the dumping cycle a large plume of dust was 

created but the low air movement allowed the 

dust to remain around the crusher for an 

extended period of time. Stratification or 

layering of the air may be causing this effect as 

the fan is suspending the dust above the crusher, 

but is ineffectual in removing it. 

Site 4 - Belt: Both the respirable and silica dust 

concentrations at the belt location are half of the 

levels at the crusher, at a distance of 

approximately 152 m (500 ft) from the crusher. 

The pDR concentration graphs from the belt 

were characterized by very consistent levels of 

dust throughout the sampling period when 

compared to the pDR graphs from other 

locations, which usually showed spiked traces 

of high and low concentrations. The pDR 

graphs in figure 4 illustrate the difference in 

dust patterns between the belt (site 4) and 

crusher (site 3) sampling stations for a typical 

day of sampling. Since the dust is well diluted 

and uniform when it reaches the end of the belt 

this indicates that the fan air is slowly moving 

the air down the entry, but not very efficiently. 

Site 5 - Return: This location behaved much the 

same as the intake location with low respirable 

and silica concentrations showing that very little 

dust is migrating from the dump/crusher back 

into the main developments on the return side of 

the crusher. Once again, these samples suggest 

that the fan is preventing dust rollback from the 

crusher toward the intake entry. 

Site 6 - Entry A: Dust levels were nearly three 

times higher than at the return sampling 

location. This indicates that dust leakage is 

occurring through the line curtains along the 

belt entry and this dust has the potential to be 

carried toward the working faces. 

0

1

2

3

4

5

6

7

8

9

10

INTAKE

DUMP

CRUSHER

BELT

RETURN

ENTRY A

Concentration, mg/cu m

Respirable Dust

Respirable Quartz

Figure 3. Average dust concentrations for 3 

shifts of sampling. 

Conclusions 

 This baseline survey was conducted to 

evaluate dust generation and migration around 

an underground crusher during normal 

production activities. Dust concentrations 

around the crusher and down the belt entry were 

higher than desired and could be reduced with 

improved dust capture. The current fan location 

is performing a function by clearing dust at the 
dump and keeping it from recirculating back to 

the main developments. Either a push-pull 

system with two auxiliary fans or a fan-powered 

dust collector is being considered and should 

provide an effective approach for reducing dust 

levels. The push-pull system would require a 

second fan to be placed outby the crusher in the 

belt entry with exhaust tubing placed as close to 

the crusher as possible to maximize dust 

capture. Tubing will then be attached to the 

blowing side of the fan to transport captured 

dust directly to the return airway. The second 

alternative would involve the installation of a 

fan powered dust collector with filtration system 

to remove airborne dust and discharge clean air. 

Either system would increase dust capture at the 

crusher, thus lowering dust levels at the crusher 

and in the belt entry. Additionally, less dust 

would leak through the stoppings into Entry A. 

0

2

4

6

8

10

12

14

16

0 50 100 150 200 250 300

TIME, min

pDR CONCENTRATION, mg/cu m

BELT SITE 4

Figure 4. pDR dust levels at site 3 (crusher) and site 4 (belt) during a typical sampling day. 

SAMPLING DURING FACE SHOTS 

Sampling Strategy 

 Dust generated from face shots can increase 

the respirable dust levels in the general mine 

atmosphere as the dust may not be well diluted 

or may have a high retention time depending on 

the ventilation patterns in the mine. After the 

blast, the dust tends to move in a cloud 

following the general mine ventilation course 

until it leaves the mine. In this study, the mine 

is planning ventilation changes by constructing 

approximately 30 curtain stoppings and 

installing two low pressure propeller fans to 

better ventilate the working faces in the mine. 

These stoppings and fans will increase the 

volume of air to the faces and establish a 

directional flow of air from the eastern to the 

western side of the mine. 

 The objective of the study is twofold: 1) to 

document respirable dust and silica generated 

from face shots; and 2) to determine the 

retention time of the dust cloud as a means to 

evaluate the mine’s air velocity. This baseline 

study was initiated to assess the current 

ventilation, particularly on the east side of the 

mine. The sampling strategy was to set up 

sampling stations in key locations in the mine’s 

air course and begin sampling before the faces 

were shot. Gravimetric samplers would be used 
to determine respirable dust and silica generated 

by face shots. Personal DataRams (pDRs) 

would provide a timed record of the dust cloud 

arrival at these selected areas which could then 

be used to verify air movement patterns and 

quantify the mine’s overall air velocity. 

Location of Samplers 

 Nine sampling locations in the mine were 

selected as shown in Figure 5. 

Figure 5. Mine workings showing location of sampling stations, stoppings, and fans. 

 Site locations 

were based on suspected air flow patterns in the 

mine and the potential of dust from the face 

shots in the working developments to pass that 

particular location and record the arrival of the 

dust cloud. All instrument packages were 

positioned on the rib approximately 1.52 m 

(5 ft) above the floor. Dust samples from shots 

were collected on two separate days. Three face 

shots took place on the first day of sampling and 

one shot on the second day. All face shots were 

on the east side of the mine as shown in figure 

5. Also shown in figure 5 are: 1) the location of 

three axial vane booster fans and their blowing 

direction. These fans are mobile and can be 

moved depending on ventilation patterns 

required for mining; 2) the proposed location of 

the low pressure propeller fan at the west side of 

the mine; 3) curtain stoppings which were built 

on the east side of the mine; and 4) curtain 

stoppings at the back and west side of the mine 

that still need to be constructed. Table 3 

identifies the types of dust samplers that were 

positioned at each sampling location. 

Results – Dust Sampling

 Gravimetric samplers were located at Sites 1, 

2 , 3, 4, and 8. Figures 6 and 7 show the 

gravimetric data collected during the two days 

of sampling. It should be noted that the 

gravimetric samplers reflect an average 

concentration for the entire sampling period. 

For this survey, this would include the dust 

generated by the shot as well as an extended 

period of sampling with little or no dust being 

generated. Consequently, the concentration 

values are lower than the dust concentrations 
being generated by the face shots. If samples 

were taken for a shorter length of time (1-2 hrs) 

as the dust cloud passed a particular location the 

concentrations would most likely be higher (as 

reflected by the pDR sampling data which will 

be discussed later). 

Table 3. Dust sampler location and description for face shot survey. 

Site

Sampling Instruments

Gravimetric pDR Description

1 2 1

Intake side – Monitor dust that may rollback into fresh air circuit 

from face shots on east side of mine

2 2 1 General air circuit - 4 sites located on inby side of the curtain 

stoppings to monitor dust migration from face shots on east side of 

mine

3 2 1

4 2 1

5 0 1

6 0 1

General air circuit – 3 sites were located on the on the western side 

of the mine on the inby side of the curtain stoppings. They 

monitored dust migration from face shots on the eastern side of the 

mine and were used to assess the effectiveness of the air circuit 

being planned

7 0 1

8 2 1

9 0 1

Return side - Monitor dust that may migrate past the location of 

the main exhaust fan and into fresh air circuit

 When examining graphs in figures 6 and 7, 

several points need to be noted. 

0

0.2

0.4

0.6

0.8

1

1.2

1 2 3 4 8

Site

Concentration, mg/cu m

Day 1

Day 2 

Figure 6. Respirable dust concentrations. 

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

1 2 3 4 8

Site

Concentration, mg/cu m

Day 1 

Day 2 

Figure 7. Respirable quartz dust 

concentrations. 

 First, site 1, 

the control intake location, had the lowest 

concentration with little variation in the 

concentration values for the two days of 

sampling. This indicates that booster fan 1 in 

the present location (see figure 5) together with 

the curtain stoppings on the east side of the 

mine are operating effectively to prevent dust 

generated by face shots from rolling back to site 

1, the intake. Second, on the first day of 

sampling, three shots took place on the east side 

of the mine as compared to the second day 

which had only one face shot. This is evident in 

the concentration values for both respirable dust 
and quartz dust which are higher on the first day 

at all locations. This should be expected since 

more shots would generate more dust. Third, 

the graph shows the increasing concentration 

from site 2 to site 4 indicating that the 

ventilation on the east side of the mine is 

moving the dust as planned. However, once the 

dust cloud was beyond the last curtain stopping 

(located two entries past station 4 as shown in 

figure 5) it began to break-up and disperse 

through the benched area on the west side of the 

mine as shown by the lower concentration at site 

8.

 Figure 8 shows the pDR concentrations at 

each location for a particular time interval, that 

being a 1 hr period during the peak arrival of the 

dust cloud produced by the face shots. 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

1 2 3 4 5 6 7 8 9

Site

Concentration, mg/cu m

Day 1: 3 Face Shots

Day 2: 1 Face Shot

 Figure 8. Average concentrations from pDRs for a 1 hour period during the peak arrival 

of dust from face shots. 

This 

time interval was selected because pDR data 

showed that most of the dust from the shots had 

past the sampling locations within this time 

frame. Therefore, figure 8 represent a snapshot 

of the dust concentrations at each location as 

well as the movement of the dust. The general 

trend of this graph is similar to the concentration 

graph from the gravimetric samples in figure 6 

and 7. Concentrations are low at site 1, the 

intake sampling location, then progressively 

increase from sites 2 through 4 on the mine’s 

eastern side, then decrease at sites 5 through 9 

where the last curtain stopping was installed. 

Again, this decrease in concentration is due to 

the dust cloud breaking-up and dispersing 

through the benched area. Once the curtain 

stoppings are complete on the west side of the 

mine and the benched area is isolated this 

should result in less dust dispersion and better 

movement of the dust from the faces. 

Results - Estimate of Air Velocities

 Limestone mines are classified as large 

opening mines where entries can exceed 12 m 

(40 ft) in width and 7.6 m (25 ft) in height. 

Commonly used air measurement techniques, 

such as anemometers and smoke tubes, are 

unable to measure extremely low air velocities 

associated with large openings. Tracer gases 

have been used successfully to assess ventilation 

patterns in large opening mines, but are very 

time consuming and costly. As a means to 

assess air velocities, dust clouds generated by 

face shots were monitored using the pDRs to 

time the movement of the dust through the 

mine. 
Stations 3 through 9 were located downwind of 

the blast area as shown in figure 5. The pDR=s 

were all set to record concentrations at 10 

second intervals. The concentrations from the 

pDRs were graphed versus time and the arrival 

of the dust cloud was observable as a peak 

concentration on the graph. Table 4 summarizes 

this information. 

Table 4. Estimated air velocities after face shots 

pDR 

Location,

Site #

Distance from 

Face Shot,

m

(ft)

Peak Concentration 

on pDR,

mg/m3

Time to Peak 

Concentration,

min

Estimated Air 

Velocity,

m/s

(fpm)

3 36.5

(120)

12.5 5 0.12

(24)

4 122

(400)

11.5 20 0.10

(20)

5 228

(750)

1.8 35 0.11

(21)

6 288

(950)

1.7 65 0.08

(15)

7 320

(1050)

1.9 70 0.08

(15)

8 335

(1100)

1.4 100 0.06

(11)

9 350

(1150)

1.9 110 0.05

(10)

Average Velocity 0.09

(17)

 As shown in table 4, estimated velocity show 

a decreasing trend with distance from the face 

shots. Peak concentration falls dramatically after 

the dust passes station 4 where the last stopping 

is located, but peak concentrations from the dust 

cloud were still apparent as the cloud broke-up 

and became diluted. An average of all the 

velocities gives a value of 0.09 m/s (17 ft/min), 

which would be an overall average air velocity 

for the entire mine. Using this value to estimate 

a retention time of the dust from face shot 

locations in the eastern section of the mine to 

the proposed location of the fan 3 (see figure 5) 

near site 8 gives a value of 1.2 hrs. From face 

locations closer to fan 3, on the west side of the 

mine, the retention time would be 

approximately 15 minutes. 

Conclusions

 Respirable dust concentrations from both the 

pDR’s and gravimetrics are low at site 1, the 

intake sampling location, then progressively 

increase from sites 2 through 4 on the mine’s 

eastern side, then decrease at sites 5 through 9. 

This decrease in concentration, beginning at site 

5, is due to the dust cloud breaking-up and 

dispersing through the benched area. This is the 

approximate position of the last curtain stopping 

and the air began to short circuit into the 

benched area at this location. Second, the 

current location of fans 1 and 2 (see figure 5) 

together with the completed stopping have 

improved air flow on the east side of the mine. 

Part B of the study will be initiated once the 

curtain stoppings are complete on the west side 

of the mine and the low pressure propeller fans 

are installed as shown in figure 5. This should 

result in less dispersion of the dust into the 

bench area and better movement of the dust 

cloud from the faces and out of the mine. Third, 

the air velocities as calculated from the pDR 

data is very consistent with an average velocity 

for the entire mine of approximately 0.09 m/s 
(17 ft/min). As a result, this value gives dust 

retention times from face shots located in the 

eastern section of the mine to the proposed 

location of the fan near site 8 of 1.2 hrs. From 

closer face locations in the west side of the 

mine, the retention time is approximately 15 

minutes. Air velocities should increase and 

retention times be reduced once the project is 

completed. 

REDUCING DUST LEVELS IN AN 

 ENCLOSED TRUCK CAB 

Background

 Underground limestone mining operations use 

various types of heavy equipment to prepare the 

faces for blasting and to load and haul the 

limestone product from the mine. Equipment 

commonly used at these operations include face 

drills, front end loaders, and haul trucks. The 

original cab designs on this equipment degrades 

through normal operation in the harsh mine 

environment and the protection initially 

afforded to the operators is compromised. 

Therefore, many equipment operators can be 

exposed to elevated levels of respirable silica 

dust. In an effort to improve the protection of 

workers exposed to harmful dusts in enclosed 

cabs, NIOSH has entered into a number of 

cooperative research efforts with mining 

companies and cab filtration and pressurization 

companies. Several studies regarding the 

effectiveness of these systems have been 

published for surface coal operations (Cecala, et 

al., 2002 and Organiscak, et al., 2000). These 

units were installed on front-end loaders and 

overburden drills to reduce both respirable coal 

and silica dust in the operator’s cab. In this 

study, NIOSH and Sigma Air Conditioning Inc. 

entered into a cooperative cost-sharing 

agreement to evaluate the impact of retrofitting 

a haul truck at an underground limestone mine 

with a new system to reduce the operator's 

exposure to silica dust. 

 The truck selected for retrofit with the new 

unit was a Euclid R-50 manufactured in 1975. 

This truck had multiple duties. For the most 

part, it was used to haul fines from the 

processing plant to one of two different 

locations. On most trips, the operator would 

dump the fines at an outside stockpile. 

Occasionally, this truck would also haul the 

fines into the mine and dump them for backfill. 

When needed, the truck would also be used to 

haul stone from the faces in the mine and dump 

them at the outside crushing facility. The truck 

was originally fitted with a heating and air 

conditioning unit that did not filter the intake air 

or pressurize the cab. The unit was functional, 

but outdated and required replacement. 

 The Euclid R-50 was retrofitted with a Sigma 

Model EC5- 0500 rooftop mounted unit. The 

new system had heating, air conditioning, air 

filtration, and cab pressurization features. The 

pressurizer is a 2-stage cyclonic and 1-stage 

particulate filter. It has a separate blower and 

motor to positively supply air to the return air 

chamber of the air conditioner. The first stage 

filter, has a 95% efficiency rating for particles 

0.5 μm. This filter is designed to remove the 

larger particles and reduce the loading of the 

second and final stage filter, a pleated spun 

polyester washable medium, which is 99% 

efficient on particles > 0.5 μm. Installation of 

the unit took two 8-hr shifts and another shift of 

resealing the cab with foam weather stripping 

around the doors and service panels and 

caulking to seal smaller cracks. A positive 

static pressure of 0.01 inches of water gauge 

was achieved after resealing. 

Sampling 

 The main objective of this study was to 

determine the impact on silica dust within the 

truck cab after the new system was installed. To 

make this assessment, gravimetric samplers 

were used to measure dust concentrations both 

outside and inside the truck cab. Baseline dust 

sampling was conducted before the unit was 

installed and sampling repeated after the 

installation of the new system. The position of 

the gravimetric samplers was the same for both 

the pre- and post-installation parts of the study. 

Their location was selected as not to interfere
with operator’s vision or operation of the truck. 

Two gravimetric samplers were positioned 

outside the cab below the front window, and 

two were positioned in the cab to the right of the 

operator, at the same height as the breathing 

zone. Sampling was conducted for three shifts 

before installation and then three shifts after 

installation. Sampling time was approximately 

6 hrs per shift. During sampling, a time study 

was conducted on the truck and dust conditions 

noted for each day. 

Results 

 Table 5 summarizes the dust concentrations 

values from the gravimetric samplers for the six 

days of sampling. 

Table 5. Pre and post installation dust concentrations 

Outside Cab Inside Cab

Respirable Dust

mg/m3

Respirable

Quartz Dust

mg/m3

Respirable 

Dust

mg/m3

Respirable

Quartz Dust

mg/m

3

Day Pre-Installation: Original AC and Heating System

1 0.401 0.033 0.271 0.020

2 0.662 0.056 0.369 0.042

3 0.213 0.015 0.197 0.008

Average 0.425 0.035 0.279 0.023

Day Post-Installation: Sigma Model EC5-0500

1 1.037 0.083 0.430 0.026

2 1.069 0.068 0.288 0.016

3 0.924 0.061 0.234 0.010

Average 1.010 0.071 0.317 0.017

 The concentration values for 

each day are the average of the two gravimetric 

samplers for that day of sampling. The 

“Average” row is the survey average. Figure 9 

graphs the reduction in respirable dust and 

respirable quartz dust for the outside versus the 

inside of cab based on pre- and post-installation 

3-day averages in table 5. The reduction of 

respirable dust and respirable quartz dust was 

3 3

34% (0.425 mg/m to 0.279 mg/m ) and 33% 

3 3

(0.035 mg/m to 0.023 mg/m ) respectively, 

before the new system was installed. After 

installation of the new system, the reduction 

3 3

improved to 69% (1.010 mg/m to 0.317 mg/m ) 

3 3

and 75% (0.071 mg/m to 0.017mg/m ), 

respectively. Figure 9 illustrates that the new 

unit afforded a greater protection to the operator 

from outside dust levels. 

 Figure 9 represent the comparison of outside 

to inside levels of dust for the pre- and postinstallation of the unit. 

34 33

69

75

0

10

20

30

40

50

60

70

80

90

Respirable

Dust

Respirable

Quartz Dust

Respirable

Dust

Respirable

Quartz Dust

Percent

Pre-installation Post-installation

Figure 9. Percent reduction of dust in cab 

(outside versus inside of cab).

However, to obtain a 

actual reduction in dust inside the cab, measured 

dust levels in the cab must be compared. It 

should be noted that the conditions for all three 

days, during post-installation sampling, were 

much dustier than during pre-installation. This 
was visually noted during sampling and 

supported by the measured dust concentration in 

table 5. The respirable dust and the quartz dust 

concentrations outside the cab during postinstallation sampling are double the values 

during pre-installation. These higher 

concentrations influence the amount of dust 

actually penetrating the cab. This needs to be 

taken into account to determine the actual 

reduction in cab dust before and after 

installation of the unit. Figure 10 takes these 

higher outside concentrations into account by 

normalizing the average concentration values 

during post-installation to the baseline values 

during pre-installation inside the cab. 

0.279

0.133

0.023 0.0085

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Pre-installation

Post-installation

Pre-installation

Post-installation

Concentration, mg/ cu m

Respirable Dust 

 In Cab

Respirable Quartz 

 Dust in Cab

52%

Reduction

63% 

Reduction

Figure 10. Reduction in dust concentrations 

in cab when normalizing values to baseline.

Using this 

analysis, a 52% reduction in respirable dust and 

a 63% reduction in respirable quartz dust is 

achieved in the cab after the new system was 

installed. This is a measure of the actual 

improvement in the cab working environment. 

 Another way to illustrate the effectiveness of 

the new system is by determining the increase in 

the cab protection factor (Heitbrink, et. al., 

1998). This factor is calculated by dividing the 

average outside concentration by the inside 

concentration given in table 5. The protection 

factor for the respirable dust before and after 

installation is 1.5 and 3.2, respectively. For the 

respirable quartz dust the protection factor 

before and after installation is 1.5 and 4.1, 

respectively. 

 At this mine, several newer Komatsu trucks 

were operating as mine trucks hauling stone 

from the faces to the outside crusher. As a final 

measure to evaluate the new system, a 5-yr old 

Komatsu 100 ton truck was sampled for one day 

to determine the effectiveness of the 

pressurization and filtration system originally 

supplied on this truck. The number of samplers 

and their positioning outside and inside the cab 

were the same as that of the Euclid R-50. The 

average outside and inside respirable 

3

concentrations were 0.71 and 0.22 mg/m , 

respectively. This gives a reduction of 70% 

from outside to inside the cab and a protection 

factor of 3.3. These values are very similar to 

the Euclid R-50 after retrofit with the new 

system (69% and 3.2) indicating that the 

retrofitted system was equivalent to the 

performance of the newer trucks. 

 

Conclusions 

 This field study on a haul truck at a limestone 

mine retrofitted with a new filtration and 

pressurization unit demonstrates that older 

model trucks can be successfully upgraded to 

protect the operator from silica dust. Studies 

have shown (Cecala, et.al., 2002 and 

Organiscak, et.al.,2000) that two key 

components for successful installation and 

operation are effective filtration and cab 

integrity. Outside air, as well as inside 

recirculated air, should be filtered through a 

high quality and high efficiency filter and the 

cab should be sealed to attain a positive 

pressure. In this study, a 63% reduction in 

respirable quartz dust within the cab was 

achieved after the new unit was installed and the 

protection factor for respirable quartz was 

increased from 1.5 to 4.1. The new filtration 

and pressurization system compared favorably 

to a newer Komatsu truck as the reduction in 

respirable dust and protection factor were very 

similar. 
SUMMARY 

 NIOSH is conducting research in an effort to 

lower the silica dust exposure of workers at 

underground limestone mines. Dust surveys 

were conducted to quantify respirable dust 

generated by an underground crusher, face 

blasts, and the load-haul-dump cycle of a haul 

truck. These surveys evaluated current dust 

controls and for the haul truck, evaluated a new 

filtration system for the enclosed cab. A 

summary of each survey follows: 

Underground crusher - Sampling results 

indicated that dumping and crushing activities at 

the underground crusher are liberating high 

levels of respirable dust. The axial vane fan 

currently located inby the crusher was shown to 

prevent liberated dust from rolling back toward 

the intake air entry. However, sampling results 

also indicate that the liberated dust is not 

effectively moved to the return entry and dust is 

leaking through stoppings designed to isolate 

the crusher entry. Additional controls (another 

auxiliary fan or dust collector) and improved 

stoppings are being planned by mine 

management. NIOSH will conduct a follow-up 

survey to evaluate the effectiveness of added 

controls. 

Face shots - Mine-wide sampling was conducted 

to quantify dust levels generated during face 

shots and monitor airflow movement/dust 

migration throughout the mine after the shots. 

The mine was in the process of installing a 

series of stoppings to provide directed 

movement of the air and dust out of the mine. 

Baseline survey results indicated that the 

stoppings constructed on the east of the mine are 

effectively moving shot-generated dust to 

sampling station 4. When the dust cloud 

reached the west area of the mine where 

additional stoppings need constructed, air 

velocities and dust levels dropped suggesting 

that the dust is dispersing throughout the 

benched entries of the mine. The mine is 

continuing to install stoppings and plans to 

install new fans to assist air movement and dust 

removal. NIOSH will conduct a dust survey in 

the near future to evaluate the impact of the new 

ventilation system on dust retention in the mine. 

Haul trucks - Dust sampling was conducted to 

quantify the respirable dust levels present inside 

an enclosed cab on an older haul truck. A 

filtration/pressurization system was then 

retrofitted on this truck and the seals on the 

enclosed cab were improved. The cab was 

sampled again to document any changes in cab 

dust levels. Results show that quartz dust levels 

in the enclosed cab were reduced by over 60% 

with the new filtration unit installed. 

 NIOSH will continue to investigate control 

technologies that can be implemented to 

effectively reduce worker exposure to silica dust 

in underground limestone mining operations. 

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Cecala AB, Organiscak JA, Heitbrink WA, 

Zimmer JA, Fisher T, Gresh R, Asheley JD, 

[2002]. Reducing Enclosed Cab Drill 

Operator’s Respirable Dust Exposure at Surface 

Coal Operation with a Retrofitted Filtration and 

Pessurization System. SME Annual Meeting, 

Phoenix, Az. Preprint 02-105, 6 pp. 

Heitbrink WA, Hall RM, Reed LD, Gibbons D, 

[1998]. Review of Ambient Aerosol Test 

Procedures in ASAE Standard S525. Journal of 

Agricultural Safety and Health, 4(4), pp. 255-

266. 

MSHA, [1999]. Sampling results from 1993 to 

1998 from the Metal/Nonmetal Mine Inspection 

Data (MNMID), available from the MSHA 

Pittsburgh Safety and Health Technology 

Center, Dust Division, Pittsburgh, PA 15236. 

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CFR Part 50, 2000. Denver, CO: U.S. 

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Administration, Office of Injury and 

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2000 from the Metal/Nonmetal Mine Inspection 

Data (MNMID), available from the MSHA 

Pittsburgh Safety and Health Technology 

Center, Dust Division, Pittsburgh, PA 15236 

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December 7-8, 1999, Cincinnatti, OH. 

Organiscak JA, Cecala AB, Heitbrink WA, 

Thimons ED, Schmitz M, Ahrenholtz E, [2000]. 

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Mining Health, Safety and Research. Roanoke, 

Va. Aug. 27-30, 2000, pp. 57-68. 

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[1987]. Fundamental Studies on the 

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1987, 179 pp.