parting type, thickness, and silica content) affect
silica content (7,8). For these analyses, silica
content was assumed as an indicator of either
work practices or geologic conditions on the
mining unit,

ASSESSMENTS OF FACE VENTILATION

Four basic types of face ventilation systems
were noted in reviews of MSHA mine inspector
reports.

1. Exhaust ventilation curtain with dust
control provided by a machine-mounted
flooded-bed dust scrubber.

2.. Exhaust ventilation curtain with water
sprays on the mining machine arranged
in either a sprayfan or other directional
face spray design.

3. Combination system using intake
curtains to ventilate faces on one side of
the section belt entry and exhaust
curtains to ventilate faces on the other
side. This system reduced the number of
drive-through check curtains between
each face and the feeder-breaker in the
belt entry. Dust control provided by a
flooded bed dust scrubber.

4. Intake or blowing ventilation curtain

with a flooded-bed scrubber for dust
control. However, this face ventilation
scheme was not widely represented in
the accumulated data and, for this
reason, was not evaluated further in this
study.

Occupational exposures were noted when
the continuous mining machine operator was
sampled concurrently with the roof bolter
operator. Table II compares minimum,
maximum, and median values for silica dust
exposure and silica content at the mining
machine operator occupation for group A and
group B operations.

This data in table III shows that all face
ventilation designs were equally represented in
group A operations while group B operations
used combination curtain face ventilation by
nearly two-to-one margin over other ventilation
schemes. Group A silica exposures generally
were much less for all face ventilation schemes
with the exception of the exhaust curtain with
dust scrubber. For group B operations, silica
exposures were less using this ventilation
scheme. Silica content levels were much less
for group A operations than for group B
operations.

Table III. Distribution of silica exposures and silica contents at continuous mining machine
operator occupations for groups A and B operations.

Silica exposures (j1g/m’) Silica contents (%)
ays Number
Ventilation Scheme f Group ~
of samples Range of Median Range of | Median
values value values value
Exhaust curtain with 59 A 16-312 90 1-17 6
scrubber 31 B 1-532 150 0-16 9
Exhaust cur main with | 57 A 2-260 100 0-14 8
dineet na’ spray 31 B 20-946 210 2-25 il
signs
Combination curtain 62 A 3-549 90 2-14 8
system 55 B 40-571 180 7-22 11



This data show that silica exposures and
silica contents at all occupations were higher for
group B operations than group A operations.
Variations in exposures and contents suggest
that these differences were attributable to
changes in dust control effectiveness, work
practices, and geologic conditions.

The single head roof bolter (helper)
possessed among the highest silica exposures
and silica contents in group A and B operations.
This suggests less effective dust controls,
inappropriate work practices, and poorer
geologic conditions at this occupation for group
A and B operations, It is also possible that the
bolter helper engaged in activities that put this
person at risk for increased exposure to silica.
These would include emptying the dust box and
cleaning the dust filters.

SUMMARY

Data on dust-control parameters, geology,
and corresponding silica exposures was
gathered for nearly forty operations that, despite
silica content in excess of 5%, were able to
maintain silica dust levels in a majority of
occupational samples at or below 100 g/m’.
Approximately forty other operations were
identified that were unable to maintain a
majority of occupational samples less than or
equal to100 g/m’. These two subsets were
termed group A and group B operations,
respectively.

Evaluations of geologic and dust control
parameters generally revealed only minor
differences between group A and group B
operations. Subsequent assessments of face
ventilation design showed that group B
operations suffered from higher silica exposures
and contents at continuous mining machine and
roof bolter occupations. However, these

, differences were minimized when using exhaust
curtain ventilation with a dust scrubber. This
face ventilation system may benefit those
operations having difficulty controlling
occupational silica exposures.

These evaluations also revealed that
occupational silica exposures and silica contents
were higher for group B operations than group
A operations. In both groups, the single head
roof bolter (helper) occupation possessed among
the highest silica exposures and silica contents.
It is possible that this occupation engaged in
activities that put this individual at risk for
overexposure to respirable silica.

REFERENCES

1, Mineral Resources. Code of Federal
Regulations Title 30, Parts 70 and 75,
U.S. Government Printing Office.

2. Tomb, TF, Gero AJ, Kogut J [1995].
Analysis of quartz exposure data
obtained from underground and surface
coal mining operations. Appl. Occup.
Environ. Hyg.10(12): 1019-1026.

3. Ainsworth SM, Gero AJ, Parobeck PS,
Tomb TF [1995]. Quartz exposure
levels in the underground and surface
coal mining industry. Am. Ind. Hyg
Assoc. J., 56(10):1002-1007.

4. Jankowski RA, Niewiadomski GE [1987].
Coal mine quartz dust control, an
overview of current U.S. regulation and
recent research results. Proceedings,
Intl. Symp. on Coal Mining and Safety,
Soeul, Korea, pp. 303-313.

5. Colinet JF, Shirey GA, Kost JA [1985].
Control of respirable quartz on
continuous mining sections. Contract
J0338033, US Bureau of Mines, June, 90

Pp.

6. Organiscak JA, Khair AW, Ahmad M
[1995]. Studies of bit wear and
respirable dust generation. Soc. Mining
Eng. Transactions 298:1874-1879.


Table I. Characteristics of operations in two sample groups

Group A Operations Group B Operations
i Nuntber Range of Median Number Range of Median
Characteristic values value values value
samples samples
Production (tons) 384 96-2700 600 226 85-2520 572
Coal thickness 318 4-108 48 162 0-126 42
(inches)
Rock thickness
(inches) 315 0-60 8 162 0-96 8
Face rai flow) 325 | 1080-49500 | 6462 | 203 | 2024-45360 | 6500
Scrubber flow (cfm) 133 3000-11600 5034 53 2580-13099 5600
Spray count 313 18-50 27 188 14-58 25
Spray pressure (psi) | 313 50-280 90 195 50-260 85

Notes:

1. Face ventilation flow is amount of air flowing to the continuous miner.
2. Spray count is the number of water sprays on the continuous mining machine.

This data shows small differences in dust
control parameters and geologic conditions for
operations in groups A and B. Group A
operations produced roughly 5% more than
Group B operations. Ranges of coal and rock
thicknesses were greater for Group B operations
than Group A operations. The range of face
ventilation values was greater for group A than
group B although median values were similar.
While the range of scrubber airflows was
similar, the median value was greater for B
operations than group A operations. Water
spray count and spray pressure were consistent
between the two groups.

Although data was available on rock
thicknesses, little information was available on
the composition of rock present at the cutting
face. Previous work (6,8) showed that the silica
content in the roof, floor, or parting material
could influence the amount of respirable silica
generated during the mining process.

Previous work showed that a number of

factors could potentially impact occupational
silica dust exposures and silica dust contents in
underground coal mining. Dust control
practices such as ventilation airflow, flooded-
bed dust scrubber quantity, water spray quantity
and pressure, and water spray configuration
influence silica dust exposures (4). Dust control
practices operating in the ranges given in table
II have more effect on silica exposures than
silica contents. With MSHA’s presence during
sampling, it is likely that good execution of the
dust control plan occurred. When evaluating the
collected data, silica exposures were assumed as
indicators of dust control effectiveness on that

mining unit.

Work practices affect silica content, for
instance, using a modified cutting scheme to
avoid grinding of parting rock (4), avoiding the
use of worn cutting bits (5), minimizing the time
the roof bolter works downwind of the
continuous mining machine (6), and
maintenance of dust control systems. Geologic
conditions at the mining unit (such as rock


These data suggest that dust control
effectiveness for group A operations generally
was superior to that for group B operations for
all face ventilation schemes except when using
exhaust curtain with a scrubber. The high silica
contents of group B show that this group of
operations likely suffered from poor work
practices or inferior geologic conditions
compared to group A operations.

Roof bolter exposures were more difficult
to assess because their position could change
with respect to the fresh airflow of the face
ventilation curtain. While being sampled by the
MSHA mine inspector, the roof bolter(s) could
move upstream or downstream of the
continuous mining machine, thus changing
environmental conditions for roof bolter
occupations. Unfortunately, information of this

type was seldom available from MSHA reports.

A comparison of silica exposures and
contents at roof bolter occupations for group A
and B operations is given in table IV. This data
show that, for all three face ventilation designs,
bolter occupation silica exposures were highest
for group B operations. The difference between
A and B operations was greatest when using
exhaust curtain face ventilation with directional
sprays or combination curtain face ventilation.
Differences were less when using exhaust
curtain ventilation with a scrubber. Silica
contents for roof bolter occupations in group A
operations were less than those measured in
group B operations. This suggests improved
work practices or geologic conditions for group
A operations.

Table IV. Distribution of silica exposures and contents at roof bolter occupations for group A and

B operations.
Silica exposures ({1g/m*) Silica contents (%)
ae Number
Ventilation Scheme f Group
of samples Range of Median Range of Median
values value values value
Exhaust curtain with 59 A 0-364 80 0-24 9
scrubber 31 B 17-381 150 5-21 11
Exhaust curtain with | 57 A 0-259 80 0-14 8
direc aS ied 31 B 20-1771 210 5-77 14
signs
Combination curtain 62 A 20-303 90 2-16 10
system 55 B 40-571 190 7-28 12

The data show that differences in group A
and group B silica exposures for mining
machine and roof bolter operator occupations
‘were minimized when using exhaust curtain
face ventilation with a dust scrubber. Using
either exhaust curtain ventilation with
directional sprays or combination curtain
ventilation, group B operations were unable to
control occupational silica exposures at either

the mining machine or roof bolter occupations.
It appears that operations able to control silica
exposures for mining machine operator
occupations were able to control exposures for
roof bolter occupations.

The data also suggest that operations

having difficulty controlling silica dust
exposures may benefit from using exhaust



7. Organiscak JA, Page SJ, Jankowski, RA
[1990]. Sources and characteristics of
quartz dust in coal mines. Information.
Circular 9271, US Bureau of Mines, 21
pp.

8. Taylor LD, Thakur PC, Riester, JB [1986].
Control of respirable quartz on
continuous mining sections. Contract
J0338077, US Bureau of Mines, 67 pp.

9. Schultz MJ, Fields KG [1999]. Dust
control considerations for deep cut
mining sections. SME Annual Mtg,
Denver, CO, Preprint 99-163, 4 pp.

10. Goodman GVR, Listak JM [1999].
Variation in dust levels with continuous
miner position. Min. Eng. 51(2): 53-58.


A REVIEW OF OCCUPATIONAL SILICA EXPOSURES ON CONTINUOUS MINING
OPERATIONS

Gerrit V.R. Goodman, Jeffrey M. Listak, and John A. Organiscak

Pittsburgh Research Laboratory
National Institute for Occupational Safety and Health

ABSTRACT

Data on dust control practices, geology, and
occupational exposures were gathered for
approximately eighty underground continuous
mining units. Despite silica contents in excess of
5%, nearly forty units successfully maintained
silica concentrations at or below 100 pg/m? ona
majority of occupational dust samples while the
remainder had difficulty maintaining this level.
These two sample sets were termed group A
operations and group B operations, respectively.

Analyses of productivities, geologies, and
dust control parameters revealed only minor
differences between these two groups.
Subsequent analyses of face ventilation design
showed considerable differences in silica
exposure and silica content between group A and
group B at the continuous mining machine and
roof bolter operator occupations. These
differences were minimal when using exhaust
curtain ventilation with a dust scrubber. This
face ventilation system may benefit operations
having difficulty controlling silica dust exposure
and silica dust content.

Finally, the collected data showed that
occupational samples from group B operations
possessed generally higher silica exposures and
silica content than similar samples from group A.

The single head roof bolter (helper) possessed
among the highest silica exposures and silica
contents in both groups.

INTRODUCTION

The Federal Coal Mine Health and Safety
Act limits the respirable dust exposure of mine
workers to a time weighted average of 2.0 mg/m’
for a working shift (1). Ifthe respirable dust
sample contains more than 5 percent silica by
weight, the dust standard is reduced according to
the formula 10 + (%silica). This maintains silica
dust levels at or below 100 pg/m’,

The process of adjusting the respirable dust
standard is discussed by others (2-4) and is based
upon the silica content of dust samples collected
by coal mine operators and by Mine Safety and
Health Administration (MSHA) coal mine
inspectors. Such sampling is conducted for eight
hours at 2.0 liters/min using a 10-mm nylon
cyclone preseparator. On continuous mining
operations, samples analyzed for silica content
are typically collected at mining machine
operator and roof bolter operator occupations.

Typically the dustiest occupation at each
production unit, mining machine operator


curtain ventilation with a dust scrubber. This
scheme is not affected as much by high intake
air velocities as is blowing curtain ventilation. -
Past work showed that high air velocities
issuing from the mouth of a blowing curtain
could disrupt capture of respirable dust by
blowing the dust cloud around the mining
machine (9). Also, positioning of the mining
machine operator is not as critical to controlling
this person’s occupational exposure when using
exhaust curtain ventilation. The operator has
some latitude in movement as long as this
person remains outby the curtain mouth.
However, the operator must remain within the
mouth of the blowing curtain to control
occupational dust exposures (10). Three,
exhaust curtain ventilation with a dust scrubber
does not produce high dust gradients around the
mining machine as can occur with the sprayfan
system. These high dust gradients also can

affect occupational exposures of the roof bolter
operator when working downwind.

ASSESSMENTS OF OCCUPATIONAL
EXPOSURES

Data on silica exposures and silica
percentages were categorized according to
worker occupation for group A and B operations
(table V). Much of the exposure data is given
for the continuous mining machine operator.
The remaining roof bolter occupations, with
exception of the single head roof bolter helper,
had similar numbers of samples in this study.
Silica data were available for other non-bolting
occupations. However, these samples were not
very numerous and, consequently, were not

reported.

Table V. Distribution of silica exposures and contents for continuous mining and roof bolting

occupations in group A and B operations.

Silica exposures (ig/m’) Silica contents (%)
. Number
Occupation f , Group - .
oF samples Range of | Median Range of | Median
values value values value
Continuous mining 321 A 1-664 90 0-37 8
machine operator 192 B 1-1149 100 0-34 9
Twin boom roof bolter 71 A 8-340 70 1-23 10
(intake side operator) 52 B 10-583 150 1-74 12
Twin boom roof bolter 70 A 10-329 70 2-21 9
“(return side operator) 54° B 4-653 200 0-27 13
Single boom roof 8% 6] A 0-263 80 0-35 8
bolter operator 67 B 1-1771 200 0-39 14
Single boom roof 22 A 7-644 90 1-17 9
bolter (helper) 20 B 4-740 280 2-21 14



samples are used to adjust the respirable dust
standard for the entire mining unit with the
understanding that the reduced standard will
protect all other occupations. Due to a variety of
factors (5), however, silica dust levels at roof
bolter occupations often can exceed silica levels
found at the continuous mining machine. To
provide additional protection, MSHA may
require more frequent sampling of roof bolter
occupations by the mine operator.

A number of studies have defined trends in
silica dust exposures for the underground coal
mining industry (2,3). These analyses revealed
that for the period July 1991 through 1992, more
than 40% of the continuous mining machine
operator and machine helper samples exceeded
5% silica. Between 25 and 30% of the operator
and helper samples exceeded 100 Fg/m’ for
respirable silica. For this same period, roughly
50 to nearly 70% of the roof bolter operator and
helper samples exceeded 5% silica. Thirty to
pry wan of the samples exceeded 100

Comparisons of two groups of operations
provided insight into potential causes of silica
dust exposures. One group, despite high silica
dust content, successfully controlled
occupational exposures. The second group could
not successfully control exposures. This
required examinations of dust control practices,
work practices, geologic conditions, and
corresponding occupational exposures for
operations in each of these two groups.

Such information was available at MSHA
field offices, Data on approved dust control
practices and work practices were part of the
dust control plan established for each
underground coal operation. Actual operating
conditions plus corresponding occupational
exposures were found in reports filed by MSHA
coal mine inspectors after sampling at these
operations. This study only used exposure data
from MSHA compliance sampling in lieu of
exposure data from mine operator compliance
sampling.

The two groups were identified through
examinations of the MSHA coal mine silica data
base that contains all compliance samples
analyzed for silica after 1981. Nearly 97,000
silica records are represented in the period 1982-
1996. Each sample record contains considerable
information, such as MSHA mine identification
number, mining unit designation, sample date,
sampling time, occupation sampled, pre and post
filter weights, and silica percentage. From this
data, respirable dust and respirable silica dust
concentrations were calculated.

Due to the large number of underground
coal mining operations represented in the
database and the need to gather dust plan and
MSHA exposure data from each operation, only
a very small subset of these operations could be
considered for the study. For this reason, several
restrictions were placed on the selection of a
particular mining operation.

Only operations in southern West Virginia,
southwestern Virginia, northeastern Kentucky,
and southeastern Kentucky were considered.
Prior to the start of this study, MSHA suggested
that these areas be considered due to their high
prevalence of silica exposure.

When this survey began, October 1997
compliance sampling data was the most current
available from MSHA. Identification of
operations for this study was based upon the
results of MSHA compliance sampling at the
continuous mining machine operator and roof
bolter operator occupations for the period
January 1997 to October 1997. Pre-1997
sampling data was not used to keep this
information as current as possible.

For this study, two groups of operations
were identified in each area. The first group
contained those operations with a majority of
samples exceeding 5% silica and having silica
dust concentrations Jess than or equal to 100
Fg/m? (group A operations). The last group
contained those operations with a majority of
samples exceeding both 5% silica and 100 Fg/m’
(group B operations). Eighty operations initially


were considered in this study, forty able to
maintain respirable silica dust levels on a
majority of compliance samples at or below 100
Fg/m’ and forty unable to maintain a majority of
samples below this level. During visits to the
various MSHA field offices, we discovered that
some operations were no longer producing.
Because current exposure data was not available
for these operations, they were dropped from
further consideration. The study examined 39
underground coal mining operations in group A
and 36 operations in group B. These operations
were selected from a population of nearly 700
operations in these geographic areas.

MSHA field offices were visited and data
gathered from the approved dust control plan for
each of the selected underground mining
operations in that district. MSHA mine inspector
reports for each operation were reviewed to note
dust control practices and to note corresponding
occupational exposures to respirable silica and
coal mine dusts (table I). The data gathered from
these visits produced a history of occupational
exposure for these mining operations from
January 1997 to an approximate end date of June
1998 to October 1998 (end dates differed
because the districts were not visited in the same
month).

Table I. Distribution of occupational exposures for two sample groups

Number of occupational Number of occupational samples
Occupation sampled samples in group A operations in group B operations
<100ug/m*> | > 100yg/m* $ 100ug/m* > 100yg/m?
Continuous miner 177 60 63 88
operator
Twin head roof bolter
(return side operator) 60 8 Ml 34
Twin head roof bolter
(intake side operator) 50 1 Ml 35
Single head roof bolter 58 R 2 39
(operator)
Single head roof bolter
(helper) 15 4 5 14
EVALUATION OF GENERAL quantity, spray count, and water pressure) as
CHARACTERISTICS measured by the coal mine inspector during
sampling.
General characteristics for group A

operations and group B operations are given in
table II. These include daily tonnage produced,
coal thickness mined, and rock thickness mined
during sampling by the MSHA mine inspector.
Also given are values for various dust control
parameters (face ventilation quantity, scrubber

Maximum, minimum, and median values
categorize the distribution of values for each
characteristic. Differences in sample size
between group A and B operations are attributed
to the number of operations in each group and
the extent of sampling at each operation.