pressurized cab air cleaning system maintained
lower dust levels with very little variation during
the work shift.

s
&

2.0

&

MiniRAM Dust, relative units
a

e
a

2
°

9:50 14:38
TIME, he:min

Figure 3 - Loader cab dust level histories,

Table I shows the average dust levels and
cab protection factors with daily ranges for the
baseline and modified cab on the CAT 980B
front-end loader. The CAT 980B front-end
loader showed a significant improvement in
operator cab dust control effectiveness with the
addition of the Red Dot and Clean Air Filter”
units to the cab enclosure and the sealing of all
the visual openings or cracks found in the cab
enclosure. The inside cab dust levels were
reduced by about one order of magnitude after
these changes were made. The enclosed cab dust
protection factors (outside/inside) was increased
on average from 1.1 to 10.1. Similarly, the
percentage of cab penetration of dust
(Ginside/outside) H100) was reduced from 87% to
10%, while the percentage of dust reduction
(Cinside ! outside)/outside) H100) increased from
-13 % to -90 % with the cab improvements.

This cab improvement is considered to be
Statistically significant. The one-tailed t-test
probability (p-value) that the inside and outside

cab dust levels are the same notably decreased
from 0.328 to 0.006 with the cab modification.
Although this average improvement was
significant, it must be noted that the day-to-day
protection factors for the modified cab ranged
from 2.1 to 50.1 (see table I), while the inside cab
respirable dust levels consistently remained under
0.07 mg/m’. This supports the work of others
that field evaluation of cabs with low exterior
dust levels tends to bias the cabs protection
factor towards lower levels because the relative
differences approach the background levels inside
the cab [Heitbrink et al. 1998; Heitbrink et al.
1999].

The particle size percentage of dust
penetration (% of particle count inside the cab as
compared to outside the cab) into the modified
loader cab by particle size count can be seen by
the OPC data collected on Aug 11 in figure 4.
This figure shows that a somewhat higher
percentage of smaller sized dust particles was
observed to penetrate the loader cab. Never the
less, the loader cab appeared to provide effective
protection throughout the particle size range,
with less than 5 % of the 1 im or larger particles
penetrating the loader cab enclosure. The
average shift cab protection factor and the
average shift percentage of cab penetration of
respirable dust measured with the personal
samplers during this particular day was 24.2 and
4.1%, showing reasonably good agreement with
the OPC data.

Although the original cab provided negligible
control of respirable dust, the original cab
enclosure kept a notable amount of the larger
sized dust particles from entering the cab. The
average mass median diameter (MMD) of the
outside dust particles and the inside dust particles
for the original cab was 27.3 im and 11.5im,
respectively, This indicates that the CAT 980B
front-end loader had a fairly good enveloped cab
structure for resealing and retrofitting with a
filtered air conditioning system. Finally, the silica
percentage of the respirable dust for the CAT
980B front-end loader was found to be very low,
commonly below 5%, both inside and outside the


0 5 10 15
Wind Speed, mph

Figure 9 - Tripod dust level and wind speed
relationship around the drill.

bod

N

Xv
°

i
rl
i
a

6:00 8:24 10:48 13:12 15:36
TIME, hr:min

Figure 10 - Drill cab dust level histories.

These improvements from the modified cab
are considered to be statistically insignificant.
The one-tailed t-test probability (p-value) that the
inside and outside cab dust levels are the same
increased from 0.042 to 0.078 with the cab
modification. Typical instantaneous dust level
histories (MINIRAM) recorded inside the drill
cab are shown in figure 10 for the March 31"
baseline shift and the Aug. 12" modified shift.

These dust level histories illustrate that no notable
inside cab dust reductions were achieved from
the drill cab modifications.

Problems encountered with the drill cab
enclosure were large openings in the cab structure
for the mechanical control linkages and a loosely
fitting bi-folding door facing the drill table.
Positive cab pressure was not achieved on the
drill because of these enclosure sealing problems.

Also, the operator usually opened the cab doors
to communicate with the helper, during drill steel
changes and drill moves. Furthermore, the wind
direction played a very important role in the dust
measurements made during these field studies,
especially for the baseline conditions. These
variations can be seen in the shift ranges of the
cab protection factors, the percentage of cab
penetration, and the percentage of dust
reductions measured (see table I). The baseline
cab protection factors ranged from 0.5 to 13.2
(percentage of cab penetration from 8 to 182 and
percentage of dust reductions from -92% to +
82). The modified cab provided less variation in
protection factors, ranging between 2.0 and 7.9
(percentage of cab penetration from 13 to 49 and
percentage of dust reductions from -87% to -
57%). Although the drill cabes clean air system
did not show an average improvement in its
effectiveness, the variation caused by wind effects
were somewhat reduced.

The particle size percentage of dust :
penetration (percentage of particle count inside
the cab as compared to outside the cab) into the
drill cab by particle size count is demonstrated by
the OPC data collected on Aug 12 in figure 4. As
can be seen in this figure, the drill had a notably
higher percentage of dust penetrating the cab as
compared to the loader. The drill cab dust
penetration was one order of magnitude higher as
compared to the front-end loader. Ten to 30%
of the 7 to 1 im dust particles, respectively,
penetrated the drill cab enclosure. The average
shift protection factor and the average shift
percentage of cab penetration of respirable dust
measured with the personal samplers during this
particular day were 2.0 and 49.3%, showing


cab during all the baseline testing and modified
cab testing.

100.0% Bee epepey

8
3
xz

8
2

PARTICLE PENETRATION

0.1% *

PARTICLE DIAMETER (Hm)

Figure 4 - Percentage of aerosol penetrating
into the cabs.

Davey M8B Rotary Rock Drill: Figure 5 shows
the Davey M8B rotary drill with the Red Dot and
Clean Air Filter’ units operating on top of the cab
enclosure during the highwall drilling operation.
The drill operated throughout most of the shift,
except for a morning break and short non-
production time periods needed for minor drill
maintenance. This drill operation utilized two
employees working as a team. One employee
operated the drill from within the cab (drill
operator) while the other employee worked
outside the cab (drill helper), changing drill steels
and clearing the cuttings from the hole.
Throughout the shift the drill operator and helper
switched work details (positions). During the
baseline testing, the two cab doors were
constantly left open all shift so the employees
could visually and verbally communicate with
each other. During the modified cab testing, the
two cab doors were typically closed during most
of the drilling activities. The bifold door facing
the drill table was opened for drill steel changes
and both doors were opened for drill placement
during the modified cab sampling shifts. The
employees open these cab doors at these

particular times so that they can observe and
communicate (visually and verbally) to each other
during the manual changing of drill steels and drill
machine placement on the bench.

Figure 5 - Davey M8B drill studied.

4 % "x, % %
“ee, hae hy ee %

2
SANPLEDATE
‘NOTE Mid winds were present around the dion Api 1 and
Agt

Figure 6 - Dust results from drill cab
evaluation.

The average respirable dust levels (taken by
3 personal samplers) measured inside and at two
locations outside the drill cab are shown in figure
6, with their respective standard error bars for the
samplers. The inside and outside cab dust
samplers were at fixed positions on the drill,
while the mobile tripod dust samplers were placed
on the downwind side of the drill hole shroud (see
figure 5). The dust levels measured inside the
operator cab were commonly lower and


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noticeably more consistent than those levels
measured at the tripod and outside of the cab
during the study. The tripod dust levels were’
more variable and typically higher than the
outside cab dust levels, during both the baseline
and modified cab testing. The tripod dust levels
were also significantly higher during 3 of the
baseline sampling shifts as compared to the
modified cab sampling shifts.

degrees

‘WIND DIRECTION STANDARD DEVIATION.

AN Soa

Figure 7 - Summary of daily wind speed and
direction measurements around the drill.

The key factors influencing these day-to-day
dust level variations were wind speed and
direction. Figure 7 shows the average wind
speed and the standard deviation of the wind
direction around the drill for the sampling shifts,
while figure 8 shows the typical wind direction
histories observed during a strong windy day
(March 31) versus a calm mild day (August 12).
As evidenced by these data, on the strong windy
days the wind direction deviations were notably
less than for the calmer days. In comparison
tripod dust levels were notably higher than the
outside cab dust levels for the strong windy days,
While they were more comparable on the calm
days (see figure 6). This most likely occurred
because the dust plume for the windy days was
more directional towards the downwind tripod
sampling location, while it was more dispersed

among the tripod and outside cab sampling
locations for the calm days (see figure 8). Figure
9 also shows the strong positive relationship
between the tripod dust level and wind speed.
The wind speed irrespective of directional
variation was also believe to be a key factor in
this tripod dust and wind speed relationship, since
notably more dust entrainment around the drill
was visually observed on the extremely windy
days during the baseline sampling in March.

6:00 8:24 10:48 13:12 15:36
TIME, he:min.

Figure 8 - Strong and mild wind direction
histories around the drill.

Table I shows the average dust levels and
cab protection factors, with daily ranges for the
baseline and modified cab on the Davey M8B
drill. The drills outside cab respirable dust levels
reported in table I are inclusive averages of both
the tripod and outside cab sampling locations, A
negligible change in operator cab dust control
effectiveness was measured by adding the Red
Dot and Clean Air Filter’ units to the cab
enclosure. The enclosed cab dust protection
factor was decreased on average from 5.4 to 2.9.

Similarly, the percentage of cab penetration of
dust was increased from 19% to 34% and the
percentage of dust reduction decreased from -
81% to -66% with the cab improvements.


FIELD ASSESSMENT OF RETROFITTING SURFACE COAL MINE EQUIPMENT CABS
WITH AIR FILTRATION SYSTEMS

by J.A. Organiscak', A.B. Cecala!, W.A. Heitbrink', E. D. Thimons',
M. Schmitz’, and E. Ahrenholtz”

‘National Institute for Occupational Safety and Health. *Clean Air Filter”

ABSTRACT

Operator cabs on a front-end loader and a
rotary rock drill were retrofitted with ceiling
mounted heating/AC units and air filtration
systems. Subsequently surface coal mine field
studies were conducted to evaluate the respirable
dust protection these retrofitted cab systems offer
to the equipment operator. A significant 10:1
respirable dust protection factor (ratio of outside
to inside cab dust levels) was measured for the
front-end loader cab with positive pressurization
of the cab interior. Whereas an insignificant 3:1
respirable dust protection factor was measured
for the drill cab without positive pressurization of
the cab interior. These results indicate that
achieving positive interior cab pressurization with
retrofitted cab filtration systems is a key element
to their dust control effectiveness.

INTRODUCTION

The Mine Safety and Health Administration
(MSHA) permissible dust exposure for coal mine
workers is a shift average of 2.0 mg of airborne
respirable coal mine dust per cubic meter of air
(2.0 mg/m’) as defined by the Mining Research
Establishment (MRE) Criteria (U. S. Code of
Federal Regulations, 1998). If the airborne

respirable dust (ARD) sample contains more than
5% crystalline silica, the dust standard is reduced
to the quotient of 10 divided by the percentage of
silica in the dust, limiting the respirable crystalline
silica exposure to a maximum of 100 i g/m*
(MRE equivalent) for the working shift.
Compliance with these respirable dust standards
is expected to significantly reduce a workers risk
of occupational lung disease over an average life
expectancy.

MSHAss dust enforcement program includes
both inspector and coal mine operator dust
sampling. MSHAss surface coal mine dust
program focuses its sampling efforts at designated
work positions (DWP:s). These are particular
areas or occupations that have been historically
shown to either exceed 1 mg/m’ of respirable dust
or have high silica exposure. The local MSHA
official has the authority to classify DWPs based
on an operationss dust sampling history or to
classify non-designated work positions (NDWPs)
based on a history of competent dust abatement.

The most frequently sampled and classified
DWPs at surface coal mines are the highwall drill
operator, bulldozer operator, refuse/backfill truck
driver, and highlift operator. MSHA dust’
exposure data from 1985-1992 showed that the
percentage of the DWP dust samples containing
more than 5% silica ranged between 81% for the


REFERENCES

Ainsworth S M, Parobeck P S, Tomb T F [1989].
Determining the quartz content of respirable dust
by FTIR. Mine Safety and Health Administration
RI 1169, 14 pp.

ASAE [1997]. Agricultural cabs--environmental
air quality. Part 1: Definitions, test methods, and
safety practices. ASAE Standards, 44" Ed.
$525-1.1 Nov97, St. Joseph, Mich.:ASAE.

Cecala, A B, McClelland J, Jankowski R A
[1988]. Substantial time savings achieved
through computer dust analysis. American
Industrial Hygiene, 3(7), pp. 203-206.

Heitbrink W A, Hall R M, Reed L D, Gibbons D
[1998]. Review of Ambient Aerosol Test
Procedures in ASAE Standard $525. Journal of
Agricultural Safety and Health, 4(4), pp. 255-
266.

Heitbrink W A, Hall R M, Reed L D, Gibbons D
[1999]. Use of ambient aerosol for testing
agricultural cabs for protection against pesticide
aerosol. American Journal of Industrial
Medicine Supplement, 1:75-76, Published by
Wiley-Liss, Inc.

Organiscak, J A, Page S J[2000]. Field
assessment of control techniques and long-term
dust variability for surface coal mine rock drills
and bulldozers. International Journal of Surface
Mining and Reclamation, 13(4), pp.165-172.

MSHA [2000]. Informational seminars on
MSHAss new noise standard, noise control
technology, and silica dust control technology.
Mine Safety and Health Administration @
http://www.msha.gov/events/noise/seminar.pdf

Tomb T F, Gero A J, Kogut J [1995]. Analysis
of quartz exposure data obtained from.
underground and surface coal mining operations.
Appl. Occup. Environ. Hyg., 10(12), December,
pp. 1019-1026.

U. S, Code of Federal Regulations, 1998, ATitle
30-Mineral Resources; Chapter I--Mine Safety
and Health Administration, Dept. Labor;
Subchapter O-Coal Mine Safety and Health, Part
70--Mandatory Health Standards--Underground
Coal Mines, Subpart B, Section 70.101; Part 71--
Mandatory Health Standards--Surface Coal
Mines and Surface Work Areas of Underground
Coal Mines, Subpart B, Section 71.101,¢ U.S.
Gov. Printing Office, Office of Federal
Regulations, July 1, 1998.


reasonably good agreement with the OPC data.

The drill cabs air cleaning system did
provide some improvement in the median dust
size that entered the cab. A negligible difference
was observed in the mass median diameter
(MMD) of dust outside the cab as compared to
inside the cab during the baseline tests (31.3 im
outside and 28.7 im inside). However, the
MMDs of the outside dust and the inside dust for
the improved cab were 32.5 im and 18.3 im,
tespectively. This indicates that the filtered air
made some improvement within the cab, but the
effect was diluted by outside air infiltration into
the cab by gaps in the structure (no positive cab
pressure) or by the door being opened frequently.

Finally, the silica percentage of the respirable
dust for the Davey M8B drill was found to be
notably higher than that of the front-end loader,
commonly above 20%, both inside and outside
the cab during all the baseline testing and
modified cab testing.

CONCLUSIONS

These field studies show that two key
elements are needed to control dust levels in
enclosed operator cabs. First, the cab needs to
have a high quality of re-circulating and incoming
filtered airflow; secondly, a cab structure needs to
be adequately sealed to achieve positive static
pressure with the incoming clean air flow. Both
of these key elements were accomplished with the
CAT 980B front-end loader, providing on
average a 10:1 cab protection factor for the
operator. The cab structure on the Davey M8B
was not adequately sealed, diminishing the overall
effectiveness of the cab air filtration system. By
not achieving positive pressure inside the drill
cab, the outside dust was able to penetrate the
cab structure.

Relative cab performance measures
determined in the field were noticeably affected
by the wind and outside dust levels. The cab
Protection factor, percentage of cab penetration,
and percentage of dust reduction measures were

observed to change noticeably during day-to-day
operations, while the respirable dust levels
remained consistently low inside the cab.
Exterior cab dust levels affected by weather
(wind and precipitation) were found to
considerably change the relative cab performance
measures, especially if the exterior cab dust levels
were very low. However, the primary goal of
enclosed cab performance should focus on
consistently achieving inside cab dust levels
below worker compliance levels, while ensuring
adequate protection from high dust levels outside
the cab.

MSHaAss dust enforcement data show that
enclosed cabs on drills, bulldozers, refuse/backfill
trucks, and high lifts (front-end loaders) continue
to be suspect in providing adequate operator
protection from silica dust at mining operations.
Therefore, some of the enclosed cabs used on
mining equipment need to be updated,
teconditioned, or better maintained. To resolve
enclosed cab performance problems and improve
mine worker health, MSHA is currently pursuing
enclosed cab seminars around the U.S. to build
partnerships between health specialists, labor,
mining companies, mining equipment
manufacturers, heating & air-conditioning
equipment manufacturers and filter media
companies [MSHA2000]. NIOSH is currently
studying field measurement quality control
procedures for prompt determination of an
enclosed cabs dust protection capabilities.

ACKNOWLEDGMENTS

The authors would like to express their
appreciation to the other partners that made
noteworthy contributions to this project. They
include Gary Hansen and Chris Coppock of Red
Dot Corporation for contributing the heating and
air-conditioning units to the project and William
E. Burton of Al Hamilton Contracting Co. for
providing the equipment and mine sites for the
field studies.


noticeably more consistent than those levels
measured at the tripod and outside of the cab
during the study. The tripod dust levels were’
more variable and typically higher than the
outside cab dust levels, during both the baseline
and modified cab testing. The tripod dust levels
were also significantly higher during 3 of the
baseline sampling shifts as compared to the
modified cab sampling shifts.

degrees

‘WIND DIRECTION STANDARD DEVIATION.

AN Soa

Figure 7 - Summary of daily wind speed and
direction measurements around the drill.

The key factors influencing these day-to-day
dust level variations were wind speed and
direction. Figure 7 shows the average wind
speed and the standard deviation of the wind
direction around the drill for the sampling shifts,
while figure 8 shows the typical wind direction
histories observed during a strong windy day
(March 31) versus a calm mild day (August 12).
As evidenced by these data, on the strong windy
days the wind direction deviations were notably
less than for the calmer days. In comparison
tripod dust levels were notably higher than the
outside cab dust levels for the strong windy days,
While they were more comparable on the calm
days (see figure 6). This most likely occurred
because the dust plume for the windy days was
more directional towards the downwind tripod
sampling location, while it was more dispersed

among the tripod and outside cab sampling
locations for the calm days (see figure 8). Figure
9 also shows the strong positive relationship
between the tripod dust level and wind speed.
The wind speed irrespective of directional
variation was also believe to be a key factor in
this tripod dust and wind speed relationship, since
notably more dust entrainment around the drill
was visually observed on the extremely windy
days during the baseline sampling in March.

6:00 8:24 10:48 13:12 15:36
TIME, he:min.

Figure 8 - Strong and mild wind direction
histories around the drill.

Table I shows the average dust levels and
cab protection factors, with daily ranges for the
baseline and modified cab on the Davey M8B
drill. The drills outside cab respirable dust levels
reported in table I are inclusive averages of both
the tripod and outside cab sampling locations, A
negligible change in operator cab dust control
effectiveness was measured by adding the Red
Dot and Clean Air Filter’ units to the cab
enclosure. The enclosed cab dust protection
factor was decreased on average from 5.4 to 2.9.

Similarly, the percentage of cab penetration of
dust was increased from 19% to 34% and the
percentage of dust reduction decreased from -
81% to -66% with the cab improvements.


was used to align the instrument for proper
azimuth readings. This instrument was also
connected to two Metrosonics 331 data loggers
to record both the speed and direction of the
wind. Again the output stored in these loggers
was downloaded to a personal computer for data
analysis [Cecala et al.1988]. The drill location
and orientation during the shift was drawn on a
map to identify the dust migration of the drill dust
with respect to the operator cab. Also, a
description of weather conditions was recorded
with wet and dry bulb temperatures taken during
the shift. Because the loader is mobile,
constantly changing its orientation with the wind,
its cab dust protection effectiveness was
anticipated to be a good long-term representative
average for the various wind directions.
Therefore, wind measurements were not taken
around the front end loader.

FIELD STUDY RESULTS

CAT 980B Front-End Loader: Figure 1 shows
the CAT 980B front-end loader loading coal
from a stockpile with the Red Dot and Clean Air
Filter’ units operating on top of the cab
enclosure. The 980B loader was used for loading
stockpiled coal into the portable crusher,
removing the coal seam from the pit, and loading
coal trucks for transport from the mine. The
loader was commonly rotated among these
multiple tasks during the same sampling shift.
The operator kept the doors and windows shut
during all the sampling shifts (baseline and
modified cab), except to enter and exit the loader
cab.

Average respirable dust levels (taken by 3
personal samplers) measured inside and outside
the loader cab are shown in figure 2 with their
Tespective standard error bars for the multiple
samplers. Results from the 3 baseline shifts
(March 29 - 31) and six controlled shifts (Aug.
11 and 12, Sept. 14 and 15, and Oct. 20 and 21)
show that the refurbished enclosed cab had
noticeably reduced respirable dust levels inside
the cab as compared to outside the cab. Wet

ground conditions on Sept. 14 and Oct. 20 from
early morning rain lowered the outside loader cab
dust levels as compared to the other four
controlled shifts with dry ground conditions.

Figure 1 - CAT 980 loader studied.

* nn he

Note: the gourd wee wet fom maning ran
on Sept. 14 and Oct. 20

Figure 2 - Dust results from loader cab
evaluation.

A typical instantaneous dust level history
(MINIRAM) recorded inside the loader cab is
shown in figure 3 for the March 31* baseline shift
and the Aug. 12" controlled shift. These dust
level histories illustrate the notable inside cab dust
reductions achieved from the loader cab
modifications. They also show that the


systems. A baseline cab dust study was initially
conducted on each piece of equipment for 3 to 4
production shifts before any cab modifications °
were made. A follow-up dust study was repeated
for 4 to 6 production shifts after the cab
modifications.

Each enclosed cab was retrofitted with a new
ceiling-mounted Red Dot air-conditioner/heater
unit (Model R-9757) with an external make-up
air fan and Clean Air Filter’ filtration system?
The external filter for the make-up air was a 2-
stage Clean Air Filter’ cartridge with a cellulose
paper medium for the first stage and a final
respirator medium as the second stage. A Clean
Air Filter” housing with cyclonic inlets contained
the filter cartridge and was connected to a
centrifugal fan for blowing make-up air into the
Red Dot air-conditioner/heater unit. A single-
stage respirator media filter was also mounted on
the Red Dot unites inside cab re-circulation inlet.

The respirator media filter performance
specifications are at least 99 percent capture
efficiency for 0.1 im of mono-dispersed sodium
chloride particles, as determined from TSI test
method 85 Ipm/dm’.

Installation of each piece of equipment for
the ceiling-mounted system took about a day,
while another half day was invested on sealing the
cab. The original floor heaters in the cabs were
either removed or disconnected from operation,
so that the Red Dot ceiling units and Clean Air
Filter’ units would consistently be used for both
heating and air-conditioning functions. Cab
enclosure structures on both pieces of equipment
had numerous holes and cracks and thus positive
inside cab air pressure was difficult to achieve.
To enhance cabin air pressure, the CAT 980B
cab enclosure cracks were sealed with silicon
caulk and the door gaps were sealed with dense
foam weather strip. A positive static cab

>Mention of any company name or
Product does not constitute endorsement by the
National Institute for Occupational Safety and
Health.

pressure of 0,01" to 0.015" water gauge was
achieved after several hours of sealing all the
visible gaps/holes in the cab. The Davey M8B
drill cab structure was in very poor condition,
with large holes in the cab for the mechanical drill
control linkages and a loosely fitted bi-folding
door on the drill table side of the drill. Because
of the numerous holes and gaps present in the cab
enclosure, positive static cab pressure was not
achieved on the Davey M8B drill. However, the
discharge of the air-conditioning/heating unit was
directed over the operator position in an attempt
to provide him with a clean-air zone within the
cab.

FIELD SAMPLING PROCEDURES

Dust sampling was conducted during
multiple working shifts inside and outside the
operator cabs to measure the enclosed cab dust
control protection factor before and after the cab
modifications were made. Data collected
included: personal respirable dust samples;
personal impactor dust mass size distributions;
optical particle counter (OPC) size distributions;
miniature real-time aerosol monitor (MINIRAM)
respirable dust levels; weather conditions (wind
speed, direction, temperature, humidity, etc.); and
qualitative documentation of equipment
operation.

Gravimetric Sampling: Area airborne respirable
dust sampling was conducted with personal
samplers located inside the operator cabs and
near the dust source outside of the cabs to assess
the protection performance of these cabs in
relation to operator dust levels, Each personal
dust sampler included a Mine Safety Appliance
(MSA) Flow-Lite pump, operating at 2.0
liters/min, with a 10-mm Dorr-Oliver nylon
cyclone classifier to collect a respirable dust
sample (U. S. Code of Federal Regulations,
1998). The respirable dust was deposited on a 37
mm MSA coal dust filter cassette. Three
personal gravimetric dust samplers were used at
each sampling location so that adequate amounts
of dust could be collected for silica analysis. One


highwall drill operator and 25% for the highlift
operator [Tomb et al. 1995]. The percentage of
these DWP dust samples that exceeded the 100

i g/m’ silica limit ranged between 77% for the
highwall drill operator and 26% for the highlift
operator [Tomb et al. 1995]. These data suggest
that overexposure to silica dust is an ongoing
surface coal mine dust problem for the highwall
drill operator, bulldozer operator, refuse/backfill
truck driver, and highlift operator.

An engineering control measure for surface
mining equipment is enclosed operator cabs with
air filtration systems. These cabs usually re-
circulate and re-condition a majority of inside cab
air with a smaller portion of the air added from
the outside as makeup air. In order for the
enclosed cab to protect the operator from the
dust generated during excavation, the inside cab
air must be efficiently filtered and the cab
enclosure must be maintained under a positive
ventilation pressure.

The agricultural industry has developed
quality performance specifications for tractor cab
enclosures. These enclosures are designed to
protect equipment operators from pesticide
exposures during their application. The premise
for this standard is that a cab must act as an
acceptable substitute for a respirator that adheres
to the Worker Protection Standard (WPS) of the
USS. Environmental Protection Agency (EPA)
[Heitbrink et al. 1998]. The American Society
of Agricultural Engineers (ASAE) Standard $525
specifies that a cab enclosure will provide a 50:1
reduction in particles (commonly referred to as a
protection factor of 50) with an aerodynamic
diameter larger than 3 1m [ASAE 1997]. This
enclosed cab performance standard is equivalent
to the protection offered by a full face respirator.

The ASAE standard also specifies a minimum
positive differential static pressure of 6 mm of
water gauge for the cab enclosure and
recommends ambient aerosol test procedures
with optical particle counters to evaluate the
performance of these enclosures [ASAE, 1997].
However, field evaluation of these agricultural
cab testing procedures has shown that low

ambient aerosol concentrations outside the cab
can notably bias the evaluation to yield lower
protection factors [Heitbrink et al. 1998;
Heitbrink et al. 1999].

Recent surface mining dust surveys
conducted by the National Institute for
Occupational Safety and Health (NIOSH) on
drills and bulldozers have shown that enclosed
cabs can effectively control the operators dust
exposure, but enclosed cab integrity problems still
exist [Organiscak and Page 2000]. The enclosed
cab protection factors measured on rotary drills
ranged from 2.5 to 84, and those measured on
bulldozers ranged from 0 to 45. Some of the
newer equipment cabs tended to be better sealed
and cleaner, while some of the older equipment
tended to be more poorly sealed and dirtier. One
of the least protective drill cabs did not include
any heating, air conditioning, and air filtration
systems. Some of the older surface mining
equipment also possesses enclosed cabs with
heating, but no air-conditioning, and/or air
filtration systems.

To evaluate the respirable dust protection
provided by retrofitting enclosed cab
improvements, NIOSH recently conducted
several dust control field studies of retrofitting
old enclosed cabs with air-conditioning, heating,
and air filtration systems. This paper describes
these field studies conduced on a CAT 980B
front-end loader and a Davey M8B rotary drill.

. RETROFITTED CAB DEMONSTRATIONS

An enclosed operator cab field dust
evaluation was conduced on a CAT 980B front-
end loader and a Davey M8B drill before and
after the cabs were retrofitted with roof mounted
air- conditioner/heater units and external filtration
units. This work was conducted as part of a mine
demonstration project to improve the dust control
integrity of enclosed operator cabs on mobile
mining equipment. The two pieces of equipment
studied originally possessed enclosed cabs with
floor heaters and no air-conditioning and filtration