Journal of Occupational and Environmental Hygiene, 2: 54–63

ISSN: 1545-9624 print / 1545-9632 online

Copyright c 2005 JOEH, LLC

DOI: 10.1080/15459620590903444

Reducing Enclosed Cab Drill Operator’s Respirable

Dust Exposure with Effective Filtration and

Pressurization Techniques

Andrew B. Cecala,1 John A. Organiscak,1 Jeanne A. Zimmer,1

William A. Heitbrink,2 Ernest S. Moyer,3 Michael Schmitz,4

Eugene Ahrenholtz,4 Chris C. Coppock,5 and Earle H. Andrews6

1Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh,

Pennsylvania

2University of Iowa, Iowa City, Iowa

3Morgantown Research Laboratory, National Institute for Occupational Safety and Health,

Morgantown, West Virginia

4Clean Air Filter Company, Defiance, Iowa

5Red Dot Corporation, Seattle, Washington

6U.S. Silica Company/Better Minerals and Aggregates Company, Berkeley Springs, West Virginia

Many different types of surface mining equipment use enclosed cabs to protect equipment operators from health and

safety hazards. The overburden removal and mining process

can be extremely dusty and can cause excessive dust exposure.

To study this issue, a cooperative research effort was established between the National Institute for Occupational Safety

and Health, U.S. Silica Co., Clean Air Filter Co., and Red

Dot Corp. in an effort to lower respirable dust levels in an

enclosed cab on an older surface drill at a silica sand operation. Throughout this research effort, a number of modifications

were incorporated into the drill’s filtration and pressurization

system, as well as in other areas, to improve its design and

performance. An average cab efficiency of 93.4% was determined with gravimetric sampling instruments when comparing

the outside with the inside cab dust levels on the final design.

Although this study considered just one operation, the goal

was to identify cost-effective improvements that could be implemented on all types of enclosed cabs to lower respirable

dust concentrations. Two critical components for an effective

enclosed cab system are having a properly designed, installed,

and maintained filtration and pressurization system, along with

a method for maintaining structural cab integrity, which allows

the cab to be positively pressurized. Another important component is maintaining cab cleanliness. Although this research

was originally directed toward the mining industry, it is also

applicable to agricultural or construction equipment.

Keywords dust control, dust exposure, enclosed cab, filtration and

pressurization system, respirable dust, silicosis

Address correspondence to Andrew B. Cecala, NIOSH, P.O. Box

18070, Pittsburgh, PA 15236; e-mail: aic1@cdc.gov.

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

INTRODUCTION

This research addresses the health component of enclosed

cabs to determine whether acceptable air quality can be

provided to operators while the equipment is being used in

extremely dusty environments. The issue becomes even more

critical when this equipment is operated in an environment

where silica dust is present. The serious health effects to workers breathing respirable silica dust and the potential for subsequently developing silicosis have been known for decades.(1–5)

The concern with this dust has been further complicated by

Monograph 68, published by the International Agency for Research on Cancer in 1997, which states that inhaled crystalline

silica from occupational sources is considered a (Group 1)

human carcinogen.(6)

Worker exposure to respirable silica dust is common in surface metal/nonmetal and coal mining operations in the United

States, since a significant portion of the overburden material

contains high silica-bearing strata. This study evaluated a surface drill at a silica sand operation where the majority of ore

being drilled had a very high silica or quartz content.

A multitask research effort has been under way to evaluate

the air quality delivered to enclosed cabs on many different

types of heavy equipment. Acceptable air quality being provided to workers in enclosed cabs is based on many different

factors, but the most critical one is the cab’s filtration and

pressurization system. Since a significant portion of surface

mining equipment in the United States is old, the cabs may not

be providing acceptable air quality to the equipment operators.

While visiting many operations over the past few years, we
have observed a wide spectrum of effectiveness of various

filtration and pressurization systems being used in the mining

and agricultural industries. Some enclosed cabs had effective

filtration and pressurization systems that provided acceptable

air quality to the operator, while others were poorly maintained

and provided marginal or unacceptable air quality. A major

factor in the degradation of these systems was the aging and

wear of many of the components on the enclosed cabs. As cabs

get older, gaskets and seals commonly deteriorate to a point

where they no longer provide adequate closure or sealing, and

positive pressurization cannot be achieved. Other cabs were

observed with poorly designed filtration and pressurization

systems not capable of providing acceptable air quality. Such

design flaws have even been identified on some new equipment.

In an effort to minimize workers’ respirable dust exposures,

this and other similar studies were concurrently performed to

evaluate and improve the effectiveness of the air quality provided to workers operating heavy equipment inside enclosed

cabs.(7–10) The National Institute for Occupational Safety and

Health (NIOSH) entered into a verbal agreement with U.S.

Silica Co. to determine the respirable dust exposure to the operators of a surface drill and then to subsequently implement costeffective methods to optimize the air quality in the enclosed

cab. Initially, this effort focused on making improvements to

the existing filtration system and improving the pressurization

by installing an add-on system. In addition, the integrity of

the cab was also improved by replacing gaskets and seals,

and by sealing holes and cracks. Eventually, a completely new

filtration/pressurization system was installed to determine the

level of improvement in air quality that could be achieved with

a new prototype system. Results from this study can be applied

to other types of enclosed cabs on different equipment across

various industries.

METHODS

The objective of this research was to initially determine

the air quality in the enclosed cab of the drill and then

compare it as modifications were implemented to improve

the cab’s filtration and pressurization system. The sampling

strategy provided a quantitative analysis of respirable dust

levels inside and outside the enclosed cab on this drill. Data

collected included gravimetric and instantaneous respirable

dust monitoring measurements, instantaneous particle counting measurements, weather conditions (wind speed, direction,

temperature, and relative humidity), and documentation of

equipment operation.

Sampling Strategy

Three main sampling locations were used for this evaluation: (1) inside the enclosed cab, (2) outside the enclosed

cab, and (3) outside the cab on a movable sample tripod positioned downwind in the dust cloud. The inside sample location

was used to determine the air quality inside the cab, which

represents the drill operator’s respirable dust exposure during

the time spent inside the enclosed cab over the workday. All

sampling instrumentation was located on a sampling rack positioned directly behind the drill operator’s chair. This sampling

rack normally comprised three gravimetric samplers and an

instantaneous respirable dust monitor.

The outside sampling location also used a sampling rack that

was attached to the drill cab directly under the window where

the operator views the drilling process. This sampling unit also

comprised three gravimetric samplers and an instantaneous

respirable dust monitor. Cascade impactors and temperature

recording devices were used at the inside and outside sample

locations, but that data will not be presented here.

The tripod sample location used three gravimetric samplers

and was manually moved by test personnel to always be positioned downwind of drilling. This sample location normally

represented the worst-case dust exposure potential, since it was

always located in the dust cloud even if the wind was blowing

the dust away from the enclosed cab.

Respirable dust measurements using gravimetric samplers

were normally taken by three individual units located side by

side on the sampling rack. MSA Escort Elf (MSA, Pittsburgh,

Pa.) sampling pumps were used in this study and calibrated

to a flow rate of 1.7 L/min before each field survey, which is

the required flow rate as established by the American Conference of Governmental Industrial Hygienists (ACGIHR

) for

the metal/nonmetal mining industry.(11) Dust samples were

collected with a 10-mm Dorr-Oliver cyclone, which classifies the respirable portion of dust, and then deposited on a

37-mm MSA filter. Filters were pre- and postweighed to the

nearest 0.001 mg on a microbalance in a temperature/humidity

controlled weighing room. All sampling pumps were also postcalibrated to ensure that an acceptable flow rate of 1.7 L/min

(±0.015 L/min) was maintained throughout testing. For every

10 gravimetric filters used in the field, a blank cassette was

used to determine a correction factor for the filter weighing process that was then applied to all the field gravimetric

measurements.

Airborne respirable dust measurements inside and outside

the enclosed cab location were taken with Personal DataRAM

(PDR) instruments (model 1200, Thermo Andersen, Smyrna,

Ga.). The PDR instruments were operated in the active sampling mode, which involved the use of the same sampling setup

as previously described for the gravimetric instruments. After

the filter was postweighed and a correction factor calculated,

it was applied to all dust data concentrations for that day of

testing with the PDR.

Two different types of particle counting instruments were

used for short evaluations during this research. These were

the Pacific Scientific Met One model 227B hand-held particle

counting instruments and Grimm PDM model 1108 optical

particle counters. The Met One instrument is a laser-based

sensor that was used in the manual mode to record 1-min measurements for 0.3 and 3.0 μm particles. The Grimm particle

count instruments were used to measure aerosol concentrations

with an omnidirectional sampling inlet in 15 different ranges

from 0.3 to 15 μm.
A number of different instruments were used to determine

environmental conditions during testing. Since this drill operates outside during a wide range of weather conditions throughout the year, environmental factors such as wind velocity,

wind direction, temperature, and relative humidity levels were

monitored during each test. Periodically throughout each day

of testing, a 9-inch (22.9-cm) sling psychrometer was used to

verify temperature and relative humidity measurements. The

drill location and orientation were manually recorded by work

personnel to provide a wind profile for the drill that was used

for data analysis calculations. Baseline testing included two

different 2-day series that were slightly over a week apart (May

10 and 11, and May 19 and 20, 1999).

Pressurization and Filtration Effectiveness

Descriptors

When evaluating a pressurization and filtration system

(comparing outside with inside respirable dust concentrations),

a number of different descriptors or measures provide a numerical value to describe the system’s effectiveness. The following

three measures can all be used:

Protection factor (PF) = Co/Ci (ratio)

Efficiency (η) = (Co − Ci/Co) (fraction—or

multiplied by “100” for percent)

Penetration (Pen) = 1 − η (fraction)

where Co = outside respirable dust concentration, Ci = inside

cab respirable dust concentration.

Comparison PF = Co/Ci = 1/1 − η = 1/Pen

Drill Description

The drill evaluated in this study was a DrilTech D40Kii

rotary percussion drill approximately 20 years old, operated

by a drill operator and drill helper. The drill was driven into

place by the drill helper from a driver’s compartment located

in the left front of the drill rig. When the drill was moved at this

operation, the drill operator would exit the drill cab and stand

outside on the driver’s side of the drill rig, providing hand

motions to the drill helper to position the drill in place for

the next hole. The driller operated the drill from the enclosed

cab located on the right back of the drill. This drill cab had a

Red Dot R-9727 12-volt air-conditioning unit located on the

roof of the cab. This unit had two internal fans that moved

approximately 320 ft3/min (9.1 m3/min) of air to the enclosed

cab, but there was no filtering of this air. The only dust filtering

performed for this system was from an external filter housing

that brought outside air into the system. At the time of testing,

the filter housing unit was substantially dented and the general

condition of the unit was poor, although this is typical in the

mining industry.

Postmodification A

A Clean Air Filter Co. cab pressurization system was added

to the existing system, providing approximately 70 ft3/min

(2.0 m3/min) of makeup air to the system. Since the volume of

the enclosed cab was approximately 45 ft3 (1.3 m3), this new

system provided approximately 1 1/2 filtered air changes to the

enclosed cab every minute. This system was composed of a

prefilter, a blower, and a respirator-medium secondary filter

(Figure 1). The prefilter was a two-stage media type filter in a

steel housing located on the negative side of the fan and was

used to remove the larger particles (greater than 20 μm). The

final filter was on the positive side of the fan housed in a 3-inch

(7.6-cm) cylindrical chamber. This filter was a near-respiratory

quality electrostatic media with a rating of 97% efficiency at

0.3 μm. After the pressurization system was installed, the cab

static pressure was measured at 0.01 in. H2O.

Another significant modification was improving the sealing effectiveness of the enclosed cab. New door gaskets were

installed, and all small cracks and holes in the shell of the

cab were plugged with silicon caulking. All control levers had

rubber boots placed around them to provide the highest quality

seal possible. A tenfold increase in the cab pressurization was

achieved through these numerous efforts and resulted in a cab

pressure of approximately 0.1 in. H2O. Achieving this level of

pressurization is important because it prevents moderate wind

velocities from forcing dust-laden air into the cab.(12) Testing

these improvements consisted of 21/2 days in November 1999

and 2 days of testing in January 2000.

Postmodification B

A newly designed low-profile plastic body Red Dot Corp.

R-9777 system was installed on the enclosed cab that offered

improved sealing capacity over the traditional metal unit. The

new system located the fan before the heater core and the

FIGURE 1. Cab filtration and pressurization system
air-conditioner evaporator to minimize the negative pressure

area within the unit. The floor heater was removed since the

overhead unit provided heating capabilities. Clean Air Filter

Co. also developed a high-efficiency pleated recirculation filter

that was placed inside this unit and located immediately before

the heating and air-conditioner core. This filter medium was

rated at 97% efficiency at 0.3 μm particles. It was an electrostatic filter medium because it provided less restriction per unit

area of filtering when compared with a standard mechanical

type filter. One goal was to maximize the amount of recirculated air within the cab to increase the efficiency of the heater

and air-conditioner by minimizing the amount of fresh air that

needed to be conditioned. The cab was once again sealed of all

possible leak areas. After all of these modifications, a positive

cab pressure of between 0.07 and 0.12 in. H2O was achieved.

A full range of temperature conditions were achieved for the

7 days of testing from April 2001 to March 2002 to evaluate

this test condition.

Statistical Analysis

A statistical analysis was performed on the inside cab respirable dust concentrations to examine the enclosed cab modification significance level, given the day-to-day variation of outside dust concentrations. An analysis of covariance (ANCOVA)

statistical technique was deemed suitable for this cab modification effect examination because it combined the features

of analysis of variance (ANOVA) and regression.(13) The ANCOVA technique augments an ANOVA model of the qualitative

effect of cab modification with the regression of the quantitative variable (or covariance) of outside cab dust concentration,

both anticipated to influence the day-to-day inside cab dust

concentrations during the study. This method is a special type

of regression analysis used to determine the level of influence

or significance of the variables (qualitative and quantitative)

on the inside cab dust concentrations.

RESULTS

Baseline

The first part of Table I indicates the respirable dust concentrations determined at all three monitoring locations using

gravimetric dust sampling equipment. Average outside respirable dust concentrations ranged from 0.71 to 6.71 mg/m3 as

compared with 0.02 to 0.08 mg/m3 for levels inside the operator’s cab. For all 4 days of testing, the outside cab respirable

dust concentrations were higher than for the tripod sample

location because the primary wind direction was from the drill

hole to the enclosed cab.

Pressure measurements taken inside the cab indicated that

there was very minimal cab pressurization at approximately

0.005 in. H2O. Despite the poor visual conditions of the filtration system and the poor structural integrity of the cab,

respirable dust levels measured inside the cab during baseline testing were much lower than expected. Since the airconditioning unit was being used for a substantial part of each

day of testing, it was believed that a significant portion of dust

TABLE I. Gravimetric Dust Sampling on Surface Drill for Various Phases of Research

Outside Cab Outside Tripod Average Outside Inside Cab

Dust Conc. Dust Conc. Dust Conc. Dust Conc. Protection Efficiency

Date (mg/m3) (mg/m3) (mg/m3) (mg/m3) Factor (%) Penetration

Baseline

5/10/1999 0.97 0.54 0.76 0.08 9.4 89.4 0.11

5/11/1999 2.21 1.57 1.89 0.03 63.0 98.4 0.02

5/19/1999 12.88 0.53 6.71 0.02 335.3 99.7 0.00

5/20/1999 0.91 0.51 0.71 0.02 35.5 97.2 0.03

Postmodification A

11/16/1999 2.74 14.74 8.74 0.38 23.0 95.7 0.04

11/17/1999 2.47 5.38 3.93 0.58 6.8 85.2 0.15

11/18/1999 11.07 4.82 7.95 1.16 6.8 85.4 0.15

1/25/2000 14.09 19.88 16.99 0.72 23.6 95.8 0.04

1/26/2000 73.33 60.09 66.71 0.54 123.5 99.2 0.01

Postmodification B

4/24/2001 3.01 1.14 2.08 0.07 29.6 96.6 0.03

4/25/2001 0.28 0.52 0.40 0.11 3.6 72.5 0.28

10/16/2001 0.31 0.14 0.23 0.02 11.3 91.1 0.09

11/1/2001 6.06 2.65 4.36 0.10 43.6 97.7 0.02

11/2/2001 29.91 36.78 31.85 0.13 245.0 99.6 0.00

3/27/2002 1.95 1.74 1.85 0.04 46.1 97.8 0.02

3/28/2002 3.83 2.3 3.07 0.04 76.6 98.7 0.01
wasremoved as the air moved through the evaporator unit, thus

increasing the filtering efficiency of the system.

After baseline testing was completed, a number of modifications were made to further improve the filtration efficiency

and pressurization of the enclosed cab. Since the R-9727 roofmounted unit appeared to be in poor working order, a maintenance overhaul and cleaning was performed on it. One problem

with the current design was that the filtration unit was under

negative pressure; thus, any leak in the system would allow dust

to enter the unit and be blown directly into the enclosed cab.

Postmodification A

To correct the problems identified, the drill was modified

with a new pressurization system, and the cab integrity and

cab pressurization were improved (postmodification A). After

the implementation of these changes, the identical dust analysis performed during baseline testing was repeated (Table I).

Originally it was anticipated that dust levels would decrease

because of the modifications and improvements made to the

filtration and pressurization system on the enclosed cab; instead, respirable dust levels inside the enclosed cab increased

seventeenfold from an average concentration of 0.04 mg/m3

(baseline) to 0.68 mg/m3 in posttesting.

In trying to determine the reason for this significant increase in respirable dust levels, the researchers examined all the

parameters that changed from baseline to postmodifications.

Since postmodification A testing took place in winter conditions with low outside air temperatures, a radiator-type floor

heater with a fan was used in the cab (Figure 2). This type of

heater is commonly used in heavy equipment during the winter

months and is well liked by equipment operators because it

keeps their feet warm. After considering all the factors in this

analysis, it was hypothesized that the floor heater in the cab

was the primary cause of the increased dust concentrations.

It was believed that dust was being generated from the drill

operator’s boots grinding and stirring up material on the floor

and from dust being blown off the operator’s clothing.

FIGURE 2. Floor heater inside enclosed cab

In an effort to verify the effects of using the floor heater, the

drill was taken into the maintenance shop and tested using two

Grimm particle-counting instruments. The first sequence was

to monitor particle levels inside and outside the enclosed cab

with the filtration and pressurization system operating along

with the recirculation system, which was the postmodification

A setup. The next sequence was to operate only the recirculation system. The third and final sequence was with both units

operating in conjunction with the floor heater. Once this test

series was completed, the Grimm instruments were switched

to minimize any instrument bias effects, and the test series

was repeated. Figure 3 shows the results from this testing.

The lowest levels recorded in the cab resulted from sequence

#1 (0.01 mg/m3), which was with the improvements to the

air filtration and pressurization system. For test sequence #2,

pretest conditions averaged a respirable dust concentration

of 0.03 mg/m3, while test sequence #3 (both units operating

with the floor heater) recorded the highest average respirable

dust concentration at 0.26 mg/m3. This represents a ninefold

increase over pretest conditions (sequence #2) and a twentysevenfold increase over postmodification A conditions (sequence #1). Outside respirable dust levels remained relatively

constant throughout this testing.(14)

Postmodification B

After completing the floor heater testing, it was decided

to make additional changes on the drill in an effort to further

improve the air quality in the enclosed cab—postmodification

B. When the drill became available after being out of production for 14 months, testing was performed to evaluate the

new filtration and pressurization system’s ability to improve

the air quality in the enclosed cab (postmodification B). Seven

different days of testing were performed over an 11-month

period to evaluate the effectiveness of the system during a

range of weather conditions when either the heater or the air

conditioner was used.

Table I shows the results for all testing performed. In addition to dust concentrations inside and outside the cab, the table

presents the protection factor, efficiency, and penetration for

FIGURE 3. Shop testing to determine increase in respirable dust

levels inside cab with floor heater
TABLE II. Cab/Filtering Performance Measures Using Met One Particle Counting Instruments

Date μm Size Outside Count Inside Count Protection Factor Efficiency Penetration

5/11/2001 0.3 48,465 2531 19 0.95 0.05

0.5 15,953 431 37 0.97 0.03

10/17/2001 0.3 233,220A 30,942 8 0.87 0.13

3.0 533 2 267 0.99 0.00

11/1/2001 0.3 849,382A 892 952 0.99 0.00

3.0 1744 2 872 0.99 0.00

11/2/2001 0.3 908,221A 437 2078 0.99 0.00

3.0 2558 1 2558 0.99 0.00

3/27/2002 0.3 320,412A 3287 97 0.99 0.01

3.0 157 11 14 0.93 0.07

Note: All testing performed under postmodification B testing. AParticle concentrations exceed the manufacturers 5% concentration level criteria of 200,000 particles/min.

each day of testing, which indicate that the cab effectiveness

measures are strongly influenced by the relative changes in

outside dust concentrations. An example of this is for the last

2 days of testing on March 27 and 28, 2002, when the inside respirable dust concentration was 0.04 mg/m3 for both days, but

the outside concentration averaged 1.85 and 3.07 mg/m3, respectively. The effect of the outside dust concentration caused

the protection factor to go from 46.1 to 77.6, the efficiency

from 97.8 to 98.7, and penetration from 0.02 to 0.1. This also

indicates the changes to each of the descriptors on a relative

basis. The point to note is that a system could have extremely

low inside dust concentrations but with high outside levels,

the performance values for protection factor efficiency, and

penetration may not appear to be significant.

Met One Cab Filtering Performance—Postmodification B

During the course of this research effort, Met One particle

counter instruments were used to evaluate cab performance

effectiveness. An initial analysis was performed on May 11,

2001, to verify that the Met One instrument provided particle

count values comparable to the Grimm instruments. For this

comparison, 0.3 and 0.5 μm particle sizes were used and indicated that similar values were obtained with both instruments.

After this initial test, all other tests used the 0.3 and 3.0 μm

levels since the large particle size is more significant in the

respirable size range. All of the subsequent tests using the Met

One instruments were static, nonproduction tests performed

during off-shift production times (testing during production

time periods would have overloaded the instrument). Table II

lists the results from this testing.

When comparing the results from the Met One and gravimetric sampling instruments, it can be seen that the Met

One cab/filtering performance measures (protection factor,

efficiency, and penetration) normally are higher than for gravimetric sampling. However, the Met One performance measures

are static tests under ideal performance conditions. In contrast,

gravimetric testing is performed continuously throughout the

workday and includes periods when the drill operator is going

into and out of the enclosed cab and may include periods when

the cab door is left open. Dust enters the cab when the door is

open and thus lowers the performance measures for gravimetric

sampling relative to the Met One instrument.

Statistical Analysis

An ANCOVA was performed for the cab data shown in

Table I to determine the significance level of the enclosed cab

changes with respect to the variation of outside dust concentrations. A natural log-transformation of the dust concentrations

was conducted to meet the ANCOVA assumptions of normality

and equal variances between treatment groups.(13) For this

statistical analysis, the inside cab dust concentration was the

dependent variable, the cab modification was the treatment

effect (main factor effect), and the outside dust concentration was considered the uncontrolled independent covariate.

Table III shows the summary statistics of the dust data for the

three different cab test conditions studied and indicates which

TABLE III. Summary Statistics of Dust Concentrations Measured

Summary Statistics of Field Testing Dust Data

Data Range

Min Max

Arithmetic

Mean

Std.

Dev.

Baseline

Outside cab (n = 4)

Inside cab (n = 4)

0.71

0.02

6.70

0.08

2.52

0.04

2.84

0.03

Postmodification A (floor heater)

Outside cab (n = 5)

Inside cab (n = 5)

3.92

0.38

66.73

1.16

20.85

0.68A

26.08

0.03

Postmodification B (new units)

Outside cab (n = 7)

Inside cab (n = 7)

0.25

0.02

31.77

0.13

6.25

0.07

11.34

0.04

AANCOVA analysis of the loge transformed dust data showed that the

postmodification A inside cab concentrations were significantly higher than

the other two periods at the 95% confidence level.
average cab dust concentration was significantly different (at

the 95% confidence level) from the others.

Results from ANCOVA analysis showed that some of the

cab modifications, along with outside dust concentrations,

were significantly associated with dust concentrations measured inside the cab. Table III indicates that a significant difference in respirable dust concentrations were measured inside the cab between the baseline and postmodification A

testing. In this case, floor heater use and increased outside

dust concentrations were both significantly associated with the

increase of inside cab dust concentrations from 0.04 mg/m3 to

0.68 mg/m3. A second evaluation comparing postmodification

A with postmodification B indicated that postmodification B

was significantly associated with reducing the enclosed dust

concentrations from 0.68 mg to 0.07 mg/m3. Postmodification

B removed the floor heater and heated the cab from the ceiling.

The third analysis compared baseline levels with postmodification B, which indicated that there was not a significant

difference for inside cab dust concentrations between these

two field studies when the floor heater was not used.

DISCUSSION

While performing this research, a number of issues were

identified that affected the air quality inside the enclosed cab. Cab cleanliness is one such area that was brought

to light by the floor heater issue. The shop testing showed a

seventeenfold increase in dust levels inside the enclosed cab

from the heater stirring up dust on the floor of the cab and from

the operator’s clothing. Although floor heaters are no longer

recommended for use within enclosed cabs, the magnitude of

this problem was exacerbated by the amount of product on the

floor in this cab. Walls and floors of cabs should be cleaned

by equipment operators on a daily basis to ensure that dust

is constantly removed from the cab interior. This work also

stresses the significance of the operator having clean work

clothing.

Another outcome from the floor heater analysis is the importance of providing effective filtering of recirculated air within

the enclosed cab. When dust is generated within a cab from

items such as the floor heater or contaminated work clothing,

the only method available to remove this dust is through the

use of a recirculation filter. Many roof-mounted filtration and

pressurization systems have inferior recirculation filters that

are incapable of removing the respirable fraction of dust. This

is a primary reason that a number of additional modifications

(postmodification B) were performed on the drill cab after the

shop testing was completed.

Another notable finding is the loss of filter efficiency for

electrostatic filter media over time, even without the filter in

use. The filter in question is the final filter on the pressurizer

system. Loss of filter efficiency was clearly noted during the

Met One particle count testing. May 11, 2001, was the first

day of testing with the Met One instruments, and these results

were used as a baseline. The drill had a major mechanical

failure shortly after this test and was out of service for the following 4 months. Shortly after the drill was repaired, Met One

testing was performed on the drill on October 17. The inside

0.3 μm particle count was very high and averaged a twelvefold

increase, even though the outside change was under a fivefold

increase. Despite this loss of efficiency, the filter had minimal

use between these two tests. For the testing on November 1

and 2 the following month, a new filter was installed and levels

once again were low, in the 0.3 μm range. When testing was

performed 4 months later, the loss of efficiency in the 0.3 μm

range became evident once again. Similar findings regarding

the loss of efficiency over time with electrostatic filter medium

have been noted by other researchers.(15–17)

In light of these findings, if an electrostatic filter is used

on enclosed cab filtering systems, it needs to be changed on a

scheduled time basis whether or not the filter is used or visually

shows signs of dust loading. In an effort to improve the filter

design, Clean Air Filter Co. is now fabricating an “absolute”

design. This incorporates a 95% efficiency mechanical filter

medium for 0.3 μm particles, with an electrostatic filter media

wrapped around the mechanical filter.

Another important finding during this research was the impact of the drill cab door being opened throughout the day. It

was noted on many occasions that the drill operator left the

enclosed cab door open when the drill was being moved and

repositioned, as well as during some nonproduction periods,

such as lunch, which allowed outside dust to enter the cab.

The impact of this can be seen in Figure 4 for a particular

day of testing when this situation occurred frequently. This

graph provides respirable dust levels inside and outside the

cab taken with PDR instantaneous dust monitors. The inside

cab dust levels are designated as both production (drilling)

and nonproduction time periods (nondrilling). As shown, the

highest respirable dust levels recorded inside the cab were

during the nonproduction time periods. The graph also shows

slightly higher dust levels inside the cab during the lunch break,

from 11:33 a.m. to approximately 12:45 p.m., in comparison

with outside dust levels.

In light of these findings, we recommend that the drill cab

door remain closed whenever possible. We also recommend

that the drill operator remain in the drill cab as much as possible

when the drill is being repositioned to the next drill hole. The

drill operator could easily communicate with the drill helper

through radio communications to position the drill over the

next hole without ever leaving the cab. This would decrease

the drill operator’s dust and noise exposure, as well as reduce

the amount of dust entering into and contaminating the interior

of the enclosed cab.

Sweep Compound Testing

After acknowledging the problem created by the floor heater

and that cab floors are commonly very soiled from the equipment operators tracking dirt inside the cabs on their work shoes,

a test was performed to determine whether dust levels in the

enclosed cab could be lowered by using a floor sweep compound. The goal of the sweep compound is to keep agglomerate

dirt and dust on the floor from becoming aerosolized. Prior
FIGURE 4. Impact of respirable dust levels inside enclosed cab when door is left open

to this testing, a number of different sweep compounds were

evaluated. For this testing, it was decided to use a canola oilbased sweeping compound, which is preferable to a petroleumbased compound because of problems associated with using

the petroleum product in a closed environment.

Presweep dust levels were already determined by using the

postmodification A dust levels. For the 5 days of testing, the

average respirable dust concentration was 0.68 mg/m3. A 1/4

inch to 1/2 inch (0.64 to 1.3 cm) thick layer of canola oil sweeping compound was applied on the floor of the drill cab, then

3 days of posttesting was performed. Testing showed that this

sweeping compound had a positive effect on suppressing dust

from the soiled work floor in this cab. The 3 days of posttesting

averaged a respirable dust concentration of 0.11 mg/m3, which

accounted for an 84% reduction in dust levels. Despite these

positive results, it must be noted that in two similar studies

performed in another drill and a dozer, the floor sweep did

not provide similar reductions as obtained in this study.(18)

However, both of these studies indicate the need to maintain

a clean cab and floor area and the potential that a floor sweep

compound can have in lowering dust levels by agglomerating dust and dirt on the floor and keeping it from becoming

airborne.

System Cost

A primary concern regarding any modification or change to

equipment is cost. The following is an approximate cost for the

various equipment installed during this phase of the research:

(1) an R-9777 heater and air-conditioner system donated by

Red Dot Corp. (approximate cost: $2200); (2) an external air

filtration and pressurization system donated by Clean Air Filter

Co. (approximate cost $1000). It took approximately 40 manhours to complete the installation of components 1 and 2, the

removal of the floor heater, and the inspection and sealing of

the enclosed cab.

CONCLUSIONS

Results from this study can be applied to other types of

equipment that use enclosed cabs, that is, in mining,

agriculture, and construction. An older surface drill at a silica sand operation was initially evaluated to determine the

air quality delivered to the enclosed cab from the existing

filtration system. After this testing was completed, a number

of modifications were implemented in an effort to improve air

quality. This research shows that there are two key components

necessary for an enclosed cab to be effective from a dust control

standpoint: effective filtration and cab integrity.

Filtration System

An effective filtration system should be composed of both

a recirculation and outside (makeup) air system. The majority

of air inside an enclosed cab should be recirculated through

a high-quality filter medium. This allows air to be heated or

cooled to the cab operator’s comfort without the major air

changes that would significantly affect the size and capability

requirements and, ultimately, the cost for conditioning the cab

air.

A major component in an effective system is to have the

makeup air positively pressurize the enclosed cab. This results in any system leakage from inside of the cab to outside,

preventing dusty air from entering the cab. It is also highly

recommended that the makeup air be positively pressurized

after being filtered to eliminate any possibility of dust-laden

air being drawn into the system. Additionally, the optimal

location for the makeup air inlet on the cab would be the

farthest location from the dust sources.(19) This reduces the

amount of loading on the filters and increases the time between

cleaning or replacement. Finally, the makeup discharge vent

into an enclosed cab should be located high in the enclosure,

preferably at the roof, which allows the clean air to be blown
FIGURE 5. Ideal schematic for effective filtration and pressurization system

down over the equipment operator’s breathing zone without

becoming contaminated by any in-cab dust sources.

One last design criterion is for the filtration component

to use a top-down approach to the clean air flow pattern. In

this study, as well as in most other systems, the intake and

discharge for the recirculation air is located in the roof of the

cab. Although this is acceptable, we believe the most beneficial

design would be to draw the recirculated air from the bottom

of the cab instead of at the roof of the enclosure. This allows

the dust-laden air to be drawn out of the cab near the worker’s

feet and away from the breathing zone. Again, the clean air

would be blown in at the roof of the enclosure and the dustladen recirculated air would be withdrawn from the floor of the

cab. We strongly recommend against the discharge of clean air

low in a cab wall because, as we observed, this can entrain

a significant amount of dust from soiled work clothes, boots,

and a dirty floor. Figure 5 represents our ideal schematic for

an effective filtration and pressurization system on an enclosed

drill cab.

Cab Integrity

Cab integrity is necessary to achieve some level of pressurization. Field testing has shown that installing new door

gaskets and plugging and sealing cracks and holes in the shell

of the cab have a major impact on increasing cab pressurization.

To prevent dust-laden air from infiltrating into the cab, the

cab’s static pressure must be higher than the wind’s velocity

pressure. Although higher static pressure requirements help

overcome outside wind speeds, a major drawback is that this

necessitates more air being delivered by the outside air unit,

causing more loading on the filters. Higher airflows through

filters increases particle penetration and decreases the filter’s

efficiency by allowing more contaminants to flow through the

filter media. Another drawback to higher airflows is that they

create more air-conditioning (heating and cooling) requirements for operator comfort, which increases the size and cost of

the unit. The key to an effective retrofit enclosed cab filtration

and pressurization system is to balance the various factors and

components discussed in this report.

Finally, because of the significant increase in dust levels

with the use of floor heaters, it is recommended that they

not be used in their present form. If needed, they should be

repositioned to a higher area in the cab where they are less

prone to recirculate dust from the floor and the operator’s

clothes. Probably the best solution would be to install a heating

and air-conditioning unit into the clean air and pressurization

system.

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