MAXIMIZING AIR QUALITY INSIDE ENCLOSED CABS WITH UNI-DIRECTIONAL FILTRATION AND PRESSURIZATION SYSTEM 

A. B. Cecala, NIOSH, Pittsburgh, PA 

J. A. Organiscak, NIOSH, Pittsburgh, PA 

J. A. Zimmer, NIOSH, Pittsburgh, PA 

M. S. Hillis, Vulcan Materials Co., Birmingham, AL 

D. Moredock, Sy-Klone International, Jacksonville, FL 

ABSTRACT 

Enclosed cabs have been used on surface mining equipment for 

many years to protect equipment operators from health and safety 

hazards. The surface mining and quarrying process is a highly 

mechanized process that generates large quantities of respirable dust. 

Due to economic factors, more older mining equipment is being used 

today than in years past. With this older equipment, it is common to 

observe deterioration of many components on the enclosed cabs 

which greatly reduce their protection effectiveness. NIOSH has 

performed a number of different field studies, as well as a 

comprehensive laboratory study evaluating various factors and 

parameters regarding enclosed cabs. NIOSH has recently completed 

a cooperative research effort with Sy-Klone International and Vulcan 

Materials Company to evaluate the effectiveness of a newly designed 

uni-directional filtration and pressurization system that brings all the 

clean filtered air in at the roof of the cab and extracts all recirculated air 

near the floor. This design provides the highest air quality to the 

equipment operator. Testing on this new system showed an almost 

10-fold reduction in respirable dust inside the enclosed cab. The unidirectional design is easy to incorporate and should be considered for 

all new and retrofit systems in surface mining equipment. 

INTRODUCTION 

Enclosed cabs have been used for many years in the mining 

industry to isolate workers from dust sources. Workers in enclosed 

cabs at these operations are surrounded by dynamic working 

conditions and highly variable dust sources. The enclosed cabs create 

a microenvironment where they can be either better protected or more 

vulnerable to respirable dust. This issue came to light from a study 

performed in 1996 and 1997 which showed an alarming prevalence of 

silicosis in the surface coal mining industry. This study was 

undertaken at eight different surface coal mining operations in central 

Pennsylvania and was performed by the Mine Safety and Health 

Administration, the Pennsylvania Department of Health, the 

Department of Health Evaluation Sciences of the Pennsylvania State 

University College of Medicine, and the CDC’s National Institute for 

Occupational Safety and Health (NIOSH). This was a voluntary 

program that screened 1,236 miners for lung diseases and determined 

that 6.7 percent of these workers were classified with at least category 

1/0 silicosis. In an anomaly that was noted in one particular county 

(Clearfield), 16 percent of the 213 participants were classified with 

silicosis [1]. This investigation noted that a number of young miners 

with relatively few years of mining experience were developing silicosis 

from using surface drills with enclosed cabs that were not providing an 

acceptable level of protection. Further, although surface drills provided 

the highest risk of overexposure, operators of other types of 

mechanized equipment such as bulldozers, loaders, and haul trucks 

were also being overexposed to crystalline silica and respirable dust. 

To address these issues, a substantial amount of research was 

initiated by a number of organizations to analyze the problem and

investigate methods and solutions to improve the air quality in 

enclosed cabs. The National Institute for Occupational Safety and 

Health entered into a number of cooperative research efforts with 

mining companies, heating and air-conditioning companies, and cabfiltration manufacturers to improve the air quality in enclosed cabs in 

the surface mining industry. The work in this report discusses one 

such study on a uni-directional filtration and pressurization system, 

which is thought to be the optimal design for the airflow pattern inside 

the enclosed cab. 

The uni-directional design is one that uses a top-down clean-air 

flow pattern in the enclosed cab. In most recirculation systems, the 

intake and discharge air vents are located at the roof of the cab. This 

design has two shortcomings. First, some of the clean air discharged 

is immediately short circuited right back into the recirculation vent 

(intake) without ever flowing through the enclosed cab. Second, dustladen air from the operator’s clothing, on the inside walls of the cab, 

and the floor, is drawn up over the operator’s breathing zone as it 

travels into the recirculation duct at the roof of the cab. A better design 

is to have all the clean filtered air brought in at the roof of the cab and 

all the recirculated air withdrawn near the floor of the cab, in a onedirectional or uni-directional design. 

This uni-directional filtration and pressurization study was a 

cooperative research effort involving NIOSH, Sy-Klone International, 

and Vulcan Materials Company, to determine the impact of retrofitting 

an older piece of mining equipment with a uni-directional pressurization 

and filtration system. One objective was to perform a worst-case 

scenario to determine the degree of improvement when retrofitting a 

poor-quality cab with a new pressurization and filtration system. 

Vulcan reviewed its internal dust sampling records and chose its worst 

cab and filtering system at one of its limestone operations near 

Birmingham, Alabama, where a pneumatic rotary drill was being used. 

The first aspect of this research study was to take baseline dust 

measurements on the drill before any changes or modifications were 

made to the unit. This drill had the heater/air conditioner unit that was 

provided by the original equipment manufacturer when the drill was 

purchased. Over time, this HVAC unit, as well as the cab’s gaskets 

and seals deteriorated to a point where they needed to be replaced 

and/or refurbished, although dust and noise levels were still within the 

acceptable standard. When completed, both Sy-Klone and Vulcan 

removed the old HVAC and filtering system and installed a new Red 

Dot 9777 HVAC unit and a Sy-Klone uni-directional filtration and 

pressurization system. Once it was determined the system was 

working properly, the post-evaluation was performed using the 

identical dust analysis performed during baseline testing. 

BACKGROUND 

Before reviewing the details of this retrofit uni-directional 

cooperative study, a brief overview of the common terminology used in 

evaluating the effectiveness of enclosed cabs, as well as a brief 

summary of some previous field studies and a recently performed 

laboratory study, will be presented. 

Effectiveness Terminology 

When evaluating the effectiveness of filtration and pressurization 

systems to provide clean air to enclosed cabs, a few different factors 
can be used. These factors are all interrelated and compare outside 

with inside respirable dust concentrations. These factors are: 

CO Protection factor (PF) = (ratio)

Ci

Ci Efficiency (η) = C − O (fraction, or multiplied by 100 for 

CO

percent value) 

Penetration (Pen) = 1 − η (fraction) 

Where: 

Co = outside respirable dust concentration 

Ci

 = inside respirable dust concentration. 

A comparison of these descriptors can be provided by the 

following: 

Co 1 1

PF = = = C 1 − η Pen i

Obviously, the higher the value for both protection factor and 

efficiency, and the lower the value for penetration that can be achieved 

the better the air quality inside the enclosed cab. For convenience, the 

term protection factor (PF) will be used in this report to evaluate the 

effectiveness of the uni-directional system. 

Field Studies 

Based on the outcome of the previously mentioned health study, 

which showed a high prevalence of silicosis among study participants, 

a number of organizations began investigating enclosed cabs to better 

understand the problem and to determine methods and solutions to 

improve the air quality and protect workers. This led to a number of 

studies in which new filtration and pressurizations systems were 

installed on older pieces of mining equipment in an attempt to improve 

the air quality inside these enclosed cabs. A few of these studies can 

be seen in Table 1, listed in ascending order of performance [2-5]. 

These studies highlighted some very important factors relevant to 

improving the air quality in enclosed cabs and ultimately protecting the 

workers. Cab integrity, and the related ability to achieve positive 

pressurization, was found to be a critical component. As seen in the 

first two studies listed in Table 1, when there was very little to no cab 

pressure detected, this resulted in minimal improvement in the cab’s 

air quality. In fact, similar filtration and pressurization systems were 

installed on a rotary drill and front-end loader, listed as items 1 and 3 in 

Table 1, with the protection factor ranging from 2.8 to 10. One notable 

difference between these two systems was that a small amount of 

pressurization was achieved in the front-end loader, whereas it was not 

possible to achieve any pressurization in the rotary drill. 

Table 1. Summary of field studies evaluating upgraded cabs. 

Cab Eval. [Ref] Cab Press. 

inches w.g. 

Avg. 

Inside Cab 

Dust 

Level, 

mg/m3

Avg. Outside 

Cab Dust 

Level, mg/m3

Protection 

Factor Out/In

Rotary Drill [2] None 

Detected 0.08 0.22 2.8

Haul Truck [3] 0.01 0.32 1.01 3.2 

Front-end Loader 

[2] 0.015 0.03 0.30 10.0

Rotary Drill [5] 0.20 to 0.40 0.05 2.80 56.0 

Rotary Drill [2, 4] 0.07 to 0.12 0.07 6.25 89.3 

Another critical factor determined was the quality and 

effectiveness of the filtration system. The various studies presented in 

Table 1 indicated substantial improvement in the interior air quality 

from effectively removing the dust particles from the outside air and 

delivering this clean filtered air into the enclosed cab. Very good air 

quality (and protection factor) was obtained in these cabs when 

sufficient pressurization was achieved, along with an effective filtration 

system. 

Laboratory Study 

From these various field studies, a number of different factors 

emerged that were relevant to the effectiveness of filtration and 

pressurization systems. In an effort to evaluate this area, a controlled 

laboratory experiment was performed to systematically examine 

multiple cab designs. Figure 1 shows the cab filtration system setup 

used for this controlled laboratory study and the various parameters 

that were evaluated [6, 7]. 

ηI 

Co

QL , l 

QI 

 ηR 

 QI + QR 

Ci

 

Vc

Where: 

PF = Protection Factor, Co/Ci

Co = Outside Cab Concentration

Ci = Inside Cab Concentration 

ηI = Intake Filter Efficiency, 

 f ractional 

QI = Intake Air Quantity 

QL = Intake Air Quantity Leakage 

l = Intake Air Leakage, QL/QI

ηR = Recirculation Filter Efficiency, 

fractional 

QR = Recirculation Air Quantity 

Qw = Wind Quantity Infiltration 

Vc = Cab Volume 

Qw

Intake Pressurizer Fan 

Recirculation 

F ()

Cab Enclosure 

Figure 1. Laboratory test setup to evaluate various operational 

parameters on filtration and pressurization system for an enclosed cab. 

The results of this laboratory study indicate that intake filter 

efficiency and the use of a recirculation filter had the greatest impact 

on improving the air quality. When considering the use of an intake air 

filter, the addition of the recirculation component significantly improved 

the air quality due to the repeated filtration of the cab’s interior air. The 

addition of an intake pressurizer fan to the filtration system increased 

both intake airflow and cab pressure significantly. The cab air quality 

was also affected by intake filter loading and air leakage. 

Throughout the course of the laboratory study, the significance of 

various filtration system factors was evaluated and the following 

mathematical model was developed [7]. It was formulated from a basic 

time-dependent mass balance model of airborne substances within a 

control volume with steady state conditions. It determines the 

protection factor in terms of intake air filter efficiency, intake air 

quantity, intake air leakage, recirculation filter efficiency, recirculation 

filter quantity, and outside wind quantity infiltration into the cab. 

C Q + Q η o I R R PF = = C Q (1−η + lη ) + Q i I I I W

Note: The above equation is dimensionless; therefore air 

quantities used must be in equivalent units. Also, filter 

efficiencies and intake air leakage must be fractional values 

(not percentage values). 

This equation allows for a comparison of how changes in the 

various parameters and components in the system impact the 

protection factor. By using this equation, operations have the ability to 

systematically achieve a desired protection factor in an enclosed cab 

to improve the air quality to safe levels and ultimately to protect their 

workers. 

TESTING 

The objective of this research was to determine the improvement 

in the air quality inside an enclosed cab of a surface drill with a unidirectional cab filtration and pressurization system. The sampling 
strategy was designed to provide a quantitative analysis of the change 

in the respirable dust concentration inside the cab relative to outside 

before (baseline) and after (post-testing) the new system was installed. 

The pre-testing analysis (baseline) was performed on the drill as 

originally found and operated without any changes. This drill was then 

retrofitted with a uni-directional flow filtration and pressurization system 

and three different series of identical post-testing analyses were 

performed. 

Four main sampling locations were used for this study: (1) inside 

the enclosed cab (top), (2) inside the enclosed cab (floor), (3) outside 

the enclosed cab, and (4) outside the enclosed cab on the far side 

pneumatic leveling cylinder. Obviously, the two inside sample 

locations were used to provide the respirable dust exposure levels that 

the drill operator would be exposed to while performing his drilling 

functions. The inside floor location was only used during post-testing 

with the new system. All dust sampling instruments were placed on 

sampling racks at each of the four sample locations. For the two inside 

locations, these sampling racks were positioned directly behind the drill 

operator. The top location was behind the operator’s head and the 

floor location was immediately above the recirculation pick-up point. 

Both sampling racks were composed of two gravimetric samplers and 

a pDR instantaneous respirable dust monitor. 

For the two outside sampling locations, the first location was 

attached to the drill cab directly under the rear window. The second 

outside location was located on the far side pneumatic leveling 

cylinder, which also had two gravimetric samplers and a pDR 

instantaneous dust monitor. The reason for two outside sample 

locations was to determine the respirable dust generated during drilling 

no matter which direction the wind was blowing. 

Two gravimetric samplers were located side by side on each of 

the sampling racks to provide respirable dust concentrations at each 

sample location. MSA Escort Elf (MSA, Pittsburgh, PA) sampling 

pumps were used 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 (ACGIH) 

for the metal/nonmetal industry [8]. 1

 

Mention of any company name or product does not constitute 

endorsement by the National Institute for Occupational Safety and 

Health. 

Dust samples were collected with 

a 10-mm Dorr-Oliver cyclone, which classifies the respirable portion of 

dust, then deposited on a 37-mm MSA filter. Filters were pre- and 

post-weighed to the nearest 0.001 mg on a microbalance in a 

temperature/humidity controlled weighing room. All sampling pumps 

were also post-calibrated to ensure that an acceptable flow rate of 1.7 

L/min (+0.015L/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. 

All instantaneous respirable dust measurements were taken with 

personal Data RAM (pDR) instruments (model 1200, Thermo Fisher 

Scientific Corp., Waltham, MA). This is a real-time aerosol monitor that 

measures the respirable dust concentration based upon the light 

scatter of particles in an internal sensing chamber. The respirable dust 

levels were recorded on an internal data logger every 10 seconds and 

were downloaded to a laptop computer at the end of each day of 

testing. All pDR units were operated in the passive mode in which dust 

particles flow through the sampling chamber without mechanical 

assistance. 

The average gravimetric respirable dust value at each sample 

location was used to determine a correction factor for the pDR 

instantaneous dust monitor at that same location. The average 

respirable dust concentration measured by the two gravimetric 

samplers was compared to the instantaneous respirable dust 

concentration as measured by the pDR monitor for the exact sampling 

time period. A correction factor was then calculated by dividing the 

pDR average concentration value into the gravimetric value. This 

calculated correction factor was then multiplied by all the individual 

 

1

dust measurements taken with the pDR device in an Excel 

spreadsheet. Using both types of respirable dust monitoring 

equipment provided a good profile of the dust concentrations 

throughout testing, as well as variations and changes in respirable dust 

concentrations throughout each day. 

All cab pressure measurements were taken with a TSI model 

8705 DP-CALC Micromanometer (TSI Inc., St. Paul, MN). These 

pressure measurements were taken every 30 seconds and recorded 

on the unit’s internal datalogger. 

Baseline testing was performed on this drill for three days, 

September 26-28, 2006. Upon the completion of this testing, the old 

air filtration system was removed and the new uni-directional filtration 

and pressurization unit was installed. Sy-Klone partnered with Red 

Dot Corporation in a design that incorporated the filtration and 

pressurization unit into a heating and air conditioning system. Figure 2 

shows a design drawing of this uni-directional filtration and 

pressurization system. 

Figure 2. Design of uni-directional cab filtration and pressurization 

system as installed on enclosed cab of pneumatic drill. 

The new system is composed of a make-up and a recirculation air 

component. The make-up air unit takes outside air and draws it 

through two parallel Sy-Klone Gideon technology powered air precleaners. Each Gideon pre-cleaner unit delivered approximately 40 

cfm of air, making the potential make-up air quantity total roughly 80 

cfm. These pre-cleaners use a centrifugal design to spin off larger 

dust particles (larger respirable range and above (> 5.0 micron)). After 

the Gideon pre-cleaners, the air passed through a canister filtering 

cartridge. Once passing through this filter, the clean air then entered a 

plenum where it was mixed with the recirculation air. At this point, the 

combined air then travels through the HVAC unit where it is 

conditioned for temperature control before being blown through air 

vents into the enclosed cab. 

The recirculation component was designed to incorporate a return

air duct, which captured the recirculation air near the floor of the cab, 

isolating it from the cab, and then transporting it to the filtering unit at 

the roof. This recirculated air then passed through a similar canister 

filtering arrangement as with the make-up air. This recirculated air 

then entered the same plenum with the make-up air, passed again into 
the HVAC unit, and then back into the cab. The approximate quantity 

of air recirculated was 300 cfm. 

For the HVAC system, the fan always operated when the drill was 

running. The drill operator manually adjusted a solid-state control with 

three fan speeds. The fan speed mainly affected operator comfort and 

had a very minor effect on cab pressurization. The unit also had a 

second control that was used to adjust the air temperature inside the 

cab to the operator’s comfort. The volume of this enclosed cab on this 

drill was measured to be slightly over 50 ft3

. 

The filter canisters used in this study had efficiency ratings of 

greater than 99 percent, (0.3 micron particles). Although intake and 

recirculation filters are normally at different efficiencies, (recirculation 

filters normally have lower efficiencies), Sy-Klone decided to use the 

same efficiency filters for both the intake and recirculation components 

in this study since it eliminates confusion when the filters need to be 

changed. Although these filters were rated at greater than 99 percent, 

there appeared to be a filter seating issue which caused dust to leak 

through the filter gasket material in some cases. This caused the filter 

to operate at a lower efficiency level. Before each post-testing 

analysis, an evaluation was performed in the shop area to ensure that 

the uni-directional pressurization and filtration system was working 

properly. Part of this analysis was to use ARTI HHPC-6 particle 

counting instruments (Hach Ultra Analytics, Grants Pass, OR) to 

determine dust particle counts inside and outside the enclosed cab. 

These particle count values were used to calculate the actual filter 

efficiency based upon this leakage component, which was determined 

to be 97 percent (0.3 micron particles). 

After the uni-directional filtration and pressurization system was 

installed and determined to be working properly, the post-evaluation 

was performed. Originally, two different post-evaluations with identical 

testing procedures as during baseline testing were scheduled to be 

performed. During the first post-evaluation performed in December 

2006, there was very limited drilling because of a lack of production 

needs. In addition, for the second day of testing on December 14, the 

drilling area was extremely wet, which minimized respirable dust 

concentrations. Because of this, it was decided to perform two 

additional post-testing surveys, which took place in March and August 

of 2007. 

RESULTS 

The main objective of this research was to determine the impact 

on the operator’s respirable dust exposure inside the drill cab by the 

implementation of a uni-directional pressurization and filtration system. 

Table 2 shows the average respirable dust concentrations obtained by 

averaging the two gravimetric dust units at each sampling location. 

The values in this table include the entire day of testing from the time 

of drilling the first hole to the completion of the last hole at the end of 

the shift. It must be noted, though, that this time frame included 

periods of downtime and breaks, which lowers the overall dust levels. 

Table 2. Respirable dust concentrations measured at inside and

outside enclosed cab sample locations during evaluation.

Outside Cab, mg/m3 3

Inside Cab, mg/m

rear left top floor

Baseline 

9/26/2006 0.84 1.06 0.43 n/a

9/27/2006 1.62 2.73 0.95 n/a

9/28/2006 1.68 0.33 0.54 n/a

Post-Test 1 

12/12/2006 

12/14/2006 

3.64 

0.54 

4.76 

1.23 

0.22 

0.06 

0.19

0.06

Post-Test 2 

3/13/2007 1.52 5.48 0.23 0.19

3/14/2007 0.70 6.12 0.23 0.18

3/15/2007 0.44 0.93 0.21 0.16

Post-Test 3 

8/21/2007 

8/22/2007 

1.01 

2.52 

2.27 

8.71 

0.09 

0.14 

0.14

0.13

The most important trend in this table is revealed by comparing 

the dust levels inside the enclosed cab between baseline and posttesting. During baseline testing, the average respirable dust 

concentration ranged from 0.43 to 0.95 mg/m3

 inside the cab for the 

three days of testing. This compares to the range of 0.06 to 0.23 

mg/m3 for the seven days of post-testing, using the identical sampling 

procedure (other than the second sample location being added near 

the floor). This definitely indicates the improvement to the air quality 

with the new filtration and pressurization unit. 

Figure 3 shows the calculated protection factor for each day of 

testing for both the baseline and post- evaluations. For the baseline 

testing, the protection factor averaged 1.8 for the three days of testing. 

This compares to an average PF of 17.7, 12.3, and 27.8, for the posttesting 1, 2, and 3, respectively, with an average of 18.3 for all the 

days of post-testing combined. 

Figure 3. Protection factors for the drill cab during baseline and posttesting analysis 

The most revealing trend in the PF results is the respirable dust 

levels inside the enclosed cab. The other component that determines 

the PF value is the outside dust concentration. Since the outside dust 

concentration can vary greatly, this can significantly impact the PF. 

This is probably most evident when comparing the values from the 

post-test 2 in March. The individual values for PF were 16.6, 16.7, and 

3.7, for March 13, 14, and 15, respectively. When considering the 

inside respirable dust concentrations (Table 2), the values were all 

very similar for all three days of testing. But when one considers the 

outside respirable dust levels, the values on March 15 were 

substantially lower than for the other two previous days of testing, 

which greatly affected the lowering of the PF value. Although it was 

the exact same system being used for all three days of testing, since 

the outside respirable dust concentration was so low on March 15, it 

could appear as though the system was not working as well, which 

was not the case. The purpose of this comparison is to indicate that 

the PF values can sometimes be misleading based on how substantial 

the differences are in outside dust concentrations. Obviously, higher 

outside dust concentrations provide higher PF levels. Conversely, it is 

impossible to achieve high PF values with very low outside 

concentrations. 

One critical area that has been determined both from the previous 

field studies and the laboratory study is the ability to seal the enclosed 

cab and achieve positive pressurization. This issue was closely 

monitored during all phases of this cooperative research effort. During 

baseline testing, the cab pressure was minimal. After the installation of 

the new uni-directional cab filtration and pressurization system and 

before any modifications were made to improve the integrity of the 

enclosed cab, the pressure was measured to be 0.045 inches w.g. 

After this measurement, a significant amount of time and effort was put 

into sealing around the door with new gasket foam, sealing around drill 

levers with a flexible foam material, and sealing all large cracks and 

holes using silicon caulking. After this was completed, the cab 

pressure increased to approximately 0.3 inches w.g. As more time and 
effort was taken to seal all minor cracks and around gauges in the 

control panel, the cab pressure was increased even further to 

approximately 0.45 inches w.g. This indicates the impact that can be 

achieved in relation to protection when the time and effort is spent to 

thoroughly seal a cab. 

During post-tests 1 and 2, cab pressures remained at levels near 

0.4 inch w.g., similar to levels achieved after the improvements were 

incorporated. During the five months from post-test 2 to post-test 3, 

the integrity of the cab started to deteriorate. For the two days of posttest 3, the cab pressure ranged from 0.1 to 0.15 inch w.g. Although 

this is still an acceptable cab pressure, this was a significant drop and 

indicates that time and effort should once again be dedicated to 

improving the cab integrity by replacing gaskets and sealing all holes 

and cracks. 

DISCUSSION 

When considering the results of this evaluation to reduce 

respirable dust concentrations inside the enclosed cab of the 

pneumatic drill with the use of a uni-directional cab filtration and 

pressurization system, it was decided to compare the actual values 

achieved during the post-testing to those derived from the 

mathematical equation formulated from the laboratory study. Once 

again, the equation derived during laboratory testing for the Protection 

Factor is: 

C Q + Q η PF o I R R = = C Q (1−η + lη ) + Q i I I I W

The following are the actual operating parameters of the new 

system: Intake Air Quantity, (QI ) = 80 cfm; Recirculation Air Quantity, 

(QR) = 300 cfm; Intake Filter Efficiency, (η ) = 0.97 (97 percent I

efficiency); Recirculation Filter Efficiency, (η ) = 0.97 (97 percent R

efficiency). 

To determine the Wind Quantity Infiltration, the follow equation is 

used: 

Wind Velocity Equivalent = 

(4000 √ Δpcab) fpm × 0.011364 mph/fpm 

@ Standard Air Temp and Pressure 

Using a positive pressure of 0.4 inch w.g., which was typical cab 

pressure during the testing for the Δpcab, the calculated wind velocity 

equivalent would be 28.7 mph. This means that a wind velocity of 

greater than this value would be necessary to blow dust-laden air from 

outside into the enclosed cab. Since the wind velocity remained below 

this value, we can assume that QW = 0. Based upon these values, 

the model provides a PF of 155. 

When this calculated value of 155 is compared to the actual 

measured average value of 18.3 for all of post-testing, it initially 

appears that the laboratory derived value does not closely approximate 

the actual value measured during field testing. But when considering 

the actual test conditions, there was a significant factor observed 

during testing that allowed dust-laden air to enter the drill cab. During 

post-test 2 in March 2007, 110-ft boreholes were being drilled. Since 

the drill steels are 25 ft long, five steels are required to drill a hole to 

this depth. Each time the drill operator would add an additional drill 

steel, he would open the cab door, lean out, and with his left arm 

manually guided the drill steel into the previous steel. The total time to 

add a new drill steel was approximately 3 minutes. Approximately 2 

minutes into the process, the drill operator opened the cab door to 

guide the next drill steel into place. The cab door was normally open 

somewhere between 30 and 45 seconds before being closed again. 

Since no drilling was occurring and no dust cloud was visible as the 

operator opened the cab door, the impact in respirable dust 

concentrations in the cab was thought to be very minor. However, 

when this issue was investigated by analyzing the pDR monitor results 

inside the enclosed cab, a substantial increase in respirable dust 

concentrations was noted during the periods when the door was open 

[9, 10]. 

The impact of this increase in respirable dust concentrations can 

be seen in Figure 4. The graph shows the average respirable dust 

concentrations inside the enclosed cab of the drill for all three days of 

testing for time periods when the cab door was closed versus open. 

The average concentration for all three days of testing was 0.09 mg/m3

with the cab door closed and 0.81 mg/m3

 with the cab door open. 

Despite there being no visible dust cloud during these time periods 

when the cab door was open, the respirable dust concentrations were 

9 times higher than when the door was closed. Further adding to the 

problem, once dust enters the enclosed cab and coats the inside 

surfaces, it also becomes a dust source to the drill operator at later 

times as it is disturbed and then becomes airborne. 

Figure 4. Average respirable dust concentration inside drill cab during 

periods with cab door closed and open for three days of testing. 

Although the problem of the cab door being open was not noticed 

until the second field evaluation because of the greater drill depth and 

constant changing of the drill steels, it was decided to analyze the 

potential impacts of this problem for all post-evaluations. Table 3 

presents the average respirable dust concentrations at both inside cab 

sample locations during periods when the cab door was closed and 

open. The information was determined from the instantaneous 

respirable dust concentrations from the pDR dust monitors. 

Table 3. Respirable dust concentrations at top and floor sample 

locations for 3 post-test evaluations for time periods when the cab door 

was closed and open.

Inside Cab, mg/m3

Cab Door Closed Cab Door Open 

 top floor top floor

Post-Test 1 

12/12/2006 0.3 0.2 0.2 0.2 

12/14/2006 0.0 0.0 0.1 0.0 

Post-Test 2 

3/13/2007 0.1 0.1 0.8 0.6 

3/14/2007 0.2 0.2 0.4 0.2 

3/15/2007 0.0 0.1 1.2 0.6 

Post-Test 3 

8/21/2007 0.0 0.1 0.9 0.9 

8/22/2007 0.1 0.1 0.6 0.5 

Averaging all the values for post-testing with the cab door open 

and closed, the average respirable dust concentration was 0.11mg/m3

and 0.52 mg/m3

, respectively. This represents an increase 4.7 times 

higher when the cab door was open during non-drilling activities, such 

as changing drill steels or to measure the depth of a borehole, than 
when compared to the actual time of drilling, when the cab door was 

closed. It also needs to be noted that the cab door was closed for a 

much greater percentage of the time as compared to when it was 

open. 

In order to achieve a protection factor of 18.3 for the actual test 

conditions, an intake air leakage l of 0.23 would need to exist. Based 

on the degree of dust leakage into the cab during the times that the 

door was open, an equivalent air leakage of 23 percent seems like a 

realistic value. Comparing the laboratory calculated value of 155 to the 

actual measured value of 18.3 stresses the importance of having a 

sealed cab with positive pressure, and also the importance of keeping 

the door and windows closed at all times other than when entering or 

exiting the cab. 

The purpose of this cooperative effort was to retrofit the enclosed 

cab of a pneumatic drill at a crushed limestone operation with a new 

uni-directional filtration and pressurization system and to quantify the 

reduction in respirable dust to the drill operator in the enclosed cab 

with this new system. Although NIOSH has been involved in a number 

of similar cooperative effort studies, this was the first formal field 

evaluation of a uni-directional design. Figure 5 shows the unidirectional design as compared to one with both intake and return in 

the roof of the cab. Since the clean air introduced into the enclosed 

cab could not short-circuit and was forced to travel down near the floor 

area, and because dust generated or leaked into the cab would not be 

drawn up over the operator’s breathing zone, it was believed that the 

uni-directional design was the optimum design in relation to the most 

effective in-cab airflow pattern. Although it is still believed to be the 

optimal design, this was not supported from actual field testing data 

described in this case study. The two sampling locations inside the 

enclosed cab did not show any significant difference between the top 

and near floor sample locations. In addition, in some limited and undocumented laboratory testing, it appears that the air discharged from 

the clean air vents at the roof of many systems may exit with enough 

air velocity to provide a complete mixing of the cab air and thus 

somewhat inhibit a uni-directional flow pattern. Although it is still 

believed that the uni-directional (roof to floor) flow pattern is the optimal 

design, study results suggest that the difference between the two 

different flow patterns may not be as significant as originally believed. 

Figure 5. Airflow pattern for typical design with intake and return at 

roof of cab and with uni-directional airflow design.

CONCLUSION 

This field study on a uni-directional cab filtration and 

pressurization system showed the system to be very effective at 

reducing respirable dust levels inside the enclosed cab of a pneumatic 

drill, thus providing the drill operator a more comfortable and safer 

work environment. During baseline testing, respirable dust 

concentrations inside the enclosed cab ranged from an average of 

0.43 to 0.95 mg/m3

 for the three days of testing. There were three 

post-test evaluations and small modifications and improvements were 

continually made during these evaluations to optimize the filtration 

effectiveness and provide the cleanest air quality to the drill operator 

inside the enclosed cab. In the third and final post-evaluation, the 

average respirable dust concentration ranged from 0.09 to 0.14 mg/m3

for the two days of testing. This comparison indicates the improved air 

quality inside the enclosed cab with the filtration system and ultimately, 

the improved protection to the drill operator. As seen in previous field 

studies as well as the laboratory study, there are a number of critical 

components for an effective filtration and pressurization system, also 

verified in this study. First, an effective filtration system needs to be 

composed of both an outside (make-up) air and a recirculation air 

component. It is also critical to establish and maintain cab integrity in 

order to achieve an acceptable level of positive cab pressurization. 

Without positive pressure in the cab, both the effectiveness of the 

system and the air quality are greatly compromised. The unidirectional flow pattern should also help to maximize system 

performance, although the result from this field analysis did not show a 

significant difference in this application. 

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