UNIDIRECTIONAL 

Clearing

the Air

UNIDIRECTIONAL 

FILTRATION/PRESSURIZATION

SYSTEM PROTECTS

EQUIPMENT OPERATORS

FROM DUST EXPOSURE

BY ANDREW CECALA AND JOHN ORGANISCAK

Miner’s exposure to dust remains 

a top concern among regulatory agencies, particularly at production 

facilities with high levels of silica. This 

respirable dust is present in almost all 

operations and is a direct cause of the 

deadly disease silicosis. Silica is one of 

the most abundant minerals on Earth 

and is a threat at all operations, especially granite and sand and gravel facilities.
The National Institute for Occupational Safety and 

Health continues to analyze silica’s threat. The institute’s 

research indicates that heavy-equipment operators have 

an increased risk to dust exposure, especially when 

drilling or breaking rock, even when working from an 

enclosed cab. However, there are methods to reduce an 

operator’s exposure further, according to recent NIOSH 

research, which is detailed in its report, Improving Air 

Quality in Enclosed Caps with Uni-Directional Filtration/ 

Pressurization System. 

NIOSH indicates that when surface mining equipment 

is new, the operator’s dust exposure is normally at acceptable levels. As the equipment ages and components such 

as gaskets and seals deteriorate, the level of protection 

can degrade to a point where it is no longer acceptable. 

However, the institute indicates that at least two critical 

factors necessary for an effective system. 

THE FIRST FACTOR is an effective cab-filtration system 

composed of both a recirculation and clean make-up air 

system. Approximately 75% of the air inside the enclosed 

cab should be recirculated through a high-quality filter 

and mixed with the make-up air before being returned. 

The make-up air is pulled from outside the cab and also 

should be filtered before being mixed with recirculated 

air. The make-up air creates the positive pressure inside 

the enclosed cab. 

The second critical factor for an effective system is cab 

integrity, which is necessary to achieve pressurization. It 

has been shown that installing new door gaskets and seals, 

along with plugging and sealing cracks and holes in the 

shell of the cab, has a major impact on achieving cab integrity, and thus, positive cab pressurization. 

The retrofit filtration and pressurization system evaluated in this study used a unidirectional design, which was a 

continuous 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. 

In theory, this conventional design has two shortcomings. First, some of the clean air discharged can immediately short circuit back into the recirculation intake vent 

without ever flowing through the enclosed cab. Second, 

dust-laden air from the inside walls of the cab, the floor, 

and the operator’s clothing is drawn up over the operator’s 

breathing zone as it travels into the recirculation pick-up 

location at the roof of the cab. A design, which may eliminate these flaws, brings all the filtered air brought in at the 

roof of the cab and expels all the recirculated air near the 

floor of the cab. (SEE FIGURE ONE) 

Figure 1 Dust-laden recirculation air

Outside filtered air

Clean enclosed cab air

Inside 

Recirculating 

Pick-up

Intake and Return 

at Roof Flow Design

Inside 

Recirculating 

Pick-up

Unidirectional

Flow Design

To test the unidirectional filtration system, NIOSH 

Physical Scientist Technician Jeanne Zimmer teamed with 

Sy-Klone International Vice President David Moredock and 

Vulcan Materials Co. Safety Representative Misty Hillis. 

The installation of a unidirectional filtration system can 

greatly reduce a worker’s exposure to harmful dust.
Vulcan reviewed its internal dust records to identify a 

cab and filtration system that was performing poorly, essentially a worst-case scenario. It chose a pneumatic rotary drill 

at one of its limestone operations near Birmingham, Ala.

The first task was to take baseline dust measurements 

on the drill before any changes or modifications were 

made to the unit. This drill still had the heating, ventilation 

and air conditioning unit (HVAC) that was provided by the 

original equipment manufacturer. 

The HVAC unit, as well as the cab’s gaskets and seals, 

deteriorated to a point where they needed to be replaced 

or refurbished. Dust levels remained within the acceptable 

federal and state standard. After baseline testing was completed, the old unit was removed and a new Red Dot 9777 

HVAC unit with a Sy-Klone International unidirectional filtration and pressurization system was installed. 

Figure 2 shows the components of this unidirectional 

filtration and pressurization system. 

Figure 2

Dust-laden recirculation air

Outside make-up air

Filtered air

Dust

HVAC Unit

Operator 

 cab

Powered air precleaners

Make-up air 

 filters

Recirculation air filter

Recirculation pick-up

Baseline testing was 

performed on this drill for three days. Upon the completion 

of this testing, the old air filtration system was removed 

and the new unidirectional filtration and pressurization 

unit was installed. 

The new retrofit 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 pre-cleaners. 

Each pre-cleaner unit delivers at least 40 cfm of air, making the potential make-up air quantity a minimum of 80 

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

dust particles larger than 5 micron. After the pre-cleaners, 

the air passes through a canister filtering cartridge before 

entering a plenum where it is mixed with the recirculation 

air. This combined air then travels through the HVAC unit 

where it is conditioned for temperature before being blown 

through air vents into the enclosed cab.

The recirculation component was designed to incorporate a return-air duct-—which captures the recirculated 

air near the floor of the cab—isolate it from the cab and 

transport it to the filtering unit. This recirculated air also 

passed through a similar canister filtering arrangement as 

with the make-up air. The actual filter efficiency for both 

the recirculation air and the make-up was determined to 

be greater than 99% for particles 0.3 micron and larger, 

as determined by field testing. This recirculated air then 

entered the same plenum with the make-up air. The 

approximate quantity of air recirculated was 300 cfm.

After the retrofit unidirectional filtration and pressurization system was installed and determined to be working 

properly, the postevaluation was performed. Three separate post-tests were performed to evaluate the retrofitted 

cab over eight months.

The postevaluations mirrored the dust analysis performed during baseline testing. Tests were taken near the top 

and the floor of the enclosed cab, on the outside of the cab at 

the far side of the pneumatic leveling cylinder and near the 

rearview mirror. The two inside sample locations were used 

to provide the respirable dust levels that the drill operator 

would be exposed to while performing drilling functions.The 

sampling racks were composed of two gravimetric samplers 

and a pDR instantaneous respirable dust monitor.

Table 1

Respirable dust concentrations measured at sampling 

locations inside and outside the enclosed cab. The 

measurements are micrograms per cubic meter.

Outside Cab Inside Cab

rear left top floor

Baseline

Day 1 0.84 1.06 0.43 n/a

Day 2 1.62 2.73 0.95 n/a

Day 3 1.68 0.33 0.54 n/a

Posttest 1

Day 1 3.64 4.76 0.22 0.19

Day 2 0.54 1.23 0.06 0.06

Posttest 2

Day 1 1.52 5.48 0.23 0.19

Day 2 0.70 6.12 0.23 0.18

Day 3 0.44 0.93 0.21 0.16

Posttest 3

Day 1 1.01 2.27 0.09 0.14

Day 2 2.52 8.71 0.14 0.13

Table 1 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 drilling the first hole to the 

completion of the last hole at the end of the shift. It must 

also be noted that this time frame included periods of downtime and breaks, which lowers the overall dust levels.

These drill operators are busy installing a new unidirectional filtration and pressurization system.

PROBABLY THE MOST important aspect to consider when 

evaluating the effectiveness of the new retrofit system is 

to compare 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 

micrograms per cubic meter inside the cab for the three 

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

cubic micrograms for the seven days of posttesting at the 

top sampling location in the cab. These results illustrate 

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 postevaluations. 

50

40

30

20

10

 Baseline Test Post Test 1 Post Test 2 Post Test 3

20.4

14.9

2.2 2.3 1.9

16.6 16.7

3.7

13.7

41.8

1 2 3 1 2 1 2 3 1 2

DAY

Figure 3 Protection Factor

Protection factor is to quantify cab performance; it is a ratio 

of the outside dust concentration divided by the inside concentration. The larger the protection factor value, the greater 

protection to the operator inside the enclosed cab. For baseline testing, the protection factor averaged 2.1 for the three 

days of testing. This compares to an average Protection 

Factor of 17.7, 12.3, and 27.8 for the first three posttestings 

with an average of 18.3 for all the days of posttesting.

When comparing the roof to floor sample locations during posttesting, as shown in Table 1, there appears to be 

very little difference between these two sample locations. 

This indicates a uniform airflow throughout the cab that prevented elevated dust levels from building up in any region 

of the cab. One potential reason for the lack of a significant 

difference could have been due to the amount of times that 

the drill operator opened the door of the enclosed cab. Each 

time the door was opened, dust to enter the cab, equally 

impacting dust levels at the roof and at the floor of the cab.

This suggests the need for additional technology and 

methods to limit an operator’s need to open the door to dust. 

Yet, these results show that harmful dust can be removed, 

assuring workers longer and healthier lives.

ACKNOWLEDGEMENT: Jeanne Zimmer, physical scientist technician with NIOSH; David Moredock, Vice-President with Sy-Klone 

International; and Misty Hillis, Senior Health and safety representative with Vulcan Materials Company, contributed to this article.