Using Ventilation Control Technology to Reduce Respirable Dust 

Exposures at U.S. Metal/Nonmetal Mining Operations 

Cecala A B 1, Zimmer J A 1, Colinet J F 1, Timko R J 1, Chekan G J 1, and Pollock D E 1

ABSTRACT 

It has long been known that ventilation is a costeffective method to lower respirable dust concentrations in all 

types of mining applications. The National Institute for 

Occupational Safety and Health has been working on a 

number of different research techniques that use ventilation as 

a critical component to lower respirable dust levels at surface 

and metal/nonmetal operations in the United States. This 

article presents five such research efforts. The first research 

area discusses how improving the ventilation flow patterns in 

an iron ore mill facility lowered respirable dust concentrations 

by 31 pct throughout the primary grinding area. The second 

research area examines how respirable dust levels were 

lowered at a dimensional stone shop by using air cleaning 

units suspended from the ceiling. A third area discusses how 

improvements in ventilation at underground limestone mines 

lowered respirable dust concentrations. The fourth area 

examines how improvements in dust filtration and 

pressurization systems significantly impacted the air quality in 

enclosed cabs of surface mining equipment. The last area 

presents a newly developed clothes cleaning technique. This 

technique uses an air spray manifold to blow dust from a 

worker’s clothing in an enclosed booth, which confines the 

dust for capture and removal by a baghouse dust collector. 

These research areas represent an array of different control 

technologies to lower respirable dust concentrations. 

Ventilation is an integral part of these control technologies. 

INTRODUCTION

The effective use of ventilation has long been known as a 

critical component for the mining industry. Fundamentally, 

ventilation is aimed at providing sufficient airflow throughout 

underground mining operations. Providing clean air to miners 

while diluting and removing contaminants and gases from 

working faces in underground mines is a critical function of 

ventilation; however, ventilation can also provide a vast array 

of other useful functions in the mining industry. 

Reducing respirable dust exposures to workers is a constant 

goal throughout the mining industry, and various applications 

of ventilation techniques can be a major contributor toward 

reaching this goal. When engineers and health and safety 

specialists investigate methods and techniques to lower dust 

levels in surface and metal/nonmetal operations, such control 

measures as water sprays, surfactants, dust collection systems, 

 

1

 National Institute for Occupational Safety and Health 

(NIOSH), Pittsburgh Research Laboratory, PO Box 18070, 

Pittsburgh, Pennsylvania 15236, USA 

cyclones, and electrostatic precipitators usually come to mind. 

Ventilation should also be viewed as a useful technique to 

improve air quality and lower respirable dust exposure in these 

types of operations. 

One benefit with the use of ventilation is that many times it 

encompasses an entire structure and thus has the potential to 

lower exposures to numerous workers. One such example was 

a study performed by the Bureau of Mines a number of years 

ago regarding the use of total mill ventilation systems, 

emphasizing the need for an effective ventilation flow pattern 

in mineral processing operations (Cecala, et al., 1993). The 

goal of a total mill ventilation system is to bring clean makeup 

air in at the base of a structure and draw this air up through the 

building, clearing major dust areas as air flows towards the top 

of the structure. This dust-laden air is then exhausted from the 

top of the walls or at the roof of the structure. 

During this research study, average respirable dust levels were 

reduced by 40 and 64 pct at two field evaluation sites. These 

significant reductions were achieved throughout the entire 

structure with only a relatively minor amount of ventilation air 

added in reference to the size of the structure. The targeted air 

quantity necessary for this total mill ventilation system is at 

least 10 air changes per hour. This technique is believed to be 

the most cost-effective method to lower respirable dust 

concentrations throughout an entire mineral processing 

structure, and it demonstrates the impact that ventilation can 

have at these types of facilities. 

This manuscript will discuss five different research studies 

conducted by the National Institute for Occupational Safety 

and Health (NIOSH) which use ventilation in some 

application to lower workers’ respirable dust exposures at 

surface and metal/nonmetal mining operations. As federal 

standards for health issues continue to require lower levels in 

the future, engineers and health and safety specialists must 

investigate novel approaches to improve air quality. The goal 

of the research discussed in this manuscript is to provide an 

array of applications where ventilation can be used to reduce 

respirable dust exposures. 

FIVE RESEARCH STUDIES USING VENTILATION 

TO LOWER RESPIRABLE DUST LEVELS 

Improving Ventilation at an Iron Ore Processing 

Plant 

Through a cooperative work effort between the Tilden Mining 

Company L.C., the United Steelworkers of America, and 

NIOSH, a number of studies have been performed over the 
past three years in an effort to lower respirable dust levels at 

these processing facilities. We will limit our discussion in this 

research area to an evaluation performed in March 2004 

examining the impact of ventilation changes on the air quality 

at the Tilden iron ore facility. This facility was located near 

Ispheming, Michigan, and was a 26,600-m2

 (940,000-ft2) 

structure with an internal air volume of 1,273,500 m3 (45 

million cubic feet). Initially, a number of different approaches 

and techniques were investigated, but as time progressed, it 

became evident that the most substantial impact on lowering 

respirable dust levels could be achieved through 

improvements in the ventilation flow pattern within a 

structure. This was achieved by changing the quantity of air 

that entered and was exhausted from the structure. Ventilation 

at this operation was performed by using roof-mounted fans in 

either the intake or exhaust mode, which is common 

throughout the iron ore industry. 

During the first day of testing, the operation’s normal 

ventilation setup was evaluated. Figure 1 shows a summary of 

this ventilation setup with the exhaust fans indicated by arrows 

exiting and intake fans indicated as arrows entering the 

structure. To evaluate this original ventilation design, the 

structure was broken down into three distinct areas: primary 

grinding (northern-most portion), secondary grinding with 

pebble mills, and filtering and flotation (southern-most 

portion). Respirable dust samplers were located in these three 

areas, with the greatest focus being on the primary grinding 

mills. This original ventilation setup included twelve exhaust 

fans in the primary grinding area, eight exhaust fans and three 

intake fans in the secondary grinding area, and nineteen 

exhaust fans in the filter and flotation area. The only other 

mechanical ventilation provided was from two intake heater 

fans located at the base of the facility, one located on the east 

and the other on the west side of the building. The volume of 

air being exhausted from the structure was 54,700 m3

/min 

(1,930,500 cfm), as compared to 17,000 m3

/min (600,000 cfm) 

being brought into the structure. These ventilation air 

volumes were calculated based upon the values provided by 

the facility and the assumption that all the fans were operating 

at their rated output. The two intake heater fans accounted for 

12,800 m3

/min (451,000 cfm), or 75 pct of the total intake 

airflow. Dust sampling under this ventilation setup was 

performed for approximately 20 hours of testing. 

The imbalance between the intake and exhaust air volumes 

with this ventilation setup was substantial; therefore it was 

decided to modify the ventilation to a more balanced design. 

It should to be noted that although the goal was to lower 

respirable dust concentrations throughout the entire facility, 

the main priority was to lower respirable dust concentrations 

in the primary grinding area. Dust levels in the primary 

grinding area were higher than in the other areas because of 

the amount of dust liberated from the numerous conveyor lines 

feeding the primary grinding mills, as well as the amount of 

dust being liberated from the mills themselves. In an effort to 

focus on this area, a ventilation change was initiated as shown 

in Figure 2. The two intake heater fans were again used to 

bring a significant portion of intake air into the facility. For 

this modified design, there were eleven exhaust fans operating 

in the primary grinding area, twelve intake fans in the 

secondary grinding area, and eight exhaust fans operating in 

the filtering and flotation area. The volume of air being 

exhausted from the structure was approximately 26,600 

m3

/min (940,500 cfm), with approximately 29,600 m3/min 

(1,045,000 cfm) brought into the structure through the roof 

intake fans and two heater fans. This ventilation design was 

again tested for approximately 20 hours. 

During this phase of testing, a smoke flare was released into 

one of the intake fans over the pebble mill. Since properly 

operating intake fans should move an air parcel 10 fan 

diameters before being reduced to 10 pct of the fan’s air 

velocity, the roof-mounted fans should introduce a parcel of 

air into the lower parts of the building (Industrial Ventilation, 

2001). These roof-mounted fans are all 1.2 m (4 ft) in 

diameter and should introduce a slug of air at least 12 to 15 m 

(40 to 50 ft) into the structure. Since this testing was 

performed during the winter months, introducing cold outside 

air into the structure should also assist the air parcel’s ability 

to flow down into the lower levels of the structure. From 

visual observations during this smoke flare release, it appeared 

that the intake fan chosen for this testing was not operating at 

the calculated airflow. The smoke remained within the top 

one-quarter of the structure and tended to drift towards the 

southern portion of the building (toward the flotation and 

filtration area). As test personnel walked through the 

secondary grinding area, it was very difficult to feel the intake 

air being delivered into the structure by the intake fans. Since 

the walkway was only 9.1 m (30 ft) below the intake fans, 

airflow should have been perceivable at this level. 

 

Table 1 

Average respirable dust concentrations in mg/m3 for three ventilation 

setups. 

Location Setup #1 

Day 

Setup #1 

Night 

Setup #2 

Day 

Setup #2 

Night 

Setup #3 

Day 

Primary #2 1.06 1.21 0.66 0.38 0.44 

Primary #4 0.72 0.70 0.59 0.59 0.45 

Primary #6 1.15 1.34 1.37 1.49 0.98 

Primary #8 1.23 1.34 1.18 1.23 0.86 

Primary #10 - 0.97 1.05 0.91 0.72 

Primary #12 0.67 0.72 0.79 0.88 0.75 

Average 0.97 1.05 0.94 0.91 0.70 

Pebble #1 0.32 0.38 0.38 0.31 0.29 

Pebble #3 0.42 0.42 0.47 0.51 0.42 

Pebble #6 0.34 0.35 0.48 0.50 0.35 

Pebble #9 0.49 0.50 0.51 0.56 0.49 

Pebble #12 0.38 0.41 0.36 0.43 0.38 

Average 0.39 0.41 0.44 0.46 0.38 

Floatation #3 0.25 0.32 0.35 0.34 0.24 

Floatation #9 0.27 0.34 0.45 0.40 0.31 

Average 0.26 0.33 0.40 0.37 0.27 

Since the intake air appeared to be drifting towards the 

flotation and filtration area, it was decided to modify the 

ventilation setup again in an attempt to provide a greater 

quantity of the intake air moving in the northern direction 

towards the primary grinding mills. Figure 3 indicates the 

second modification to the ventilation design which was 
evaluated for approximately 8 hrs during the last day of 

testing. This ventilation setup was composed of twenty-four 

exhaust fans and twelve intake fans, along with two base 

heater intake fans. The volume of air exhausted from the 

structure was approximately 32,000 m3/min (1,128,000 cfm), 

with 29,600 m3

/min (1,045,000 cfm) being brought into the 

structure through the roof intake fans and two base heater fans. 

The amount of intake air was identical to the previous test, but 

the exhaust was changed in an attempt to decrease the flow 

toward the flotation and filtration area while increasing the 

airflow to the primary grinding mill area. 

Table 1 shows the average respirable dust concentration as 

determined by the instantaneous respirable dust monitors used 

in this study and provides a comparison of the three different 

ventilation designs tested. Obviously, the last ventilation 

design was the most effective at lowering respirable dust 

levels, particularly in the primary grinding area, which was the 

main area of emphasis. When compared to the first two 

ventilation designs, this ventilation design lowered respirable 

dust concentrations in the primary grinding area by 31 and 25 

pct, respectively. This is a very significant reduction when 

one considers it was achieved by using the existing fans and 

simply varying the intake and exhaust flow patterns. 

Respirable dust levels were also slightly lower in the pebble 

mill and the flotation and filtration areas with the last 

ventilation setup. This study indicates the impact that 

ventilation can have to lower respirable dust levels throughout 

a large-volume iron ore processing structure. 

 

Air Cleaner Units at Dimensional Stone Operation 

This study evaluated small air cleaning units suspended from 

the ceiling at a dimensional stone operation. These units were 

installed to reduce respirable dust levels to workers during the 

winter months. Dimensional stone operations sell a variety of 

products for decorative applications, as well as a vast array of 

tabletops and countertops. After the stone is removed from 

quarries in large blocks and slabs, it is cut using various sized 

saws to fabricate the numerous products. Although the actual 

stone type varies from operation to operation, much of this 

dimensional stone contains silica-bearing material, which 

makes the dust generated during the cutting and shaping even 

more hazardous to the workers. During warm temperatures, 

the shop doors where this stone cutting takes place are open to 

the outside atmosphere, which provides natural ventilation to 

dilute the dust generated. During cold weather, these doors 

are closed to maintain acceptable work temperatures for 

employees and to keep water lines from freezing. This causes 

dust levels to significantly increase. 

To lower respirable dust levels during the winter season, one 

operation installed a number of small air cleaning units that 

were suspended from the ceiling. NIOSH was evaluating 

ways to lower respirable dust levels in these types of 

operations and one part of this study was to determine the 

effectiveness of these small air cleaner units. The stone being 

processed at this operation had an 85-90 pct silica content. 

The air cleaning units were positioned in and around one 

particular processing shop. These units were designed to take 

57 m3

/min (2000 cfm) of dust-laden air from within the 

facility, filter it, then return it to the shop area. These units 

were rectangular shaped boxes that housed two filters, a prefilter and final (high efficiency) filter. 

On the first day of testing at this operation, respirable dust 

levels were evaluated in the process shop area with the 

existing filters in the air cleaner units. Plant management 

stated that the filters on the cleaning units had been in use for 

three to four months. During this testing, air velocity 

measurements were taken with a vane anemometer, indicating 

that only a minimal amount of airflow was being cleaned 

through these units. The air volume in each of the six units 

was below 14 m3/min (500 cfm) and it appeared that the units 

were not able to handle the additional pressure created by the 

dust loading on the filters. Figure 4 shows the six different 

dust sampling locations along with the location of the six air 

cleaner units. These sampling locations provided a profile of 

respirable dust levels throughout the entire processing shop 

area. This figure also indicates the location of the shop 

machinery: a computerized saw, a polisher machine, and a 

chop saw. 

After this initial day of testing, new pre-filters and final filters 

were installed into six air cleaning units. The objective was to 

determine the maximum air cleaning potential of the units 

with new filters and to compare the results to those achieved 

with the filters that had three to four months of use. Once all 

new filters were installed, the identical dust measurements 

were again performed for the second day of testing. Airflow 

measurements were then taken on a number of air cleaning 

units to determine the airflow quantity with the new pre-filters 

and final filters. Periodically throughout the day, additional 

airflow measurements were taken to determine relative 

changes. From these measurements, it was determined that as 

the filters loaded with dust and the pressure differential across 

the filters increased, there was a drastic decrease in the air 

quantity flowing through the unit. 

Figure 5 indicates the average respirable dust concentration 

obtained with the instantaneous dust monitors at the six 

sample locations for this study. With the new filters, there 

was a 48 pct reduction in respirable dust levels when 

compared to the filters with three to four months of use. 

Based on the results of this study, it appears that the increased 

airflow through the air cleaning units was the major factor in 

reducing respirable dust concentrations in the processing shop. 

In an effort to verify this, a series of airflow measurements 

were taken on the discharge port of two of the air cleaner 

units. For the first day of testing with the used filters, the air 

volume was determined to be 8.5 m3

/min (300 cfm). When 

the used filters were removed, the unit delivered 73 m3

/min 

(2570 cfm). After new filters were installed, the airflow was 

reduced to 55 m3

/min (1950 cfm), which is within 2.5 pct of 

the rated output of the unit. After a full day of testing with the 

new filters (day 2), a 20 pct loss in airflow was determined 

with a resulting air volume of 44 m3

/min (1550 cfm). 
Figure 1 - Baseline or Normal Ventilation Setup. 

Figure 2 - Ventilation Setup #2.

Figure 3 - Ventilation Setup #3.
At the end of the second day of testing with the new air filters 

(day 3), there was a 57 pct loss in airflow with a resulting air 

volume of 24.8 m3

/min (840 cfm). After two more days, or the 

fourth day of using the new filters, the air volume was reduced

by 77 pct to an air volume of 12.7 m3/min (448 cfm). The

manufacturer of the air cleaner unit was contacted and 

informed of the problem of reduced air flow through the unit as 

the filters loaded with dust. A modification to the unit is being

considered at the present time. An improved blower that is 

able to handle the increased pressure without the drastic falloff 

in air flow is also currently being evaluated.

Although these results were disappointing based on the rapid 

decline in air volume with filter use, this testing indicated the 

potential these air cleaning units could have to improve the air 

quality in dimensional stone processing shops, especially

through the winter months when the shops are closed. As 

previously stated, there was a 48 pct reduction in respirable 

dust concentrations when new filters were used in the air 

cleaning units and they were able to clean and deliver their 

rated volume of air. The individuals working in the processing

shop indicated that they were able to see a noticeable change in 

the air quality at this level. This work also indicated how some

technology ultimately does not supply the intended level of

protection. The dimensional stone operation had good

intentions when it purchased these units to improve the air

quality in its processing shop. The operation was unaware that 

the air cleaner unit performance decreased drastically after 

minimal dust loading and only provided a minimal 

improvement to the air quality for a relatively short time

period. These air cleaner units were not designed to handle the 

level of dust encountered in these types of dimensional stone 

operations. In working with the manufacturer, we are hoping

that the units can be modified with an improved blower that is 

able to handle the pressure drop from the dust loading on the

filters and will provide a more reasonable time period before 

the filters need to be cleaned or changed. 

Evaluating Ventilating Air Movement in Underground 

Limestone Mines

Underground limestone mines typically have large entries,

ranging in cross-sectional areas from 46 to 223 m2

 (500 to 2400 

ft2

). Air quantities flowing through these large-opening entries 

result in low velocities, typically less than 0.13 m/s (25 fpm). 

Because of the low air velocities, the air may become stratified 

and readily affected by seasonal patterns in natural ventilation.

In addition, limestone mines may have minimal or no 

mechanical ventilation and often do not have stopping lines to 

control air movement throughout the mine. Consequently, it

can be difficult to measure air velocities and define airflow 

patterns by using conventional airflow measuring equipment.

Figure 4 - Air cleaner units and dust sampling locations. 
Figure 5 - Respirable dust concentrations in mg/m3 for filters that had been in operation for 3-4 months (day 1) and new filters 

(day 2) on six air cleaning units. 

A research effort was performed to evaluate air movement and 

flow patterns in underground limestone mines by measuring

respirable dust levels generated from production shots.

Production shots generate a considerable volume of respirable

dust and serve as a distinctive point source of dust. In this 

study, respirable dust was monitored with an instrument that

uses light-scattering technology to log dust concentrations in

real-time. Respirable dust (< 10 microns) that becomes

airborne after the shot can remain entrained in the air at very

low velocities. This “dust cloud” moves with the general

airflow pattern in the mine and the dust monitor records the 

dust concentration, generating a time-dependent curve. The 

shape of the curve is directly related to the distance of the 

monitor from the production shot. As the distance between 

shot and dust monitor increases, the time interval for 

measurable dust increases but the peak concentration becomes

less. Multiple shots in the same location creates a dust cloud 

that is easily distinguishable. This sampling also identified the 

length of time that was needed for the ventilation in each mine

to clear the dust from the mines after the shots. The original

study involved a number of different limestone mines; however

only one example will be presented in this report to show the 

applicability of this technique to evaluate ventilation flow 

patterns in large-opening mines (Chekan, et al., 2004). 

Figure 6 shows the mine workings and ventilation setup. The 

mine originally relied solely on natural ventilation without the 

assistance of booster fans. Airflow to the faces, especially in 

the northern and southern areas of the mine, was minimal and

air velocity and direction were difficult to characterize with 

Figure 6 - Mine workings for case study, showing 

location of dust monitors, curtain line, and fans. 
anemometer or smoke tube measurements. Dust from shots 

tended to migrate through the mine with little directional flow. 

 

Mine management eventually began a more systematic 

approach to mining, as shown in the future developments 

projections. These projections required that some extensive 

rehabilitation work be performed, which included major 

upgrades in the ventilation system to improve airflow at the 

faces. This ventilation upgrade included: 1) two Hartzell Series 

10S, 3.66-m (12-ft) dia., high-volume, low-pressure propeller 

fans each rated at 212 m3/s (450,000 cfm), which established a 

directional airflow from the southern portals to the fan; 2) a 

Spendrup Model 1000-50-26H , 1.37-m (4.5-ft) diameter axial 

vane booster fan rated at 92.7 kW (125 hp) and 42.4 m3

/s 

(90,000 cfm), which provided localized ventilation to the 

working faces; and 3) the installation of approximately 50 

curtain stoppings on the southern and eastern side of the mine 

to provide improved airflow to the active faces. 

Due to the vast workings and the need to provide sufficient air 

to the new working faces, the two propeller fans were installed 

inside the mine, rather than at the portal (typical location). On 

the exhaust side of the fan was a large abandoned benched area. 

The plan was to dump the exhaust air into this area where the 

dust would dilute and eventually exit the mine at portals to the 

west. 

The study was conducted after the new ventilation system 

installation had been completed. The dust monitoring locations 

chosen for this study were based on smoke tube measurement 

testing, resulting in locations near faces and also spaced to 

cover areas of interest where the airflow was consistent. 

Table 2 summarizes the results of the dust surveys for the two 

days of sampling. Shown in the table are the: (1) number of 

shots for each sampling day, (2) station number, (3) station 

distance from the shot, (4) peak and average concentrations, (5) 

elapsed time needed for the dust cloud to arrive, the time of 

peak concentration, and the time to the end of the dust cloud. 

The three shots on day 1 were designed to monitor the 

movement of dust from nearby faces located east of the fan and 

determine if the ventilation was sufficient to prevent the dust 

from migrating to other areas of the mine. One shot was 

located near station 6 and two shots were located near station 5. 

The location and time spacing of shots (the first shot near 

station 6 was four minutes before the two simultaneous shots 

near station 5) provided two different signatures on the dust 

monitors. The dust monitors showed that dust from the first 

shot migrated past station 6 to station 8, then to station 9 at the 

fan. Dust from the next two simultaneous shots migrated past 

station 5, then to station 9 at the fan. The dust monitor at 

station 9 (fan) showed only one peak value, indicating that the 

dust from the different shot locations merged and reached the 

fan at approximately the same time. 

 

Table 2 

Estimated velocity of dust cloud for case study. 

Day 1 – 3 Shots 

Station Approximate distance 

from shot 

(m) 

Peak conc 

mg/m3

Average conc

mg/m3

Arrival time

(min) 

Peak time 

(min) 

End time 

(min) 

Estimated average 

velocity 

(m/s) 

6 61.0 7.9 1.4 1.2 6.8 152.8 0.34 

8 260.0 3.2 1.2 12.8 25.0 211.7 0.18 

5 198.0 22.2 3.4 4.5 8.0 66.2 0.31 

9 – Fan 365.8 13.2 3.5 10.3 18.0 74.2 0.34 

10 579.1 4.0 1.8 13.3 27.0 198.5 0.38 

1 No dust recorded from shots 

2 No dust recorded from shots 

3 No dust recorded from shots 

4 No dust recorded from shots 

7 No dust recorded from shots 

Day 2 – 3 Shots 

2 122.0 18.8 5.0 5.2 24.3 284.0 0.16 

3 487.7 1.9 1.1 30.5 58.8 237.2 0.15 

4 731.5 1.2 0.8 36.8 61.0 292.0 0.19 

5 1188.7 1.2 0.6 55.2 75.7 325.7 0.23 

9 – Fan 1371.6 1.1 0.7 60.0 85.5 328.7 0.24 

10 1585.0 0.7 0.4 72.8 119.3 354.7 0.23 

1 No dust recorded from shots 

6 No dust recorded from shots 

7 No dust recorded from shots 

8 No dust recorded from shots 
Since dust was recorded at station 8, some dust was obviously 

passing the fans. However, the lower velocity of 0.18 m/s 

(34.7 fpm), lower peak value, and longer duration of the dust 

cloud in comparison to the other stations all indicate that most 

of the dust from all three shots made a direct path to the fans. 

Station 10, located approximately 152 m (550 ft) from the fan 

on the exhaust side, showed that velocities remained high but 

dilution was occurring, as evidenced by the lower peak and 

average concentrations and the longer duration of the dust 

cloud. No dust was recorded at stations 1, 2, 3, 4, or 7, 

showing that the established ventilation was sufficient to keep 

the dust from migrating to the northern and southern areas of 

the mine. 

Day 2 was designed to evaluate airflow patterns from distant 

face locations in the southern section of the mine. There were 

three shots that were approximately 1432 m (4700 ft) from the 

main fan. All three faces were shot simultaneously in the same 

general area of the mine. The results from this test were 

notably different from day 1 results, but even from this 

distance, the fans did show a distinguishable airflow pattern. 

As expected, station 2, located 122m (400 ft) from the shots, 

showed the highest peak and average concentrations (18.8 and 

5.0 mg/m3

, respectively ), but the duration of the dust cloud 

was long (278 minutes), indicating that the nearby booster fan 

was moving air to these headings but not effectively enough to 

remove the dust. The leading edge of the dust cloud then 

moved in sequence to stations 3, 4, and 5, located in the general 

air circuit, and then moved on to station 9 at the main fans. 

The peak and average values at station 2 were high in 

comparison to those at the remaining stations. Stations 3, 4, 5, 

and 9 all showed a similar pattern of long duration and low 

average and peak concentrations of the dust cloud. Even at 

these low values, the signature of the dust cloud was evident at 

all of the stations. These low values may indicate that a 

majority of the dust took a more direct path to the main fans; 

but since the arrival times at each successive station are in 

sequence, the data more likely suggest that the dust cloud was 

being diluting in the southeastern area of the mine and 

gradually migrated to the main fans. 

 The calculated velocities at each station were consistent, 

increasing slightly as the dust cloud approached the fan. The 

velocity on the exhaust side of the fan (station 10) was also 

consistent. No dust was recorded at station 1, indicating that 

no dust was migrating back towards the intake portals. Also, 

no dust was recorded at stations 6, 7, or 8, showing that the 

dust was not migrating past the fans to the northern areas of the 

mine. 

The study showed that the new ventilation system provided an 

improved directional airflow in the overall mine air circuit. 

Day 1 of sampling gave higher velocities than day 2, showing a 

relation of airflow with proximity to the fans. The average 

velocity of the dust cloud from the two days of sampling was 

0.25m/s (48.7 fpm). The retention time or the length of time it 

took the dust cloud to clear the fans was estimated by using the 

“end time” at station 9. This gave dust retention times on day 1 

and day 2 of 1.2 hrs and 5.5 hrs, respectively. 

Filtration and Pressurization Systems for Enclosed 

Cabs 

Many different types of mobile equipment used in surface 

mining utilize enclosed cabs to protect equipment operators. 

The overburden removal process is extremely dusty and can 

cause excessive exposure to respirable dust, especially 

crystralline silica. After equipment is used for years, many 

components of the enclosed cab deteriorate and the cab 

effectiveness at protecting the worker from dust is greatly 

reduced. In an effort to retrofit older enclosed cabs to provide 

acceptable air quality to the equipment operator, an effective 

retrofitted filtration and pressurization system is essential. 

NIOSH believes that there are two key components necessary 

for an enclosed cab to be effective from an air quality 

standpoint: 1) a well-designed filtration system, and 2) cab 

integrity. A 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 conditioned 

(heating or air conditioning) to the cab operator’s comfort 

without major air changes that significantly impact the size, 

capability requirements, and ultimately the cost for 

conditioning the cab air. Another consideration is to have 

separate fans for makeup and recirculating air. 

 

A major component in an effective filtration system is to have 

the makeup air positively pressurize the enclosed cab. This 

results in any system leakage to traveling from the 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 

makeup air should optimally be located on the cab the furthest 

practical distance from the dust sources (Organiscak and Page, 

2001). This reduces the amount of loading on the filters and 

increases the amount of time between cleaning or replacement. 

Finally, the discharge for makeup air into an enclosed cab 

should be located high in the enclosure, preferably at the roof. 

This allows the clean air to be blown down over the equipment 

operator’s breathing zone without becoming contaminated by 

in-cab dust sources. 

 

One last design criterion is a top-down approach to the clean 

air flow pattern. In most current systems, the intake and 

discharge for the recirculation air is located in the roof unit. 

Although this is acceptable, the most beneficial design is to 

blow in clean air from the roof and draw the recirculated air out 

from the bottom of the cab. This allows the dust-laden air to be 

drawn out of the cab near the worker’s feet and away from the 

breathing zone. Discharging clean air low in the cab can 

entrain a significant amount of dust from soiled work clothes, 

boots, and a dirty floor (Cecala, et al., 2001). Figure 7 

represents an ideal schematic for an effective filtration and 

pressurization system on a drill cab. 
Figure 7 - Schematic for effective filtration and 

pressurization system. 

The second component for dust control effectiveness is cab 

integrity. Cab integrity is necessary in order 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 pressures 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 air flows through 

filters can also decrease 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 requirements (heating and cooling) for operator 

comfort, which increases the size and cost. 

When a loss of cab pressure is determined, this indicates that 

there is either a filter loading problem or a cab integrity failure. 

Filter maintenance should be performed periodically when a 

predetermined pressure loss occurs over time. A sudden 

increase in pressure normally indicates a major failure in one of 

the filters and this problem needs to be immediately corrected. 

It has been shown through numerous NIOSH field studies that 

effective air filtration and pressurization systems can reduce the 

equipment operator’s respirable dust exposure, as well as 

provide a working environment that is more controllable and 

comfortable. In one study, a drill operator’s respirable dust 

exposure was reduced from 0.64 mg/mP

3

P

 to a level of 0.05 

mg/mP

3

P

 with a new filtration and pressurization system. This 

represents a 79 pct reduction in respirable dust concentrations 

when outside dust measurements were normalized because pretesting levels were significantly higher than post-testing. When 

considering the reduction in silica exposure, the drill operator 

had an average silica exposure of 57 μ/day for pre-testing, as 

compared to 3.5 :/day for post-testing. 

Air filtration and pressurization systems can be very effective 

in removing the respirable size range of dust particles (less than 

10 μm), which are harmful to a worker’s lungs. The systems 

can also provide pressurization to the enclosed cab many times 

without requiring any changes to the enclosure. The 

pressurization of 50 to 100 Pa (0.2 to 0.40 in. water gauge) 

ensures that the wind does not blow dust from outside into the 

cab. 

Clothes Cleaning Technique 

A cooperative research effort between UNIMIN Corporation 

and NIOSH has resulted in an improved method for cleaning 

dust-soiled work clothing at mineral processing operations. 

This new clothes cleaning process was implemented in the 

milling facility at UNIMIN’s Marston, North Carolina 

operation. From testing at this facility, it has been shown that 

this newly developed technique is a quick, safe, and effective 

method to clean dust-soiled work clothing. 

Currently in the United States, the only approved method to 

perform clothes cleaning by the federal mining regulatory 

agency is to use a HEPA-filter vacuuming system. To perform 

this technique, a worker uses the vacuum hose and manually 

moves the nozzle over his/her soiled clothing in an attempt to 

remove the contamination. This is a very difficult and timeconsuming task to perform. Because of this, few workers 

actually use this technique and prefer to use a single 

compressed air hose to blow dust from their work clothing, 

even though this is not an approved method of cleaning. 

Normally this is performed at high pressures and in the open 

work areas, which not only contaminates the worker, but coworkers as well. While investigating a new approach to 

perform this clothes cleaning process, it was critical to be able 

to meet the federal regulations and standards and come up with 

a process that workers would want to use. 

The initial step of the clothes cleaning process design was to 

develop a safe area to clean clothing. An enclosed booth was 

purchased from a manufacturer and installed at the Marston 

facility. This booth provided the worker sufficient space to 

effectively perform the cleaning operation. Above the door 

was an open grate that provided an intake for the ventilation 

airflow. A return air plenum located on the bottom-back wall 

of the booth was ducted to the baghouse dust collector system, 

which provided a constant flow of air through the enclosure. 

Since the booth is under constant negative pressure, it proved 

to be an effective area for clothes cleaning because it did not 

allow any dust leakage to the workplace. 
Figure 8 - Clothes cleaning technique design. 

Table 3 

Amount of dust remaining on coveralls after cleaning time for cotton and 

polyester/cotton blend coveralls. 

Cotton Poly/Cotton 

Cleaning blend 

method Dust on 

coveralls 

(grams) 

Clean 

time 

(seconds) 

Dust on 

coveralls 

(grams) 

Clean 

time 

(seconds) 

Vacuuming 63.1 398 45.5 346

Air hose 68.8 183 48.4 173 

Manifold 42.3 17 21.9 18

The next critical step was to develop an effective method to 

remove the product from the clothing. To do this, air nozzles 

were installed in a spray manifold that used compressed air to

blow the dust from the worker’s clothes. A considerable

amount of design effort went into determining the most 

effective spray nozzle manifold configuration, with numerous

laboratory tests conducted at the NIOSH Pittsburgh Research

Laboratory. Through this laboratory testing, researchers

evaluated the impact of varying cleaning distances, clothing 

type, nozzle types, nozzle spacing, air pressure, and spraying 

duration to optimize the cleaning effect. 

The final design was composed of an air spray manifold

fabricated from 3.8-cm (1-1⁄2 inch) schedule 80 plastic PVC 

pipe that was capped at the base. The air spray manifold was 

actuated by the worker performing the cleaning process by

operating a timer-set pneumatic valve located on the top of the 

manifold. The pneumatic valve had a safety interlock option 

which would automatically shut the air supply to the manifold 

if the exhaust ventilation system failed to keep the booth under 

sufficient negative pressure. Twenty-six (26) flat-fan air 

nozzles were mounted along the manifold, spaced on 5.1-cm

(2-in) centers. The bottom nozzle was a circular design located

15.2 cm (6 in) from the floor. This nozzle was used in 

coordination with a ball-type adjustable fitting that was 

directed downwards to clean the individual’s work shoes or

boots. 

At a pressure of 2.1 kg/cm2

 (30 psi), the air spray manifold

system expels 4.7 m3

 (166 ft3) of air for the typical cleaning 

period. In order to supply this compressed air volume to the air 

nozzles for effective cleaning, a 0.45-m3

 (120-gallon) air 

reservoir tank was necessary. This tank was installed at the 

operation and was typically pressurized to the 10.5-kg/cm2

(150-psi) level which holds approximately 5.1 m3 (180 ft3) of

air at this pressure. The air reservoir was located directly 

behind the cleaning booth and hard-piped to the air spray

manifold located inside the booth. Supply air to the manifold

was regulated down to 2.1 kg/cm2

 (30 psi). The air regulator

was located in a lock-box enclosure to prohibit anyone from

tampering with the air pressure. Figure 8 shows the cleaning 

booth, air reservoir, and air manifold configuration.

Figure 9 - Effectiveness of three different clothes 

cleaning techniques. 
A matrix of tests was performed at the Marston facility to 

evaluate the effectiveness of this newly developed technique. 

For this field testing, the new clothes cleaning technique was 

compared to the vacuuming system and the single handheld 

compressed air nozzle. In addition, two different coverall 

types were tested, being 100 pct cotton and a cotton-polyester 

blend. Prior to each test, the coveralls were soiled with 

limestone dust to a degree that represented an extreme case of 

soiling. Results of testing indicated that the manifold cleaned 

the clothes 10 times faster and removed 50% more dust than 

the single air nozzle or vacuuming methods. 

Table 3 provides the cleaning times and the remaining dust 

weights on the coveralls from the three different techniques 

evaluated. These values represent averages calculated for two 

NIOSH test personnel for a total of 96 tests. Figure 9 shows 

the effectiveness of the cleaning techniques tested. 

The worker performing the cleaning process is required to wear 

a half-mask fit-tested respirator with an N100 filter, hearing 

protection, and full-seal goggles. Respirable dust samples 

taken inside the respirator of the test personnel performing the 

clothes cleaning process showed minimal to no respirable dust 

exposure. In more than half of the 48 tests performed with the 

air spray manifold, the test subject’s respirable dust 

concentration remained at 0.00 mg/m3

 inside the half-mask 

respirator. In the remainder of the tests, the value remained 

very low with an overall average respirable dust concentration 

of 0.02 mg/m3

. 

Another factor evaluated during this study was the cleaning 

effectiveness of the process on two different coverall fabrics. 

Table 3 also indicates that there was a significant improvement 

with the cleaning effectiveness of the polyester/cotton blend 

coveralls when compared to the pure cotton type. This issue 

should also be considered by operations implementing this new 

clothes cleaning process. 

The new clothes cleaning process proved to be very efficient 

since the worker only needed to don the required Personal 

Protective Equipment (PPE), enter the booth, actuate the 

automatic valve, slowly spin in front of the air spray manifold 

(taking roughly 17 seconds), and exit the booth with clean 

clothing. This process has been demonstrated to be a much 

more effective method to remove dust from a worker’s clothing 

than methods currently used by workers. The clothes cleaning 

process is not currently approved for use in the mining industry 

in the United States and companies wishing to use this 

technique must file a petition for modification to the Mine 

Safety and Health Administration. Although this process was 

designed for workers in the mining industry, it is applicable to 

any industry where contaminated work clothing is a problem. 

CONCLUSION 

The purpose of this report was to provide five different 

applications where ventilation improved the air quality in the 

surface and metal/nonmetal mining industry. The technology 

discussed ranged from improving the ventilation design in a 

large iron ore processing facility to ventilating a small booth 

used to remove dust from a worker’s clothing. In all cases, the 

ventilation component of this research provided a crucial 

component in improving the air quality and lowering respirable 

dust exposures to workers in the mining industry. One function 

of this manuscript was to broaden the view on ventilation by 

highlighting some recent technology that has proven to be 

effective in the NIOSH dust control research program. 

REFERENCES 

Cecala, AB, Klinowski, GW and Thimons, ED, 1993. 

Reducing Respirable Dust Concentrations at Mineral 

Processing Facilities Using Total Mill Ventilation System. 

BuMines RI 9469. 

Cecala, AB, Organiscak, JA and Heitbrink, WA, 2001. Dust 

Underfoot – Enclosed cab floor heaters can significantly 

increase operator’s respirable dust exposure. Rock Products,

104(4):39-44. 

Chekan GJ, Colinet JF, Grau, RH [2004]. Evaluating 

Ventilating Air Movement in Underground Limestone Mines 

by Monitoring Respirable Dust Generated from Production 

Shots. Proceedings of the 10th U.S. Mine Ventilation 

Symposium. Anchorage, Alaska, May 17-19, 2004. 

Industrial Ventilation – A Manual of Recommended Practice – 

24th Edition – 2001. American Conference of Governmental 

Industrial Hygienists. 1330 Kemper Meadow Drive, 

Cincinnati, Ohio 45240-1634. 

Organiscak, JA, and Page, SJ, 2001. Improved cab air inlet 

location reduces dust levels and air filter loading rates. NIOSH 

Technology News No. 485.