Lower Respirable Dust and

Noise Exposure With an Open

Structure Design

RI 9670

REPORT OF INVESTIGATIONS/2006

Department of Health and Human Services

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health
Report of Investigations 9670 

Lower Respirable Dust and Noise Exposure 

With an Open Structure Design 

By Andrew B. Cecala, James P. Rider, Jeanne A. Zimmer, Robert J. Timko, 

 and Earle H. Andrews 

DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Pittsburgh, PA 

November 2006 
This document is in the public domain and may be freely copied 

or reprinted. 

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external to NIOSH do not constitute NIOSH endorsement of the sponsoring organizations 

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these Web sites. 

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DHHS (NIOSH) Publication No. 2007–101 

November 2006 

SAFER • HEALTHIER • PEOPLETM
CONTENTS 

 Page 

Abstract............................................................................................................................................1 

Introduction......................................................................................................................................2 

Testing..............................................................................................................................................3 

 Test equipment...........................................................................................................................3 

 Dust control..........................................................................................................................3 

 Real-time aerosol dust monitors ....................................................................................3 

 Gravimetric dust sampling.............................................................................................3 

 Noise control........................................................................................................................4 

 Structures evaluated...................................................................................................................5 

 Structure No. 1.....................................................................................................................5 

 Structure No. 2.....................................................................................................................6 

 Structure No. 3.....................................................................................................................9 

Results............................................................................................................................................10 

 Respirable dust.........................................................................................................................10 

 Noise ........................................................................................................................................12 

Discussion......................................................................................................................................14 

Conclusions....................................................................................................................................17 

References......................................................................................................................................18 

ILLUSTRATIONS

1. Dust and noise sampling instrumentation setup at masonry structure No. 1..............................6 

2. A researcher inspects sampling equipment at open structure No. 2 ...........................................7 

3. Dust and noise sampling instrumentation setup at open structure No. 2....................................8 

4. Dust and noise sampling instrumentation setup at steel-sided structure No. 3...........................9 

5. Sound pressure levels versus distance in structure Nos. 1 and 2..............................................13 

6. Comparison of walled and open structure design for same size mineral processing 

 building ..................................................................................................................................17 

TABLES 

1. Average respirable dust concentrations for the three product sizing structures .......................10 

2. Production levels for the evaluation period for the three product sizing structures .................11 

3. Calculated respirable dust concentration per ton of product processed for the three 

 structures evaluated................................................................................................................11 

4. Sound pressure levels of structure No. 1: reverberant environment........................................12 

5. Sound pressure levels of structure No. 2: open environment ..................................................12 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

db decibel mg milligram 

ft foot mg/m3 milligram per cubic meter 

ft3 cubic foot min minute 

hr hour mm millimeter 

Hz hertz sec second 

L/min liter per minute μm micrometer 
LOWER RESPIRABLE DUST AND NOISE EXPOSURE 

WITH AN OPEN STRUCTURE DESIGN 

By Andrew B. Cecala,1 James P. Rider,2 Jeanne A. Zimmer,3 Robert J. Timko,4

and Earle H. Andrews5

ABSTRACT 

Many different types of structures and materials have been used to build mineral 

processing facilities over the past few decades. Although the structure type and building 

material were not viewed as significant factors affecting the health of employees in these 

facilities when they were built, the National Institute for Occupational Safety and Health 

performed an evaluation to determine to what extent building types could impact respirable dust 

and noise levels. This report discusses the evaluation of three different types of product sizing 

silica sand structures: a masonry design, a steel-sided design, and an open structure design. The 

data obtained in this study indicate that the open structure design (no walls) was superior from 

both a dust and noise (health) standpoint compared to the other two structures. The open 

structure design should also be beneficial from a cost standpoint because of lower material and 

construction costs. Companies and design engineers should consider this open design when 

building new mineral processing facilities in climates where it could be applicable. Some 

companies may also want to consider modifying existing structures with a more open design to 

further reduce dust and noise levels. As the trend continues in lowering allowable dust levels for 

federal health standards in the U.S. mining industry, the open structure design may be an 

approach for some companies to consider for their operations. 

 

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

Pittsburgh, PA. 

 2

Operational research analyst, Pittsburgh Research Laboratory, National Institute for Occupational Safety and 

Health, Pittsburgh, PA. 

 3

Physical science technician, Pittsburgh Research Laboratory, National Institute for Occupational Safety and 

Health, Pittsburgh, PA. 

 4

Supervisory research physical scientist, Pittsburgh Research Laboratory, National Institute for Occupational 

Safety and Health, Pittsburgh, PA. 

 5

Manager, Occupational Health and Safety, U.S. Silica Co., Berkeley Springs, WV (retired). 
2

INTRODUCTION 

 Workers at mineral processing facilities are often exposed to high levels of respirable 

dust and noise. Federal compliance records have shown that workers at these facilities have 

some of the highest respirable dust and noise exposure rates in the entire metal/nonmetal mining 

industry [NIOSH 1999; Stephenson and Merry 1998; Watts and Parker 1995]. Since many of 

these operations process ore containing some percentage of silica, the health risk to these 

workers is even greater because of the possibility of developing silicosis [Kreiss and Zhen 1996; 

Hnizdo and Sluis-Cremer 1991; Ng and Chan 1994; Rice and Herring 1995; Steenland and 

Brown 1995]. For many years, the National Institute for Occupational Safety and Health’s 

(NIOSH) Pittsburgh Research Laboratory has been performing research to lower dust and noise 

exposures to workers at mineral processing operations. The vast majority of this research has 

been directed at source control techniques for a particular job function [Cecala and Covelli 1991; 

Cecala et al. 1984; Cecala et al. 2000; Hennings 1980; Pokora et al. 1984; Rubin et al. 1982; 

U.S. Bureau of Mines 1983; Volkwein and Gaynor 1985]. Although this research has been 

successful for a particular job function or application, there has been very little control 

technology developed to lower exposures to multiple workers or job functions throughout an 

entire structure. 

 One such study occurred back in the early 1990s with the development of the total mill 

ventilation system for mineral processing facilities [Cecala et al. 1993]. This system was 

effective at lowering respirable dust concentrations throughout an entire structure in a costeffective manner. A total mill ventilation system consists of a calculated number of exhaust fans 

that are placed on the roof or very high on the exterior walls of a structure to induce a ventilation 

flow pattern up through the building. Make-up air inlets should then be located along the base of 

the building and should consider two main factors. First, the intake air inlets need to provide 

clean (non-dust-laden) outside air into the structure. Second, these air inlets should be 

strategically located to provide an effective sweeping of the major dust-laden areas as the intake 

air moves up through the structure. Respirable dust reductions ranging from 40% to 65% were 

recorded throughout these structures during a number of field studies to evaluate this technique. 

 This study provided the impetus to broaden NIOSH’s research to consider more global 

techniques that could be used to improve the health of workers at mineral processing operations. 

Through various discussions with industry safety and health specialists, it was hypothesized that 

the open structure design may provide some overall improvements to dust and noise levels 

throughout an entire structure when compared to conventional walled structures. This 

hypothesis led to the development of this research study to evaluate the different types of 

building structural designs and materials used at mineral processing operations to determine the 

potential impact of each on dust and noise levels on a comparative basis. The primary goal of 

this research was to provide design engineers and companies with information to consider when 

designing new mineral processing facilities. If the open structure design proved to be 

advantageous from a health standpoint, some operations could also contemplate modifying 

existing structures by removing wall panels in an effort to lower dust or noise levels. 
3

TESTING 

A test plan was established to provide a valid comparison of respirable dust and noise 

levels at the three different structures evaluated in this study. This section describes the 

equipment used in this evaluation, the physical characteristics of each of the three structures, and 

the equipment setup at each structure. 

 A significant factor affecting respirable dust and noise levels in these facilities was the 

production rate during each evaluation. Under normal conditions, higher production rates 

correlate with higher dust and noise levels, although there may be some extenuating 

circumstances that may cause this to change. Production rates at these facilities were recorded 

by operations personnel, and this information was then provided to NIOSH. 

Test Equipment 

Dust Control 

 Respirable dust measurements were taken using both real-time aerosol monitors 

(RAM–1) and gravimetric samplers. Both of these dust instruments use a 10-mm Dorr-Oliver 

cyclone to classify the respirable portion of dust, usually considered to have aerodynamic 

diameters of 10 μm or less. 

Real-time Aerosol Dust Monitors 

 RAM–1 dust monitors (MIE, Inc.) were used to sample respirable dust concentrations for 

each of the three structures evaluated. The RAM–1 monitor measures instantaneous respirable 

dust concentrations by drawing an air sample into a sensing chamber and passing it through a 

light beam. A detector in the chamber produces an electrical signal proportional to the amount 

of dust in the air stream [Williams and Timko 1984]. These instruments were used to measure 

dust concentrations continuously in this study, except during brief periods of data transfer and 

calibration. The RAM–1 has been used for many years in dust research and has proven to be a 

very reliable and accurate device. The dust data from the instruments were stored on Metrosonic 

331 or Telelog 2101 dataloggers that averaged the dust concentration at predetermined periods 

(either 30-sec or 1-min averages for this testing). Approximately every 8 hr, the information 

recorded on the dataloggers was downloaded and stored on a laptop computer. The RAM–1 dust 

monitors were recalibrated during this time. Once all testing was complete, the dust data stored 

on the computer were taken back to the laboratory and analyzed using commercially available 

software packages. 

 Testing was performed daily, with midnight considered as the start and stop time for each 

day of testing. 

Gravimetric Dust Sampling 

 A gravimetric dust sampling package was used along with the RAM–1 dust monitor at 

most of the dust sampling locations for the various evaluations. Each gravimetric sampling 

package was composed of three or four individual sampling units. Each individual sampling unit 

was operated by a Mine Safety Appliances (MSA) Escort ELF sampling pump preset at a flow 
4

rate of 1.7 L/min. MSA established this flow rate for use with the 10-mm Dorr-Oliver cyclone. 

This was then mandated by the Mine Safety and Health Administration for respirable dust 

sampling for the metal/nonmetal mining industry (30 CFR6 56.5001). 

 Respirable dust was classified by the 10-mm cyclone and deposited on a 37-mm-diam 

MSA dust filter cassette. The filters were pre- and postweighed to the nearest 0.001 mg on a 

Cahn C–31 microbalance. The dust concentration of each individual sampling unit was 

calculated based on its own run time, then the concentrations of three or four units were 

averaged together to determine the average respirable dust concentration for the entire 

gravimetric dust sampling package. 

 For approximately every 10 gravimetric filters used during a field test, one control filter 

cassette was set aside for calibration purposes. These control filter cassettes remained unused 

during the dust sampling period, but were pre- and postweighed to determine any biases in the 

weighing process. A correction factor was determined based on the average differences between 

all the pre- and postweighed control filters. This correction factor was then applied to the final 

value for all field gravimetric measurements. 

 The gravimetric samples were only operated on a single-shift basis per day, which 

normally occurred during the daylight shift. In addition to providing the average dust 

concentration over the sampling period, the gravimetric sampling package provided a dust range 

to verify that the RAM–1 devices were recording properly. The gravimetric and RAM–1 dust 

averages were not expected to be identical because of variations in sampling times, but the dust 

values obtained using both methods were expected to be similar, assuming everything was 

working properly. 

Noise Control 

 Experiments to measure the spatial distribution of sound levels were conducted in two 

screening towers (structure Nos. 1 and 2). In order to minimize background noise, plant 

operations were suspended while acoustical tests were conducted. Tests to measure the acoustic 

environment were performed using an external sound source with a known sound power level. 

A Brüel & Kjær (B&K) 4205 Sound Power Source unit was used for all testing. The unit 

consisted of two separate components: a noise-generating system and an external sound device. 

The noise generation system was composed of amplifiers, octave band filters, attenuators, and 

sound intensity controls. Two loudspeakers and associated crossover networking were used as 

the external sound source. The sound power output was controlled with an attenuator in 10-dB 

increments with a 40-dB range. Sound levels were measured and recorded with a B&K 

microphone and preamplifier as input to a Nagra recorder. 

 Tests conducted at the two screening towers (structure Nos. 1 and 2) used the following 

experimental protocol. The external sound source was placed on the ground floor of both 

structures in a central location as far away from the plant equipment as physically possible. 

A sound power level of 100 dB was selected for all experiments. A measuring microphone was 

mounted on a tripod approximately 66 inches above the floor and positioned at seven different 

locations throughout each of the structures. Tests were initiated by energizing the noise source 

on the ground floor, and sound levels were stabilized for approximately 1 min. Noise data 

were measured and recorded for 45 sec at each of the seven octave bands ranging from 125 to 

 

6

Code of Federal Regulations. See CFR in references. 
5

8,000 Hz. A minimum of three data points were measured at each of the seven microphone 

locations. Noise data from the two screening towers were subsequently analyzed in the 

laboratory with a digital frequency analyzer. A 32-sec linear averaging time was used to 

calculate 1/3-octave band spectra from 63 to 8,000 Hz. 

Structures Evaluated 

 Although five different structures were evaluated throughout the entire course of this 

research effort, only three of the structures where product sizing was performed will be 

considered in this report, as discussed below. All of these structures were processing silica sand 

material. Structure Nos. 1 and 2 used the screening technique for product sizing, whereas 

structure No. 3 used an air separation technique. The other two structures evaluated in this 

research were grinding mills, but their production levels were orders of magnitude greater than 

the product sizing facilities and thus not readily comparable for this evaluation. Following is a 

brief description of each of the three structures evaluated in this report. Acoustical evaluations 

were performed only in the first two structures since personnel and budgetary restrictions did not 

allow for testing at the third facility. 

Structure No. 1 

 Structure No. 1 was a nine-story building with steel framing and a masonry block 

construction. This building was a three-tier design with a depth of 32 ft for the entire structure. 

The first tier was approximately 50 ft high and 33 ft wide, the second was 63 ft high and 25 ft 

wide, and the third was 108 ft high and 42 ft wide. The third tier sat on product storage silos for 

bulk loading that were approximately 50 ft high, making the actual inside height of the building 

58 ft. The volumetric capacity of the structure was calculated to be 204,000 ft3

. 

 Dust testing was performed for 4 days. After this initial test series was completed and 

the data analyzed, 1 additional day of testing was performed. Both RAM–1 and gravimetric 

samplers were used at all dust monitoring locations. Because plant operations had to be 

suspended during the noise study, all testing took place on off days or during scheduled 

maintenance. Microphones recorded noise data for at least a 45-sec time period at the seven 

sampling locations and for each octave band ranging from 125 to 8,000 Hz. Figure 1 shows the 

locations for dust and noise instrumentation in this structure. 
6

Structure No. 2 

Structure No. 2 was a steel beam shell using an open structure design with no walls. 

Figure 2 shows a monitor being inspected by a researcher at this open structure processing 

operation. This structure was 46 ft long, 22 ft wide, and approximately 75 ft high. A weather 

vane directional anemometer located on the very top of the structure was used to record wind 

direction and speed over the sampling period. 

 Dust testing was also performed at this facility for 4 days. There were four locations 

where both gravimetric and RAM–1 respirable dust measurements were obtained. There were 

three additional locations where only RAM–1 respirable dust measurements were performed. 

The noise data were once again measured and recorded for a 45-sec time period at various 

locations throughout the open structure. Because of the difference in the open and closed 

Figure 1.—Dust and noise sampling instrumentation setup at masonry structure No. 1. 
7

structures, the microphones were located in different positions in the open structure compared to 

structure No. 1. Although the microphones were located in different positions throughout the 

two structures, the overall distances from the sound source to each sampling location were 

comparable in both structures. Figure 3 shows the test locations for dust and noise sampling 

instrumentation. It should be noted that the dust and noise tests were performed during separate 

time periods at this facility because of scheduling conflicts among the various researchers. 

Figure 2.—A researcher inspects sampling equipment at open structure No. 2. 
8

Figure 3.—Dust and noise sampling instrumentation setup at open structure No. 2. 
9

Structure No. 3 

 Structure No. 3 was a five-story steel-framed metal-sided building. This building was 

somewhat unique from the two previous structures in that both product crushing and sizing were 

performed in the same building. This structure was 130 ft long, 65 ft wide, and 25 ft high. 

A 15- ft by 15-ft tower extended up out of the center of the structure an additional 22 ft. The 

volumetric capacity of this building was calculated to be 238,000 ft3. Dust testing was 

performed for 4 days at various sample locations, as shown in Figure 4. The majority of the dust 

sampling instruments were located toward the eastern side of the building and in the tower 

section because most of the production activities were in these areas. The western part of the 

building was mainly an access way for plant personnel and forklifts moving supplies and pallets 

of bagged product material. There were no noise measurements taken at this structure. 

Figure 4.—Dust and noise sampling instrumentation setup at steel-sided structure No. 3. 
10

RESULTS 

Respirable Dust 

 To review, structure No. 1 was a masonry structure using the screening technique, 

structure No. 2 was an open structure also using the screening technique, and structure No. 3 was 

a steel-sided structure using an air separator technique. Table 1 compares the three structures 

based on the average respirable dust levels measured with both the RAM–1 and gravimetric 

sampling instrumentation for 4 days of testing. Once again, the RAM–1 results are based on 

round-the-clock testing, whereas the gravimetric results are single-shift measurements. 

Table 1.—Average respirable dust concentrations 

for the three product sizing structures

Sample location

RAM–1 dust monitor 

dust concentration, 

mg/m3

Gravimetric dust 

concentration, 

mg/m3

Structure No. 1 – Masonry: 

 1st floor ...................................... 0.06 0.02

 3rd floor...................................... 0.10 0.04

 5th floor ...................................... 0.23 0.16

 7th floor ...................................... 0.42 0.29

Structure No. 2 – Open design: 

 Ground floor ............................... 0.06 —

 1st floor – north .......................... 0.06 0.02

 1st floor – south.......................... 0.07 —

 2nd floor – north......................... 0.03 0.03

 2nd floor – south ........................ 0.07 —

 3rd floor...................................... 0.08 0.06

 31⁄2 floor ...................................... 0.05 0.04

Structure No. 3 – Steel-sided: 

 1st floor – north .......................... 0.35 0.24

 1st floor – south.......................... 0.45 0.37

 2nd floor ..................................... 1.01 0.40

 3rd floor...................................... 2.22 1.81

 4th floor ...................................... 3.61 1.78

 5th floor ...................................... 2.12 1.57

 With any closed-wall structure, there is normally an increase in dust levels as one moves 

up through the structure. This was the case for structure Nos. 1 and 3 as dust levels generally 

increased at each level up through the structure. This occurs because either the mechanical or 

natural ventilation within the structures induces an upward flow convection, which entrains the 

dust and moves it up through the structure with the ventilating airflow. This occurrence was also 

identified during field testing of the total mill ventilation technique described earlier. 

 The one exception to this dust increase was at the fifth-floor sampling location for 

structure No. 3, as seen in Figure 4. A fan located at the top of the structure pulled a good 

portion of the supply air from the open door on the opposite side of the tower at the fourth floor. 

As the air came in from outside and swept across the tower to the fan, it traveled across the fifthfloor dust sampling location. Although this had a positive impact on lowering dust levels at the 

fifth-floor sample location, this is not an effective ventilation flow pattern for the entire structure. 

Dust levels below the fifth-floor location were not well ventilated, which caused respirable dust 

concentrations in this area to climb. 
11

 One significant benefit from the open structure design in structure No. 2 was that there 

was no dust gradient as one moved up through the different floors. Dust levels varied slightly 

from floor to floor at this open structure, with no consistent pattern. In addition, respirable dust 

levels were extremely low compared to levels measured at the other two structures. It should be 

noted that no visible dust plume was ever observed flowing from this open structure during the 

entire evaluation period. The minimal amount of dust that was not contained by the primary 

dust control systems at this operation, thus liberated into the air, migrated from the structure 

based upon the prevailing wind direction and speed. This dust should be quickly diluted to 

undetectable levels within a close proximity to the structure and should have no impact on other 

plant personnel, nearby communities, or the environment. 

When considering respirable dust levels measured during the 4-day evaluations at each 

structure, there were two major factors affecting dust generation: (1) hours of operation, and 

(2) the amount of material processed (production) during the sampling period. Periods of 

nonproduction were removed from the data and not included in the dust averages for any of the 

three structures evaluated. Table 2 shows production levels for each day over the sampling 

period and the total production value for the entire test period. Structure Nos. 1 and 2 had 

similar production levels for the 4 days of evaluation at 5,437 and 6,905 tons processed, 

respectively. Both of these production levels were significantly higher than the production level 

at structure No. 3, in which 160 tons was processed during the evaluation period. 

Table 2.—Production levels for the evaluation period 

for the three product sizing structures

Tonnage Structure Day 1 Day 2 Day 3 Day 4 Total

No. 1: Masonry................ 1,893 1,762 1,278 1504 5,437

No. 2: Open design........... 1660 2,602 2,796 1847 6,905

No. 3: Steel-sided............ 39.1 39.6 39.2 42.6 160.5

1

Partial day of testing. 

 In an effort to give a more accurate comparison of the three structures, a normalized dust 

concentration was calculated for each operation (Table 3). This calculated value provides a dust 

concentration based on an equivalent production rate for each facility. The results from these 

normalized calculations further indicate how effective the open structure design was at 

minimizing respirable dust levels. This comparison shows that respirable dust levels at structure 

Nos. 1 and 3 were 4.3 and 1,379 times higher than those measured at structure No. 2 when 

normalized for equivalent production levels. 

Table 3.—Calculated respirable dust concentration 

per ton of product processed for the 

three structures evaluated

Structures

Dust 

concentration, 

mg/m3

Production 

rate, 

tons

Normalized dust 

concentration, 

mg/m3

No. 1............. 0.20 5,437 3.7 E–5

No. 2............. 0.06 6,905 8.7 E–6

No. 3............. 1.87 160.5 1.2 E–2
12

 It should be noted that part of this study was a ventilation analysis for the walled 

structures, since ventilation is such a critical component for minimizing respirable dust levels at 

mineral processing facilities. Tracer gas studies were performed at both structure Nos. 1 and 3 to 

provide a more in-depth ventilation analysis. Although the results of these ventilation studies 

will not be presented in this report for purposes of brevity, some of the information learned will 

be noted in the “Discussion” section. 

Noise 

 From a noise standpoint, the results of the experiments conducted in structure Nos. 1 

and 2 are summarized in Tables 4–5. Octave-band sound pressure level (SPL) data are presented 

as a function of distance (and location) relative to the noise source located on the ground floor. 

Data from some of the more distant measurement locations (as shown in Figures 1 and 3) were 

not used in the final analysis due to background interference. To better visualize trends in the 

data, SPL data versus distance from the two structures are plotted on a log scale at selected 

frequencies (125, 1,000, and 4,000 Hz) in Figure 5. In addition, the SPL data have been overlaid 

with a straight line having a slope of 6 dB per doubling of distance, which is characteristic of a 

noise source located in a free field. 

Table 4.—Sound pressure levels (in decibels) of structure No. 1: reverberant environment

Octave-band 

center 

frequency, 

Hz

2nd floor, 

location 

No. 3, 

28 ft from 

source

2nd floor, 

location 

No. 2 

38 ft from 

source

3rd floor, 

location 

No. 1, 

45 ft from 

source

3rd floor, 

location 

No. 2, 

60 ft from 

source

2nd floor, 

location 

No. 1, 

60 ft from 

source

4th floor, 

location 

No. 2, 

68 ft from 

source

4th floor, 

location 

No. 1, 

79 ft from 

source

125.................. 84.3 84.8 82.7 81.3 79.3 77.9 77.8

250.................. 81.3 82.6 81.4 78.1 78.9 76.7 76.0

500.................. 80.7 79.4 78.8 76.3 76.8 77.3 72.9

1,000............... 80.1 77.7 76.6 73.8 73.4 70.9 68.5

2,000............... 77.4 74.8 75.7 70.8 71.2 69.6 65.7

4,000............... 76.3 73.4 72.9 68.5 69.4 66.6 63.6

8,000............... 75.2 71.4 71.9 65.6 66.6 66.7 59.1

Table 5.—Sound pressure levels (in decibels) of structure No. 2: 

open environment

Octave-band 

center 

frequency, 

Hz

2nd floor, 

location 

No. 3, 

23 ft from 

source

2nd floor, 

location 

No. 2 

27 ft from 

source

2nd floor, 

location 

No. 1, 

42 ft from 

source

3rd floor, 

location 

No. 2, 

44 ft from 

source

3rd floor, 

location 

No. 1, 

56 ft from 

source

125.................. 78.8 78.8 75.4 74.9 74.1

250.................. 78.4 78.0 74.1 72.1 72.3

500.................. 76.3 75.9 72.3 69.8 69.7

1,000............... 75.1 73.4 71.7 67.6 67.9

2,000............... 74.3 74.6 70.8 66.1 65.6

4,000............... 72.7 73.5 68.0 64.4 62.2

8,000............... 70.5 70.6 64.2 58.2 55.6
13

Figure 5.—Sound pressure levels versus distance in structure Nos. 1 and 2. 
14

DISCUSSION 

This report analyzes and compares the differences in respirable dust and noise levels at 

three silica sand product sizing structures that were each evaluated for 4 days. These facilities 

are dynamic environments that are constantly changing relative to time. By sampling around the 

clock, we tried to minimize some of the shift fluctuations and variations. We realize there is a 

long list of variables that affect the magnitude of dust and noise levels within these structures. 

Although the information obtained in this study is being used to compare the differences among 

these facilities, we understand that these results provide only a small picture of the respirable 

dust and noise levels during the actual test period and that measurements taken during another 

time would most likely produce different levels. 

 With this acknowledgement, considering all of the information and data, the most 

effective structural design of the three product sizing buildings was the open structure design 

(structure No. 2). Respirable dust concentrations were significantly lower with the open 

structure design than at the other two facilities because the environment acts as the best source of 

ventilation to dilute and carry away dust generated and liberated during the product sizing 

process. Respirable dust levels at structure No. 2 were more than four times lower than those at 

structure No. 1 when compared with normalized production levels. Both of these plants had 

similar screening processes, production levels, and overall structural designs. 

 Respirable dust levels at structure No. 2 were also much lower than those measured at 

structure No. 3, where product sizing was performed using the air separator technique. 

Production levels for structure No. 3 were only 2.5% of those for structure No. 2 during this 

evaluation, yet respirable dust concentrations were 1,379 times higher when the data were 

normalized for equivalent tonnages. 

 Although the open design of structure No. 2 was the most effective from a dust 

standpoint, it is worth evaluating key features in each of the wall structure designs (structure 

Nos. 1 and 3). With regard to structure No. 1, respirable dust levels were relatively low 

considering that the building was, for the most part, naturally ventilated. A tracer gas test at this 

facility indicated that there was an effective flow pattern up through the structure during the test. 

The problem with natural ventilation is that it changes significantly as the outside-to-inside air 

temperature differential changes over time. These changes can be substantial when one 

considers the variation in temperatures throughout the course of a year. 

 An interesting event was identified during the evaluation of structure No. 1 that supports 

the effectiveness of an open structure design. During the original tracer gas test, a problem 

occurred that caused the test to be invalid and necessitated that it be repeated some 21⁄2 months 

later. During this followup test, it was decided to operate the dust monitoring instruments 

concurrently to correlate the results of the tracer gas survey with respirable dust concentrations 

throughout the structure. In fact, the results from this final day of testing replaced the original 

fourth-day evaluation because it was believed to be a much more representative sample since 

there was a significant amount of downtime during the original last-day test. 

 During this retest, one significant change was that a large bay door on the fifth floor of 

structure No. 1 was open. This open door provided a significant amount of natural ventilation to 

the structure and significantly lowered the dust levels in this area of the building. With this open 

bay door, respirable dust levels were 59.3% and 70.9% lower at the fifth- and seventh-floor 

sample locations, respectively, compared to the first 3 days of testing. This also supports the 
15

impact that a more open design can have on lowering respirable dust levels in a portion of the 

structure, indicating the potential effectiveness of modifying existing structures. 

 Two possible ways to lower respirable dust levels at structure No. 1 are to (1) install 

powered exhaust fans in the upper portion of the building, as discussed in the total mill 

ventilation research [Cecala et al. 1993], and (2) provide a more open structure design, as 

evidenced by the open bay door during the retest of the tracer gas study. Exhaust fans would 

provide a consistent ventilation rate to the structure and thus reduce respirable dust levels. 

Unlike natural ventilation, which can be significantly affected by outside air temperature 

changes, the benefit of a mechanical ventilation system is its consistency throughout the year. 

A more open design would require removing portions of the walls of the structure. When 

considering both of these options, it seems that installing mechanical fans would be more 

appropriate and easily accomplished. 

 With regard to structure No. 3 (steel-sided), the majority of dust liberated within 

the structure was drawn up through the tower portion of the building, thus concentrating dust 

levels in this area of the structure. Dust levels ranged from 0.35 mg/m3 on the first floor to 

3.61 mg/m3 on the fourth floor. A better approach to ventilate this structure and lower overall 

dust levels would have been to install two or three roof-powered exhausters positioned equally 

along the peak of the structure. This would have drawn the dust more equally from the building 

and not allowed an accumulation of high dust concentrations in the tower section of the 

structure. It would also be beneficial to close the door across from the fifth floor sampling 

location and have an air supply inlet at the base of the tower structure. This would provide for a 

more uniform flow through the tower portion of the building. 

 From a noise standpoint, the SPL data measured during the experiments at the two 

screening tower facilities show that the sound field intensity in both structures decreases with 

increasing distance from the source. The open structure design (structure No. 2) appears to 

approximate an acoustic free-field environment. The level of the sound field measured in the 

reverberant environment of structure No. 1 tends to be about 5–8 dB higher than the level in 

structure No. 2 at comparable distances from the source. 

 Examining the sound field measured in structure No. 1 shows that the sound level, after 

an initial increase near the source, falls off with increasing distance much like in the free-field 

environment. The behavior of the sound field is qualitatively consistent with the results of 

previous studies to predict the propagation of sound in a fitted room [Hodgson 1990]. In 

general, when a large number of machines or fittings are distributed in an enclosed space, such 

as a plant or factory, the resulting noise level is higher near the source, but lower at greater 

distances from the source. The fittings serve to positively influence the absorption in the space 

by creating a more diffuse sound field with many opportunities for the sound field to interact 

with absorbent surfaces. 

 As the results have shown, respirable dust and noise levels were significantly lower in the 

open wall structure than in either of the two walled structures. It must be noted that with any 

open wall structure, there are a number of issues that have to be addressed that are not normal 

concerns with walled structures. First, great effort must be made to provide safety railings and 

guards to minimize the potential for any personnel falling from the structure. Another problem 

is the protection of equipment and personnel from environmental elements such as rain, snow, 

sleet, and hail. It seems that this technique would be better suited for southern climates, 

although the open structure design evaluated in this study was located in central Pennsylvania, 

which has relatively cold winters. This plant has been in operation for many years and has been 
16

very successful with the open structure design. It must also be noted that there was no water 

application within this structure, which eliminated the concern for freeze-up problems during 

periods of below-freezing temperatures. 

 Another factor to keep in mind when considering building a new structure or modifying 

an existing structure to a more open design is the proximity of the building to residential 

dwellings or other businesses. This open structure should only be considered when there is a 

substantial buffer zone around the building unless other solutions can be designed to overcome 

this, such as walls built along highways to minimize noise levels to adjacent dwellings. 

 Still another factor to consider is the number of variations in an open structure design. 

Structure No. 2 in this study was a totally open design. One modification that could be 

incorporated into the design to protect the structure from inclement weather would be a roof with 

a sufficient overhang. Figure 6 shows a conceptual drawing of a typical walled processing 

facility, then an identically sized facility with an open design structure and a roof. A roof would 

provide more protection from the natural elements than the totally open design. 

 While looking for similar types of research studies performed during the background 

information search, very little information was found that was applicable to the study described 

in this report. When considering the ventilation within a building or structure, the two main 

reference books most often used are Industrial Ventilation: A Manual of Recommended Practice, 

published by the American Conference of Governmental Industrial Hygienists [ACGIH 2001], 

and the ASHRAE Fundamentals Handbook, published by the American Society of Heating, 

Refrigerating and Air-Conditioning Engineers, Inc. [ASHRAE 1989]. Both publications provide 

a wealth of information to design engineers, but neither discusses the open structure design. The 

ASHRAE Handbook identifies a number of aspects that are applicable in a section entitled 

“Natural Ventilation Guidelines,” which appears in chapter 23, “Infiltration and Ventilation.” 

This section discusses the importance of designing structures with regard to topography, 

landscaping, surrounding buildings, and natural wind directions to take advantage of natural 

breezes to ventilate buildings. 

 It must be noted that if an open structure design is being considered, it should be viewed 

as a secondary design. The first approach to lower dust and noise exposures to workers is by 

properly designed and installed primary dust and noise control techniques. From a dust control 

standpoint, this means having a good primary dust control plan that minimizes dust generation 

and liberation and captures major dust sources at their point of origin before they are allowed to 

flow out into the plant, contaminating plant personnel. From a noise standpoint, this means 

using equipment that has been designed with noise-dampening components and providing 

acoustical dampening material when applicable around significant noise-producing equipment 

and functions. It is also important to have a good maintenance program to constantly repair 

equipment, as well as a good housekeeping program. The open structure design should then be 

evaluated as a secondary method to further improve health concerns at mineral processing 

operations. In this manner, the open design would be very similar to the total mill ventilation 

system in that it has the potential to be a global approach to lower dust and noise levels 

throughout an entire building or structure. 
17

CONCLUSIONS 

This research determined that the open structure design for mineral processing operations 

was superior to walled structures when considering both interior dust and noise levels. From a 

dust standpoint, the open structure was beneficial because there was no dust gradient when 

moving up through the building. From a noise standpoint, the open structure was beneficial 

because it eliminated the reverberation effects of sound waves bouncing off the walls. In the 

open structure design, the natural environment acts as an effective method of diluting and 

carrying away dust generated and liberated during the product sizing process, as well as 

dampening the sound waves as they travel out from the structure. Nevertheless, the open 

structure design should be viewed as a secondary control technique for both dust and noise. 

Operations must have effective primary dust control techniques to capture and contain dust at the 

 Figure 6.—Comparison of walled and open structure design for same size 

mineral processing building. 
18

point of generation. The most widely used method for these types of operations is capturing the 

dust sources with negative ventilation and filtering the dust through a collector device. No dust 

plume should ever be visible at an open structure design. Also, if dust plumes are visibly 

flowing from an open structure, then better primary dust control measures should be 

implemented. Similarly, major noise sources should be dealt with by using engineering controls 

to eliminate or dampen significant sources. Engineering out the primary noise sources with 

equipment is the primary avenue for lowering noise levels. It should be noted that some state 

and local governments may prohibit the release of dust or noise from an open structure, even 

though it should be extremely minimal. 

 This research effort has highlighted some of the advantages of an open structure design 

and why it should be considered in some applications by designers and engineering companies 

when building new structures. It is obvious that the open structure design will be more costeffective because there would be less material and construction costs. Some companies may also 

want to consider modifying their existing structures with a more open design to further reduce 

dust and noise levels. If operations are considering the open structure design, dust and noise 

controls should be impeccable to minimize any plant, community, or environmental impact. 

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199710. 
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