IICI9089 

Bureau of Mines Information Circular/1988 

Impact of Background Sources on Dust 

Exposure of Bag Machine Operator 

By Andrew B. Cecala and Edward D. 1himons 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Information Circular 9089

Impact of Background Sources on Dust

Exposure of Bag Machine Operator

By Andrew B. Cecala and Edward D. Thimons

UNITED STATES DEPARTMENT OF THE INTERIOR

Donald Paul Hodel, Secretary

BUREAU OF MINES

Robert C. Horton, Director
library of Congress Cataloging in Publication Data:

Cecala, Andrew B

Impact of background sources on dust exposure of bag machine

operator.

(Information circular/Bureau of Mines; 9089)

Supt. of Docs. no.: I 28.27:9089.

1. Mineral industries- Hygienic aspects. 2. Bagging- Hygienic aspects. 3. Mineral

dusts- Measurement. 1. Thimons, Edward D. II. Title. III. Title: Dust exposure of bag

machine operator. IV. Series: Information circular (United States. Bureau of Mines); 9089.

TN295.U4 (TH7697.M56J 622 s (622'.8J 86-600121
CONTENTS

A"bstract...................................................................... .

Introduction. .

Testing setup.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Results... .. ... . .... ..... . ,...... . . . . . . . . . . . . . . . . . . . . . .

Discussio-n. . ....... .... ..... . . . ...... . .... ...... . .... .

Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Appendix. --Calculations to determine TLV exposure time.......................

ILLUSTRATIONS

1.

2.

Sampling setup to monitor a worker's dust exposure........................

Case 1: Operator's exposure after becoming soiled with product from fill

nozzle that did not shut off.........................................

Case 2: Operator's exposure after becoming soiled with product while

changing the saddle height.........................................

Bag operator after becoming soiled with product.....................

3.

4.

5.

6.

Case 3: Operator's exposure from blowing clothes with compressed air.

Operator's exposure from broken bag at fill station. Case 4 (A) and

case 5 (H)...............................................................

Case 6: Operator's exposure from broken bag during conveying process..

Case 7: Operator's exposure from bulk loading outside.................

Case 8: Operator's exposure from bag hopper overflowing...............

Case 9: Operator's exposure from coworker sweeping floor one level down..

7.

8.

9.

10.

TABLE

1. Nine cases of background dust exposure to bag operator....................

Page

1

2

2

3

7

9

10

3

4

4

5

6

6

6

7

7

7

8
UNIT OF MEASURE ABBREV IATI ONS USED IN THIS REPORT

h hour mm millimeter

hId hour per day mg/m3 milligram per cubic meter

Ib pound min minute

l.m micrometer pct percent
IMPACT OF BACKGROUND SOURCES ON DUST

EXPOSURE OF BAG MACHINE OPERATOR

By Andrew B. Cecala1 and Edward D. Thimons2

ABSTRACT

The Bureau of Mines has recorded a number of different background dust

sources that significantly contaminated the air breathed by bag machine

operators. These background sources, observed at five different plants

over the past 2 years, can expose bag machine operators to more significant dust concentrations than the filling process itself. To keep bag

operator exposure within acceptable dust levels established by the Mine

Safety and Health Administration, U. S. Department of Labor, the significance of these background sources must be realized, and the sources must

be identified and controlled. The purpose of this work is to identify

common background dust sources and show the significant effects that

they have on the bag operator's overall dust exposure.

1 Mining engineer.

2supervisory physical scientist.

Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA.
2

INTRODUCTION

The purpose of this report is to identify some common background dust sources

in and around the bag-filling area. Although these background dust sources are

often unrecognized, they can be more significant sources of contamination than

the dust generated from the bagging

process.

The bagging of mineral products into

50- and 100-lb paper bags is common

throughout the entire mineral processing

industry. Bagging is performed by a bag

operator who si ts in front of a two- or

four-station fill machine. The bagger's

function is to attach empty bags to the

fill nozzles as filled bags are ejected

from the machine. If the product being

bagged is extremely fine (smaller than

200 mesh), the bag operator's dust exposure is usually one of the highes t in the

plant.

The bag operator is exposed to two primary dust sources. The first is product

blowback during the bag-filling cycle.

This occurs as excess pressure is released from around the fill nozzle during

fi 11 ing. As the excess air forces its

way out of the bag, product is forced out

wi th it. This blowback of product leaving the bag creates a considerable amount

of dust. The second maj or source is the

"roos te r tai 1" of product thrown f rom the

bag valve and fill nozzle as the bag is

ej ected from the fil 1 machine. Since the

bag is pressurized as it leaves the machine, product spews out of the valve for

a few secDnds.

These two known dust sources must be

controlled to reduce the exposure of the

bag operator. However, even when this is

done, the operator's exposure often remains over the threshold limi t value

(TLV) established by the Mine Safety and

Health Administration (MSHA). This is

due to background dust sources that,

in most cases, are not recognized by

plant personnel. Gravimetric dust measurements, which are the standard dust

measurements taken at mos t plants, only

indicate the overall dust exposure of

the worker averaged over an 8-h day. They do not identi fy the different

sources or the magni tude of the exposure

for specific periods during the workday.

Since respirable dust is not visible to

the unaided eye, many background dust

sources are difficult to identify. To

get an indication of the significance of

each source, an instantaneous dust monitor must be used. Then, as different

background-dust-producing events take

place during the workday, increased dust

concentrations and their related effects

can be determined.

This report presents common plant occurrences that increase the bag operator's dust exposure over the standard

baseline for dust generated at the fill

station (blowback and rooster tail).

These background sources were observed at

five different plants.

TESTING SETUP

Bag operators were monitored to determine their dust exposure at the work station. The dus t monitor used in all cases

was a real-time aerosol monitor (RAM-I)

deve loped by GCA Corp. under a Bureau of

Mines contract. This instrument uses a

light-scattering device to determine the

dust concentration of a sample drawn in

from the environment through a 10-mm cyclone. It is sensitive to changes in the

dust content (size, shape, refractive

index), but if calibrated to a specific

dust i its accuracy is within plus or

minus 10 pct of gravimetric samplers similarly equipped with the standard lO-mm

cyclone. 3

The 10-mm cyclone was used to sample

the respirable fraction of dust. This

cyclone is used in the Uni ted States for

dust compliance sampling. The cyclone

classifies "respirable" size particles

3Wiiliams, K. L., and R. J. Timko.

Performance Evaluation of a Real-Time

Aerosol Monitor. BuMines ie 8968, 1984,

20 pp.
that reach the alveolar regions of the

lungs, and are usually considered to have

aerodynamic diameters (AED) of 10 ~m or

less.

The cyclone was at tached to the bag operator's lapel to give an accurate indication of his dust exposure. The cyclone

was connected to the dust monitor

by tubing to allow the operator to work

with minimal interference (fig. 1).

The signal from the RA-I dust monitor

was fed into a strip-chart recorder, a

continuous trace of the bag operator's

dust exposure was recorded relative to

The first three cases involve increased

exposure of bag operators due to their

clothes becoming contaminated by product,

which is emitted as they perform their

work. Figure 2 shows a case in which the

bag operator became soiled with product

from a fill nozzle that did not shut off

after the bag had ejected from the fill

machine (case 1). The operator's recorded dust concentration before this

happened was approximately 0.1 mg/m3;

this increased to 1.01 mg/m3 after becoming soiled with product. Figure 3 shows

a similar occurrence when the bag operator became contaminated with product when

accidentally hitting the fill button

while changing the saddle height (case

2). Since there was no bag on the nozzle, product flowed from the fill nozzle,

soiling his clothes and increasing his

dust exposure level from 0.07 mg/m3 to

1.15 mg/m 3. In both cases, the background dust source was the operator's

work clothes. Figure 4 shows an operator

soiled with product. In a case such as

this, clothes should be changed or

cleaned using a vacuum system. Figure 5

shows a case where an operator finished

bagging and cleaned his clothes using

compressed air (case 3), which is not an

acceptable cleaning technique. His dust

exposure increased substantially, from

0.19 mg/m3 to 0.45 mg/m3 as he dusted

off. In all three cases, the dust source

was considered background because it was

not being generated by the bag-filling

operation. The operator was contaminated

originally from the bag-filling process,

3

time. The times of different dustproducing occurrences throughout the

plant were noted. When an occurrence appeared to increase the operator's dust

exposure, that time segment was analyzed

using a planimeter to calculate the area

under the curve for that time period, and

an average concentration for that period

was then calculated. The increase in the

bag operator's exposure due to the background source was established by comparing the operator's baseline dust concentration with the increased exposure

associated with the background source.

RESULTS

but if the clothes were properly cleaned,

his dust exposure would be minimized.

FIGURE 1.-Sampllng setup to monitor a workets dust exposure.
4

It 2.0

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TIME, min

FIGURE 2.-Case 1: Operator's exposure after becoming soiled with product from fil nozzle that did not shut off.

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FIGURE 3.-Case 2: Operator's exposure after becoming aolled with product while changing saddle height.

These next three cases deal with increased dust exposure from broken bags of

product material, two cases of a broken

bag at the fill station, and one of a

broken bag during the conveying process.

In most cases, bags break because of

flawed bags delivered from the manufacturer and not due to ripping by the operator or the conveyor. Figure 6 (panels A

and B) shows cases 4 and 5, in which the

bag operator's exposure was from a broken

bag at the fill station. In case 4, dust

exposure was increased from 0.07 mg/m3

to 0.40 mg/m3 immediately following the

bag breaking; in case 5, the operator's

exposure went from O. i i mg/m3 to 0.35

mg/m3 immediately following the break.

In figure 7, the bag operator's dust exposure was from a broken bag during the

conveying process (case 6). Dust exposure increased from 0.07 mg/m3 before to

0.48 mg/m3 after the bag broke. In this

case, even though the bag broke during

conveying, dust contaminated the air in

the mill substantially, then flowed into

the bagging room where the operator is

located. This occurs because the exhaust

ventilation system in the bag loading

area creates a negative pressure that

draws background air in from the mill.

The next three cases are all somewhat

different, except that in each case the

bag operator was totally unaware of the

additional activity or background source
5

FIGURE 4.-Bag operator after becoming soiled with product.
4 6

TIME, min

FIGURE 5.-Case 3: Operator's exposure from blowing

clothes with compressed air.

6

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10

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that was increasing his dust exposure.

In case 7, a truck was being bulk loaded

outside the mill where bagging was being

performed. The dust generated from this

bulk-loading process traveled through an

open door into the mill, contaminating

the bag operator (fig. 8). The bag operator's dust exposure of 0.17 mg/m3 before

bulk loading began outside and increased

to 0.42 mg/m3. Over the period of the

day, there can be a substantial number of

trucks bulk loaded at this position,

depending upon customer orders. In the

second instance, the bag hopper feeding

the bag machine overflowed with product

(case 8). As the product fell to the

floor, a substantial amount of dust was

dispersed (fig. 9). The operator's exposure was increased from 0.06 mg/m3 to

0.73 mg/m3 after the hopper overflowed.

In the last case, a coworker in the mill

was sweeping the floor one level below

with a push broom, and the operator's

exposure increased from 0.03 mg/m3 to

0.17 mg/m3 (fig. 10). Sweeping is an

2.0

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TIME, min

1.5

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TIME, min

FIGURE 6.-0perator's exposure from broken bag at fil sta"

tlon. A, Case 4; S, case 5.

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FIGURE 7.-Case 6: Operator's exposure from broken bag

during conveying process.
unacceptable

floor because

method of

of the dust

cleaning the

it generates.

7

Washing the floor with water, or vacuuming are the only two acceptable methods.

DISCUSSION

The results showed that a number of

different background sources can significantly increase the bag operator's dust

exposure. For bag operator exposure to

remain within acceptable dust levels as

established by MSHA, dust from the bagging process and background dust sources

must be controlled.

The TLV for respirable dusts containing

quartz (silica) is calculated by the following equation:

pct sii~ca + 2 = mg/m3.

In practice, personal samplers are attached to worker's breathing zone for

a normal 8-h workday. The respirable

1.5

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TIME, min

FIGURE 8.-Case 7: Operatots exposure from bulk loading

outside.

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8 12 16 ~o

TIME, min

FIGURE 9.-Case 8: Operatots exposure from bag hopper

overflowing.

particles are collected on a filter and

weighed to determine the average 8-h

exposure. This quantity is then compared

with the TLV calculated from the above

equation. The percent silica is determined by testing the material retained

on the filter, generally by X-ray defraction. A sample having 100 pct silica on

the filter will establish a TLV for the

operator of 0.1 mg/m3. Many times at

silica sand operations, although pure

ground silica is being bagged, the silica

content of the respirable dust is in the

50- to 80-pct range; thus, the TLV for

these plants is in the 0.20- to 0.12-

mg/m3 range. With these low TLV dust requirements, every effort must be made to

eliminate the background dust sources.

Table 1 shows the bag operator's exposure for each case showing the before and

after dust concentration with reference

to the background event, and the amount

of increase over previous concentrations

(before), which show as increase factor.

The TLV exposure time represents the

length of time necessary for the operator

to exceed his 8-h TLV standard when exposed to the background dust source at

the measured dust level. This is assuming a 50-pct silica content, which corresponds to a 0.2 mg/m3 standard. The

method used to calculate this TLV exposure time is presented in the appendix.

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TIME, min

FIGURE 10.-Case 9: Operatots exposure from coworker

sweeping floor one level down.

o 4
8

TABLE 1. - Nine cases of background dust exposure to bag operator

Dust conc, mg/mj Increase TLV exposure

Case Description Before After factor time using

0.2-mg/m3 std

1. . . Malfunctioning fill nozzle 0.10 1.01 10.1 1 h 35 min

2. . . Start button hit.......... .07 1.15 16.4 1 h 23 min

3. . . Blowing clothes........... .19 .45 2.4 3 h 33 min

4. . . Broken bag-fill station... .11 .35 3.2 4 h 34 min

5. . . . . .do. . . . . . . . . . . . . . . . . . . . . .07 .40 5.7 4 h o min

6. . . Broken bag-conveying...... .07 .48 6.9 3 h 20 min

7. . . Bulk loading outside...... .17 .42 2.5 3 h 48 min

8. . . Bag hopper overflowing.... .06 .73 12.2 2 h 11 min

9. . . Sweeping floor............ .03 .17 5.7 ....9 h 24 min

What must be considered with this TLV

exposure time is the duration of each occurrence. In case 3, when the operator

is blowing dust from his clothes with

compressed air, the dust exposure duration would be relatively short, since it

is unrealistic that an operator would

blow dust from his clothes for 3.5 hId.

On the other hand, there are examples

where one occurrence can overexpose an

operator (cases 1 and 2) where his

clothes have become contamnated with

product. The bag operator's dust exposure in both cases remain at significant

concentrations 30 min after becoming contamnated. If the operator decided to

finish loading the truck, or to wait until break time to clean his clothes, he

would be overexposed in 1.5 h, which emphasizes the importance of not wearing

previously soiled clothes. This would be

especially relevant in cold weather when

workers are wearing jackets that may not

be cleaned for long periods of time.

Another factor in the TLV exposure time

is the amount of air that is contaminated

by the background source. This is most

obvious with the three cases involving

broken bags. When the bags broke at the

fill station (cases 4 and 5), the product

fell through a wire mesh conveyor into a

hopper to recycle the product. The exposure time ranged between 5 and 15 min.

When the bag broke on the conveyor, the

significant exposure time was much longer

because the breakage contamnated a

greater volume of air. All the product

either remained on the belt or fell to

the floor, generating a substantial

amount of dust and contaminating

volume of air. Because of this,

po sure time doubled from that of

breakage at the fill machine.

It must be realized that the bag operator can be overexposed when a very small

number of these background sources occur.

The following are a few common background

sources observed and an estimate of the

number of occurrences necessary for the

operator to be overexposed:

a large

the exthe bag

Number of

occurrences

Soiled clothing.............

Bag breakage:

During loading............

During conveying..........

Bulk loading outside........

Bag hopper overflow.........

1

14-18

6-10

3- 4

3- 4

It must be remembered that these dust

sources are site related. A significant

background source at one plant might be

insignificant at another.

It is evident from table 1 that each of

the background dust sources had significant effect on the bag operator's dust

exposure and the importance of controlling these sources is clear. In each of

these cases, the background dust exposure could be eliminated or reduced.

An operator who becomes soiled with product should immediately vacuum or change

his clothes. He should not blow dust off

them with compressed air (case 3). If

broken bags are a problem, the answer might be to go to a better bag, for
example, a bag using stress kraft paper

or possibly a bag with additional plies.

A solution for contaminated mill air

might be to draw the air into the bag

room from a location away from the

loading activity, or to filter the

air using a canopy air curtain device.4

Contaminated air from óutside the mill

should never be permitted to flow into a

building and to contaminate workers as in

9

case 7 where bulk loading was being performed. Also, sweeping should not be

performed.

These case studies are designed to show

the extent and seriousness of background

dust sources in terms of the bag operator's overall dust exposure. A number of

dust control techniques are available to

deal with these background sources.5

CONCLUSION

The bagging of mineral products into

paper bags is common throughout the mineral processing industry. The bag operator's dust exposure at these plants is

usually one of the highest recorded for

the plant, especially when the product

being bagged is very fine (smaller than

200 mesh). Frequently, events not directly related to the bagging operation may be more significant sources of

contamination than the bagging process

4U.S. Bureau of Mines. Dust Free Work

Station for Minerals Processing. Technolo News, No. 230, Oct. 1985, 35 pp.

itself, sometimes increasing the operator's exposure 10 times normal. In cases

where the operator's TLV is low because

of high silica content, these background

sources have been shown to overexpose an

operator in less than 2 h. To effectively keep bag operator exposure at acceptable dust levels, these background dust

sources must be controlled.

SNational Industrial Sand Association.

Guidance and

Levels in the

ica Product.

35 pp.

Solutions to Reducing Dust

Bagging of Whole Grain Si lSilver Spring, MD, 1977,
10

APPENDIX. --CALCULATIONS TO DETERMINE TLV EXPOSURE TIME

To calculate the workers' TLV exposure

level, the following equation is used:

10 = mg/m3.

pct silica + 2

1. Assume 50 pct respirable silica

content, which calculates to be a iLV

value of 0.192 mg/m3. For convenience,

this value will be rounded off to 0.2

mg/m3.

2. This means that the workers' average dust exposure over the 8-h day, must

not exceed 0.2 mg/m3.

8 h x 60 min 480 min

US GOVERNMENT PRINTING OFFICE 1986~605-017140.064

480 min x 0.20 mg/in = 96 mg min/in.

3. By knowing the dust concentration

for a particular occurrence, for example

the 1.01 mg/in in case 1, and by dividing

this 1.01 value into 96, the TLV exposure

time can be determined.

96 mg min/in t 1.01 mg/in

95 min, or 1 h 35 min.

4. This means that at a dust concentration of 1.01 mg/in, in 1 hand 35 min,

the worker will reach his TLV level.

INT.-BU.OF MINES,PGH.,PA. 28320