Exposure of employees to man-made mineral fibers: Mineral wool production

Exposure

of Employees to Man-Made Mineral Wool Production

NUKTAN DIANE

Mineral

Fibers:

A. ESMEN, YE:HIA Y. HAMMAD’, MORTON CORN, WHITTIER, NANCY Korslto, MARTIN HALLER* AND RUSSELL A. KAHN

This paper reports the results of a study in which employee exposure to mineral wool fibers in the mineral wool production industry was measured. Data for this study were collected in five plants operated by three different corporations. The results of the study indicate that the average exposure is about 0.1 to 0.5 fibers/ml. The results also indicate that despite operational diversities among the plants the size and length distributions of airborne fibers were consistent. It was also found that there is an excellent correlation between average total suspended particulate matter and average fiber exposure for types of work activity found in the plants.

INTRODUCTION

The man-made mineral fibers are used extensively in a wide variety of industrial and consumer products, but there is relatively little information on the exposure of workers to man-made mineral fibers. In previous reports (Corn and Sansone, 1974: Corn et al.. 1976) we focused on concentrations of airborne fibers and particulate matter in three fibrous glass manufacturing plants and two mineral wool manufacturing plants. In these two reports the data were reported on a plant by plant basis. Because these investigations are a part of a larger and longer term environmental-epidemiological investigation of the health experiences of workers in the man-made mineral fiber industry. and because many similarities and differences in the entire industry are essential for understanding the character of the exposure of workers to man-made mineral fibers, this report presents a detailed analysis of worker exposure to mineral wool in the mineral wool production industry. The analysis is based on data gathered in five plants which are operated by three different corporations. DESCRIPTION OF FACILITIES

Approximately 80 persons are employed in this plant: 80% of these are associated with production and the rest with services. The plant was established in 1937 and since 1968 three production lines, each with its own cupola. have been in operation. The three lines are producing fibers with the same physical and chemical characteristics, but different final products emerge from each line. Slag and coke are charged to the top of the cupola where it is melted at about I Present 2 Mellon

address: Institute.

Tulane University. Carnegie-Mellon

0013-9351/78/0152-0262$02.00/O

New Orleans. LA. University. Pittsbur-gh.

PA

EXPOSI‘KE

‘1’0

MAN-bl.-ZDE

hllNEK.41.

FIBERS

263

2600°F. The melted slag is allowed to flow in open channels to the rotary spinners, where fiber attenuation is achieved by blowing steam under high pressure around the spinners. The formed fibers are vacuum drawn on a steel screen to form a blanket. In the forming section. a phenolic-type binder and an oil emulsion are sprayed simultaneously on the fibers. The fiber mat is passed through the curing oven where it is cured at about 400°F. The first line produces felt of high density that is used for insulation in industrial processes. The second line produces insulation batts of low density for home insulation. The third line produces granulated wool to be used either as blowing wool or for a special product called “Spray On”. It consists of wool, clay, cement. as well as other special admixtures which together form a slurry that is sprayed on steel columns for insulation. Plcirlt B

This plant operates on a three shift basis and employs approximately 160 persons, of which about 80%’ are associated with production. Since 1966, three lines producing the same fibers, each having its own cupola, have been in operation. Slag and coke are charged to the top of the cupola where they are melted at about 2500°F. The melted slag is allowed to flow in open channels to the rotary spinners, where fiber attenuation is achieved by blowing steam under high pressure around the spinners. The formed fibers are vacuum drawn on a steel screen to form a blanket. In the forming section, a phenolic-type binder and an oil emulsion are sprayed simultaneously on the fibers. The amount of binder sprayed on the fibers depends on the type of product. The fiber mat is passed through the curing oven where it is cured at about 400°F. The product of the first line is packaged bags of granulated fibers without binder which are used as blowing wool. The second line produces high-density insulation boards that are trimmed to the proper width and length by circular saws. The trimmings and rejects from this line are chopped and used as pouring wool. The third line produces insulation batts, with or without craft paper or aluminum foil facing. Pltrt1t c

Approximately 400 persons are employed at this facility, with approximately two-thirds of these associated with production and the rest with services. During the period of the survey, the plant operated with four shifts on a 24 hr basis. The forming section of the plant consists of five cupolas which operate at approximately 2600°F to melt a rock and coke charge; the latter are charged in layers. At the base of the cupolas centrifugal blowers “blow down” the melt to form wool fiber: unmelted “shot” falls by gravity to a wetted down conveyor screen which separates it from fiber. “Shot” is used for landfill. Wool conveyed from the forming process can be baled for storage or sent to a wool bin. Binder, newsprint, clay, and water are mixed in a mixing tank to form a slurry. Wool and perlite are introduced by mixing prior to delivering the final slurry to the Fourdrinier, a machine which forms wet board composed of approximately SO”r fiber. Perlite is pneumatically conveyed to the mixing area from a

264

ESMEN

ET AL.

perlite expander facility in an adjoining building. Drying ovens are utilized to produce dryboard with a predetermined moisture content. Finishing operations include cutting, planing, painting, and packaging. The two main product lines are panel and tile. Finishing operations are visibly dusty. Plarzt D

This plant operates over three shifts and employs approximately 90 persons of which about 80% are associated with production. The raw materials used are iron slag, iron ore, and coke. The raw materials are charged to the cupola where melting occurs at about 2400°F. The melted slag flows from the bottom of the cupola to a water cooled spinner; steam is used to attenuate the fibers. There are three production lines, with each line being fed by its own cupola. The three main products produced are bailed wool, blowing wool, and batts. The bailed wool contains fibers without binder. It is shipped to other company plants where it is processed into ceiling tiles. Blowing wool without binder is used to make batts for acoustical and thermal insulation. Batts are produced in thicknesses ranging from 11% to 6 inches and may or may not have a paper backing, vapor barriers, or aluminum foil. The batt forming line is equipped with binder sprays and a curing oven to apply a phenolic-type binder. Plant E

This plant operates on a three-shift basis with a total of about 130 persons, of which three-fourths are involved with production. However, there is a significant difference in the number of employees between day and night shifts. A combination of raw materials, including shot from cupola preparation of rock wool, slag from steel preparation, scrap from the plant, and fly ash, are mixed and delivered by conveyor to one of two reverbatory furnaces. The material is molten at approximately 2300°F. Melt is rotary spun into fiber, blown out with steam and oil is added. Two production lines can be operated with fiber formed from each furnace. Only the first line from one furnace is equipped to spray binder on the fiber before a fiber mat is formed; the mat is cured in an oven to form board. The two lines from the second furnace are not equipped for spraying and curing. If fiber is not cured, but contains binder and oil, it is called green felt: green felt can be produced on two of the four lines. Board, a relatively rigid material, is packaged for use with or without screen facing. If less binder is added to the mat prior to curing, a product referred to as “blanket” is formed. Blanket can also be faced with metal screen. Green felt is edge-trimmed by sawing and chopping into lengths before being loaded onto carts to be taken to the molded forming, curing, and packaging area. Fiber with oil but without binder is used to form “tufts” for P.V. Block production. Tufts, binder, and clay are mechanically mixed in a wet slurry to form the starter material for P.V. Block. The slurry is delivered to a continuously moving horizontal chain belt, where it drains to leave a wet block. The wet block is pressed and cut to size prior to curing in an oven. The product, P.V. Block, is a rigid block which may, for special applications. be further trimmed in the packaging area.

ESPOSI’RE

TO

MAN-hf.&DE

MINERAL

FIBERS

265

Green felt produced in the operation described above is stored on carts and then placed by hand into molding machines which consist of molded half-round forms on carts with wheels; the latter are rolled into a molding oven after being loaded with green felt. There are 24 molding ovens, each serviced by two operators. The maximum number of ovens in operation at one time is 12. Ceramic fibers produced elsewhere, clay, and binder are mixed to form a slurry. This slurry is drained and the residue is hand cut and molded to form wet ceramic fiber blocks. The blocks are cured and packaged. Trimming operations may be performed on odd shaped blocks prior to packaging. SAMPLING Sampling

AND DATA ANALYSIS Methods

and Analysis

METHODS of Samples

In this study personal samples were collected from within the breathing zone of the employees. These are obtained by having the worker wear a filter with holder and pump while he performs his paperwork tasks. Particular attention was paid to not permitting personal samples to be contaminated with dust from worker garments. It was not possible to sample all individuals in each area; therefore, names were randomly selected from job sites which comprised the allocated number of samples to be obtained in a given zone. Prior to the day on which sampling was scheduled, the men and/or women selected to wear personal samplers were “briefed” as a group for 15-30 min by one of the survey team. They were informed of the purpose of the survey, the function of the instrument they would wear, how the sample would be analyzed, etc. Any questions they raised were answered. Cooperation of employees was excellent in this investigation. Samples of total suspended particulate matter were collected on 37-mm diameter membrane filters with a pore size of 0.8 Frn. Before use, the filters were desiccated for 24 hr in a glove box and then weighed to t 0.01 mg in the glove box. The filters were then placed in completely enclosed plastic filter holders. For sample collection one part of the filter holder was removed, which allowed the filter to be used in the open-face mode. To prevent leakage into the filter holder, a cellulose band was allowed to shrink tightly around the circumference of the filter holder, thus, effectively preventing contamination of the sample. Samples of air were drawn through the filter at a flow rate of 2 liters per min by means of a pump calibrated against a soap bubble meter for volumetric rate of air flow with the filter and sampling line in place. In use, the filter was oriented face down. During sampling, air-flow rate was checked periodically and noted on the field sampling data sheet. After collection, the filter was sealed in its holder until analyses could be performed. The sample and filter were desiccated and reweighed in the laboratory. From the gain in weight and air volume sampled, airborne concentrations of total suspended particulate matter, expressed as milligram per cubic meter, were calculated. Fiber counting and sizing techniques were based on the method described by Edwards and Lynch (1968) for analysis of asbestos in air samples. This method

266

ESMEN

ET AL.

served as the basis for procedures recommended by the National Institute for Occupational Safety and Health for asbestos evaluation in the Criteria Document for AsbestoG. and subsequently adopted by the Occupational Safety and Health Administration. All of our sampling and evaluation procedures adhered to the guidelines offered by the Occupational Safety and Health Administration for asbestos (OSHA Standard 29 CFR 1910.93). The sensitivity of our procedure for fiber detection. that is, the lowest detectable fiber concentration by optical phase microscopy, was 0.0012 fibers/ml. Although it was possible to discern particles of about 0.3 pm size, resolving power was determined to be approximately 0.56 pm. Filters were cut in half; one half of the filter was used for analysis by optical phase microscopy, while the other half was placed in a coded glass Pyrex boat. The boat and filter were placed in a Low Temperature Asher. where ashing occurred at a constant pressure and oxygen flow rate. Reagent grade isopropyl alcohol was poured into the glass boats containing the ashed sample. The boat was then placed in an ultrasonic bath and agitated for approximately two min. Afterward, the sample was refiltered on a 0.22 pm pore size Millipore membrane filter. The filter containing the collected sample was placed, particulate side up, on a clean glass slide. The edges of the filter were fastened to the glass slide using transparent tape. The assembly was exposed to acetone vapor to fuse the filter by placing it on a wire mesh bridge in a petri dish containing acetone. Fusion of the membrane filter was considered complete when the filter became transparent. The fused filter assembly was then placed in a vacuum evaporator and the collection side of the filter was coated with carbon. Using a petri dish as reservoir, a piece of polyurethane sponge was immersed in acetone and coded 200 mesh. 3-mm diameter EM grids were placed on the sponge. Squares about the size of the grid were cut from the fused and coated filter and placed. particle side up, on the EM grids. The polyurethane sponge provided a porous platform for gentle dissolution of the original membrane filter, leaving the particulate adhering to the insoluble carbon film, which was in turn supported by the EM grid. Sample grids were observed in a Phillips Model 200 electron microscope operated at 80 kV and a magnification of 10,000; minimum beam intensity was utilized to minimize beam effects on samples. Eight specific grid fields on each grid. or a total of 16 fields for the two grids prepared from each ashed sample, were evaluated. All fibers less than 0.2-pm diameter in the tield of view were subjected to electron beam diffraction to determine if they were crystalline or amorphous. Electron beam diffraction was also performed on some larger fibers which had a thin edge. The remaining fibers were determined to be man-made mineral fibers by their morphology. For example, amorphous fibers do not contain “thickness contours” which are characteristic of crystals and are visible with many crystalline fibers. Amorphous fibers were photographed at 5000 magnification. The microscope was calibrated with a diffraction grating having 54,864 lines per in.

l.tSI’()SL’Rt:

TO

The total fibers sampled were

~\IAN->l.3.L)F

calculated

MlNEK.4L

using the equation

No. of fibers per milliliter

where:

N A Df (I F 1’ P

= = = = = = =

number area of dilution area of number volume portion

267

I-IBERS

below:

N .A .Df = F n . v .p

(1)

of fibers counted. filter used for refiltration after ashing of sample ( 182.7 mms). factor. field of view ( 10404 ,um’). of fields evaluated (16). of air sampled (cm:‘). of original sample filter used for ashing (0.5. 0.25, or 0.13).

Because the area of the EM field and area of filter used for refiltration constant. the above equation can be simplified to: No. of fibers per milliliter

= 17.55

x

N Of F.V.P

were kept

(2)

The sensitivity of these procedures for fiber detection. that is, the lowest detectable fiber concentration by electron microscopic methods. was 0.0023 fiber/ml based on an approximately eight hr personal sample collected at a flow rate of 2.0 liter/min. This sensitivity corresponds to the observation of one fiber in the sample when the methods described above are used. The method of transfer and analysis described were subjected to rigorous verification by spiked samples: the precision of the method was found to be within 3% of the mean fiber concentration. During the initial phase of the study the electron microscope analysis of the samples was made by a different method. (Carpenter and Spolyar, 1945) The method described is a distinct improvement over the earlier methodology. Analysis of Data When exposures to an agent are studied within a given plant, the variation of the exposure data obtained depends, among other factors, upon the habits of the workers, the process. the availability of control measures and the characteristics of the agent. When exposures to a given agent occurring in different plants are compared. there is added variability because of the differences in the abovementioned operational characteristics in the different plants. In the study reported here. where work practices in facilities operated by different corporations are associated with the fiber exposure data, the variations in the data may obscure efforts to focus on the common characteristics of airborne fibers and their concentrations. In order to avoid this difficulty, the comparisons of data from different facilities are made partially on the basis of the similarity of the tasks performed by employees during air sampling. Prior to assigning a data point to an activity classification category, the field data sheets are carefully examined to pinpoint the task of the employee as closely as possible; the tasks are classified in nine categories shown in Table I. Although these categories may or may not reflect the occupational titles recorded by the companies (or by the unions). they are based on the observations carried out during sampling: hence. they reflect a realistic classification of the tasks performed.

Classification Fiber

forming

Duties All hot-end workers, charging operators

cupola

operators.

batch

mixers,

transfer

operators,

Production

A

Cold-end workers cutting. sawing, stuffing operators,

who are in direct contact with fibers but are not involved in sanding. or finishing operation. Workers such as bailers, machine tenders

Production

B

Workers

Production

C

Workers involved in general manufacturing operations, such as painting finished boards. molding drier ovens, handling boxed and/or packaged material and similar jobs where direct contact with fibers is less likely than Production Aor B.

involved

in trimming,

of package

sawing.

material,

cutting

fork

truck

and finishing

Shipping

Transportation ators

Janitorial and clean-up

Jobs involve sweeping floors. cleaning dust collectors, machinery. general cleaning within the plant. These workers are usually employed on machines that are not operating on that particular day or they are at entry level.

Maintenance

Maintenance the production

Quality

control

Workers

Isolated

jobs

Maintenance and unlikely to have supervisors. etc.

workers who repair production area on need basis.

sample

product

RESULTS

shop direct

and perform personnel contact

operators,

machinery

tests to ascertain who work with fibers.

shipping

yard

and generally product

in isolated areas boiler operators.

oper-

work

in

quality which are shipping

AND DISCUSSION

of’ Airborne Dust nithin Plants The results of measurements of airborne particulate matter collected on filters are presented in Tables 2-4. Concentrations are expressed as either Total Suspended Particulate Matter (TSPM), milligram per cubic meter, or fibers per cubic centimeter (fibersicm3). The TSPM concentration averages varied considerably within plants, as well as between plants. The overall average concentration of TSPM ranged from a low of 0.6 mg/m3 to a high of 3.98 mg/m3. The fiber concentrations also varied considerably within and between plants. Most striking, perhaps. is the recognition that average exposure concentrations in different plants can differ by fivefold or more when similar processing operations are performed. The overall total fiber concentrations are approximately 0.1 fibers/ml; the overall averages indicate that the highest exposures occur in Production B. Quality Control and Janitorial areas (Table 3). This is not unexpected, because these jobs entail machining of the finished product, cleaning of product scraps, and taking samples out of product lines. Surprisingly, relatively low values of concentration were measured in Production A where the workers are in intimate contact with the fibers produced. The explanation for this observation lies in the production process itself. Where the fibers are produced and conveyed to points of bailing and packaging, large amounts of air are moved to capture the fibers from Concentmtions

Resp. Total Resp.

Total Resp. Total Resp. Total Resp. Total Resp. Total Resp. Total Resp.

A Production B

Production C Shipping

* Numbers

Quality control Isolated jobs

Maintenance

0.122 0.055 0.065 0.036 0.146 0.074 0.080 0.043 0.192 0. I?? 0.033 0.017

0.067 -

0.071 0.042 0.165

Average**

in parentheses

Total Resp.* Total

Janitorial

0.45 1.08 0.09

indicate

0.044 0.025 0.077 0.043 0.013 0.004

0.051 0.027 0.041 0.017

0.011

0.033 0.023 0.017

Plant

0.134 0.069 0.307 0.201 0.059 0.034

0.188 0.079 0.088 0.054

(7)

(2)

(6)

(I)

(5)

16)

(19)

(9)”

the number

-

--

- 0.278 -

- 0.120 - 0.073 - 0.630

Range

A

COSCESTRA~IOS

the number

- 3.58 (6) - 1.98 (2) - 2.14 (7)

0.91 0.86

2.28 1.34 2.09 2.05 I.53 0.96

indicate

- 3.24 (9)” - 9.47 (18) - 5.59 (6) - 2.51 (5)

0.94 0.26

Range

A

1.65 2.53

in parentheses

Fiber forming Production

a Numbers

Fiber forming Production A Production B Production C Shipping Janitorial Maintenance Qual. Control Isolated Jobs

Average+

Plant

of samples.

0.022 0.009 0.026 0.010 0.022 0.012 0.018 0.007 0.025 0.009 0.020 0.01 I

0.013 0.056 0.025

0.015 0.011 0.028

Average

Range

B

KA I IONS

-0.053 0.040

0.030 0.012 0.046 0.020 0.028 0.015 0.042 0.020

- 0.04 -

* Respirable

0.004 0.002

0.013 0.007 0.008 0.004 0.015 0.008 0.008 0.003

0.004

- 0.024 - 0.016 - 0.109

Range

Plant B

0.009 0.007 0.004

average

fibers

(5)

(I)

(IO)

(3)

(8)

(4)

(1)

(22)

(5)

AS DE.TERMISED

* Arithmetic

0.44 1.08 (5) 0.02 - 8.05 (22) (I) 0.01 - 0.52 (4) 0.13 - 1.01 (8) 0.23 0.91 (3) 0.02 - 0.92 (IO) 0.03 - 1.47 (5)

Plant

COS~.LNI

OF FIBERS

of samples.

0.76 0.67 0.39 0.26 0.66 0.58 0.54 0.09 0.57

Average

Wttt:~~

2

3

is defined

0.240 0.185 0.154 0.128 1.104 0.827 0.196 0.126 0.077 0.068

0.190 0.524 0.399

0.146 0.119 0.235

Average

(14)

(13)

with

- 0.101 - 0.091

0.017 0.006 0.005 0.004 0.008 0.000 0.008 0.003 0.022 0.017 0.002 0.001

0.004

0.064 0.055 0.169 0.150 0.145 0.102 0.074 0.057 0.131 0.065 0.168 0.115 3.0 pm.

--

- 0.074 -

- 0.195 - 0.13 - 0.153

Range

(IO)

(2)

(9)

(5)

(7)

(6)

(17)

(IO)

0.077 0.056 0.032 0.026 0. I I I 0.082 0.089 0.058 0.035 0.029

0.050 0.193 0.148

0.578 0.365 0.083

Average

average.

0.011 0.010

0.005 0.002 0.019 0.008 0.025 0.012 0.011 0.006

0.005 0.038 0.025

-

0.472 0.349 0.054 0.044 0.277 0.215 0.239 0.167

- 0.067 - 0.056

--

- 0.263 - 0.953 - 0.708

(4)

(I) (12) (8) (22) (7) (3) (14)

(5)

(13)

(3)

(6)

(22)

(9)

(13)

4.10 - 2.26 2.24 - 3.6X - 2.83 - 2.09 - 6.88 - 1.41

Range

E

- 0.374

Range

E

0.67

4.10 0.14 0.51 0.52 0.72 0.46 0.05

Plant

0.014

Plant

4.10 1.34 0.96 1.27 1.31 1.08 1.96 0.90

Average

** Arithmetic

(FlB~RS/tIIm) D

0.006 0.003 0.007

less than

0.038 0.025 0.049 0.037 0.056 0.031 0.032 0.018 0.076 0.041 0.045 0.030

0.030 -

0.092 0.073 0.049

Average

Plant

MICROXOPY

1.79 (5) ~ 1.78 (6) - 2.24 (5)

-

0.21 0.06 0.43

-

0.87 1.04 I.19 1.63 1.25 1.38

Range

D

1.16 - 5.7b(lO) 0.09 - 4.71 (17)

Plant

2.18 1.08

Average

t (mgim’)

diameter

(2)

0.496 0.241 (7) 0.276 (5) 0.244 I.72 (41 1.29 - 0.65 (II) 0.367 -

- 0.532 1.26 - 0.958

- 0.231 - 0.207 - 0.650

Range (7)

- 7.79 (2)

CONRAS~

-~ -

Dra

- 5.43 (7) - 3.00 (13) 14.00 (13) - 3.77 17) - 3.80 (5) - 23.64 (4) - 22.40 (9)

Range

c

TOTAL

by fibers

0.053 0.045

0.180 0.087 0.101 0.069 0.144 0.117 0.041 0.031

0.057 0.206 0.185

0.068 0.035 0.075

Plant

BY PHASE c

0.53

0.66

TABLE

1.31 1.51 1.74 1.00 1.00 4.50 0.73

Plant

2.18 2.05 5.49 2.14 1.77 14.16 4.99

Average

OF AIKBORN~

TABLE

E CA

2

$

E

i 2

%

7 z

5

2

270

E:SMEN

ET AL.

PLANT 0 t 0 A 0

PERCENT

FIG:. I. Diameter

LESS

distribution

THAN

INDICATED

of airborne

fibers

c E A D B

SIZE

(all plants).

the fiberizer units and deposit them on conveying mechanisms. represents a built-in ventilation system for the operation.

This process

In the five plants studied there were significant variations in the processes of manufacturing and handling of material. However, the size and length distributions of the airborne fibers were consistent. as shown in Fig. 1-4. The size distributions of the fibers were calculated by matching the distributions determined by light and electron microscopes. The correct concentration of fibers less than 1 pm in diameter was assumed to be the one determined by electron microscopy: the correct concentration of fibers greater than I pm in diameter was assumed to be the one determined by light microscopy. The size distributions obtained indicate that, on the average, approximately 50-6052 of the fibers are less than 3 pm in diameter. This calculated result is confirmed by the data obtained from microscopic analysis (Table 3). About 809 o f the fibers were less than 50 pm in length and about 10% were less than 10 pm in length (Fig. 2). As shown in Fig. 3, there was little change in the fiber length distribution for airborne dusts collected in different zones in a given plant. Data from Plant D is utilized in Fig. 2 to present a representative distribution of fiber lengths between zones. Similarly, fiber diameters determined for fibers in airborne dust collected in different zones did not change appreciably. Fig. 6 utilizes Plant A data to illustrate a representative distribution. “Respirable” fibers were defined as all fibers less than 3.0 pm in diameter: about 8Oq of all fibers and about 705Y of fibers less than 3 grn were less than 50 pm in concentrations presented are somewhat conlength. Therefore, the “respirable” servative. Table 4 indicates that relatively few fibers were less than 1 pm in diameter. The concentrations and the characteristics of the fibers determined in this study

271

PLANT l c + E 0 A n D 13 B

01

I

5

PERCENT

FIG.. 2. Length

10

LESS

20 THAN

distribution

40

60

80

INDICATED

of airborne

SIZE

fibers

tall

plants)

can be compared to previously reported results based on similar measurements performed directly on samples of fibrous glass and. to a certain extent, to measurements of asbestos. Mineral fibers resemble fibrous glass with respect to both concentrations measured and the respirable and nonrespirable fiber fractions. The overall airborne concentrations of both types of fibers are on the order of 0.1 fibers/cmj, and the respirable fraction accounts for about 5OV of the fibers. The fraction less than 1 pm in diameter is about one order of magnitude lower in concentration than the fraction containing fibers greater than 1 pm in diameter. This finding contrasts sharply with reported observations of asbestos in air, where

-* em

PERCENT

FIG.. 3. Length

LESS

distribution

THAN

INDICATED

of airborne

WORK A + x 0 D 0 0 0

AREA I 2 4 5 6 7 B 9

SIZE

fibers

(all

plant\)

212

ESMEN

ET AL.

WORK A +

2

AREA I 2

+XaDY

05

5

20 PERCENT

FIG:. 4. Diameter

40 LESS

distribution

60 THAN

80 INDICATED

of airborne

95

99

99.8

SIZE

fibers

(plant

A).

submicron diameter fiber concentrations, as determined by electron microscopy. are 10 to 400 times the concentration of fibers with diameters larger than I Frn. Also, asbestos fiber concentrations in places where major amounts of asbestos are used are very rarely as low as the concentrations observed for fibers of mineral wool, unless very sophisticated control devices are used. This observation is readily explained by the breakage characteristics of man-made mineral fibers. The man-made fibers tend to reduce in length upon breakage whereas asbestos fibers tend to separate to minimal fibrils. thus, generating large amounts of submicron diameter fibers (Assuncao and Corn, 197.5). The motivation behind this industry-wide survey was not only to determine the existing levels of exposure to mineral fibers, but also to characterize the exposure in a manner that would permit reconstruction of historical exposures to mineral fibers from the existing data. The historical reconstruction of the exposures is of importance in studying the relationship between employment and long-term illness. Historically unknown or unclassifiable exposures introduce marked dilution factors which affect the findings relative to a cohort group in an epidemiological investigation. For example, if in an epidemiological study the total plant populations are used as the exposed group with the exposure taken to be at fiber concentrations found in a current survey, then the results of the study are diluted in the sense that enormous excesses of effect would be needed to see the specific hazards. On the other hand, an effect due to very high historical exposures may be mistaken as an effect due to much lower exposures of the current study. These weaknesses may be minimized if the survey data are used to reconstruct a historical exposure scheme. In this study there was sufficient internal consistency of the data to perform such a reconstruction.

ESPOSYRE

‘I.0

MAN-MADE

XINF.RAL

FIRFRS

273

274 032 I

.

WEIGHT

FIG:. 5a. pressed

Relationship

as fibers/millimeter

between and

fiber weight

CONCENTRATION,

concentrations concentrations.

MC/M”

determined

by

phase

contrast

microscopy.

ex-

(mg/m2).

As noted above, average TSPM concentrations varied considerably within plants and between plants. Furthermore. when TSPM concentrations were compared to fiber concentrations on a single-sample basis the correlation between TSPM and fiber concentration was found to be very weak. In other words, if the TSPM concentration and fiber concentration of each sample are compared. the correlation between the two measurements is poor. A typical result for measurements obtained in one facility is shown in Fig. 5. However. if average TSPM concentrations for work activity classes are compared to average fiber concentrations for the same classes, then the correlation is excellent (1’ = 0.934), as shown in Fig. 6. The explanation for this result is that the fibers constitute a very small portion of the totai dust. and for each sample the contribution of fibers to the total weight of dust is small. However, when TSPM, which is a measure of “dustiness. *’ is taken as a measure of work practices. then it is reasonable to expect that a correlation will exist between fiber exposure and general “cleanliness” of the operation. Historically. this is an important point because the only reported concentration measurements available in these types of facilities prior to the 1970’s were those of total dust concentration. The analysis of the TSPM data, as a measure of work practices. was performed by investigating the internal consistency of the data among work activity classes. TSPM was taken as a measure of the “cleanliness” of the work activity, and a ranking order was assigned to each activity across the plants. In other words. a

275

026

0.2c \

0"

.

:: 0.16

. ..

ooe

0.04

*

:

.

.

. . a* .

.

- . . : * .

C

I

2 WEIGHT

FIG.. 5b. Relationship .tc fibers/millimeter

and

between weight

FIG.. 6. Relationship exposure.

between

plant

TSPW

work

4

CONCENTRATION,

fiber concentrations concentrations (mg/m’).

AVERAGE

_

3

BY

practices

5 MG/M3

determined

WORK

AREA

measured

6

by electron

-

microscopy.

expressed

MO/H’

as average

TSPM

and

average

fiber

276

ESMEN

Plant A (TSP:FIB) Fiber forming Production A Production B Production C Shipping Janitorial Maintenance Quality control Isolated jobs Rank

correlation

Plant B (TSP:FIB) S:S 5:5

4:‘l I:’ -

3:3 5:s

i:t 22 2:2

5:s

2: 3 I: I

5:s 5:5 3:3

3:4

5:s

coefficient.

ET AL.

Plant C (TSP:FIB)

Plant D (TSP:FIB)

Plant E (TSP:FIB)

23 4:4

3:3

414 4:3

?:2 3:3 3:4 4:3 3:2

-

3:4 4:4 2:2

4: 1

l:?

2:3

22 2: I I:1 2: I 1:I I: I 1:l

I:1

I’ = 0.82

specified activity was compared between plants and the plants were ranked in The plant that had the lowest TSPM for a given activity order of “cleanliness.” was ranked as one and the plant with the highest activity was ranked as five. A similar rank ordering was performed for each plant on the basis of the average fiber exposure for the work activities. The results of the ranking exercise are displayed in Table 5. These two observations, close correlation between average TSPM and average fiber concentration and close correlation in ranking, may be utilized to equate the TSPM measure to a relative measure of work practices, housekeeping, and ventilation in a plant. From a historical perspective, there is no reason to assume that the work practices as measured by TSPM concentrations were not correlated to the fiber concentration. Therefore, the correlation found in this study may be used to estimate past exposures to fibers. The results of this study indicate that (Fig. 6) the fiber exposure level may be approximated by: Fiber concentration as determined by phase contrast microscopy equals (TSPM)

x (0.08 + 0.04)

The above formula applies to mineral fiber only and is based exclusively on averages of dust concentrations. An additional difficulty in estimating historical concentrations is associated with the correlation between gravimetric analysis of airborne dust and impinger counts as a measure of “dustiness.” Most of the historical data are in the form of impinger counts. We have made a rough estimate of the equivalency between weight concentration of dust determined by filter collection and number concentration determined by counting particles (fiber or otherwise) on filters. If an 80% particle collection efficiency is assumed for the Greensburg-Smith impinger, this equivalency is about 1.2 + 6 mg/m3 per million particles per cubic foot (mppcf) for the mineral wool plants. Carpenter and Spolyar t 1945) reported impinger counts taken in 1935 with concentrations about 12 to 26 mppcf and about 5 to 10 mppcf, subsequently. These results suggest that the average historical exposure to airborne fibers was about 1.2 to 2.5 fibers/ml. Even with minimal engineering control

EXPOSURE

TO

MAN-MADE

MINERAL

277

FIBERS

devices during the late 1940’s and early 1950’s. the exposure concentrations are suggested to have been, in general. about 0.5 to 1.0 fibers/ml. It is again important to note that there is consistency in the results when the measured values of dust concentration are treated in terms of averages, rather than single values. Further analysis has permitted the differentiation between historical exposures for each work activity. However, the current data suggest that the variation between work activity classes is small for this specific industry: therefore, further analysis does not result in calculated significant deviations from the estimated overall historical exposures. REFERENCES Assuncao.

.I. and

Corn.

M. (1975).

chrysotile asbestos fibers. Carpenter. J. L. and Spolyar. 1. .IllC’d. .I. Corn, M. and

Sansone.

fiber concentrations Corn. M.. Hammad. and

G. H. and of asbestos

Amc,r. L. W.

E. B. t 1974).

effects

fntl.

of milling

Hy#.

(1945).

on diameters

and

lengths

of fibrous

glass

and

./. 36, 81 I.

Negative

Determination

chest of total

X-ray suspended

findings

in a mineral particulate

matter

wool

industry.

and

airborne

at three fibrous glass manufacturing facilities. Ertt,iro~. Res. 8, 37-52. Y. Y.. Whittier. D.. and Kotsko. N. (1976). Employee exposure to airborne

total particulate Criteria for a Recommended partment of Health. Edwards. tion

The

in two mineral wool facilities. Erwirr~7. Rcs. 12. 59-74. Standard Occ~rf~xrri~~/ttr/ Erpc~.srrre to A.shr.cto.t. NIOSH. 1972. pp. VIII-I to VIII-g.

fiber

matter

Lynch. .I. R. t 1968). dust on membrane

The filters.

method

used

AJI/I. Ckcrlp.

by the

HJ,~.

Public Health 11. l-6.

Service

U.

S. De-

for enumera-