reduction of occupation exposure to perchloroetylene and trichloroethylene in metal degreasing

7/30/2019 Reduction of Occupation Exposure to Perchloroetylene and Trichloroethylene in Metal Degreasing...

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Reduction of occupational exposure to perchloroethylene andtrichloroethylene in metal degreasing over the last 30 years: influences

of technology innovation and legislation

JULIA VON GROTE, CHRISTIAN HU RLIMANN, MARTIN SCHERINGER AND KONRAD HUNGERBU HLER

Institute for Chemical and Bioengineering, Swiss Federal Institute of Technology, Zurich, Switzerland

Occupational exposure to trichloroethylene (TRIC) and perchloroethylene (PERC) in metal degreasing is analyzed by calculating airborne concentrations

for a large set of possible exposure scenarios (Scenario-Based Risk Assessment, SceBRA). Different types of degreasing machines ranging from open-top

machines used until the 1980s to closed-loop nonvented machines used since the 1990s are investigated; the scope of the study is Germany.

Concentrations are calculated for different kinds of releases (emissions from open baths, leakage, release of contaminated air during loading and

unloading) with a dynamic two-box model for the near-field and the far-field. The concentration estimates are in good agreement with measured data.

The airborne concentrations are compared to maximum workplace concentrations (MAK values). The full set of scenarios shows for which situations

MAK values were exceeded and how the transition to newer degreasing machines reduced the occupational exposure by more than one order of

magnitude. In addition, numbers of exposed workers are estimated for different years. While more than 25,000 workers in the near-field were exposed to

TRIC and PERC in 1985, the number is below 3000 since 1996, which is mainly due to technology changes, rationalization, automatization, and

replacement of TRIC and PERC by nonchlorinated solvents.

Journal of Exposure Analysis and Environmental Epidemiology (2003) 13, 325340. doi:10.1038/sj.jea.7500288

Keywords: mass-balance models, occupational exposure, metal degreasing, trichloroethylene, perchloroethylene, risk screening, risk assessment

Introduction

Efficient control of human exposure to the solvents

trichloroethylene (TRIC) and perchloroethylene (PERC) is

important because these solvents cause a variety of toxic

effects, possibly including cancer (BUA, 1993, 1994;

ECETOC, 1994, 1999), and because they are widely used

as cleaning and degreasing agents with a potentially high

number of exposed workers.

Assessments of the occupational exposure to solvents such

as TRIC and PERC are impeded by the complexity of the

activity patterns of workers and the temporal and spatial

variability of workplace conditions (Matthiessen, 1986;

Jayjock and Hawkins, 1993; Fehrenbacher and Hummel,

1996). One difficulty is that it is often impossible to ascertain

exactly the conditions that determine the exposure in a

specific case. A second problem is that the variability of a

broad range of workplace conditions has to be included.

With respect to the second problem, which is in the focus of

the present study, sufficiently flexible methods for covering

the wide range of possible exposure situations are required.

To this end, we adapt the method Scenario-based risk

assessment (SceBRA), which has been developed for

characterizing exposure to solvents used in a variety of

applications (Scheringer et al., 2001), to the assessment of the

occupational exposure to TRIC and PERC in metal

degreasing. Metal degreasing is performed in highly variable

settings; for metal parts of different sizes and shapes; and

with machines of different capacities and technological

standard. Therefore, the variety of scenarios employed in

the SceBRA method is a suitable approach to characterize

this technology.

The objectives of this paper are (i) to quantify reliably the

exposure levels resulting from the use of TRIC and PERC in

various metal degreasing machines, ranging from highly

emissive machines developed in the 1950s and 1960s to

closed-loop machines used since 1990, and (ii) to demon-

strate the effect of technology development and stricter

legislation on the two dimensions of risk considered in

SceBRA, namely the risk quotient, that is, the ratio of

airborne concentrations and an effect level, and the number

of exposed workers. The scope of the study is GermanyReceived 20 November 2002; accepted 18 April 2003.

1. Address all correspondence to: Dr. Martin Scheringer, ETH Hoeng-

gerberg, HCI G127, CH-8093 Zu rich, Switzerland.

Tel.: +41-1-632-30-62. Fax: +41-1-632-11-89.

E-mail: [email protected]

Journal of Exposure Analysis and Environmental Epidemiology (2003) 13, 325340

r 2003 Nature Publishing Group All rights reserved 1053-4245/03/$25.00

www.nature.com/jea


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because this country has one of the strictest legislations

regulating the use of TRIC and PERC and because data on

machine types and numbers as well as working conditions

could be obtained.

The metal degreasing technology is described and theSceBRA method is introduced in the Materials and methods

section. The mass-balance models employed and the model

parameters describing metal degreasing settings are defined in

the section Model and Exposure Scenarios. The results

obtained for airborne TRIC and PERC concentrations,

risk quotients, and numbers of exposed workers for

different machine types and years are presented in the

Results section.

Materials and methods

Metal Degreasing

Metal degreasing can be carried out either with aqueous

systems, with hydrocarbons, or with chlorinated solvents.

Chlorinated solvents are often used in vapor degreasing

equipments for difficult cleaning tasks such as metal parts

that need to be totally dry, are very small, are temperature

sensitive, or have lots of cavities (Leisewitz and Schwarz,

1994).

The fact that TRIC and PERC are both suspected to cause

cancer (BUA, 1993, 1994; ECETOC, 1994, 1999) led to

several revisions in the German law regulating the use of

PERC and TRIC. The German 2nd Federal Immission

Protection Directive of 1986 (Zweite Bundes-Immis-sionsschutzverordnung : 2nd BImSchV) (BImSchV, 1986)),

the 2nd BImSchV of 1990 (BImSchV, 1990), and the

amendment of the 2nd BImSchV 1990 in 2001 (BImSchV,

2001) prescribe not only the chemicals to be used in different

applications but also technological standards of metal

degreasing machines, which have changed substantially overthe last 30 years.

The technological development of metal degreasing

machines can be grouped into five machine types (see

Figure 1). Information on these types was taken mostly

from sales brochures (Manufacturers, 19602001). Type I

machines are fully emissive open-top machines with water-

cooling at 151C. They consist of several solvent baths and a

vapor bath and have a suction device at the rim of the baths.

Type I machines are followed by open-top machines with

electro-cooling at 21C (type II). In type III, the baths are

encased. Type IV machines are the first one-chamber metal

degreasers (there for the first time the solvent is brought to

the metal parts and not vice versa) with an integrated

recirculated air dryer condensing the solvent and a recycling

loop. The parts are dried with refrigeration-cooling at

temperatures between 201C and 401C. Type IV machines

and some upgraded type III machines fulfill the 2nd

BImSchV of 1986. The modern type V machines, which

are in use today, are closed one-working-chamber machines,

with closed-loop drying and recycling systems with refrigera-

tion-cooling where the cooled air is additionally directed over

activated carbon before re-entering the working chamber to

dry the metal parts. Type V machines do not release any

exhaust air to the environment. The chamber concentration is

monitored continuously and the metal parts are only releasedif the concentration in the cleaning chamber is below 1 g/m3.

bath 1 bath 2 vapor bath 1 bath 2 vapor

types I and II type III vent

tankrefrigeration

working

chamber

vent

drying cycle

type IV

activated

carbon refrigeration

working

chamber

drying cycle

type V

Figure 1. Technological progress of metal degreasing machines represented by five different machine types.

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326 Journal of Exposure Analysis and Environmental Epidemiology (2003) 13(5)


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This technological standard is prescribed in the 2nd

BImSchV of 1990. The amendment of this regulation of

2001 requires type V machines that are operated under

vacuum (type V B machines) for the use of TRIC.

The fully open type I machines were first built in theearly 1950s and type II machines in the late 1950s. The

encased type III machines were developed in the middle of

the 1960s. These three types of machines were in use

until the 2nd BImSchV was enacted in 1986. Type IV

machines that fulfill the standard of 1986 came up

in the middle of the 1970s. After 1986 and after the

transitional regulation, only type IV machines and

upgraded type III machines were allowed. In the late

1980s, type V machines were developed. The regulations of

1990 (BImSchV, 1990) require closed-loop type V

machines for all metal degreasing installations using PERC

and, after the amendment of 2001, closed-loop type V B

machines with integrated vacuum system for degreasers using

TRIC.

Scenario-based Risk Assessment (SceBRA)

The SceBRA method is intended to represent the potential

health risk through exposure to chemicals used in

many different applications during different stages of the

products lifetime such as production, formulation,

transport, and end-use (Scheringer et al., 2001). In SceBRA,

various scenarios for possible exposure situations are

investigated.

The resulting risk is then characterized by two indicators:

(i) the risk quotient, r, which is the ratio of concentrationand effect level, and (ii) the number of exposed workers,

N. Here, inhalative exposure to volatile solvents is

calculated with mass-balance models for the vicinity of the

machine (near-field) where the machine operators are

situated, as well as for the far-field where other work is

carried out. The long-term airborne concentration

C, calculated as time weighted average (TWA) for an 8-h

working day, is compared with the German MAK

(maximum workplace concentration) value, corresponding

to threshold limit values (TLVs), which leads to the risk

quotient r C/MAK. The MAK values of 50 ppm (0.345 g/

m3) for PERC and of 50 ppm (0.270g/m3) for TRIC have

not been changed since 1982 for PERC and since 1970 for

TRIC (BUA, 1994, 1993).

In the SceBRA calculations, good working practice is

assumed, as there is hardly any possibility to abuse the

machines that are in use today. For older machines, however,

working practice in some instances may have differed largely

from good working practice; it has been reported that under

time pressure, cleaned parts had been taken out before being

entirely dried (Mannheim et al., 1979; Leisewitz and

Schwarz, 1994). However, because of lack of data, it was

not possible to include bad working practice for older

machines.

Model and exposure scenarios

Mass-balance Model

As there are very few measured data reported in the past and,

in addition, the available data often lack a detaileddescription of the machine technology and the workplace

surrounding, it is necessary to simulate the airborne

concentrations in metal degreasing facilities (Nicas and

Jayjock, 2002). To this end, indoor air quality models based

on mass-balance equations that account for incoming and

outgoing material are used. The mass-balance equation for a

one-box model reads

dCt

dt

.

E

V kCt 1

where C(t) is the pollutant concentration in air (g/m3), t is the

time (h),.

Eis the emission rate (g/h), Vthe box volume (m3),

and k is the overall loss rate constant (h1

). If all productionand loss mechanisms are constant with time, Eq. (1) may be

integrated to give

Ct C0ekt

.

E

Vk1 ekt 2

where C0 (g/m3) is the concentration of the pollutant at time

t 0 and.

E=Vk C1 is the steady-state concentration.

In many cases, however, a simple one-box approach is

inadequate. For room volumes greater than 500 m3, which

are common in metal degreasing facilities, complete mixing is

often not given (Gmehling et al., 1989). Therefore, some

studies have recommended the use of mixing factors to

account for noncomplete mixing (Drivas et al., 1972;Matthiessen, 1986; Jayjock, 1988). Another approach is to

use a two-compartment mass-balance model (Cherrie et al.,

1996; Furtaw et al., 1996; Keil, 1998, 2000). Nicas (1996)

discusses the advantages of two-box models versus the use of

mixing factors.

In a two-box model, the concentrations in the vicinity of

the source (near-field) and further away (far-field) can be

distinguished. This results in two coupled linear differential

equations, see Eq. (3). A two-box model may still be

insufficient to describe complex situations such as the

concentration in the breathing zone during spray painting.

Under these conditions, additional boxes may be introduced

to account for the complexity or computational fluid

dynamic models may be applied (Flynn and Sills, 2000;

Nicholson et al., 2000), but the need for adequate measure-

ments or estimates of the transfer coefficients precludes the

use of full, multibox models for many problems of practical

interest (Ryan et al., 1988).

In this study, a two-box mass-balance model is used to

calculate the airborne concentrations for the near-field and

the far-field of metal degreasing devices (see Figure 2). In the

simplest case, there is a constant mass flow.

EA (g/h) from the

machine into the fictitious box A. The air exchange between

box A with volume VA (m3) and concentration CA (g/m

3),

Occupational exposure to PERC and TRIC in metal von Grote et al.

Journal of Exposure Analysis and Environmental Epidemiology (2003) 13(5) 327


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and box B with volume VB (m3) and concentration CB (g/

m3) is given by the exchange rates kA (h1) and kB (h

1). kBand kA are related by kB (VA/VB)kA. For the air exchange

of box B with the environment, denoted by kL (h1), it is

assumed that the background concentration of the fresh air

can be neglected. The mass-balance equations for the two

boxes are

dCA

dt

.

EA

VA

CAkA CBVB

VA

kB

dCB

dt CA

VA

VBkA CBkB kL

3

Equation (3) represents a system of two coupled first-order

linear inhomogeneous differential equations that can be

solved for CA(t) and CB(t).

Different emission factors can be used to characterize the

variability of the release of chemicals from intermittent

sources (Franke and Wadden, 1987; Wadden et al., 1991;

Keil, 1998). Here, the exposure models for the use of TRIC

and PERC in metal degreasing include three different

emission factors: (i) a continuous factor.

Ec1 (g/h) for diffuse

emissions from open bath surfaces (open-top machines) or

from leakage of sealing (closed machines); (ii) a second

continuous factor.

Ec2 (g/h) for emissions from cleaned parts

and metal parts with cavities, and (iii) a periodic factor Ep(g/batch) for repetitive emissions from chamber air during

loading and unloading (Wadden et al., 1989; Wadden et al.,

1991; Scheff et al., 1992).

For the two continuous emission factors, the inhomoge-

neous differential equation system is given by Eq. (3) where.

EA is replaced by.

Ec1 or.

Ec2. For the periodic emission

factor, the homogeneous differential equation system is used,

which means that Eq. (3) has no continuous source term.

EA.

Instead, periodic emissions Ep (g/batch) are added at each

time when the machine is unloaded so that the new

background concentrations C0p;A;i1 for box A are obtained:

C0p;A;i1 Cp;At itb Ep

VA4

where i indicates the number of unloads and tb the batch

time. With these three sets of equations, the airborne

concentrations are calculated for an 8-h working day with

the degreasing devices continuously in use (see Figure 3). The

concentrations in boxes A and B are obtained as

CAt Cc1;At Cc2;At Cp;At

CBt Cc1;Bt Cc2;Bt Cp;Bt5

Metal Degreasing Scenarios

To represent the different exposure situations in metal

degreasing, a range of possible and plausible user scenarios

are defined. The scenarios are described by machine

parameters (such as sizes, technology types, and loads of

machines) and emission factors. Another factor that

influences the occupational exposure is the workplace

surrounding, which is described by workplace parameterssuch as room size and air exchange rates.

Machine Parameters The machine technology has

changed fundamentally since the 1950s. Therefore, five

types of machines are defined according to information

from sales brochures (Manufacturers, 19602001) (see the

section Metal Degreasing). To account for small differences

in each type due to different manufacturers, subtypes are

defined which vary in the number of baths (types I and II), in

the cooling temperature (type IV), or in the existence of a

vacuum drying device (type V) (see Table 1). Each machine

fresh air

machine

box A

box B

emission

exhaust air

box A and box B

EA

concentrationCB

kA

kB

kLair exchange betweenkL

concentrationCA

.

Figure 2. Two-box model with fictitious box A for the near-field of the machine and box B representing the far-field.




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type is manufactured in different sizes adjusted to specific

purposes. For this study, the five most common sizes are

chosen. They range from small size 1 machines with a loading

of 4050 kg to large size 5 machines with loadings of more

than 1000 kg (Manufacturers, 19602001). Among the

different sizes, size 3 machines are most commonly used.

The batch time, tb, depends on the material, the geometry,

and the weight of the parts to be cleaned (during vapor

degreasing, the parts are heated till they have the same

temperature as the vapor, then condensing stops and the

vapor degreasing process is finished (Mannheim et al.,

1979)). Therefore, size 5 machines have longer batch times

than size 1 machines (see Table 2) (Manufacturers, 1960

2001).

Emission Factors The diffuse emission factor.

Ec1 (g/h)

describes a continuous discharge out of the machine into box

A. For open-top machines (types I and II), this correspondsto releases from open bath surfaces, which emit as long as the

baths are heated. In fully emissive machines, the solvent

vapor layer at the height of the air-cooler coils separates the

vapor phase of the bath and the workplace air. Additionally,

a rim exhauster directs the ascending hot vapor toward the

side panel where the coolers are situated; the recirculation

flow draws in fresh air from the workplace. In addition to the

temperature of cooling and the capacity of the rim exhauster,

also the width of the baths and the freeboard rate, which is

the ratio of freeboard height and bath width, influences the

emission mass flow. Owing to lack of adequate information

on the volumetric flow rate and the dimensions and locationof the exhauster inlet, an exact calculation of the

emission mass flow is not feasible (Heinsohn, 1991).

However, a simplified calculation makes it possible to

determine reasonable estimates. The emissions from open

bath surfaces are approximated with Ficks first diffusion

law:

Jz DDC

Dh6

where Jz (g/(m2 h)) is the diffusion mass flow in the z

direction, D (m2/h) the molecular diffusion coefficient, DC

(g/m3) the concentration gradient between the saturation

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 2 4 6 8 10 12

PERCconcentration(g

/m3) dynamic concentration box A,CA(t)

dynamic concentration box B,CB(t)

long-term concentration box Along-term concentration box B

MAK value: 0.345 g/m3

time (h)

Figure 3. Dynamic concentration of PERC used in a type III, size 5 metal degreasing machine. Parameter values are: Ep: 117 g/batch,.

Ec1: 17.2g/h,.

Ec2: 73.4g/h, a: 0.75h, tb: 0.75h, V: 400m3

, kL: 5.5 h1

, kA: 6 h1

(for parameter definition, see the section Metal Degreasing Scenarios).

Table 1. Machine parameters: characteristics of types and subtypes.

Types and

subtypes

Characteristics

I A Two baths

I B Two baths and vapor degreasing

II A Two baths

II B Two baths and vapor degreasing

III No subtypes

IV A Cooling temperature: PERC 201C, TRIC 301C

IV B Cooling temperature: PERC 401C, TRIC 401CV A No vacuum drying

V B Vacuum drying

Table 2. Machine parameters: load and batch time.

Machine size Load (kg) Batch time tb (h)

1 4050 0.080.12

2 5060 0.10.17

3 120150 0.140.2

4 Approx. 600 0.330.5

5 Approx. 1000 0.51




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concentration in the cooling zone and the air in box A, and

Dh (m) the diffusion length. Dh was fitted so that the mass

flow equals the results reported by CEFIC (1984), which

state that if common freeboard rates of 0.5 for open-top

degreasers are increased to 0.75, solvent emissions can bereduced by 2530%. As a cross-check, mass flows based on

the fitted Dh were calculated for different cooling

temperatures. The results agree with the finding that

emissions can be reduced by 50% if the cooling

temperature in open-top machines is reduced from 51C to

301C (CEFIC, 1984). Multiplication of Jz with the bath

surface area results in the continuous emission factor.

Ec1.

For encased and closed machines (types IIIV),.

Ec1 is taken

as a leaking rate proportional to the solvent volume present

in the machine (CerclAir, 1993).

The cleaned parts that are taken out of the machine after

washing and drying are sometimes not completely dry,

especially if parts with cavities or cupped parts are cleaned.

Emissions from cleaned parts depend on the drying time, the

surface of the cleaned parts, and on the existence of cavities

and cupped areas (Scheff et al., 1992).

In degreasing, metal parts vary considerably in size,

weight, and shape (e.g., from small needles to whole car

bodies). Therefore, a range of standard metal parts is defined

here with different amounts, sizes, and arrangements of

plates and closely packed spheres. Between the closely packed

spheres there are cavities where residual solvent can remain;

this amount determines the emissions from cavities. Together

with the amount that is dragged out by parts without cavities,

this leads to the total amount of residual solvent Ec2 (g). As aworst-case scenario, it is assumed that all of the residual

solvent will evaporate continuously over a certain period a

(h), which lies between 0.5 and 1.5 times the batch time. The

quotient of Ec2 and a leads to the emission factor.

Ec2 (g/h).

The residual PERC amounts on noncupped parts dragged

out of fully emissive open-top machines are assumed to be

3.94.3g PERC per m2 of metal surface after a solvent bath

and 3.23.6 g/m2 after vapor degreasing. The lower boiling

point of TRIC and therefore the shorter time till the metal

parts have achieved the boiling temperature is the reason that

metal parts that are cleaned with TRIC drag out smaller

amounts of residual solvent. These figures correspond well

with reported amounts of solvent that had to be refilled in

open-top machines (Mannheim et al., 1979).

For the spheres it is assumed that there are some cavities

between the spheres, which are not easily dried. It is assumed

that for types IIII machines, 5% of the sphere surface can

be taken as a cupped area, which drags out 32.535.7 g

PERC per m2 of sphere surface (Mannheim et al., 1979).

Each time when the machine is loaded or unloaded with a

basket of metal parts, solvent-charged air from the chamber

volume is exchanged with workplace air, which is described

by the periodic emission factor Ep (g/batch). The concentra-

tion in the working chamber can be calculated for the open-

top machines as the solvents saturation concentration at the

height of the cooling device. During loading, the vapor

volume proportional to the volume of the metal parts is

displaced; during unloading, the basket volume minus the

volume of the parts is dragged out. The rim exhauster oftypes I and II machines captured 50% of the solvent,

whereas for types III and IV 90% could be kept back from

emitting into the workplace (Mannheim et al., 1979; CEFIC,

1984; Leisewitz and Schwarz, 1994). In type V machines, the

cleaned parts are released only if the chamber concentration

is below 1 g/m3 (BImSchV, 1990). The emission factor is

calculated as intermittent emissions that occur at the very

moment when the door is opened.

In Table 3, the three different emission factors for each

machine size are shown for PERC as an example. Note that

the emissions through loading and unloading contribute most

to the overall emissions (Ep and.

Ec2).

Wadden et al. (1991) report emission factors of 16.9 g

TRIC/basket of parts for an open-top degreaser where

emissions from cleaned parts (.

Ec2) were not taken into

consideration. The described machine is similar to a type II

and size 3 machine. In the SceBRA calculations, the amount

of Ep added to the amount of.

Ec1 is 18.521.0 g/basket,

which shows good correspondence.

Workplace Parameters Not only do the degreasing

equipment and the operation of the machine influence the

occupational exposure, but also the workplace surrounding.

The airborne concentration is dependent on the room volume

and the air exchange rates. Production hall volumes can varyfrom small volumes of about 300 m3 up to several thousand

cubic meters. Here, relatively small volumes of 400 and

600m3 are assumed for the whole workplace. The volume is

divided into a fictitious working box of a volume of 100 m3

and the rest of the volume, where other work not related to

the degreasing is carried out. According to Gmehling et al.

(1989) this is an appropriate assumption. Typical air

exchange rates in industrial facilities lie between 4 and 15

times per hour (Hayes, 1991; Recknagel et al., 2000).

Wadden et al. report an air exchange of 6 h1 for metal

degreasing with TRIC (Wadden et al., 1989) and 5.1 h1 in

an offset printing shop without local exhaust (Wadden et al.,

1995). Workplaces for metal degreasing are equipped with

local exhaust devices. Therefore, an efficient and complete

mixing for the near-field (box A) can be assumed. On the

other hand, air exchange rates above 8 h1 are not likely, as

there would be an uncomfortable draught for the workers.

Here, kL values of 6 and 6.5 h1 are used for open-top types I

and II machines, whereas for encased and closed machines

values of 5.5 and 6 h1 are employed. The air exchange rate

between box A and box B kA is assumed to be greater than

the overall exchange rate kL because particularly the older

machines are heat sources and create an upward flow of

warm air (Heinsohn, 1991; Bach et al., 1992). Therefore, the




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kA values are assumed to be 7 and 7.5 h1 for open-top

machines and 6 and 6.5 h1 for types IIIV machines.

Number of Exposed Workers In addition to the risk

quotient, the number of exposed workers is estimated in

SceBRA. For this purpose, a distinction between the two

boxes with their different concentrations must be made. The

number of exposed workers in the near-field can be estimated

from the number of machines in use in different years. It is

assumed that between 1.5 and 3 workers (average of 2.25)

were required to handle the mostly manually operated open-

top types I and II machines. The encased and fully closed

machines are automated and therefore only between 1 and

1.6 workers (average of 1.3) are needed (Nader, 2001).

Furthermore, it is assumed that the workers operating the

machines are present in the near-field during the whole 8-h

working day.

The estimation of the number of workers in the far-field is

more difficult. As metal degreasing machines are used all over

in the metal processing industries, workplaces can vary

considerably from small production halls with only a

few workers to whole production lines. As the open-top

machines were mostly used in times when the production

processes were not automated, it is assumed that for

types I and II machines 310 workers work in the far-field,

whereas for types IIIV machines the number is set at 28

workers.

In Table 4 the industrial estimates of the numbers of metal

degreasing machines using PERC are listed for the years

1985 (only former West German states, 1 year before the 2nd

BImSchV of 1986) (Adams and Jeker, 1986), 1991 (only

former West German states, during transitional regulation

for 2nd BImSchV 1990) (Adams, 1993), 1996 (after the

2nd BImSchV 1990) (Adams, 1997), and 1999 (Nader,

2001).

Table 3. Emission factors for PERC.

Type Size 1 Size 2 Size 3 Size 4 Size 5

Diffuse emission factor.

Ec1 (g/h)

IA Min 1.27E+01 1.82E+01 2.26E+01 4.12E+01 1.03E+02Max 1.40E+01 2.00E+01 2.49E+01 4.53E+01 1.13E+02

IIA Min 7.05E+00 1.01E+01 1.26E+01 2.29E+01 5.72E+01

Max 7.76E+00 1.11E+01 1.38E+01 2.52E+01 6.29E+01

III Min 2.12E+00 3.03E+00 3.77E+00 6.86E+00 1.72E+01

Max 2.33E+00 3.33E+00 4.15E+00 7.55E+00 1.89E+01

IVA Min 1.50E+00 2.17E+00 2.92E+00 3.33E+00 8.33E+00

Max 2.40E+00 3.47E+00 4.67E+00 5.33E+00 1.33E+01

VA Min 1.14E+00 1.65E+00 2.22E+00 2.53E+00 6.33E+00

Max 1.50E+00 2.17E+00 2.92E+00 3.33E+00 8.33E+00

Emission factor from cleaned plates.

Ec2 (g/h) for perioda

IA Min 3.20E+02 3.21E+02 4.00E+02 6.07E+02 7.05E+02

Max 1.06E+03 1.06E+03 1.32E+03 2.00E+03 2.33E+03

IIA Min 3.20E+02 3.21E+02 4.00E+02 6.07E+02 7.05E+02

Max 1.06E+03 1.06E+03 1.32E+03 2.00E+03 2.33E+03III Min 3.34E+01 3.34E+01 4.17E+01 6.32E+01 7.34E+01

Max 1.10E+02 1.10E+02 1.38E+02 2.09E+02 2.42E+02

IVA Min 8.34E+00 8.35E+00 1.04E+01 1.58E+01 1.84E+01

Max 2.75E+01 2.76E+01 3.44E+01 5.22E+01 6.06E+01

VA Min 4.17E+00 4.17E+00 5.21E+00 7.90E+00 9.18E+00

Max 1.38E+01 1.38E+01 1.72E+01 2.61E+01 3.03E+01

Periodic emission factor from chamber air Ep (g/batch)

IA Min 1.47E+00 3.05E+00 8.55E+00 4.59E+01 2.70E+02

Max 1.79E+00 3.73E+00 1.05E+01 5.61E+01 3.30E+02

IIA Min 8.14E01 1.70E+00 4.75E+00 2.55E+01 1.50E+02

Max 1.06E+00 2.20E+00 6.18E+00 3.32E+01 1.95E+02

III Min 6.36E01 1.32E+00 3.71E+00 1.99E+01 1.17E+02

Max 8.12E01 1.69E+00 4.74E+00 2.54E+01 1.50E+02

IVA Min 1.77E01 3.68E01 1.03E+00 5.53E+00 3.25E+01Max 2.82E01 5.89E01 1.65E+00 8.85E+00 5.21E+01

VA Min 3.53E03 7.36E03 2.06E02 1.11E01 6.51E01

Max 7.06E03 1.47E02 4.12E02 2.21E01 1.30E+00




8/16

Table 4. Estimates of the number of metal degreasing machines using PERC in Germany for the years 1985*, 1991*, 1996, and 1999 (*only former West

German states).

Machine type Sizes 1985* (a) 1991* (b) 1996 (c) 1999 (d)

1 1444 6482 149 67

IA 3 122 55

4 50 22

5 6 3

1 1444 649

2 149 67

IB 3 122 55

4 49 22

5 5 2

1 0 9

2 452 35

IIA 3 379 44

4 341 165 38 2

1 0 9

2 451 35

IIB 3 379 44

4 341 16

5 38 2

1 18 88

2 72 353

III 3 90 442

4 32 157

5 4 20

1 13 272 52 106

IVA 3 13 133

4 0 47

5 0 6

1 1 27 24

2 6 106 94

IVB 3 1 133 118

4 0 47 42

5 0 6 5

1 44 70 75

2 177 282 300

VA 3 221 352 375

4 79 125 133

5 10 16 17

1 23 38

2 94 150

VB 3 117 188

4 42 67

5 5 8

Total 6261 4028 1408 1351

Vapor degreasing only 3280 3127 1408 1351

Sources: (a) Adams and Jeker (1986); (b) Adams (1993); (c) Adams (1997); (d) Nader (2001).




9/16

Results

The two-box model allows us to specify the dynamic and the

long-term concentration of each single scenario. By combin-

ing all independent input parameters, up to 2160 possible andplausible exposure scenarios can be obtained for each type of

machine. From these, the maximum and minimum scenarios

are selected for each size of machine. Thereby the number of

scenarios is reduced to 20 for each machine type (two values

for five sizes and two subtypes). On this basis, the five types

of machines can be compared, and characteristic differences

between the two solvents can be depicted. Finally, for

selected years the number of exposed workers, N, is

combined with the risk quotients, r, of the different types

and sizes of machines. On this basis, the changes in the

number of exposed workers and the magnitude of the risk

quotient can be compared over a period of 15 years.

Dynamic Concentration for a Single Scenario

With one set of input parameters for a specific machine

(machine parameters, emission factors, workplace para-

meters), the dynamic and the long-term concentrations over

an 8-h working day can be calculated for a single scenario. In

Figure 3, these concentrations are shown for PERC used in

an encased type III and size 5 metal degreasing machine as an

example. In this scenario, metal plates are washed with a

batch time tb of 0.75 h. Every time when the basket with

metal plates is unloaded, a peak concentration can be

observed. The long-term concentration is calculated as the

integral of the concentration curve over one batch time,divided by tb. After the second unloading (after 1.5 h), the

steady state has been achieved; thereafter a regular pattern of

subsequent peaks follows. After 8 h, the operation of the

metal degreasing device stops and the airborne concentration

decreases to the initial value. The long-term concentration

lies at 0.42 g/m3 in box A and at 0.11 g/m3 in box B, which

means that for this specific scenario the long-term concentra-

tion in the near-field, the working area, is 1.2 times the MAK

level of 0.345 g/m3, whereas the long-term far-field concen-

tration reaches only 32% of the MAK level.

Parameter Combination

For one machine of type II A and size 5, the contribution of

the different input parameters to the range of airborne

concentrations is depicted in Figure 4 by systematically

combining minimum, average, and maximum values of all

input parameters. The basic scenario indicated by the circle in

Figure 4 represents the minimum values of the emission

factors, the room volume, the air exchange rate, and the

batch time and the case that plates are cleaned. The black

diamonds show the concentrations obtained if only a single

parameter is increased from the minimum to the maximum

value (Ep,.

Ec2, tb: also average values). Changing several

parameters at a time results in the combination of the

individual effects; these scenarios are represented by the gray

marks. Parameters that vary only sparsely between

different scenarios have a relatively small influence on the

long-term concentration. Such parameters are the continuous

emission factor from open baths and leakage of

closed machines (.

Ec1) and the room volume (V). This is

because.

Ec1 represents emissions caused by machines in

standby mode with only small fluctuations. The room

volume has a small effect because it influences mainly the

concentration level in the far-field and the air exchange from

box B to box A (kB).

Parameters that show stronger effects are primarily the

batch time tb, the kind of cleaned parts, the coupled air

exchange rates kL and kA as well as the coupled periodic

emissions Ep (g/batch time) and.

Ec2 (g/h). The length of tbinfluences the long-term airborne concentration most

strongly, as it determines the spacing between the emission

peaks. The difference between the two kinds of metal parts is

due to the different volume and surface ratio of plates and

spheres. The variations in Ep and.

Ec2 represent the

differences in the work process; they depend on the

saturation concentration in the machines, the geometry and

2.5

2

1.5

1

long-termT

RIC

concentration(g/m3)

Ep and Ec2,

av

Ep and Ec2,max

Ec1, av

Ec1, max

V, max

kL and kA, max

spheres

instead

of plates

tb, max

tb, av

.basic scenario

scenarios

.

.

.

Figure 4. Effects of changes in different model parameters on the long-term concentration in box A of a type IIA, size 5 metal degreasingdevice using TRIC. Starting from the basic scenario (circle), thedifferent parameters are varied from their minimum to maximumvalues (emission factors and batch time: also average values), whichleads to the concentrations indicated by black diamonds. The graymarks indicate the concentrations obtained by combinations of two ormore parameter changes. For this machine, the minimum and themaximum concentrations are 2.2 and 0.9 g/m3.




10/16

5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1

I II III IVA

5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1

I II III IVA IVB VA VB

5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1

I II III IVA

5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1 5 4 3 2 1

I II III IVA IVB VA VB

10.000

1.000

0.100

0.010

0.001

10.000

1.000

0.100

0.010

0.001

types and sizes

MAK-value: 0.270 g/m3

TRIC PERC

concentration(g/m3)

Figure 5. Comparison of near-field concentrations (logarithmic scale) of the five types (and relevant subtypes) subdivided into five sizes of metal degreaArrow: machine characterized in Figure 4.

334

JournalofExposureAnalysisandEnvironmentalEpidemiology(2003)13(5)


11/16

Table 5. Exposure measurements of TRIC used in metal degreasing.

Year Country Machine

type

Machine description Measurement Sample Number of

machines

Number of

samples

Range

(ppm)

Range

(mg/m3)

Avera

(ppm

1942 USA I n.d. During normal

operation

Personal 108 n.d. n.d. n.d. 128.9

194759 Denmark I n.d. n.d. Air n.d. 200 n.d. n.d. 120.9

1956 n.d. I n.d. n.d. Air n.d. n.d. 5600 273240 n.d.

1955* Switzerland I and II Mostly open-top

degreasers

510 min samples Air n.d. 84 n.d. n.d. 49.8

196096 Denmark I or II n.d. n.d. Air n.d. 50 n.d. n.d. 59.6

1972 Germany I or II Vapor degreasing

with exhaust

n.d. Personal 1 n.d. 2247 119254 n.d.

1972 Germany I or II Vapor degreasing

without exhaust

n.d. Personal 1 n.d. 65.0165.0 351891 n.d.

197079 Denmark I or II n.d. n.d. Air and

personal

n.d. 116 n.d. n.d. 55.9

1984 Western

Europe

I and II Open-top equipment n.d. Air n.d. n.d. 90%-pct. 100 90%-pct. 540 n.d.

75%-pct. 50 75%-pct. 270

198089 Denmark II and III n.d. n.d. Air andpersonal

n.d. 371 n.d. n.d. 13.0

198591 Germany III and IV n.d. TWA and range

measurements

n.d. 267 748 90%-pct. 101.3 90%-pct. 547 15.4

199095 Germany IV and V Manually operated TWA Personal 7 14 90%-pct. 56.3 90%-pct. 304 7.6

95%-pct. 109.6 95%-pct. 110

199095 Germany IVand V Automatic without

exhaust

TWA Personal 18 23 90%-pct. 52.2 90%-pct. 282 4.1

95%-pct. 104.8 95%-pct. 566

199095 Germany IVand V Automatic with

exhaust


95%-pct. 67.4 95%-pct. 364

199095 Germany IV and V n.d. o1 h Personal 22 30 50%-pct. 44.6 50%-pct. 241 n.d.

90%-pct. 131.7 90%-pct. 711

95%-pct. 168.9 95%-pct. 912

199297 Germany IV and V n.d. TWA and onlyfor 1992

range measurements

n.d. 97 179 90%-pct. 52.4 90%-pct. 283 11.3

1997* Australia IV and V n.d. TWA Personal n.d. 5 n.d. n.d. 8.9

2000 Germany V n.d. Information from

manufacturers

Air n.d. n.d. 2.05.0 10.827.0 n.d.

n.d. not defined; *no year of measurement given, year of publication used; pct. percentile.


335


12/16

Table 6. Exposure measurements of PERC used in metal degreasing.

Year Country Machine type Machinedescription

Measurement Sample Number ofmachines

Number ofsamples

Range(ppm)

Range(mg/m3)

Ave(ppm

198285 Finland n.d. n.d. n.d. Air 3 n.d. 1.85.0 12.427.0 3.0

1985 Germany n.d. n.d. n.d. Air n.d. n.d. 12.520 86138 n.d

1988 Germany IV n.d. n.d. Air n.d. n.d. 7.210 5069 n.d

1988 Germany IV n.d. TWA Personal n.d. n.d. n.d. n.d. 8.4

1991 Germany IV and V n.d. n.d. Air n.d. n.d. 1.36.2 943 n.d

1991 Germany IV and V n.d. TWA Personal n.d. n.d. 1.42.0 1014 n.d

1 990 95 Germany IV a nd V Manually

operated

without exhaust


95%-pct. 116.4 95%-pct. 803

199095 Germany IV and V Automatic

without exhaust


95%-pct. 73.2 95%-pct. 505199095 Germany IV and V Automatic

without exhaust


95%-pct. 78.8 95%-pct. 544

199095 Germany IV a nd V n.d. o1 h Personal 22 30 50%-pct. 5.1 50%-pct. 35 n.d

90%-pct. 39.6 90%-pct. 273

95%-pct. 46.2 95%-pct. 319

1996 Finland IV and V n.d. n.d. Air 1 n.d. 110 769 5.2

1996 Finland IV and V n.d. n.d. Air 1 n.d. 3.816.4 26113 10.1

1993 Germany V n.d. n.d. Air n.d. n.d. o0.7 o5 n.d

1993 Germany V n.d. TWA Personal n.d. n.d. n.d. n.d. 2.8

1998 Germany V n.d. Control of TWA

according to 2nd

BImSchV 1990

Air 9 52 95%-pct. 10.1 95%-pct. 70 0.5

n.d. not defined; * no year of measurement given, year of publication used: pct. percentile.

336



13/16

volume of the metal parts, and the amount of solvent that is

dragged out. The air exchange rates kL and kA directly

influence the removal of the pollutant.

In addition to the effects depicted in Figure 4, the major

impact on the airborne concentration within one machine

type is caused by the size of the machine, see concentration

ranges given in Figure 5 for all types and sizes. The machinesize directly influences the bath volumes and bath surfaces,

the volumes of the baskets and, therefore, the surfaces of the

cleaned metal parts.

Comparison of Machine Technologies

The different machine technologies can be compared by

analyzing the concentration ranges for each size of machine.

In Figure 5, where the airborne concentrations are given for

all five machine types (and for different subtypes if they are

discernible in terms of concentrations), it can be seen that for

both solvents, the concentration ranges decrease with

decreasing machine size, and are reduced by more than two

orders of magnitude from fully emissive open-top type I to

closed-loop type V B machines.

Fully emissive types I and II and encased type III

machines using TRIC show a bigger variation between the

different sizes of machines than machines operated with

PERC. This can be attributed to the fact that the vapor

pressure of TRIC is higher for the temperature range in

which these machines are used. Therefore, higher concentra-

tions are achieved by increasing the bath surface and the

volume of the basket from small size 1 to size 5 machines.

In Tables 5 and 6, measured concentration data are listed

for TRIC and PERC. In general, the measurement reports

contain little information about the machines that were

measured; for types IV and V machines, relatively good

information with indicated number of samples, measurement

ranges, and long-term concentrations is available (Thoulass,

1998; BK-Report, 1999; Bock et al., 1999). In this situation,

it is not possible to assign the measured data to machine sizes

or even individual scenarios. However, if the ranges of themeasured concentrations are compared to the ranges spanned

by the model results obtained for all machine sizes of a

certain type (IV), a good agreement of these ranges is

observed (data in Tables 5 and 6 and graphs in Figure 5). All

measured average concentrations lie within the SceBRA

ranges.

Exposed Workers

Finally, the average risk quotients of the different machine

types and sizes that were in use at a certain time are combined

with the number of workers exposed to TRIC and PERC in

metal degreasing and compared for different years. Figure 6

shows a cumulative representation of the results for the near-

field (box A) over 15 years. In this graph, each point (log r;

Ncum) of the curve shows the total number of exposed

workers, Ncum, with a risk quotient at least as high as

indicated by log r. This kind of plot combines the informa-

tion from all scenarios of a solvent for a specified year in a

single line and thus several years and both solvents can be

compared in one plot. For each year, the total number of

exposed workers and the number of exposed workers with a

risk quotient above 1 (or any other particular value) can be

derived from the plot. The total number of exposed workers

in the near-field in 1985 is 13,800 for PERC and 10,830 for

0-2 -1 0 1

log risk quotient

numberofexposedworkers

16000

14000

12000

10000

8000

6000

4000

2000

-2.5 -1.5 -0.5 0.5 1.5

1985 PERC

1985 TRIC

1991 PERC

1991 TRIC

1996 PERC 1999 PERC

1996 TRIC1999 TRIC`

Figure 6. Cumulative log r, N-plot of PERC and TRIC in the near-field for the years 1985*, 1991*, 1996, and 1999 in Germany (*only former WestGermany).




14/16

TRIC compared to 1760 for PERC and 391 for TRIC in

1999.

In 1985, 1 year before the 2nd BImSchV was enacted in

1986, mainly open-top types I and II and encased type III

machines were in use, whereas in 1991 all machines had tofulfill the 2nd BImSchV of 1986. In 1996, after the transition

period for the amended regulations, and in 1999, all

machines had to conform to the 2nd BImSchV of 1990,

meaning that only closed-loop type V machines were

allowed.

In 1985, 53,000 tonnes of PERC and 34,000 tonnes of

TRIC were sold (only former West German states), but only

4500 tonnes PERC and 5100 tonnes TRIC in 1999 (Nader,

2002). This is a reduction by 91.5% for PERC and by 85%

for TRIC. The reduction in the solvent amount used in metal

degreasing is due to changes in environmental legislation,

which led to: (i) technology changes (closed-loop machines

with integrated recycling, where the solvent has a residence

time in the machine of about 1 year (Nader, 1993), whereas

Mannheim et al. (1979) report for open-top types I and II

machines a total solvent loss of 0.9 kg/h for PERC and

0.7 kg/h for TRIC, which means that solvent had to be

refilled once or twice a week); (ii) solvent substitution; and

(iii) more efficient processing in metalworking industry with

less greasing and degreasing.

Discussion

The core part of the calculations with SceBRA presented inthis paper is the airborne concentrations obtained with the

two-box model. As far as a comparison with measured data

is possible, these model results are in agreement with

concentrations measured in metal degreasing facilities, see

the values listed in Tables 5 and 6 and the model results

shown in Figure 5. Owing to highly variable workplace

conditions, the measured data show some scatter that is not

reproduced by the model, but the model results match the

average values of the measured data and there are no

systematic deviations.

All of the model parameters are based on empirical

information and none of them was adjusted in order to

increase the correspondence of calculated and measured

concentrations. A local sensitivity analysis for the model

input parameters indicated that the size of machine, the batch

time tb, and the air exchange rate kA are the most influential

parameters for the airborne concentration in box A. The

machine sizes are taken from sales brochures and are

therefore well known. The batch time depends on the loads

of the machine, that is, the weight, the material, and the

shape of the parts. A specification of the parts and therefore

more specific batch times would only make sense if the

different parts cleaned in the region under consideration

(Germany) and at a certain time could be described in more

detail. As this is not possible in the context of a screening

method, it is reasonable to use a realistic range of batch

times, which was also the result of discussions with machine

manufacturers.

The air exchange rate between boxes A and B is anassumption specifically made for the two-box model. More

specific air exchange rates for certain workplaces could be

estimated by tracer gas measurements (Sohn and Small,

1999). However, as we want to estimate the concentrations

for a broad range of industry facilities, it seems to be

sufficient to take reasonable kL values for rooms with local

exhaust and to assume that for older machines, which are

stronger heat sources, the assumed value of kA is somewhat

higher than kL.

Parameters with a medium influence are the emission

factors.

Ec2, Ep, and.

Ec1. Taken together, the three emission

factors correspond well with total solvent losses for the open-

top machines reported by Mannheim et al. (1979). Better

estimations could be achieved if emission factors are

measured for different machine technologies, which, how-

ever, is not easily feasible as in Germany today only type V

machines are in use. Parameters that have hardly any

influence on the near-field concentration are Vand kL.

Overall, the concentrations obtained with the scenarios

correspond to the range of measured concentrations. This

means that the scenarios cover a relevant range of exposure

situations. A strategy for selecting a reasonable set of

scenarios is to define the scenarios bottom up by setting

up all reasonable parameter combinations (here a total of

2160 scenarios for each machine size) and then to analyzewhich scenarios lead to very similar concentrations. On this

basis, redundant scenarios can be omitted and those that

span the relevant concentration range can be selected for

further investigation.

Using a broadly defined set of scenarios makes it possible

to include the high variability of metal degreasing facilities

into a comprehensive exposure analysis. In the present

application of the SceBRA method, this variability is

described by clusters of scenarios defined by the parameter

values, for example, machine size, types of parts cleaned, etc.

(see Figure 4), but not by frequency distributions. (Note that

the set of scenarios can be seen as a discrete frequency

distribution of concentrations resulting from identical

frequencies for all model input parameter values.) In a

companion study on dry cleaning, which deals with a much

less heterogeneous functional unit (textiles), we derived

frequency distributions for the different model input para-

meters from a questionnaire providing information on the

parameters describing typical dry cleaning facilities.

In conclusion, the SceBRA method is suitable for

characterizing the occupational risk through TRIC and

PERC in metal degreasing. The numbers of exposed workers

and the risk quotients in Figure 6 clearly indicate the

combined effect of technological innovation and stricter




15/16

legislation, leading to strongly reduced airborne concentra-

tions and also much lower numbers of exposed workers. The

reduction in the number of workers is also due to

automatization and a replacement of TRIC and PERC by

nonchlorinated solvents. A similar assessment for theapplication of nonchlorinated solvents would also be

desirable.

Acknowledgments

We thank Dow Europe S.A., the Swiss Reinsurance

Company, and the Swiss Federal Office of Public Health

(BAG) for financial support of this study. Thanks go to

H.-N. Adams, Dow Europe, and Jo rg Pastre for helpful

discussions.

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reduction of occupation exposure to perchloroetylene and trichloroethylene in metal degreasing

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