AMERICAN JOURNAL OF INDUSTRIAL MEDICINE 39:351±360 (2001)

Cumulative Exposure to Styreneand Visual Functions

Laura Castillo, MD, MSc,1,2 Mary Baldwin, MSc, SM,1 Marie-Pascale Sassine, MSc,1

and Donna Mergler, PhD1�

Background Results from a 1990±1992 longitudinal study of several reinforced plasticsplants showed that for those workers whose styrene exposure had decreased, color vision(CV) improved; while near visual contrast sensitivity (CS) was poorer.Methods In 1999, we retested these visual functions in 18 workers with good visualacuity. A cumulative exposure index (CEI), corrected for respirator use, was calculatedfor each worker.Results Intra-individual comparison of mandelic acid (MA) showed a signi®cantdecrease over time (Friedman; P � 0:015), but current values were not related to CEI.For CV, no signi®cant difference was observed between 1992 and 1999; present resultswere not related to MA or CEI. The CS pro®le decreased over time, with signi®cantdifferences at 3 cpd (Friedman; P < 0:05). CS did not vary with MA levels, but wassigni®cantly depressed at the intermediate frequencies among those in the upper CEIcategory (Kruskal±Wallis; P < 0:05).Conclusions These ®ndings suggest that CS loss increases with long-term cumulativeexposure, re¯ecting chronic damage to the neuro-optic pathways. Am. J. Ind. Med.39:351±360, 2001. ß 2001 Wiley-Liss, Inc.

KEY WORDS: vision; contrast sensitivity; styrene; cumulative exposure; colorvision; mandelic acid; reinforced plastics; workers

INTRODUCTION

Several studies have shown that styrene has ophthal-

motoxic properties (Gobba et al., 1991; Fallas et al., 1992;

Chia et al., 1994; Campagna et al., 1995; Eguchi et al., 1995;

Mergler et al., 1996). Most of this research focused on

acquired dyschromatopsia, which has been observed at

relatively low levels of exposure (< 125 mg/m3) [Chia et al.,

1994; Eguchi et al., 1995; Campagna et al., 1996]. It has

been suggested that styrene-related color vision loss may be

reversible over a period of more than 1 month, but less than

2 years [Gobba et al., 1991; Mergler et al., 1996]. Near

visual contrast sensitivity loss, which has been observed

among workers exposed to organic solvent mixtures

[Frenette et al., 1991; Donoghue et al. 1995], has also been

reported for styrene-exposed workers in reinforced plastics

manufacturing plants [Campagna et al., 1995; Mergler et al.,

1996].

In 1990 and 1992, our research group conducted a

longitudinal study of several reinforced plastics plants in

QueÂbec. This study was carried out in cooperation with joint

union±management occupational health and safety commit-

tees, and occupational health professionals from the

Department of Community Health and the Center for Local

Community Services (CLSC). In the 1990 study, the rela-

tionship between internal and external parameters of styrene

exposure and the visual functions of chromatic discrimina-

tion and near visual contrast sensitivity were examined. It

was shown that workers with higher end-shift, end of work-

week urinary mandelic acid (MA) had poorer color vision

1Centre d'�etude des interactions biologiques entre la sant�e et l'environnement (CINBIOSE),Universit�e du Qu�ebec�a Montr�eal, Montr�eal, Qu�ebec, Canada

2Centro de estudios de la salud de los trabajadores (CEST), Universidad de CaraboboContract grant sponsor: Conf�ederation des syndicaux nationaux�Correspondence to: Dr. Donna Mergler, CINBIOSE, Universit�e du Qu�ebec �a Montr�eal,

CP 8888, succ. Centreville, Montr�eal, Qu�ebec, Canada H3C 3P8E-mail: mergler.donna@uqam.ca

Accepted12 November 2000

ß 2001Wiley-Liss, Inc.

and depressed contrast vision sensitivity in the intermediate

and higher frequencies (12 and 18 cycles � degreesÿ1)

[Campagna et al., 1995]. Immediately following the 1990

study, recommendations were made to the joint health and

safety committees to decrease styrene exposure levels. A

follow-up study was carried out in 1992, after one plant had

made changes in work practices and ventilation improve-

ments. For those workers whose exposure had decreased,

performance on several neurobehavioral tests and color

vision improved; however, near visual contrast sensitivity

was poorer [Mergler et al., 1996].

In 1999, we re-tested visual function in a subset of the

original cohort located at the plant which had changed work

practices in 1991, in order to explore changes in visual

function in relation to longer term exposure and to examine

chronicity. Our hypothesis was that cumulative styrene

exposure contributes to the degradation of near visual

contrast sensitivity, while color vision would re¯ect current

exposure. Thus, one objective was to construct styrene

exposure pro®les for these workers in order to create a

measure of cumulative exposure. The second objective was

to examine visual functions in relation to current and

cumulative styrene exposure.

MATERIALS AND METHODS

Participants

The participants in the 1999 study were recruited from

those who had participated in both the 1990 and 1992 study

and were still employed at the plant where changes had been

made. Of the 32 male workers who had participated in the

two previous surveys, 30 were still working at the plant and

24 (80%) participated in the present study. Consent forms

were signed by all participants. Since the group of workers

was relatively small, rather than adjusting for near visual

acuity loss, those with poor visual acuity (minimal reso-

lution of 1 min of arc at 0.37 m) in each eye with use of the

National Optical Visual Chart (n� 6) were excluded to

maintain homogeneity. Thus, the present analyses comprise

18 persons.

Plant and Process Description

The plant under study manufactures showers and bath-

tubs using the open or contact mold process. The work

process begins with the painters, who spray a layer of

pigmented resin containing styrene (gelcoat) on a mold.

Then, choppers spread a mixture of ®berglass and styrene

containing resin on the coated mold, either manually or with

the help of a hand-held spray-gun. In the next step, lami-

nators, using rollers and their hands, ensure an even layer of

resin with no air bubbles. These ®rst three stages constitute

the principal emission sources of styrene vapor. Following

lamination, the pieces are placed to dry and then unmolded;

a series of operations on the ®nished pieces (®nishing)

completes the process. Additional jobs include foreman,

mold repairs, and warehousing.

In 1990, the painters and choppers worked in semi-open

areas, which were served by local exhaust systems. There

were no fresh air inlets for most of these enclosures. These

workers wore half-face respirators with charcoal cartridges

during most of their operations. The laminators generally

used neither respirators nor gloves, because they claimed the

respirators blocked their vision and the gloves decreased

their manual dexterity. Styrene evacuation in this area was

by general dilution ventilation, and there was no local ex-

haust system. Few other workers wore respiratory protec-

tion.

Following the 1990 study, the ®rst changes were made

in this plant, with additional ventilation; training was given

in respirator use, and there was more stringent use of

respirators, with better provisions for changing the organic

vapor cartridges, although there was no formal Respiratory

Protection Program (RPP).

Plant history and work practice changes were obtained

from interviews with company and union representatives,

senior workers, and exposure reports.

Exposure Information Sources

Airborne Styrene

Sampling data from the 1990 and 1992 studies were

available. In these two surveys, industrial hygienists and

technicians from the CLSCs carried out on-site measure-

ments for environmental exposure. Airborne styrene was

determined by personal sampling, using two passive dosi-

meters (3 M, 3500), worn consecutively for a full shift (8 h).

Analysis was performed by gas chromatography with a

¯ame ionization detector at the Quebec Institute for

Research in Occupational Health and Safety (IRSST,

1990a), and the Time Weighted Average (TWA) for the

shift calculated.

Additional styrene exposure survey reports conducted

by community health authorities and external consultants

were obtained. These contained personal sampling data

covering the period 1987±1998. In 1999, it was not possible

to gain entry to the plant to obtain personal samples for

airborne styrene.

The criteria for extracting styrene sample values from

these reports were that they were analyzed quantitatively

and documented as to sampling and analysis method, samp-

ling time, job group/task, date, and purpose. Indicator tube

sampling data were not included, but noted as a guide. Not

all samples were full-shift. Based on the review of Nor-

wegian styrene sampling data in the reinforced plastics

industry carried out by Lenvik et al., [1999] and the work of

352 Castillo et al.

Severi et al., [1994] on half-shift sampling, and preliminary

examination of the data, the few short-term samples of less

than 1-h duration were excluded. All samples of 1 h or more

were included as integrated exposures representative of the

workday for the tasks of the various job groups in the plant.

It is noteworthy that all styrene analyses had been carried

out at the IRSST, which ensured analytical quality control.

The ®nal styrene exposure database contained 286 personal

sampling results from identi®ed job groups.

Biological Indicator of Exposure

End-shift, end of work-week urinary mandelic acid

(MA) data were available from the 1990 and 1992 studies.

In these two studies, end-shift urine samples were collected

on the same day as personal environmental monitoring took

place. In 1999, end-shift, end of work-week urinary samples

were collected, and analyzed for MA. Urinary samples in all

these three studies were analyzed at the IRSST for MA

using high-pressure liquid chromatography with a UV

detector [IRSST, 1990b], the results being corrected for

urinary creatinine [IRSST, 1989]. No other reports contain-

ing MA values were located.

Treatment of Exposure Data

The environmental styrene exposure data were log

normally distributed. The 1990 study, which had identi®ed

various job groupings: choppers, laminators, painters, all of

whom were highly exposed, and unmolders, ®nishers, and a

mixed group of low exposure jobs [Truchon et al., 1992]

formed the basis for the present classi®cation. Data from

within each year were examined by analysis of variance

(ANOVA), and where there was no signi®cant difference

between job groups, data for the year were combined to

describe larger groupings. Although there was often no

signi®cant difference between ®nishers and the mixed low

exposure group in a given year, ®nishers were maintained as

a separate well-de®ned group for the following reasons:

there were considerable data for this group; their work was

in well-de®ned locations; they represented a stable group to

monitor general plant conditions over time.

Job groups were then examined between years to

determine whether signi®cant change had occurred, and

again combined when there was no signi®cant difference

between years on the log transformed data. Decisions for

points of change, and years with no data were made taking

into account information on plant history. The arithmetic

mean of the data for a job grouping and period was then used

as the exposure value for the period concerned, as the

arithmetic mean is considered preferable when cumulative

exposure is being estimated [Seixas et al., 1988]. The ratio

between job groups for some highly exposed groups

(painters, choppers, laminators) for an appropriate year

with complete data was used to estimate missing values in

years where data existed for at least one of the three groups,

according to the method outlined by Stewart et al. [1996].

The worker with most seniority commenced in 1983.

For the period prior to 1987 where there were no reports, the

1987 values were back-extrapolated on the assumption that

conditions were unlikely to have been better in the past. This

is a conservative assumption and is supported by the survey

of Lemasters et al. [1985] of U.S. data from government

sources of plants in the reinforced plastics industry. That

survey included three plants in tub and shower manufactur-

ing, where laminators showed a mean styrene exposure

value of 239 mg/m3 styrene. The assumption is also sup-

ported by exposure data from a mortality study of the

European reinforced plastics industries [Welp et al.,1996].

Correction for Use of Respirators

In the 1990 study it was noted that both painters and

choppers wore masks much of the time and laminators

rarely; however, there was no formal RPP in place at that

time, and cartridges were not regularly changed. Lamina-

tors, choppers, and painters in many years had similar

magnitude styrene exposure levels from environmental data.

To account for the modifying effect of respirator usage on

the styrene absorption of these three heavily exposed groups,

correction factors were developed on a job-group basis.

One approach to correction for respirator use would

have been to consider the percentage of time wearing res-

pirators. While questionnaire data for the percentage of time

using a respirator was collected in the 1990 study, this did

not account satisfactorily for the MA values of those using

half-face organic vapor cartridge respirators. The effective-

ness of respiratory protection was variable, and did not

apply uniformly across job groups as a simple percentage of

time wearing respirator [Truchon et al., 1992].

The correction for respirator use in the present study

was derived from the data from the 1990 study of three

plants (including the current plant), which showed a strong

linear relationship between end-shift, end of work-week

MA, and environmental styrene values from same-day per-

sonal sampling for those workers who did not use a chemical

cartridge mask on the day of assessment. The following

equation, provided by Campagna et al. [1995], describes this

relation:

y � 0:0024 X� 0:0114; r s � 0:95 �P < 0:001�N � 68 �1�

where

y�mandelic acid (mmol/mmol creatinineÿ1)

x� styrene concentration (mg/mÿ3)

Cumulative Exposure to Styrene and Visual Functions 353

It should be noted that all of the participants in the

present study were included in the dataset used to generate

Equation (1).

For each job group using respirators (e.g., painters), the

styrene value corresponding to the mean of end-shift, end of

work-week urinary MA values for all painters in 1990 was

calculated using Equation (1). This calculated value was

then compared with the mean airborne styrene value for

painters in 1990 obtained from the personal sampling data.

The ratio of the calculated styrene to the measured styrene

expressed as a percentage was used as the factor to correct

for reduction in exposure due to the use of respirators. The

year 1990 was considered representative of conditions prior

to changes implemented in this plant, and the 1990 factors

were applied to earlier data. A similar procedure was app-

lied to the 1992 data, using Equation (1), to develop a

second set of correction factors. Changes were implemented

in 1991, and in 1992 there was some training in respirator

use, and fresh cartridges were provided, but there was still

no formal RPP in place until the end of 1998. The factors

derived from the 1992 data were applied from 1991 to 1998.

Work History Information Sources

Work history information was available from self-

administered questionnaires from the 1990 study and the

1992 follow-up. An update questionnaire was given during

the 1999 study, followed by a brief individual interview for

clari®cation. Veri®cation points for the job histories were

obtained from several of the environmental styrene exposure

survey reports where speci®c workers sampled were identi-

®ed by name as well as job position.

Construction of CumulativeExposure Index

Work histories of individual workers from 1983 to 1999

were combined with styrene exposure values estimated for

each job group and year using appropriate correction factors

by job group and year for use of masks. The cumulative

exposure index (CEI) was calculated for each worker in the

study, using the styrene exposure for the job-group year as

calculated from sampling data modi®ed by the appropriate

mask correction factor, summed over the individual's

speci®c years of work history. These calculations were

carried out blinded to and independent of the analyses of the

data from the visual function tests.

Visual Function Assessment

The protocol for visual function assessment was iden-

tical for the three testing periods: 1990, 1992, and 1999. In

1999 visual functions were assessed on Saturday morning,

at least 12 h after the last exposure to styrene. All visual

functions, including near acuity, were examined mono-

cularly in a darkened room, under standardized illumination

provided by a `̀ daylight'' 1150-lux ¯uorescent lamp posi-

tioned 0.3 m above the target. Participants with a pre-

scription for eye glasses wore their own untinted eyeglasses

or contact lenses.

Color vision was assessed with the Lanthony D-15 hue

desaturated panel (Luneau Ophthalmique, Paris, France), as

described for the previous studies [Campagna et al., 1995;

Mergler et al., 1996]. The results of the test were expressed

as the Color Confusion Score (CCS), calculated using a

program developed by Huie [1984]. The mean value for the

two eyes was used in the present analyses.

Near visual contrast sensitivity was evaluated using the

Vistech 6000 card system according to methods described

for the earlier studies [Campagna et al., 1995; Mergler et al.,

1996]. The same cards were used for the three studies. Near

visual contrast sensitivity threshold was determined for each

eye at spatial frequencies of 1.5, 3.0, 6.0, 12.0, and 18.0

cycles per degree (cpd) and mean values for both eyes were

calculated.

Statistical Analyses

Both styrene exposure data and MA data were log

normally distributed and analyses were performed on the

transformed data. Analysis of variance (ANOVA) was used

for within-year comparisons of job grouping exposures,

comparison of means was done using Student's t-test. Non-

parametric statistics were used to compare repeated meas-

ures over time (Friedman's Test) or differences in group

categories (Kruskall±Wallis). The signi®cance threshold of

0.05 was used throughout. Analyses were performed using

Statview 5.0 (SAS Institute Inc.).

RESULTS

Table I describes the socio-demographic characteristics

of the workers who participated in this study.

The distribution of the monitoring information for each

year where data existed and over job groups at the time of

sampling is shown in Table II. It was possible to validate 64

points in time for correspondence between work histories as

provided by the self-administered questionnaires ®lled out

by the workers and the information for particular individuals

derived from the exposure reports. There was concordance

for 60 speci®c jobs (94%); the remaining four were in

closely related jobs.

Based on the job histories, the following job groupings

were examined to create the CEI: painter, chopper, lami-

nator, ®nisher, and other (low exposure). Figure 1 shows the

pro®le of mean airborne styrene levels for these job groups

over the period 1987±1998. The increase in exposure levels

354 Castillo et al.

TABLE I. Socio-Demographic Characteristics of the18Workers From a Reinforced Plastics Plant, Canada

Mean SD Median Range

Age (years) 38.3 7.6 35 27^51Education (years) 11.3 1.8 11.0 7^15Alcohol intake (g/week) 155 250 41 0^994Seniority (years) 13.3 2.4 13.0 10^20

TABLE II. Exposure Information Data by Job and Year in a Reinforced Plastics Plant

Year

Job 1987 1988 1989 1990 1991 1992 1995 1998 Total

Paint/laminator 9 23 14 ö ö ö ö ö 46Laminator ö ö ö 17 ö 10 6 ö 33Chopper ö ö ö 9 ö 8 6 ö 23Laminator/chopper ö ö ö ö ö ö ö 12 12Paint/unmolder ö ö ö 10 ö 4 7 9 30Paint/lamin/chopper ö ö ö ö 12 ö 7 ö 19Unmolder ö ö ö 3 ö 2 4 ö 9Finisher 3 7 4 31 ö 12 7 3 67Other (lowexposure) ö ö ö 25 ö 10 9 3 47

Total 12 30 18 95 12 46 46 27 286

FIGURE1. Mean styrene concentrations for job categories from1987 to1998.

Cumulative Exposure to Styrene and Visual Functions 355

following 1987 was associated with increase in production.

For example, for 1989, painters and laminators did not differ

and the measured exposure levels ranged from 528 to 1035

mg/m3 (n� 14); the geometric standard deviation (GSD) for

that year was 1.21. Exposure for laminators remained

relatively constant from 1992 until the most recent survey in

1998, with values ranging from 209 to 458 mg/m3 (n� 34);

the GSD for this period ranged from 1.23 to 1.30. Finishers'

exposure has decreased slightly over time, with the highest

mean value in 1989 (61 mg/m3), ranging from 50 to 84 mg/

m3 (n� 4) and the lowest values in 1998 (mean: 12 mg/m3),

ranging from 10 to 15 mg/m3 (n� 3).

The low exposure levels observed for all groups in 1991

were the combined effect of reduced production due to

economic conditions (many workers were laid off) and

the introduction of ventilation and work practice changes.

Exposures remained stable over the 1992±1994 period for

all groups. In late 1994, the Quebec TWAEV was lowered

from 420 to 215 mg/m3 [Editeur, of®ciel du QueÂbec, 1987,

1995], and further changes in ventilation and work practices

decreased airborne exposure levels of choppers and

painters; however, laminators' airborne exposure level did

not change.

The distribution of urinary MA for the workers in the

present study is shown in Figure 2 for 1990, 1992, and 1999.

Comparison of each worker to his previous values showed

that there was a decrease in exposure over time (Friedman

Rank; w2� 8.4; df� 2; P � 0:015). The most important

decreases were for the job categories with the highest MA

levels. Current MA levels for the different job categories are

presented in Table III; laminators and choppers now rotate

jobs throughout the day and are grouped into one category.

Table IV shows the percent correction factors for

respirator use developed from the end of shift, end of work-

week urinary MA values with respect to environmental

styrene exposure. For the laminators in 1990, mask use was

totally ineffective as estimated from MA, supporting the

observations that workers removed their masks for most of

the time as the respirator impeded their work. Although the

changes in work practices and use of personal protective

equipment resulting from the 1990 study substantially

reduced the effective exposure for laminators and choppers

in 1992, the effectiveness of mask usage for painters chan-

ged little.

For the 18 workers retained in these analyses, the CEI

ranged from 198 mg/m3 � years to 6,022 mg/m3 � years.

Because of the small number of participants for visual test-

ing, the workers were equally divided into three cumulative

exposure groups: low CEI < 1; 000 mg/m3 � years (n� 6;

mean age: 35 years� 7); medium CEI � 1,000< 2; 500

mg/m3 � years (n� 6; mean age: 39 years� 8); and high CEI

� 2,500 mg/m3 � years (n� 6; mean age: 40 years� 7).

Although the least exposed were somewhat younger, the

differences are not signi®cant (Kruskal±Wallis; P � 0:22).

No inter-group differences were observed for alcohol

consumption (Kruskal±Wallis; P � 0:42). There was also

no signi®cant difference in current exposure, re¯ected by

urinary MA levels (Kruskal±Wallis; P � 0:11).

The in¯uence of the potential co-variables: age educa-

tional level, and alcohol intake, were examined. Color

FIGURE 2. Percentile distribution of urinary MA for the 18 participating workers in 1990,

1992, and1999.The lines indicate: 10th, 25th, 50th, 75th, and 90th percentile scores and the

circles are outliers.

TABLE III. Mandelic Acid (mmol/mmol creatinine) levels by Job Category for 1999 for Workers in a ReinforcedPlastics Plant, Canada

Job category n Mean SD Geometric mean Range

Laminators/choppers 5 0.099 0.031 0.095 0.067^0.143Painters 5 0.119 0.090 0.094 0.041^0.241Finishers 3 0.036 0.037 0.026 0.013^0.079Others 4 0.046 0.036 0.038 0.020^0.098

TABLE IV. Percentage Correction Factor for Respirator Use forWorkers in aReinforced Plastics Plant, Canada

Painter Laminator Chopper(%) (%) (%)

1990 44 100 531992 48 13 28

356 Castillo et al.

confusion scores and the higher spatial frequencies of the

near-visual contrast sensitivity were associated with age,

but not educational level or alcohol intake. The scores were

adjusted for age for each measurement year using the

method of co-variance.

The distribution of the age-adjusted color confusion

scores 1990, 1992, and 1999 are shown in Figure 3. Paired

analyses showed that color vision improved between 1990

and 1992 (Wilcoxon; P < 0:05), re¯ecting the results shown

with the larger group, reported by Mergler et al. [1996]. No

signi®cant difference was observed for the period between

1992 and 1999. The 1999 results were not related to cur-

rent exposure, evaluated by MA levels, or the cumulative

exposure groups.

The mean age-adjusted near visual contrast sensitivity

pro®le for 1990, 1992, and 1999 showed a tendency to

decrease over time, with signi®cant intra-individual differ-

ences at 3 cpd (Friedman; P < 0:05) (Fig. 4). The 1999

pro®le did not vary with MA levels. When examined with

respect to the CEI, the mean age-adjusted contrast sensi-

tivity pro®le was depressed among the group with the

highest CEI, and signi®cantly lower (P < 0:05) at the spatial

frequencies 3, 6, and 12 cpd (Fig. 5). This same pattern was

observed using the continuous variable CEI with age-

adjusted near visual contrast sensitivity scores. The relations

were signi®cant at 3 cpd (r2� 0.41; P < 0:004) and 6 cpd

(r2� 0.30; P < 0:02), while at 12 cpd, the relation was just

above the signi®cance level (r2� 0.18; P � 0:08); for 1.5

and 18 cpd, the relations were not signi®cant (P � 0:13 and

0.10, respectively).

DISCUSSION

The results from our previous 2-year follow-up study of

workers in a reinforced plastics plant showed that reduction

of styrene exposure was associated with improvement on

some neurofunctional tests, notably color vision [Mergler

et al., 1996]. On the other hand, near visual contrast sensi-

tivity decreased during this interval in spite of overall

reduction in exposure levels. This raised the question of the

possible effects of cumulative exposure, which we were able

to investigate in a subset of the same workers, 6 years later.

A primary concern in using historical measurement

data to estimate cumulative exposure is the question of the

representativeness of the data to the true long-term exposure

[Stewart et al., 1996]. Here, individual work histories were

combined with group data considered representative of the

exposure at a particular job category over time. The 1990

study, for which there are the most measurements, was not

designed to evaluate speci®c job groupings, and each parti-

cipating worker was sampled once. The distribution of

measurements re¯ects the workforce distribution of parti-

cipants during a period of high production, which accounts

for the larger number of samples on ®nishers, a low ex-

FIGURE 3. Percentile distribution of age-adjusted Color Confusion Score (n�17) in1990,1992, and1999.The lines indicate: 10th, 25th, 50th, 75th, and 90th percentile scores and the

circles are outliers.

FIGURE4. Age-adjustednear visual contrastsensitivityprofiles for the18workers in1990,

1992, and1999.The asterisk indicates a significant decrease at P < 0:05; Friedman's Rank

Test.

FIGURE 5. Age-adjusted near visual contrast sensitivity profiles with respect to cumula-

tive exposure index (low, n� 6), medium (n� 6), and high (n� 6).The asterisk indicates a

significant decrease at P < 0:05; Friedman's RankTest.

Cumulative Exposure to Styrene and Visual Functions 357

posure group, relative to the number of samples in each of

the three high exposure groups.

The 1992 study, which sampled a subset of the initial

group, was a follow-up study to evaluate the effect of

de®ned changes in ventilation and use of respirators. Again,

each worker was sampled once, and the distribution re¯ects

the ongoing workforce which participated in the follow-up.

There had been substantial attrition of the workforce due to

economic conditions. In both the 1990 and 1992 studies,

personal sampling and biological monitoring were con-

ducted on the same day, on the ®nal full workday of the

work-week, and are considered representative.

Sampling carried out by the community health organi-

zation and consultants was more frequently for compliance

evaluation purposes, and tended to include fewer samples of

less exposed job groups. The 1995 survey data followed the

reduction in the Quebec TWAEV for styrene in late 1994

from 420 to 215 mg/m3 [Editeur, of®ciel du QueÂbec, 1987,

1995].

The pro®le of external styrene levels for laminators in

this plant is not dissimilar to that shown for a large group of

European laminators [Welp et al., 1996], although exposure

levels in the Quebec plant are higher. The results here are

similar to those reported in a 1999 study in the New England

boat-building industry, where lapel sampling showed mean

values of 294 and 299 mg/m3 for spray gun operators and

400 and 434 mg/m3 for roller operators [Malek et al., 1999],

jobs similar to choppers and laminators.

The current styrene levels in this Quebec plant are also

comparable to those observed in a recent study by Gobba

et al. [2000] in a small Italian plant manufacturing rein-

forced plastics products, where half-shift monitoring over a

2-week period of a group of seven hand-rollers and sprayers

gave mean styrene exposure levels ranging from 169.2 to

335.7mg/m3.

The process used in the present study for the correction

of environmental styrene exposure for respirator use is

similar to that used by Gobba et al. [2000] to evaluate the

effectiveness of respirator use during a complete half-shift,

using urinary styrene (StU) as the biomarker. These authors

found wide inter- and intra-individual variability, with a

mean 50±60% decrease in StU and a progressive reduction

in the effectiveness of the respirators as the week progressed.

As our estimates were based on the ®nal full work-day of the

work-week, they are conservative estimates of the protec-

tion afforded various work groups. No formal RPP was in

place until late 1998, and it is likely that the effectiveness of

respirators decreased over the work-week. LoÈf et al. [1993]

in an evaluation of respirator use in Swedish boat-building

operations, found that use of respirators for 52% of the time,

which corresponded to times when workers were exposed to

high concentrations of styrene during spraying, laminating,

and painting operations, reduced styrene exposure during

the entire workday by 56±92%, with a mean of 76%. The

respirators used in the Swedish study included both half-

facepiece air-purifying respirators with charcoal ®lters, used

during laminating, or full-face air-supplied respirators with

positive pressure used during spraying. In neither of these

two studies were measures identi®ed more closely with

speci®c operations.

The correction factors by job group used in the present

study provide a similar range of styrene reduction to those

reported by LoÈf et al. [1993], when respirators were worn,

but take into account work practices and conditions within

the three highly exposed job groups, and demonstrate the

effect of changes in work practice introduced in the plant.

For the laminators in our 1990 study, mask use was totally

ineffective as estimated from the MA values, con®rming the

observations made at that time about work practices and

supporting the use of the bioindicator to correct for res-

pirator usage. The level of reduction for choppers improved

from 47 to 72%, and the effectiveness of respirator usage for

painters did not change. Using urinary MA as a bioindicator

of effective exposure showed that respiratory protection in

this plant has been only moderately successful in controlling

styrene exposure.

Although control of environmental levels of styrene in

this plant has improved as a result of changes to meet more

stringent regulatory standards, the jobs of chopper, painter,

and laminator in this type of operation remain highly

exposed. The situation in this Quebec plant is probably

typical of many other small plants in the reinforced plastics

industry.

To constitute the cumulative exposure index, job

histories retrospectively reported by the workers were used.

The question of recall and the accuracy of workers' reports

is always a key issue and a potential source for misclassi-

®cation. For this study, questionnaires on job history were

completed in 1990, with updates in 1992 and 1999, and a

brief interview to clarify. The lapsed time for recall, and

number and duration of assignments, which have been

found to affect recall in job histories [Bond et al., 1988] was

thus divided into three shorter periods. Although company

records were not available in 1999, we were able to verify

the workers' self-reported histories at several points in time.

All jobs of all the workers participating in the 1999 study

were identi®ed in the sampling that took place in 1990 and

in 1992, and other sampling reports contained additional

veri®cation points. Cross-checking of workers' names and

speci®c jobs indicated in the sampling records with

workers' retrospective reporting showed agreement for

94%. The few that were not in complete agreement were

in jobs with similar exposure levels. These results are

similar to those obtained by Bourbonnais et al. [1988], who

found 89% concordance for job title between workers' self-

reporting and administrative records in a single plant, while

Bond et al. [1988] found from interviews that workers were

able to recall their usual work area assigment 70.8% of the

358 Castillo et al.

time. The history of changes in the plant which could affect

exposure levels was established through multiple sources

documentation in the reports, focus group meetings and the

interviews with the participants, and company and union

representatives.

Most studies of color vision among workers exposed to

styrene have demonstrated dose-dependant loss with current

exposure [Gobba et al, 1991; Fallas et al., 1992; Chia et al.

1994; Campagna et al; 1995]. Some authors have suggested

that the threshold for color vision impairment is very low

(< 30 ppm) [Chia et al., 1994; Campagna et al. 1996] and

may be reversible over time [Mergler et al., 1996]. In the

present study, MA levels were all inferior to 0.25 mmol/

mmol creatinine, which corresponds to the approximate

equivalent of 25 ppm (103 mg/m3) of styrene. These low

levels of actual exposure probably account for the absence

of a dose-dependant relation between color vision loss and

exposure.

Near contrast sensitivity was lowest in the group with

the highest cumulative exposure, with signi®cant loss in the

intermediate spatial frequencies (3±12 cpd). All workers

had good visual acuity, as was the case for the subset of

former microelectronics workers with a history of exposure

to a mixture of organic solvents who likewise presented

depressed near visual contrast sensitivity pro®les in the

intermediate spatial frequencies [Frenette et al., 1991]. A

similar contrast sensitivity loss among microelectronics

workers was con®rmed by Broadwell et al. [1995].

Donoghue et al. [1995] reported decreased far visual

contrast sensitivity in the same frequency range (3±12

cpd) in 16 patients diagnosed with occupational organic

solvent-induced chronic toxic encephalopathy. The group

included two styrene boat builders with 6 and 8 years

exposure.

It is interesting that in the present study there was no

relation between bioindicators of current exposure and

contrast sensitivity loss. This would suggest that contrast

sensitivity loss re¯ects long-term cumulative exposure and

chronic damage to the neuro-optic pathways. There is no

evidence of reversibility in this follow-up study or in any

previous studies which examined visual contrast sensitivity.

Further follow-up studies are needed of workers with org-

anic solvent-induced contrast sensitivity loss to con®rm

irreversibility, and to examine the progression and implica-

tions of the loss of visual functions.

Although it would have been interesting to follow a

reference group over the same period, this was not possible

here. Further follow-up studies should strive to include a

control group that could be assessed parallel to the exposed

workers.

The strengths of this study lie in the extensive data that

were available to construct a cumulative index of exposure.

Earlier studies by our research group, coupled with the work

of the local occupational health professionals and con-

sultants in this industry, where over-exposure to styrene has

been an important issue as the permissible levels have

decreased over time, provided valid measures over the past

12 years. The weakness lies in the small number of workers

in the study. It is noteworthy that even with these small

numbers, the relation between cumulative exposure and

near visual contrast sensitivity loss is signi®cant and

consistent with previous studies as well as the mechanistic

hypothesis that loss in the intermediate range, which re¯ects

the functioning of the neuro-optic pathways would be

affected by styrene exposure, while the higher frequencies,

which represent visual acuity, remain intact.

The effects seen for near visual contrast sensitivity

represent long-term chronic exposure at high levels, as all

the workers had a minimum of 6 months exposure in the

plant at the time of the 1990 study.

ACKNOWLEDGMENTS

We are grateful for the collaboration and participation

of the workers at this plant. The authors acknowledge the

support of Dr. Ginette Truchon of the QueÂbec Institute for

Research in Occupational Health and Safety (IRSST) in the

laboratory analyses, as well as Ms. Marie-Ludivine ChaÃteau-

Degat for her assistance with ®eldwork. We are particularly

appreciative of the support of Johanne Leduc and Sylvie

Champoux. We thank the Association of Universities and

Colleges of Canada and the Canadian International Devel-

opment Agency for their ®nancial support for Dr. Laura

Castillo.

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