287 

Ann. occup. Hyg., Vol. 47, No. 4, pp. 287–295, 2003

© 2003 British Occupational Hygiene Society

Published by Oxford University Press

DOI: 10.1093/annhyg/meg046

Respiratory Symptoms and Occupational Exposures in 

New Zealand Plywood Mill Workers

WOUTER FRANSMAN1,2, DAVE McLEAN1*, JEROEN DOUWES1,2,

PAUL A. DEMERS3, VICTOR LEUNG3 and NEIL PEARCE1

*Author to whom correspondence should be addressed.

Tel: +64 4 380 0609; fax: +64 4 380 0600; e-mail: 

d.j.mclean@massey.ac.nz

1Centre for Public Health Research, Massey University Wellington Campus, Private Box 756, 

Wellington, New Zealand; 2Institute for Risk Assessment Sciences (IRAS), Division of Environmental 

and Occupational Health, Utrecht University, The Netherlands; 3School of Occupational and 

Environmental Hygiene, University of British Columbia, Vancouver, BC, Canada

Received 29 July 2002; in final form 10 December 2002

Objectives: To study work exposure and respiratory symptoms in New Zealand plywood mill

workers.

Methods: Personal inhalable dust (n = 57), bacterial endotoxin (n = 20), abietic acid (n = 20),

terpene (n = 20) and formaldehyde (n = 22) measurements were taken and a respiratory health

questionnaire was administered to 112 plywood mill workers.

Results: Twenty-six percent of the dust exposures exceeded 1 mg/m3, however, none of the

samples exceeded the legal limit of 5 mg/m3 [geometric mean (GM) = 0.7 mg/m3, geometric

standard deviation (GSD) = 1.9]. Workers in the composer area (where broken sheets are

joined together) were significantly (P < 0.01) more highly exposed. Endotoxin levels were low

to moderate (GM = 23.0 EU/m3, GSD = 2.8). Abietic acid levels ranged from 0.3 to 2.4 μg/m3

(GM = 0.7 μg/m3, GSD = 1.8) and were significantly (P < 0.05) higher for workers in the

composer area of the process. Geometric mean levels of α-pinene, β-pinene and ∆3-carene were

1.0 (GSD = 2.7), 1.5 (GSD = 2.8) and 0.1 (GSD = 1.4), respectively, and α-pinene and β-pinene

levels were significantly (P < 0.001) higher for workers in the ‘green end’ of the process, up to

and including the veneer dryers. Formaldehyde levels ranged from 0.01 to 0.74 mg/m3 [GM =

0.08 mg/m3 (= 0.06 p.p.m.), GSD = 3.0]. Asthma symptoms were more common in plywood mill

workers (20.5%, n = 112) than in the general population [12.8%, n = 415, adjusted OR (95%

CI) = 1.5 (0.9–2.8)]. Asthma symptoms were associated with duration of employment and were

reported to lessen or disappear during holidays. No clear association with any of the measured

exposures was found, with the exception of formaldehyde, where workers with high exposure

reported more asthma symptoms (36.4%) than low exposed workers [7.9%, adjusted OR (95%

CI) = 4.3 (0.7–27.7)].

Conclusions: Plywood mill workers are exposed to inhalable dust, bacterial endotoxin,

abietic acid, terpenes and formaldehyde, and they appear to have an increased risk of developing work-related respiratory symptoms. These symptoms may be due to formaldehyde

exposure, although a potential causal role for other exposures cannot be excluded.

Keywords: asthma; exposure; woodworkers

INTRODUCTION

A number of studies have reported that workers in

wood processing industries are exposed to relatively

high levels of dust in their working environment

(Norrish et al., 1992; Mandryk et al., 1999; Teschke

et al., 1999; Cormier et al., 2000; Demers et al.,

2000) and that they report higher rates of both lower

and upper respiratory tract symptoms than do nonexposed controls (Norrish et al., 1992; Demers et al.,

1997). A recent study in New Zealand has reported

an increased prevalence of asthma and cough symptoms and of eye and nose irritation for sawmill

workers (Douwes et al., 2001). Whereas studies in

the sawmill industry are plentiful, only a few studies

have been conducted specifically in plywood mills.
288 W. Fransman et al.

Previous studies in plywood mills have reported that

workers are exposed to terpenes in the veneer drying

process and to formaldehyde in the pressing section,

causing irritation of eyes, nose and throat (National

Institute of Safety and Health, 1981, 1982; Paustenbach et al., 1997) and that, in addition, exposure to

formaldehyde is associated with decrements in baseline spirometric values and various respiratory symptoms (Malaka and Kodama, 1990; Makinen et al.,

1999), as well as an increased risk of nasopharyngeal

cancer (Vaughan et al., 2000). The relationship

between respiratory symptoms and exposure to both

bacterial endotoxin and abietic acid has not yet been

studied in the plywood mill industry. However,

previous studies in sawmills have shown that sawmill

workers are exposed to both bacterial endotoxin

(Mandryk et al., 1999; Cormier et al., 2000; Douwes

et al., 2000) and abietic acid (Demers et al., 2000),

which could both be related to the occurrence of

respiratory symptoms.

In the current study, we measured exposure in New

Zealand plywood mill workers to a range of agents

with known or suspected potential to cause respiratory symptoms [i.e. inhalable dust, bacterial endotoxin, abietic acid, terpenes (α-pinene, β-pinene, ∆3-

carene) and formaldehyde] and related these to the

prevalence of respiratory symptoms. In addition, we

compared the prevalence of respiratory symptoms

with general population symptom prevalence rates in

the same area of New Zealand.

MATERIALS AND METHODS

Plywood manufacturing process

The plywood mill that we studied is located in the

central North Island of New Zealand and employs

200 workers. A single pine species (Pinus radiata) is

used in the process. This process consists of four

main activities: (i) ‘green end’—peeling veneer from

steam preconditioned and debarked logs and cutting

the veneer into sheets; (ii) ‘dryers’—drying the sheets

in high temperature (200°C) veneer dryers, with

broken sheets being joined together at the ‘composers’;

(iii) ‘pressing’—glueing (phenol-formaldehyde adhesive) and hot pressing (140°C) the plywood sheets

together; (iv) ‘finishing end’—sawing sheets to size,

filling knot holes and sanding, grading, stacking and

banding of finished plywood sheets. Compressed air

and dry sweeping are used to remove dust and dirt

from machinery and floors. Extraction fans are

provided for the veneer dryers, hot presses and

sanders, but extensive fugitive emissions contribute

to the emission of a ‘blue haze’.

Study participants

As the workers at the mill were approached

through the site managers or team leaders we were

unable to calculate the exact number of workers that

were asked to participate in the respiratory health

questionnaire. However, based on personnel records

this number was estimated to be 170. Of the total

number of workers approached, 112 (66%) agreed to

participate in the study. All subjects gave written

informed consent and the study was approved by the

Massey University Ethics Committee (MUHEC, PN

Protocol 01/24).

Exposure measurements

Inhalable dust. Sixty full-shift (8 h) personal inhalable dust samples were collected over a range of job

titles in the sections ‘green end’, ‘dryers’, ‘composers’,

‘pressing’ and ‘finishing end’. Within these job titles,

workers were randomly selected and the measurements were taken over 8 work days. Dust was

sampled on glassfiber filters at a flow rate of 2.0

l/min using portable pumps (Casella) and IOM heads.

One field blank was taken each measurement day.

The filters (taken out of the cassette) were pre- and

post-weighed on an analytical balance (0.01 mg

sensitivity).

Bacterial endotoxin and abietic acid. Twenty of the

inhalable dust samples were selected from a range of

job titles in the mill (representing all the sections of

the mill) for endotoxin analyses and using the same

approach another 20 samples were selected for

abietic acid analyses. The selection of samples within

a job title was random. Endotoxin extraction and

analysis were performed as described previously

(Douwes et al., 1995), with a limit of detection of 0.5

EU/m3. Abietic acid analyses were performed using

GC/MS with a limit of detection of 0.5 μg/m3, as has

been described previously (Demers et al., 2000).

Terpenes. Twenty full-shift personal monoterpene

samples were collected using SKC (model 575-003)

diffusion samplers. The samples were taken on two

consecutive work days from 10 job titles in the

sections ‘green end’, ‘dryers’ and ‘pressing’, which

were expected to have the highest levels of volatiles

in the mill. No samples were taken in the ‘finishing

end’, where only dried veneer is processed. One field

blank was taken for each measurement day. Terpene

analyses were performed as described previously

(Demers et al., 2000). Limits of detection were 0.60

μg for α-pinene, 0.55 μg for β-pinene and 0.59 μg for

∆3-carene.

Formaldehyde. Twenty-two personal formaldehyde samples were taken using silica gel tubes

impregnated with 2,4-dinitrophenylhydrazine (DNPH)

(flow rate 1 l/min) (NIOSH method 2016). The

sampling time of each sample was 15 min. The

samples were desorbed in acetonitrile and analyzed

for formaldehyde using high performance liquid
Respiratory symptoms in plywood mill workers 289

chromatography. The limit of detection for formaldehyde was 0.03 mg/m3 (= 0.03 p.p.m.).

Questionnaire

Questionnaires were administered face-to-face and

consisted of three parts, including: (i) personal and

work characteristics; (ii) respiratory health symptoms; (iii) smoking habits. They were the same as

those used in a previous study of sawmill workers

(Douwes et al., 2001). As well as analyses of the

responses to individual questions, overall asthma

prevalence was estimated using the ECRHS definition (Burney et al., 1994), which is based on the

proportion of subjects answering ‘yes’ to: (i) woken

by shortness of breath in last 12 months; (ii) asthma

attack in last 12 months; or (iii) current asthma medication.

General population data

We have previously conducted a random population survey (Lewis et al., 1997) in several areas of

New Zealand. We compared the asthma symptom

prevalence in the plywood mill with that in the previously collected general population data for the same

area.

Statistical analysis

The data were analyzed using SAS statistical software (version 8.01; SAS Institute, Cary, NC). In situations where sample values were less than the limit of

detection (LOD), the LOD divided by 21/2 was substituted for sample values to calculate average exposure

levels. All measured air contaminants approximated

a log-normal rather than a normal distribution.

Pearson correlation coefficients were calculated to

examine the relationship between the different agents

using natural log-transformed data. Differences in

exposure levels among areas in the plywood mill

were tested using the t-test procedure on natural

log-transformed data. Symptom prevalences were

compared in workers with ‘high’ and ‘low’ exposure

to various factors (terpenes and formaldehyde).

Crude and adjusted prevalence odds ratios (and 95%

confidence intervals) to describe the association

between exposure and the occurrence of symptoms

were calculated by means of logistic regression

analysis (Rothman and Greenland, 1998). For the

comparison with the general population, the analyses

were only adjusted for age, gender and ethnicity,

since smoking data were not available for the general

population survey.

RESULTS

Characteristics of study participants

Table 1 shows the characteristics of the study

participants. The plywood mill workers were slightly

older than the general population comparison sample

(mean ages of 34.5 and 32.5 yr, respectively) and

included a higher proportion of males (71 and 44%,

respectively) and Polynesians (67 and 39%, respectively). The key analyses were therefore adjusted for

age, gender and ethnicity.

Exposure results

Inhalable dust. Three out of 60 inhalable dust

samples were lost due to pump failure. All of the

remaining 57 samples were above the LOD (0.06

mg), and of these 15 (26%) had dust concentrations

>1 mg/m3 (a recommended standard for exposure to

softwood dust) (Table 2). None of the samples

exceeded the New Zealand workplace exposure

standard for inspirable dust of 5 mg/m3. The

geometric mean (GM) of all samples was 0.7 mg/m3

[geometric standard deviations (GSD) = 1.9] and

workers in the composer area were significantly (P <

0.01) more highly exposed to inhalable dust (GM =

1.6, GSD = 2.1).

Bacterial endotoxin. All of the endotoxin samples

were above the LOD (0.5 EU/m3). Of the 20 samples,

seven (35%) had levels higher than 50 EU/m3, a

recommended exposure limit in The Netherlands

(Heederik and Douwes, 1997; Dutch Expert

Committee on Occupational Standards, 1998). The

correlation between inhalable dust and endotoxin

exposure was moderate with a Pearson correlation

coefficient of 0.54 (P = 0.01, n = 20). Endotoxin

exposure levels appeared to be higher in the

composer area (not statistically significant), but that

was based on only two samples (Table 2).

Table 1. Characteristics of study participants

a

In mill.

bIn current job title.

c

Maori or Pacific Islander.

General 

population

Plywood mill

n 415 112

Mean age (SD) 32.5 (7.2) 34.5 (9.1)

Male (%) 183 (44%) 80 (71%)

Female (%) 232 (56%) 32 (29%)

Duration of employmenta (SD) 4.7 (3.5)

Duration of employmentb (SD) 2.7 (2.6)

Hours/week of work (SD) 44.8 (4.4)

Smoker

Current smokers (%) 43 (38%)

Ex-smokers (%) 13 (12%)

Never smokers (%) 56 (50%)

Ethnicity

Polynesian (%)c 163 (39%) 75 (67%)

Non-Polynesian (%) 252 (61%) 37 (33%)
290 W. Fransman et al.

Abietic acid. Abietic acid was above the LOD (0.5

μg/m3) in 10 of the 20 samples. Exposure to abietic

acid was significantly (P < 0.05) higher for job titles

in the composer area compared with the other areas in

the mill (Table 2). Dust levels were highly correlated

with abietic acid levels (r = 0.86, P < 0.001, n = 20).

Terpenes. All α-pinene and β-pinene analyses were

above the LOD (0.60 and 0.55 μg, respectively). ∆3-

Carene was above the LOD (0.59 μg) in six samples

(Table 3). Exposure to α-pinene and β-pinene was

significantly (P < 0.001) higher in the job titles

before the veneer dryers (workers in the ‘green end’

and the dryer infeed operator) with GM of 2.8 (GSD

= 2.0) and 4.4 (GSD = 2.1), respectively, than in job

titles after the veneer dryers (dryer table hand, dryer

tender and workers in the ‘pressing section’) with

GM of 0.5 (GSD = 1.2) and 0.7 (GSD = 1.2), respectively.

Formaldehyde. Seventeen out of the 22 formaldehyde analyses were above the LOD (0.03 mg/m3).

None of the 22 samples exceeded the New Zealand

workplace exposure standard for formaldehyde of

1.25 mg/m3 (= 1.00 p.p.m.) (Table 3). Formaldehyde

levels were not significantly higher for job titles in

the pressing section [where the glue (which contains

formaldehyde) is added to the process] compared

with the other areas. One major outlier was found in

the dryers section for the worker that worked closest

to the pressing section. When this outlier was

included in the pressing section, the pressing section

had significantly (P < 0.05) higher formaldehyde

levels compared with the other areas.

Respiratory symptoms

Reported ‘attacks of shortness of breath with

wheezing in the last 12 months’ were significantly

higher in plywood mill workers than in the general

population, with an OR of 1.8 (95% CI = 1.0–3.1).

All other asthma symptoms were also elevated (but

did not reach statistical significance), with the exception of ‘asthma attack in last 12 months’ (Table 4).

‘Shortness of breath with wheezing in last 12

months’, ‘woken by shortness of breath in last 12

months’ and ‘asthma’ were significantly (P < 0.05)

higher in workers with a duration of employment

longer than 6.5 yr, when compared with the general

population (adjusted for age, gender and ethnicity)

(Table 5). Workers with a work history of <2 yr in the

Table 2. Personal exposure levels (GM and GSD) by work area for inhalable dust, bacterial endotoxin and abietic acid

EU, endotoxin units; LOD, limit of detection.

aP < 0.01 compared with the other areas. bP < 0.05 compared with the other areas.

Inhalable dust (mg/m3) Endotoxin (EU/m3) Abietic acid (μg/m3)

N N > LOD GM (GSD) N N > LOD GM (GSD) N N > LOD GM (GSD)

All samples 57 57 0.7 (1.9) 20 20 23.0 (2.8) 20 10 0.7 (1.8)

Area

Green end 7 7 0.8 (2.7) 3 3 23.2 (2.9) 3 1 0.8 (2.4)

Dryers 20 20 0.6 (1.7) 6 6 34.2 (2.9) 8 3 0.5 (1.6)

Composers 6 6 1.6 (2.1)a 2 2 75.7 (1.1) 2 2 1.7 (1.6)b

Pressing 9 9 0.6 (1.4) 3 3 16.6 (2.2) 4 4 0.8 (1.5)

Finishing end 12 12 0.8 (1.9) 6 6 12.2 (2.3) 3 0 0.4 (1.0)

Table 3. Personal exposure levels (GM and GSD) by work area for terpenes and formaldehyde

LOD, limit of detection.

aP < 0.05 compared with the other area. bP < 0.01 compared with the other area.

Terpenes (mg/m3) Formaldehyde (mg/m3)

α-Pinene β-Pinene ∆3-Carene N N > LOD GM (GSD)

N N > LOD GM (GSD) N > LOD GM (GSD) N > LOD GM (GSD)

All samples 20 20 1.0 (2.7) 20 1.5 (2.8) 6 0.10 (1.4) 22 17 0.08 (3.0)

Area

Green end 4 4 2.4 (2.6)a 4 3.9 (2.6)a 0 0.10 (1.0)

Dryers 8 8 1.3 (2.7) 8 2.0 (2.9) 5 0.12 (1.4)a 14 11 0.07 (3.2)

Composers 2 0 0.03 (1.0)

Pressing 8 8 0.5 (1.2)b 8 0.7 (1.2)b 1 0.08 (1.1)b 5 5 0.16 (2.7)

Finishing end 1 1 0.04 (–)
Respiratory symptoms in plywood mill workers 291

plywood industry appeared to be less symptomatic

compared with the general population (Table 5).

Symptom prevalences by level of exposure were

calculated. While the numbers of exposure measurements taken were not sufficient to adequately characterize the exposure of every job title, we used the

following assumptions to classify workers as to high

or low exposure. Terpenes showed a clear pattern of

higher exposure in those parts of the process up to the

veneer dryers (i.e. workers in the ‘green end’ and the

dryer infeed operators), with much lower exposure

levels (P < 0.01) in job titles after the veneer dryers

(i.e. dryer table hand, dryer tender and job titles in the

‘pressing section’). We therefore used the questionnaire results of workers in all job titles in the plywood

mill for the terpene analyses, although some of these

job titles had not been sampled. However, forklift

drivers and employees with no fixed workstation

among all job titles in the plywood mill were not

included, due to uncertainty about their work area.

None of the other compounds measured (inhalable

dust, abietic acid, endotoxin and formaldehyde)

showed any clear pattern of exposure, although

workers in the composer area had the highest levels

of exposure to inhalable dust, endotoxin and abietic

acid. However, only a few workers are working in

the composer area and, therefore, job titles in the

plywood mill were divided into ‘low’ and ‘high’

exposure categories based on the median exposure. In

this case, questionnaire results only from workers

with the same job titles as those actually sampled

were used to compare symptom prevalences between

exposure groups.

Table 6 shows symptom prevalences and adjusted

ORs by level of exposure to terpenes and formaldehyde. Symptoms were more common in workers with

high exposure to formaldehyde (Table 6), with a

significantly (P < 0.05) higher prevalence of ‘woken

by shortness of breath in last 12 months’ [adjusted

OR of 9.5 (95% CI = 1.2–74.7)] (Table 6). Also,

prevalences of ‘asthma medication in last 12 months’

and ‘asthma’ were elevated (not statistically signifiTable 4. Unadjusted asthma symptom prevalence (%) and adjusted prevalence odds ratios (OR) with 95% confidence intervals 

(CI) for plywood mill workers compared with the general population

aAdjusted for age, gender, ethnicity.

bSOB, shortness of breath.

c‘Woken by SOB in last 12 months’ or ‘asthma attack in last 12 months’ or ‘current asthma medication’.

dP < 0.05 compared with general population.

Symptoms in last 12 months General population (n = 415) Plywood mill (n = 112)

Percent ORa Percent ORa (CI)

Wheezing 25.1 1.0 29.5 1.2 (0.7–2.0)

Wheezing without a cold 15.2 1.0 17.9 1.1 (0.6–2.1)

SOBb with wheezing 15.0 1.0 24.3d 1.8 (1.0–3.1)d

Woken by SOBb 8.7 1.0 15.2d 1.5 (0.8–2.9)

Asthma attack 6.8 1.0 2.7 0.4 (0.1–1.3)

Asthma medication 7.2 1.0 8.9 1.3 (0.6–3.0)

Asthmac 12.8 1.0 20.5d 1.5 (0.9–2.8)

Table 5. Unadjusted asthma symptom prevalence (%) and adjusted prevalence odds ratios (OR) with 95% confidence intervals 

(CI) stratified for duration of employment in the plywood mill compared with the general population

aAdjusted for age, gender, ethnicity.

bSOB, shortness of breath.

c‘Woken by SOB in last 12 months’ or ‘asthma attack in last 12 months’ or ‘current asthma medication’.

dP < 0.05 compared with general population.

Symptoms in last 12 months General population 

(n = 415)

Duration of employment in plywood mill

<2 yr (n = 34) 2–6.5 yr (n = 39) >6.5 yr (n = 39)

% ORa % ORa % ORa % ORa

Wheezing 25.1 1.0 17.7 0.5 (0.2–1.2) 35.9 1.4 (0.6–3.0) 33.3 1.2 (0.6–2.7)

Wheezing without a cold 15.2 1.0 8.8 0.4 (0.1–1.6) 20.5 1.4 (0.6–3.6) 23.1 1.8 (0.7–4.3)

SOBb with wheezing 15.0 1.0 20.6 1.1 (0.4–3.0) 18.0 1.0 (0.4–2.7) 34.2d 2.6 (1.1–5.8)d

Woken by SOBb 8.7 1.0 8.8 0.8 (0.2–3.0) 12.8 1.6 (0.5–5.1) 23.1d 3.8 (1.4–10.0)d

Asthma attack 6.8 1.0 0.0 2.6 0.3 (0.0–2.2) 5.1 0.6 (0.1–2.8)

Asthma medication 7.2 1.0 2.9 0.3 (0.0–2.3) 7.7 1.0 (0.2–3.8) 15.4 2.1 (0.7–6.1)

Asthmac 12.8 1.0 11.8 0.5 (0.2–1.7) 15.4 1.0 (0.3–2.7) 33.3d 3.1 (1.3–7.2)d
292 W. Fransman et al. Table 6. Unadjusted symptom prevalence (%) and adjusted prevalence odds ratios (OR) with 95% confidence intervals (CI) for workers in the plywood mill by level of exposure to terpenes and formaldehyde

aAdjusted for age, gender, ethnicity and smoking.bDaily for at least part of the year.cSOB, shortness of breath.d‘Woken by SOB in last 12 months’ or ‘asthma attack in last 12 months’ or ‘current asthma medication’.eP < 0.05 compared with low exposed workers.

Plywood mill Total terpenes FormaldehydeTotal (n = 112)Low (n = 71) High (n = 24) Low (n = 38) High (n = 11)% % OR % OR (CI) % OR % OR (CI)

Cough symptoms in last 12 months

Coughb 40.2 40.9 1.0 41.7 1.3 (0.5–3.9) 44.7 1.0 45.5 0.8 (0.2–3.6)Cough up phlegmb 28.6 29.6 1.0 29.2 1.2 (0.4–3.7) 26.3 1.0 18.2 0.5 (0.1–3.2)Woken regularly due to cough 24.3 25.4 1.0 16.7 0.8 (0.2–2.9) 26.3 1.0 45.5 1.9 (0.4–9.7)

Asthma symptoms in last 12 months

Wheezing 29.5 25.4 1.0 29.2 1.7 (0.5–5.4) 21.1 1.0 36.4 2.0 (0.4–10.4)Wheezing without a cold 17.9 18.3 1.0 16.7 0.9 (0.2–3.5) 13.2 1.0 18.2 1.6 (0.2–13.2)SOBc with wheezing 24.3 25.4 1.0 26.1 1.0 (0.3–3.2) 21.1 1.0 45.5 3.5 (0.6–19.1)Woken by SOBc 15.2 12.7 1.0 16.7 1.7 (0.4–7.2) 7.9 1.0 36.4e 9.5 (1.2–74.7)eAsthma attack 2.7 0.0 1.0 12.5 5.3 1.0 0.0Asthma medication 8.9 5.6 1.0 20.8e 3.0 (0.6–14.3) 10.5 1.0 9.1 1.0 (0.1–15.3)

 Asthmad 20.5 16.9 1.0 29.2 2.0 (0.6–6.8) 15.8 1.0 36.4 4.3 (0.7–27.7)

Wheezing, SOBc or chest tightness in

relation to work

21.4 21.1 1.0 16.7 1.0 (0.3–4.0) 15.8 1.0 9.1 0.4 (0.0–5.4)

Nose, eye and skin symptoms

Runny nose, sneezing >1/week 35.7 33.8 1.0 37.5 1.5 (0.5–4.3) 28.9 1.0 36.4 1.3 (0.3–6.8)Itching watering eyes >1/week 25.0 23.9 1.0 25.0 1.0 (0.3–3.4) 26.3 1.0 9.1 0.2 (0.0–2.3)Itchy skin, skin rash >1/month 19.6 21.1 1.0 25.0 1.5 (0.4–5.0) 23.7 1.0 36.4 1.1 (0.2–5.7)

Allergy/sensitivity symptoms

Sensitive to house dust, food, animals orgrasses/plants

39.3 35.2 1.0 37.5 1.5 (0.5–4.3) 31.6 1.0 45.5 2.4 (0.5–11.8)
Respiratory symptoms in plywood mill workers 293

cantly) in workers with high exposure to terpenes,

with adjusted ORs of 3.0 (95% CI = 0.6–14.3) and

2.0 (95% CI = 0.6–6.8), respectively, compared with

workers with low terpene exposure.

Symptom prevalence did not show a very strong

relationship to any of the other measured exposures

(inhalable dust, abietic acid or endotoxin). However,

prevalences of nose, eye and skin symptoms appeared

to be slightly higher in high exposed workers for all

types of exposure (data not shown), but these differences were not statistically significant.

A high proportion of workers with respiratory

symptoms (53–83% for the different symptoms)

reported that these symptoms lessened or disappeared

during holidays or days off (data not shown).

DISCUSSION

This study has found that plywood mill workers are

exposed to a number of airborne contaminants known

to have the potential to cause respiratory symptoms

(Demers et al., 1997; Heederik and Douwes, 1997;

Dutch Expert Committee on Occupational Standards,

1998; Mandryk et al., 1999). Workers in one area

(composers) had a significantly higher exposure to

inhalable dust compared with the rest of the workers

in the plywood mill, probably due to band saw and

chipper operations (two sources of wood dust) in this

area. Inhalable dust levels did not differ markedly in

the remaining areas, probably due to numerous

sources of dust in the different sections, the use of

compressed air for cleaning and limited ventilation

throughout the plywood mill to reduce dust levels.

Levels of inhalable dust in the plywood mill were

similar to those previously found in sawmills

(Mandryk et al., 1999; Teschke et al., 1999; Cormier

et al., 2000; Demers et al., 2000; Douwes et al.,

2000). In contrast, bacterial endotoxin levels were

lower than those found in sawmills (Mandryk et al.,

1999; Cormier et al., 2000; Douwes et al., 2000),

most likely due to a combination of factors, including

the steam preconditioning of the logs used to make

plywood and the shorter time period between cutting

veneer and drying and processing the sheets,

allowing bacteria less time to grow. Abietic acid and

terpene levels were lower than those found in

sawmills and joinery shops (Eriksson et al., 1996,

1997; Demers et al., 2000) as expected, because

sawing tends to rupture wood cells and therefore

releases the cell contents, including abietic acid and

terpenes. Previous studies in sawmills concluded that

wood trimmers exposed to inhalable dust, organic

dust (molds), terpenes and endotoxins may develop

airway symptoms (Hedenstierna et al., 1983, 1986;

Dahlqvist et al., 1992). Formaldehyde levels were

lower than those measured before in plywood mills

(Malaka and Kodama, 1990). Sawmill workers are

not exposed to formaldehyde and thermal decomposition products.

Although the numbers of samples analyzed for

bacterial endotoxin, abietic acid and formaldehyde

were too few to show a clear pattern of exposure for

the different job titles in the plywood mill, this study

has shown that employees in a plywood mill are

exposed to significant levels of these compounds.

Exposure to terpenes was significantly higher for

job titles involving work before and during the

veneer drying, which suggests that terpenes are

mainly emitted during the veneer production and

veneer drying processes. Levels of terpenes were

similar to those measured in plywood mills by the US

National Institute for Occupational Safety and Health

(1981, 1982) and these studies also showed a similar

pattern of higher terpene exposure in those areas of

the process up to and including the veneer drying.

Studies in Swedish sawmills and joinery shops

(Eriksson et al., 1996, 1997) showed much higher

levels of terpenes (range 10–214 mg/m3). The lower

levels of terpenes found in Canadian sawmills (range

<LOD–6.5 mg/m3) (Teschke et al., 1999; Demers et

al., 2000) may be due to species and process differences.

In this study we found an increased prevalence of

asthma symptoms in plywood mill workers, when

compared with the general population. The findings

should be interpreted with caution since we examined

seven different asthma symptom measures and it is

therefore possible (albeit unlikely) that some findings

are due to chance. Nevertheless, our findings are

consistent with previous studies (National Institute of

Safety and Health, 1981, 1982; Malaka and Kodama,

1990) in that they indicate that working in a plywood

mill is associated with an increased prevalence of

respiratory symptoms and eye and nose symptoms.

These findings are similar to those of a recent study

in sawmill workers in the same geographical area

using the same questionnaire (Douwes et al., 2001)

and, with the exception of ‘shortness of breath with

wheezing’, there was little indication that plywood

mill workers experienced more respiratory symptoms

than sawmill workers. However, both groups showed

more respiratory symptoms than the general population.

Misclassification of exposure might have occurred

in this study as workers were classified as to high or

low exposure based on a small number of measurements. Furthermore, measurements for the classification of exposure were taken in the same period as

the questionnaire survey, whereas any work-related

asthma symptoms are more likely to result from

exposures in the past. Although we do not think that

the exposures have changed due to changes in the

process over recent years, workers may have shifted

to different areas and job titles in the mill and therefore misclassification may have biased the results.
294 W. Fransman et al.

Although the relatively low response rate (66%) may

have biased the results, the prevalences of asthma

symptoms were similar to those found in sawmill

workers, with a similar process and exposure levels

to the plywood mill. Therefore, it is not likely that the

results in this study were biased.

Plywood mill workers reported that a high

percentage of their respiratory symptoms (range 53–

83%) lessened or disappeared during holidays or

days off. This is higher than we have previously

observed in sawmill workers (Douwes et al., 2001).

Furthermore, workers with a long duration of

employment reported more asthma symptoms than

short-term employees, and a relationship of symptoms with exposure to formaldehyde was observed.

However, symptom prevalence did not show a very

strong relationship with any of the other measured

exposures (inhalable dust, abietic acid, endotoxin and

terpenes), which may be due to the relatively small

number of study participants and exposure measurements. Alternatively, other exposures, such as fungi

and fungal β(1,3)-glucans, may have played a role

(Alwis et al., 1999; Mandryk et al., 2000). In any

case, our data suggest that respiratory symptoms are

work related.

Symptoms appear to occur at levels well below the

current occupational exposure limit for softwood dust

(5 mg/m3), used in New Zealand and other countries

including the USA, since the majority of the exposure

measurements did not exceed 1 mg/m3 and none of

the samples exceeded 5 mg/m3. This provides support

for the recommendation by Demers et al. (1997) to

lower the current standard for softwood dust exposure

to 1 mg/m3.

The inhalable dust levels in the plywood mill probably consist of a mixture of (wood) dust, dirt and

exhaust fumes and are unlikely to be only wood dust.

The proportion of wood dust in the inhalable dust

fraction is not known and might vary between the

different sections in the plywood mill. A comparison

with the exposure standard for wood dust might

therefore not be justified.

Furthermore, symptoms seem to appear well below

the occupational exposure limit for formaldehyde

(1.25 mg/m3). For the other compounds, no occupational exposure limits have been set in New Zealand.

However, seven out of the 20 endotoxin measurements exceeded 50 EU/m3, an exposure limit recommended by the Dutch Expert Committee on

Occupational Standards (Heederik and Douwes,

1997; Dutch Expert Committee on Occupational

Standards, 1998). None of the terpene measurements

exceeded the present Swedish occupational exposure

limit of 150 mg/m3.

In conclusion, plywood mill workers are exposed

to significant levels of inhalable dust, bacterial endotoxin, abietic acid, terpenes (α-pinene, β-pinene and

∆3-carene) and formaldehyde, and they appear to

have an increased risk of developing work-related

respiratory symptoms. The results show an association between formaldehyde exposure and asthma

symptoms. However, no strong association with any

of the other measured exposures was found. Workers

with long-term exposure have a higher respiratory

symptom prevalence than workers exposed for

shorter periods, and a high proportion of symptoms

were reported to be work related.

Acknowledgements—The authors are indebted to the management and workers of the plywood mill for their participation.

Basic costs of the study were covered by the plywood mill

concerned. The Centre for Public Health Research is supported

by a Programme Grant and D.M. is supported by a Public

Health Research Training Fellowship from the Health Research

Council of New Zealand. J.D. is supported by a research

fellowship from the Netherlands Organization for Scientific

Research (NWO).

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