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66 Scand J Work Environ Health 2008, vol 34, no 1
Original article
Scand J Work Environ Health 2008;34(1):66–72
Asthma incidence in wood-processing industries in Finland in a register-
based population study
by Pirjo Heikkilä, PhD,
1
Rami Martikainen, MSc,
1
Kari Kurppa, MD,
1
Kirsti Husgafvel-Pursiainen, PhD,
1
Antti Karjalainen, MD
1
Heikkilä P, Martikainen R, Kurppa K, Husgafvel-Pursiainen K, Karjalainen A. Asthma incidence in wood-processing
industries in Finland in a register-based population study. Scand J Work Environ Health. 2008;34(1):66–72.
Objectives
This register-based population study determined incidence rates of clinically verified asthma among
woodworkers, other blue-collar workers, and administrative personnel employed in wood-processing industries
in Finland. Exposure to wood dust was under special scrutiny.
Methods
All Finns employed in wood-processing industries were followed for asthma incidence via record
linkage in the years 1986–1998. Incident cases included people with asthma reimbursed for medication by the
national health insurance or registered as having occupational asthma. Age-adjusted incidence rates and relative
risks (RR) by gender were estimated for wood workers, other blue-collar workers, and administrative employees
(referents) in wood industries.
Results The relative risk of asthma was increased for all woodworkers among both genders [men: RR 1.5, 95%
confidence interval (95% CI) 1.2–1.8; women: RR 1.5, 95% Cl 1.2–1.7]; a similarly elevated risk was also found
for other blue-collar workers (men: RR 1.5, 95% Cl 1.2–1.8; women: RR 1.4, 95% Cl 1.2–1.6) in the same wood
industries. Statistically increased relative risks were found for low and medium exposure to wood dust, but not
for high exposure. Altogether 217 of the 4074 clinically verified asthma cases were reported as occupational
asthma in the Finnish Register on Occupational Diseases.
Conclusions The incidence rates for asthma were significantly increased both among the woodworkers and the
other blue-collar workers in wood industries but without a clear dose–response. Cases recognized as occupational
asthma accounted for only a small part of the total asthma excess, indicating that much of the work-related asthma
excess remains unrecognized in these industries.
Key terms attributable fraction; exposure; occupational disease; relative risk; wood dust.
1
Finnish Institute of Occupational Health, Helsinki, Finland.
Correspondence to: Dr P Heikkilä, Finnish Institute of Occupational Health, Topeliuksenkatu 41 aA, FI-00250 Helsinki,
Finland. [E-mail: pirjo.heikkila@ttl.fi]
Asthma is a common disorder; 5–10% of the general
population in many countries is affected by the disease.
A recent Finnish study indicated that the attributable
fraction of work in adult-onset asthma is 29% for men
and 17% for women (1). Other studies suggest that the
proportion of new or exacerbated asthma cases due to
workplace exposures ranges between 9% and 26% (2–6).
Although over 250 agents have been documented as
being responsible for sensitizer-induced occupational
asthma, our knowledge about the multiplicity of caus-
ative factors and the pathogenic mechanisms involved
in the development of occupational asthma still remains
incomplete (7).
As concerns work in wood processing, asthma has
been attributed to dust from many different tree species,
on the basis of case reports and epidemiologic studies (8,
9). The proportion of all cases of occupational asthma
due to wood dust has been reported to be some 6–11%
(8). Asthma induced by western red cedar has been
frequently documented for sawmill workers, with an
apparent dose–response relationship (10–14). The stud-
ies proposing a relationship between exposure to other
softwood species and asthma have used self-reported
asthma (11) and clinically diagnosed asthma (15, 16)
as outcomes.
Finland provides an opportunity to study the varia-
tion in asthma incidence in a country in which the cost
of asthma medication for clinically verified asthma is
reimbursed and registered by a national health insurance
scheme that covers the entire population. A personal
identification code allows linkage of the register infor-
mation with data on employment and occupation avail-
able from the Finnish Population Census. The results
from such data linkage have identified a large number
Scand J Work Environ Health 2008, vol 34, no 1 67
Heikkilä et al
of occupations with significant excesses of asthma in-
cidence rates (1).
Specific bronchial challenge testing of individual
workers has shown that wood dusts common in Finn-
ish industries (spruce, pine, birch) cause asthma. A few
cases a year are registered in the Finnish Register on
Occupational Diseases. Yet it is not known how high
the risk actually is and which industrial sectors may be
particularly at risk. Therefore, our present analysis com-
pared the incidence rate of asthma in a variety of Finnish
wood-processing industries. We used the data from the
national reimbursement register for asthma medication,
the Finnish Register on Occupational Diseases, and
the Finnish Population Census for the occupation and
industrial sector of employment from three census co-
horts (1985, 1990, and 1995) to estimate the incidence
rates of asthma for woodworkers in wood-processing
industries.
Study population and methods
Study population and design
All Finns who were 20–59 years old, without preexist-
ing asthma on 31 December 1985, 1990, and 1995, and
who were employed according to the population census
in 1985, 1990, or 1995 in certain industrial sectors were
followed for asthma incidence rates during the years
1986–1990, 1991–1995, and 1996–1998, respectively.
The industrial sectors included in the asthma follow-
up were as follows (the codes are those used by the
Statistical Classification of Economic Activities in the
European Community, NACE Rev 1, 1990): (i) forestry,
logging and related services (02.0), (ii) sawmilling and
planing of wood and impregnation of wood (20.1), (iii)
manufacture of veneer sheets and manufacture of ply-
wood, laminboard, particle board, fiber board and other
panels and boards (20.2), (iv) manufacture of builders’
carpentry and joinery (20.3), (v) manufacture of wooden
containers (20.4), (vi) manufacture of other products
of wood and manufacture of articles of cork, straw and
plaiting materials (20.5), (vii) building and repairing of
ships and boats (35.1), (viii) manufacture of furniture
(36.1), (ix) building of complete constructions or parts
thereof and civil engineering (45.2), and (x) building
completion (45.4).
On the average, about 158 000 blue-collar workers
(141 187 men and 16 937 women) and about 13 000
administrative
workers (4628 men and 8211 women) in
the three census cohorts were employed in these wood-
processing industries.
The follow-up was performed using the unique
personal identification code in accessing two national
registers, the Drug Reimbursement
Register of the Social
Insurance Institution of Finland and the Finnish Register
on Occupational Diseases, housed in the Finnish Insti-
tute of Occupational Health. Duplicate notifications in
two registers were identified using the indivitual per-
sonal identification codes. For each follow-up period,
the cases and person-years were calculated according
to occupation and the sector of economic activity in
which the person was working at the start of the fol-
low-up. People could contribute to the person-years of
several
follow-up periods, provided they had not been
defined as an
incident case of asthma before the start
of the follow-up period
and were still employed in the
selected industrial sectors at the start of the follow-up
period. Therefore, the number
of persons under follow-
up is given as an average of the
three follow-up periods
and not as an absolute number of the
different persons
followed. The occupation was known for 99% of those
employed in each of the three censuses. The study
protocol was approved by the ethics committees of the
participating institutions.
Classification of occupation of the persons and eco-
nomic activity of their employers
Information on the professional status, current occupa-
tion, and
economic activity of employees is recorded for
every employed
person in the Finnish Population Cen-
sus. The occupational codes are based on a version of
the International Standard
Classification of Occupations,
1988 (ISCO-88 COM). The economic activity codes
partly follow an extended classification of the Standards
Industrial Classification of all Economic Activities by
the United Nations in 1968 and partly the Statistical
Classification of Economic Activities in the European
Community, 1990 (NACE Rev 1). All of the economic
activity codes were converted to NACE Rev 1.
Information on occupation was cross-sectional from
the time
of the census. Those aged <20 years were not
followed
because a change of occupation and migration
between education
and worklife make the employment
status and occupation at
the start of the follow-up less
reliable indicators of the occupational
status during the
5-year follow-up. Similarly those aged >59 years were
not followed
because most of those employed at the start
of the follow-up
would no longer have been employed at
the end of the follow-up; the time period when they left
the labor market was not known.
Occupational exposure to wood dust
The occupations in the selected 10 wood industries were
grouped into the following three categories: woodwork-
ers (with direct exposure to wood dust), other blue-col-
lar workers (with possible exposure to wood dust), and
unexposed persons (administrative workers; reference
68 Scand J Work Environ Health 2008, vol 34, no 1
Asthma incidence in wook-processing industries
group). The jobs coded as woodworkers were forestry
workers and lumberjacks, timber men, sawyers, plywood
and fiberboard workers, construction carpenters, wooden
boat builders, coach-body builders, bench carpenters,
cabinetmakers and joiners, woodworking machine op-
erators, wooden surface finishers, woodworking oc-
cupations not elsewhere classified, and cleaners. The
group of other blue-collar workers in wood-processing
industries consisted of workers with such occupations
as technicians, painters, welders, assemblers, machine
mechanics, forklift and other drivers, and packers. Some
persons in this group have also been exposed to wood
dust, including mechanics and assemblers when repair-
ing wood-processing machines. Those employed in
administrative, managerial, and clerical
work in the
selected industries were considered to be unexposed and
were used as a reference group. The incidence rates for
asthma in these three occupational groups in the selected
10 industrial sectors were followed.
Incidence rates were also calculated according to the
estimated level of wood dust exposure. Occupational
exposure to wood dust was assessed in 120 job-indus-
try combinations as an annual mean of the total dust
concentration. [Total dust concentrations correspond
to approximately 50% of the inhalable dust concentra-
tion.] The workers in the 10 selected industrial sectors
were classified into the following five categories: high,
medium or low exposure to wood dust, unknown expo-
sure, and reference group (table 1). The classification of
exposure levels was based on measurements carried out
by the Finnish Institute of Occupational Health in the
selected industrial sectors (17).
Definition and ascertainment of cases
Incident cases were collected from the Medication
Reimbursement Register of the Social Insurance Institu-
tion. In Finland, asthma is classified as a chronic disease
for which the cost of medication can be reimbursed at
a higher than ordinary level. To receive the compensa-
tion, the patient has to apply for the entitlement from
the Social Insurance Institution, submitting a medical
certificate signed by a pulmonary specialist, along with
the application. For compensation to be granted, the
disease has to fulfill the diagnostic and severity criteria
of asthma (1), including objective data of reversible
bronchial obstruction and a persistent pattern of disease.
The health insurance, which includes the drug-reim-
bursement system, covers the entire Finnish population,
and consequently all Finnish patients with asthma fulfill-
ing the criteria are identifiable in the register. The only
exceptions are patients with recognized occupational
asthma, who receive full compensation for medication
from the Statutory Accident Insurance. The recogni-
tion of occupational asthma necessitates a diagnosis of
asthma made by a pulmonary specialist and his or her
evidence of a specific causal association between the
person’s occupational exposure and the disease.
In our current study, a person was defined as having
clinically verified asthma if he or she received rights for
reimbursement for asthma medication from the Social
Insurance Institution or was registered as having occu-
pational asthma in the Finnish Register on Occupational
Diseases. In this study, the concept “persistent asthma”
is defined as asthma needing regular medication and
fulfilling the criteria for asthma medication reimburse-
ment rights. The date of the application (Social Insur-
ance Institution) or the date of the registration report
(Finnish Register on Occupational Diseases) was used
as a proxy for the date of diagnosis. As our follow-up
was based on the attribution of special reimbursement
rights for asthma medication, a person became classi-
fied as a case of asthma only when the criteria of these
rights were fulfilled, even if he or she may have had
asthma-like symptoms before. It could also be that those
who had had childhood asthma and who had recovered
before 1970 when asthma became eligible for special
reimbursement rights and who again were afflicted with
asthma during the follow-up after 1985 were classified
as incident cases of adult-onset asthma.
Statistical analyses
The men and women were analyzed separately since
the overall incidence of adult onset asthma is higher
for women than for men, and this difference applies to
Finland as well (18, 19). In addition, men and women
may perform different tasks in a given occupation, and
so their exposure patterns may differ. The incidence
rate for the exposure variables, wood dust exposure
category, and industrial sector was calculated by sum-
ming up the cases and person-years from the three
census cohorts and dividing the total number of cases
by
the accumulated person-years under follow-up in each
respective group.
Incidence rate ratios [relative risks
(RR)] and their 95% confidence
intervals (95% CI) for
the wood-dust exposure categories and woodworkers in
Table 1. Wood-dust exposure categories for woodworkers and the
number of job–industry combinations in each category.
Category Total dust concentration Number of job–industry
(mg/m
3
) combinations
Low 0.02-<0.5 68
Medium 0.5-<1.5 40
High
a
≥1.5 12
a
Jobs classified as high exposure (the codes refer to economic activity
codes; see the Classification of Occupation of the Persons and Economic
Activity of their Employers section for details): timber men (45.4), saw-
yers (36.1 & 45.4), woodworking machine operators (20.3 & 36.1),
bench carpenters, cabinetmakers, joiners, etc (36.1 & 45.4), cleaners
(20.1, 20.2 & 36.1).
Scand J Work Environ Health 2008, vol 34, no 1 69
Heikkilä et al
the selected industries were estimated by a log–linear
model (Poisson distributed), using person-years as an
offset variable. Age at the start of the follow-up (20–29,
30–39, 40–49, and 50–59 years) was included in the
model. Those employed in administrative, managerial,
and clerical
work in the selected industries were used as
a reference group in both models. SAS software (SAS
Inc, Gary, NC, USA) was used in the analyses.
The attributable fraction (AF) of the asthma cases
caused by occupational exposure in wood-processing
industries (ie, the work-related fraction of asthma among
woodworkers and other blue-collar workers) was esti-
mated according to the equation AF = (RR-1)/RR (20);
the 95% confidence intervals of the attributable fractions
were calculated from the 95% confidence intervals of
the corresponding relative risks. The number of work-
related asthma cases among the woodworkers and other
blue-collar workers was determined as N × AF, where N
was the number of all asthma cases in a group.
Results
The incident rates and relative risks for clinically veri-
fied asthma among woodworkers and other blue-collar
workers in the wood-processing industries in Finland are
presented in table 2. Out of all the blue-collar workers
(141 197 men, 16 937 women) in 10 selected wood-pro-
cessing industries, 34% men and 26% women, respec-
tively, were classified as being exposed to wood dust.
The relative risk for asthma was significantly increased
for all of the woodworkers among both genders, and an
elevated risk was also found for other blue-collar work-
ers employed in the same wood industries. Classification
by the intensity of wood dust exposure indicated that, of
the male workers, 4% were persons with high exposure,
8% had medium exposure, and 23% had low exposure.
The corresponding values for the women were 5%, 11%,
and 10%. No clear dose–response to wood dust exposure
was indicated. Nevertheless, significantly increased rela-
tive risks for asthma were found for the groups with low
or medium exposure, but not for the group with high
exposure (table 2).
From the various sectors of the wood-processing
industries, the risk was significantly increased for male
woodworkers in 7 of the10 industrial sectors studied
(table 3). The asthma risks were highest in the manu-
facture of wooden containers (RR 2.5, 95% Cl 1.3–4.6),
and in sawmilling and wood planing (RR 1.9, 95% Cl
1.5–2.5). For the female woodworkers, the risk was
significantly increased in 4 of the 10 industrial sectors,
the highest risk being determined for sawmilling and the
impregnation of wood (RR 1.8, 95% Cl 1.3–2.4). The
asthma incidence per 1000 person-years did not differ
significantly between the woodworkers (1.7 for the men
and 3.2 for the women) and other blue-collar workers
working in wood-processing industries (1.8 for the men,
2.9 for the women) (table 2).
The calculated fraction of asthma cases attributable to
work-related factors was 31% (95% CI 15–45) for both
the male and female woodworkers and 30% (95% CI
12–43) and 26% (95% CI 13–38) for the other male and
female blue-collar workers, respectively (table 2).
On the basis of the attributable fractions estimated in
table 2, a total of 1213 of all 4074 asthma cases identi-
fied in the 10 selected wood industry sectors would be
in excess (ie, attributable to work-related factors). How-
ever, only 217 were registered in the Finnish Register
on Occupational Diseases as occupational asthma, varia-
tion occurring between industries. In addition to wood
dust, other exposures, such as to formaldehyde, epoxy
Table 2. Incidence rates (per 1000 workers × year
–1
), relative risks (age-adjusted), and work-related attributable fractions (AF) for asthma
among male and female workers. The woodworkers have been subdivided according to the exposure-level categories. (RR = relative risk,
95% CI = 95% confidence interval)
Categories Men Women
Cases Popula
-
Incidence RR 95% Cl AF Cases Popula- Incidence RR 95% Cl AF
tion
a
rate (%) tion
a
rate (%)
Referents
b
86 4 628 1.4 1.0 ·· ·· 223 8 211 2.0 1.0 ·· ··
Woodworkers
c
1189 50 218 1.7 1.5 1.2–1.8 31 286 6 503 3.2 1.5 1.2–1.7 31
Low exposure 785 33 226 1.7 1.4 1.1–1.7 · 110 2 536 3.1 1.4 1.2–1.8 ·
Medium exposure 317 11 905 2.0 1.7 1.4–2.2 · 134 2 742 3.7 1.6 1.3–2.0 ·
High exposure 87 5 087 1.3 1.2 0.9–1.6 · 42 1 225 2.6 1.2 0.8–1.6 ·
Other blue-collar workers
d
2181 90 979 1.8 1.4 1.1–1.8 30 418 10 434 2.9 1.4 1.2–1.6 26
All blue-collar workers 3370 141 197 1.8 1.4 1.1–1.8 30 704 16 937 3.0 1.4 1.2–1.6 29
a
Arithmetic mean of the population under follow-up in 1986–1990, 1991–1995, and 1996–1998,
b
Administrative, managerial, and clerical workers in the selected wood industries.
c
Woodworkers in the selected wood industries included the occupations in which direct exposure to wood dust was estimated to occur.
d
Other blue-collar workers included the occupations in which exposure to wood dust was unknown.
70 Scand J Work Environ Health 2008, vol 34, no 1
Asthma incidence in wook-processing industries
compounds, isocyanates, microorganisms, or glues, have
been registered in the Finnish Register on Occupational
Diseases as causative agents for occupational asthma in
wood-processing sectors (table 4).
Discussion
We found significantly elevated risks of asthma both
for wood workers and other blue-collar workers em-
ployed in wood-processing industries. In our study,
the case ascertainment was accomplished through two
national registers that rely on diagnoses made by pul-
monary specialists based on strict clinical criteria for
a persistent form of asthma (asthma needing regular
medication). This definition is rather specific but, at the
same time, more restrictive than those typically used
in epidemiologic studies. Consequently, using such a
definition as physician-diagnosed persistent asthma
will result in high specificity and low sensitivity, and it
may in fact underestimate the frequency of asthma. The
excess observed in association with work may repre-
sent two types of disease, new-onset persistent asthma
without a history of asthma or preexisting mild disease
that the work-related exposure may have aggravated
so that it deteriorated sufficiently to fulfill the criteria
Table 3. Incidence rates (per 1000 workers × year
–1
), relative risks (age-adjusted) for asthma in wood industries. (RR = relative risk, 95%
CI = 95% confidence interval)
Categories Men Women
Cases Popula- Incidence RR 95% Cl Cases Popula- Incidence RR 95% Cl
tion
a
rate tion
a
rate
Referents
b
86 4 628 1.4 1.0 ·· 223 8211 2.1 1.0 ··
Woodworkers
c, d
1189 50 218 1.7 1.5 1.2–1.8 286 6503 3.2 1.5 1.2–1.7
Forestry and logging (02.0) 320 13 145 1.8 1.5 1.2–1.8 8
c
256 · · ·
Sawmilling (20.1) 173 5 647 2.3 1.9 1.5–2.5 55 995 4.0 1.8 1.3–2.4
Manufacture of wooden boards (20.2) 55 2 006 2.1 1.7 1.2–2.5 76 1541 3.7 1.6 1.3–2.1
Manufacture of builders’ carpentry (20.3) 103 4 375 1.7 1.6 1.2–2.2 45 922 3.7 1.7 1.2–2.5
Manufacture of wooden containers (20.4) 11 327 2.7 2.5 1.3–4.6 1
c
60 · · ·
Manufacture of other wood products (20.5) 15 652 1.7 1.5 0.9–2.6 3
c
123 · · ·
Building and repairing ships and boats (35.1) 15 1 113 1.0 0.9 0.5–1.5 7
c
290 · · ·
Manufacture of furniture (36.1) 65 4 084 1.2 1.1 0.8–1.5 26 952 2.0 0.9 0.62–1.40
Building of complete constructions (45.2) 412 17 973 1.7 1.3 1.1–1.7 62 1301 3.4 1.5 1.1–2.0
Building completion (45.4) 20 895 1.8 1.7 1.1–1.8 3
c
63 · · ·
a
Arithmetic mean of the population under follow-up in 1986–1990, 1991–1995, and 1996–1998.
b
Administrative, managerial, and clerical workers in the selected wood industries.
c
If the number of cases was <10, the incidence rates and relative risks have not been reported.
d
Code of the economic activity (see the Classification of Occupation of the Persons and the Economic Actvitiy of their Employers) in parentheses.
Table 4. Estimated work-related asthma cases and reported occupational asthma cases among the workers in the 10 wood industry sec-
tors in the Finnish Register on Occupational Diseases in 1986–1998.
Industrial sectors
a
Finnish Register on Occupational Diseases
Cases Coded causative agent
(N)
Forestry, logging and related services (02.0) 328 99 1 Animal epithelium
Sawmilling and planing of wood and impregnation of wood (20.1) 228 107 18 Spruce; wood not specified
Manufacture of wooden and fiber boards (20.2) 131 52 39 Isocyanates; formaldehyde; molds; pine; spruce; birch;
glues
Manufacture of builders carpentry and joinery (20.3) 148 58 28 Isocyanates; formaldehyde; pine; obeche; wood not
specified
Manufacture of wooden containers (20.4) 12 6 –
Manufacture of other products of wood (20.5) 18 4 9 Formaldehyde; pine; obeche; wood not specified
Building and repairing of ships and boats (35.1) 22 – 1 Teak
Manufacture of furniture (36.1) 91 4 13 Formaldehyde; spruce; beech
Building of complete constructions (45.2) 474 123 10 Isocyanates; paints; pine; obeche
Building completion (45.4) 23 10 1 Epoxy paints; obeche
Woodworkers 1 475 468 119 ·
Other blue-collar workers 2 599 745 98 ·
a
Code of the economic activity (see the Classification of Occupation of the Persons and the Economic Actvitiy of their Employers) in parentheses.
b
The estimated cases have been calculated on the basis of the work-related attributable fractions and the numbers of cases in table 3.
Cases Estimated
(N) work-
related
cases
b
(N)
Scand J Work Environ Health 2008, vol 34, no 1 71
Heikkilä et al
for reimbursement established by the Social Insurance
Institution of Finland.
In the wood-processing industries, the asthma in-
cidence rates were equally elevated among the wood
workers and other blue-collar workers when they were
compared with an unexposed (white-collar) reference
group from the same industries. The wood workers in
this study were grouped according to the intensity of
wood dust exposure. However, part of other blue-col-
lar workers has also been exposed to wood dust, and
other respiratory irritants and allergy-causing airborne
contaminants are present in these industries. Both the
woodworkers and other blue-collar workers may have
been exposed to such other exposures. This possibility
could mean either that even a rather low exposure to
wood dust can increase the risk of developing persistent
asthma or it could be that, in the group of other blue-
collar workers, also other (nonwood dusts) industrial
exposures play an important role. There is increasing
evidence that asthma and other respiratory symptoms
occur already at low wood dust levels (11, 16, 21). The
relation between occupational exposures and asthma is
complicated. Generally, a single causal agent is identi-
fied as a causative agent in occupational asthma, whereas
wood dusts and emissions from fresh wood are complex
mixtures of chemicals, particles, and biohazards that
make it more difficult to ascribe causality.
Although the relative risk for wood workers was
significantly elevated and the analysis by intensity of
wood dust exposure revealed significantly increased
asthma risks in the low- and medium-exposure groups,
no dose–response relationship between the level of wood
dust exposure and asthma was found. However, there are
several factors that may have affected the dose–response
relationships. In addition to evidence for respiratory
health effects at low wood dust levels, early in employ-
ment, workers may choose to quit when ill or when work
conditions are perceived to cause illness such as asthma.
It has been shown that changing jobs because the current
one is causing respiratory symptoms is frequent before
the diagnosis of asthma (22). Finally, the nondifferential
misclassification of exposure may cause bias away from
the null in the medium category, whereas the overall
exposure–response trend will usually be biased towards
the null (23).
The differences between the risk estimates in differ-
ent industries were small, although the exposure patterns
of the workers differed. Wood handled in sawmills and
in wooden board industries is fresh, whereas it is dry, for
example, in the furniture industry. Over 95% of the wood
used in Finnish sawmills is either pine or spruce, the rest
being birch (24). Fresh softwood wood can emit mono-
terpenes (25) and resin acids (26). Resin acids are well-
known allergens; these causative agents are believed to
be peroxides formed from abietic acid in intense heat
(27–30). Several authors have postulated that some of
the respiratory symptoms reported for woodworkers
could be due to exposure to microbe-related agents
(eg, endotoxins) (31–33). In other, questionnaire-based
studies, sawmill workers exposed to coniferous tree spe-
cies and to other exposures have also reported current
asthma more often than referents (11, 13). In Finland,
the occupational disease register (Finnish Register on
Occupational Diseases) lists a few well-documented
cases of occupational asthma caused by exposure to
dusts from spruce or pine (table 4). Such diagnoses have
been clinically verified by bronchial challenge tests us-
ing inhalable preparations of wood dust.
An obvious question in considering the findings
of our study is whether the comparison of the asthma
incidence of the blue-collar workers with that of the
administrative employees (managers, clerks) is valid.
If the risk of asthma were clearly related to the social
class of the employees, the finding would be distorted
by confounding. Many administrative employees belong
to a higher social class than blue-collar workers do. Our
analysis is part of a larger study of asthma incidence in
the employed workforce of Finland, which used admin-
istrative, managerial, and clerical work as reference (1,
34). To examine the possible effect of social class on
the results, we performed separate analyses using as
reference only the lower-level administrative employees
(secretaries, office clerks, bookkeepers, and similar oc-
cupations). The incidence rate of the lower-level admin-
istrative employees was similar to that of administrative
workers in general and did not change the relative risks
of the exposed occupations. We have also considered
potential confounding by smoking differences and con-
cluded that it is unlikely that such differences would
have significantly biased our risk estimates (1, 34).
In conclusion, we found an increased incidence
of asthma in wood-processing industries, not only for
wood workers directly exposed to wood dust but also
for other blue-collar workers employed in the same
wood industry sectors. In both of these groups, the
attributable fraction of the work-related factors of the
asthma incidence was about 31%. Cases recognized as
occupational asthma accounted for only a small part of
the total asthma excess. This finding indicates that much
of the work-related asthma excess remains unrecognized
in these industries.
Acknowledgments
We are indebted to Marianne Johnson from Statistics
Finland, Timo Klaukka from the Finnish Social Insur-
ance Institution, and Anja Saalo and Ilpo Mäkinen from
the Finnish Register on Occupational Diseases for their
assistance in creating the data files.
72 Scand J Work Environ Health 2008, vol 34, no 1
Asthma incidence in wook-processing industries
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Received for publication: 6 March 2007
