RESPIRATORY MEDICINE (1998) 92, 316-324
7- and S-year-old
and related conditions
rates and risk factor pattern
E. RONMARK*+*, B. LUNDBACK*+, E. JONSSON* AND T. PLATTS-MILLS'
“Respirafory Epidemiology Unit, Division of Allergy, Department of Occupational Health,
Nafional Institute for
Working Life, Umek and Solna, Sweden
‘The OLIN Study Group, Deparfmenf of Medicine, Cenfral Hospital of Norrboffen, Lulei-Boden,
*Lulei Healfh Care Cenfre, LuleB Sweden
‘Asthma and Allergic Diseases Center, Deparfmenf of Medicine, University of Virginia,
As a first step in an intervention study of asthma and allergic diseases among school children, a cross-sectional study
was performed during Winter 1996 in three towns (Kiruna, Lules and Pitei) in the northernmost province of
Sweden, Norrbotten. The cross-sectional study aimed to measure the prevalence of asthma, type-l allergy and
allergic diseases in order to make it possible to measure the incidence of the diseases, conditions and symptoms
related to the diseases. Another aim was to perform a screening for possible risk factors. All children enrolled in the
first and second classes at school, 7 and 8 years old, were invited to take part in this study. The ISAAC questionnaire
with added questions about symptoms, morbidity, heredity and environment was distributed by the schools to the
parents. The response rate was 97%, and 3431 completed questionnaires were returned. The children in two of
the municipalities were also invited to skin test, and 2149 (88%) were tested with 10 common airborne allergens. The
results showed that 7% of the children were currently using or had used asthma medicines during the last 12 months.
Six percent had asthma diagnosed by a physician, and 4% were using inhaled corticosteroids. The prevalence of
wheezing during the last 12 months was 12%, rhinitis without colds 14%, and eczema 27%, while 21% had a positive
skin test. The respiratory symptoms and conditions were significantly greater in boys and, further, they were most
prevalent in Kiruna in the very north, though not significantly. Type-l allergy and asthma had different risk factor
patterns. The main risk factors for asthma were a family history of asthma (ORz3.2) followed by past or present
house dampness (OR= 1.9), male sex (OR= 1.7) and a smoking mother (OR= 1.6). In Kiruna, when none of these
three risk factors were present, none of the children had asthma, but when all three were present, 38% of
these children were using asthma medicines.
RESPIR. MED. (1998) 92, 316-324
During the last three to four decades, the prevalence of
asthma and allergic diseases has increased considerably in
many countries, probably due to increased diagnostic inten-
sity, altered diagnostic criteria and also a true increase in
prevalence. Today, asthma and allergic diseases comprises
the greatest group of diseases in children, adolescents and
young adults in many parts of the world. The increase was
first documented in Australia, New Zealand and the U.K.,
countries in which the increase also seems to be most
Received 13 October 1997 and accepted 22 October 1997.
Correspondence should be addressed to: E. Riinmark, The OLIN
Study Group, Lulel Health Care Center, Timmermansgatan 31,
S-972 41 Luleb, Sweden.
pronounced (l-6). Today, high prevalence of asthma and
respiratory symptoms has been shown in children in later
industrialized countries such as Chile (7) and Singapore (8).
Prevalence rates in North America (9,lO) and Europe
(1 l-l 3), particularly Central Europe (7,l l), are generally
low in comparison with Australia and New Zealand.
The incidence of asthma and allergic diseases reaches a
peak in children of school age and younger (6,14-16).
Asthma is strongly associated with type-l allergy (17).
Type-l allergy shows a different sensitization profile in
different environments and in different parts of the world.
The dominating allergens may be pets such as cats or dogs
(10,18), house-dust mites (19,20), pollen (21), moulds (22)
and cockroaches (23,24). In the indoor environment,
tobacco smoke (25-27) and dampness at home (25,27-29)
are associated with an increased risk of asthma in children.
0 1998 W. B. SAUNDERS COMPANY LTD
ASTHMA, TYPE-I ALLERGY AND RELATED CONDITIONS 3 I7
The role of outdoor air pollution as a risk factor for asthma
is controversial (30-32); however, some studies report air
pollution from traffic to be a risk factor for asthma (33-35).
Other potential causes such as diet and respiratory infec-
tions (36) are under debate. Although an increasing number
of studies show similarities in the risk factor pattern for
asthma, there is still no general agreement about why there
has been such a marked increase.
In Sweden, the prevalence of asthma has also increased,
mainly in children (37) adolescents (15,16) and young
adults (38). Comparative studies have pointed out that
asthma is most prevalent in the north of the country
(37,38), where mites and moulds hardly exist and conven-
tional air pollution is low. This area is of particular interest
in studying risk factors of the conditions and causes of the
increasing prevalence of asthma.
An intervention study aiming to reduce the incidence and
morbidity of allergic diseases, mainly asthma, among
school children in Northern Sweden started in 1996. The
aim of this paper is to report prevalence rates of allergic
conditions, mainly asthma, and risk factors for the
Materials and Methods
All school children in the towns and municipalities of
Kiruna, Lulea and Pitea in Norrbotten, the northernmost
province of Sweden, in the first and second classes at school
were invited to take part in this study, which is planned to
last for 8 years. The intervention focused on cleaning of the
schools, active and passive smoking, and to increase the
knowledge of allergy prevention according to a programme
developed by the Swedish Institute of Public Health.
The study started in February-April 1996 with a cross-
sectional survey in order to estimate the prevalence of
asthma, respiratory symptoms, rhinitis, eczema and type-l
allergy, and to screen for risk factors for the conditions.
The study was approved by the Ethical Committee at the
University and the University Hospital of Northern Sweden
The municipalities consist of both urban and rural areas.
Two of them, Lulea and Pitea, are located on the Baltic Sea,
and Kiruna is situated in the inland mountains north of the
Arctic circle (Fig. 1). The climate in the two coastal
municipalities is similar, with an average temperature of
2°C (January - 11°C July 15°C) while the corresponding
value for Kiruna is - 2°C (January - 15°C July 12°C).
The relative humidity varies considerably between winter
and summer. The winter in Kiruna lasts for 7-8 months,
more than 1 month longer than in the coastal areas.
The city of Lulea is the administrative, commercial and
cultural centre of the province, and in 1996, it had 71 000
inhabitants, of which 50 000 lived in the urban area.
Furthermore, it has the largest steel industry in Sweden.
Pitea had 41 000 inhabitants, of which approximately one
half are living in the town of Pitea. Pitea has wood and
FIG. 1. Map of Sweden and the province of Norrbotten
with the towns Kiruna, Lulea and Pitea.
paper-pulp industries. Kiruna includes a large area with
mountains and had 26 000 inhabitants in 1996, of whom
20 000 lived in the town. The biggest iron mine in Western
Europe and airspace research are located to Kiruna.
All 3525 school children enrolled in the first and second
classes were invited to participate in the study. They were
7-8 years old with few exceptions, and 51% were boys. In
all, 20 schools in Kiruna, 35 in Lulea and 26 in Pitea were
enrolled. The study population and the participation rate
are described in Table 1.
A questionnaire with a covering letter explaining the aim of
the study was distributed to the children’s parents by the
school teachers. The response rate was 97%. In Kiruna and
Lulea, the children were invited to have skin prick tests, and
2114 (86%) were tested. In addition, 35 children had been
skin tested prior to the study at the paediatric policlinics in
Kiruna and Lulea. These results [total 2149 (88%)] have
been included in the analysis.
318 E.RONMARK ET AL.
TABLE 1. Study population: number of children who were invited and who participated in the
questionnaire study and the skin test by area and sex
Kiruna Lulea Pitea B G
B, boys; G, girls.
The questionnaire consisted of the core questions about
wheezing and asthma, rhinitis and eczema from the ISAAC
protocol (39). Additional questions about symptoms, use of
medicines, family history of asthma or allergic diseases,
demographic characteristics, and past and present living
and environmental factors were added. Questions about
mothers’ and fathers’ past and present smoking habits, past
and present pets or other animals at home, and signs of
dampness were included, as well as questions about hobbies
and sport activities.
The skin tests were carried out at school by two trained
nurses. The tests were performed according to the standards
developed by the European Academy of Allergology and
Clinical Immunology (40). The allergen extracts used were
from ALK and included a Swedish standard panel; birch,
timothy, mugworth, dog, cat, horse, two mites (Dermato-
phagoides farinae and D. pteronyssimus) and two moulds
(Cladosporium and Alternaria). Histamine 10 mg ml ~ r and
glycerol were used as positive and negative controls,
respectively. A positive reaction was recorded if the mean
diameter of the weal was >3 mm.
The definitions were based on the answers to the following
questions or combinations of questions:
Ever wheeze: ‘Has your child ever had wheezing or whistling
in the chest at any time in the past?’
Wheezing last 12 months: ‘Has your child had wheezing or
whistling in the chest in the last 12 months?’
Ever asthma: ‘Has your child ever had asthma?’
Physician-diagnosed asthma: ‘Has the child been diagnosed
by a physician as having asthma?’
Use of asthma medicines: ‘sometimes, often/periodically, or
every day’ to the question ‘During the last 12 months, how
often has your child taken medicines for asthma?’
Current asthma: subjects reporting physician-diagnosed
asthma and either symptoms related to asthma during the
last 12 months or use of asthma medicines during the last
Asthma projle: subjects with ‘current asthma’, or wheezing
during the last 12 months, or use of asthma medicines
during the last 12 months.
Ever rhinitis: ‘Has your child ever had a problem with
sneezing or a runny or blocked nose when he/she did not
have a cold?’
Rhinitis last I2 months: ‘In the past 12 months, has your
child had a problem with sneezing or a runny or blocked
nose when he/she did not have a cold?’
Physician-diagnosed rhinitis: subjects answer to the question
‘Has the child been diagnosed by a physician as having
allergic rhinitis or hayfever?’
Ever eczema: ‘Has your child ever had eczema?’
Eczema last 12 months: ‘Has your child had an itchy rash at
any time in the last 12 months?
Physician-diagnosed eczema: ‘Has the child been diagnosed
by a physician as having eczema?
Chi-square test was used for bi-variate comparisons. One-
way ANOVA (analysis of variance) was used for test for
trends. Odds ratios (OR) for various independent variables,
which according to uni- or bi-variate analyses were associ-
ated with increased or decreased risks for asthma or type-l
allergy, were calculated by using multiple logistic regression
ASTHMA AND WHEEZING
‘Ever wheeze’ was reported by 21.2%, while wheezing
during the last 12 months was reported by 11.7%. Wheezing
without colds was reported by 5.8%, wheezing at or after
physical exertion by 6.2%, sleep disturbed by wheezing by
5.1%, and frequent periods of wheezing, more than 12 times
per year, by 1.2%.
Asthma diagnosed by physician was reported by 5.7%,
while 7.1% reported they were using or had used asthma
medicines during the last 12 months. The prevalence of the
ASTHMA, TYPE-I ALLERGY AND RELATED CONDITIONS
TABLE 2. Prevalence of wheezing, use of asthma medicines and asthma by area and sex
Symptom or condition
Kiruna Lulea Pitea B
G Total Area
Wheezing in last 12 months
Sleep disturbed by wheezing in last 12 months
Wheezing on or after exercise in last 12 months
Wheezing without colds in last 12 months
Use of asthma medicines
Difference (P value) by area and sex.
B, boys; G, girls.
children with physician-diagnosed asthma and, further,
either use of asthma medicines or symptoms related to
asthma during the last 12 months, ‘current asthma’, was
5.3%. A condition with ‘current asthma’ or, during the last
12 months either wheezing or use of asthma medicines,
‘asthma profile’, was reported by 12.8%.
Asthma, use of asthma medicines and all respiratory
symptoms were significantly more common in boys. There
was a trend for asthma and most of the respiratory symp-
toms, showing that they were most prevalent in the
northernmost area, Kiruna. In Kiruna, 6% of the children
were using or had used inhaled corticosteroids during the
last 12 months, compared to 3.9% and 4.2% in Lulea and
Pitea, respectively. Prevalence rates of asthma, wheezing
and use of asthma medicines are described in Table 2.
The prevalence of physician-diagnosed asthma also
expresses the life time cumulative incidence of asthma.
Presuming that the onset of asthma is rare before the age of
1 year, the results indicate a mean annual incidence of
asthma in this cohort of approximately 1 per 100 per year.
The proportion of the subjects with physician-diagnosed
asthma who had rhinitis during the last 12 months was
46.7% (in boys, 46.8%, in girls, 47.9%). The corresponding
figures for the proportion of subjects having rhinitis during
the last 12 months who had physician-diagnosed asthma
were 19.2”/ (in boys, 21.3”/0, in girls, 16.5”/0). Of the subjects
with physician-diagnosed asthma, 46.7% had eczema dur-
ing the last 12 months, similar in boys and girls. The
proportion of subjects who had had eczema in the last
12 months who had physician-diagnosed asthma was 9.9%
(in boys, 13.0%, in girls, 6.7%1).
RHINITIS AND ECZEMA
‘Ever rhinitis’ was reported by 16.5%, rhinitis during the
last 12 months by 14.0%, physician-diagnosed allergic rhini-
tis or hayfever by 6.5%, and 9.1% were using medicines
against rhinitis or hayfever. The conditions with rhinitis
were significantly more common in boys, while only small
differences were found between the three areas.
‘Ever eczema’ was reported by 35.3%, eczema during the
last 12 months by 27.20/o, physician-diagnosed eczema by
13.4%, and 19.6% were using medicines against eczema.
Eczema was significantly more common in girls. There were
no significant differences between the three areas.
Wheezing, rhinitis or eczema were reported by 38.9%,
similar in boys and girls, and were most prevalent in
Kiruna, 44.1% (P=O.O59). All three conditions were
reported by 2.6% (in boys, 2.5%, in girls, 2.7%) and the
prevalences were similar in the three areas.
Skin tests were performed in Kiruna and Lulea. The
proportion of the children sensitized against cat was 13,40/o,
dog 8.7%, birch 7.9%, timothy 6.7%, horse 6.1% and
mugworth 0.9%. Further, 16.3% were sensitized against any
of the three animals, and 12.8% against any of the three
pollens. Only 1.4% were sensitized against any of the two
moulds. In Kiruna, no girls and one boy were sensitized
against house dust mites; the boy was not born in Kiruna
but had moved to Kiruna in early childhood. In Kiruna,
20.9% of the children were sensitized against at least one of
the 10 allergens. The corresponding figure for Lulea was
20.5%. A gender difference, though not significant (P=O.O6)
was shown; 22.3”/0 of boys and 19.0% of girls had at least
one positive skin test. Skin test results are shown in Table 3.
Of those with physician-diagnosed asthma, 49% were
sensitized against any of the 10 allergens. The correspond-
ing figure for those with ‘current asthma’ was 52%, users of
asthma medicines 48”/0, and users of inhaled corticosteroids
53%. Of those with rhinitis during the last 12 months, 47%
were sensitized against any of the 10 allergens, while the
corresponding figure for those having physician-diagnosed
allergic rhinitis or hay fever was 75%, and physician-
diagnosed eczema was 38%. Of the 3% with wheezing,
320 E.RC~NMARK ETAL.
TABLE 3. Prevalence of positive skin test by area and sex
Lulea M F Total
Any allergen 20.9
20.5 22.3 19.0 20.6 0.860 0.062
Difference (P value) by area and sex.
B, boys; G, girls.
Use of inhaled steroids in last 12 months
No symptoms in last 12 months
20 30 40
50 60 70 80
FIG. 2. The proportion of the children with asthma, rhinitis, eczema, or without any allergy-related condition, who are
sensitized to at least one of 10 common allergens.
rhinitis and eczema, 58% were sensitized against any of the
10 allergens. Of those without any of the three symptoms or
conditions during the last 12 months, 12% were sensitized
against any of the 10 allergens. The proportions of sensi-
tized children among those with different conditions and
symptoms are shown in Fig. 2.
Of the 20.6% who were sensitized against any of the 10
allergens, 15.3% reported that they had asthma diagnosed
by a physician, 26.4% physician-diagnosed allergic rhinitis
or hay fever, 27.1% physician-diagnosed eczema, and 48.1%
either asthma, rhinitis or eczema diagnosed by a physician.
When calculating risks by using multiple logistic regression
analysis for different dependent variables expressing con-
ditions related to asthma, the Odds Ratios (OR) increased
when specificity increased. Highest risks were found when
the condition ‘current asthma’ was used as the dependent
variable. Significant risk factors were a family history of
asthma (OR 3.2) past or present house dampness (OR 1.9),
mother who smoked (OR 1.6), and male sex (OR 1.7)
(Table 4). The condition ‘asthma profile’ showed a similar
ASTHMA,TYPE-I ALLERGY AND RELATED CONDITIONS 321
4. Risk factors for ‘asthma profile’, use of asthma medicines and ‘current asthma’ by multiple logistic regression
Asthma med. ‘Current asthma’
Variables Categories OR
P value OR
P value OR
Pets at home
Area of domicile
Risks in odds ratios (OR).
5. Risk factors for any positive skin test by multiple logistic regression analysis
Independent variables Dependent variable
Any positive skin test
Variables Categories OR
Family history of allergic disease
Animals at home
Area of domicile
Risks in odds ratios (OR).
risk factor pattern, however, the risks were lower. Resi-
dence in the northernmost located Kiruna was associated
with an elevated risk, though not significantly (OR 1.3).
Past or present experience of cats, dogs or other pets at
home showed significantly lower risks for all outcome
variables related to asthma.
Of the factors used in the model, only a family history of
either asthma, rhinitis or eczema (OR 1.7), and urban
residence (OR 1.4), were associated with significantly
elevated risks for type-l allergy (Table 5). Past or present
experience of cats, dogs or other pets at home were also,
as for asthma, not associated with elevated risks for
The risk of developing ‘current asthma’ (expressed in OR
for the children with a family history of asthma, past or
present problems with dampness in their homes and a
smoking mother) was 8 compared with the children without
these risk factors (Fig. 3). That risk factor pattern was most
322 E.RONMARK ETAL
h+ h+ h+ h+
;r ;'- ;g ;'+ ;r ;' jy $'+
FIG. 3. Risks for asthma in relation to a family history of
asthma (h), a smoking mother (s), and dampness at home
(d). The dependent variable is ‘current asthma’. Odds
ratios (OR) calculated by multiple logistic regression
pronounced in Kiruna, where 38% of the children with the
three abovementioned risk factors. were currently using
asthma medicines, while none of those without any of these
three risk factors had asthma diagnosed by a physician or
were using asthma medicines.
In order to collect basic data for a longitudinal intervention
study on allergic diseases, mainly asthma, an extensive
screening of symptoms, diseases and different background
data was performed. The ISAAC questionnaire (39) was
chosen, as it gives good possibilities to make comparisons
with other studies. Further, the questionnaire has been
validated (1 l), it has been translated to Swedish, and it has
been used in Sweden (41). The very high response rate and
the use of the ISAAC questionnaire contribute to the
validity of the results, and establish a good start for the
Asthma diagnosed by a physician was reported by 6%,
while 7% reported that they had used asthma medicines and
12% reported that they had wheezed during the last 12
months. The prevalence of physician-diagnosed asthma in
children of similar ages in Scandinavia has recently been
reported to be 448% in Sweden (37,42), S-9% in Norway
(16,42), and around 4% in different parts of Finland (42,43).
In lo-year-old children in West and East Germany, the
prevalence has been reported to be 9 and 7% respectively
(1 l), while in Australia (3,5,7) and Chile (7) a prevalence of
more than 20% has been reported. The big differences in the
prevalence rates may, in part, be related to differences in
study methods and also to different diagnostic criteria. In
the present study we found the ratio between the prevalence
of wheezing during the last 12 months and asthma to be 2.0.
The corresponding figure from Norway was 1.3 (16) and
Australia 0.96 (7) which strongly suggest that different
diagnostic criteria have been used in different countries.
The prevalence of ‘current asthma’, which included sub-
jects with physician-diagnosed asthma and either use of
asthma medicines or wheezing during the last 12 months,
was 5%. These criteria give a high specificity but lower
sensitivity, and consequently the prevalence of asthma will
be somewhat underestimated by using the criteria for
estimating prevalence. The situation is the opposite for the
condition ‘asthma profile’, which results in a high sensitivity
but a low specificity, and the prevalence of the condition
was 13%. When comparing the results with Australian data,
‘asthma profile’ may be a suitable condition for compari-
son. As a consequence of this discussion, the authors
believe that a ‘true’ prevalence of asthma following
Scandinavian diagnostic practice in this cohort is close to
that defined by use of asthma medicines, or around 7%. The
prevalence is similar to that which was found in somewhat
older children in Kiruna in 1991 (37), and reported from
Sundsvall on the Baltic See in Northern Sweden 550 km
south of Lulea (44).
A family history of asthma was found to be the strongest
risk factor for current asthma (OR=3.2), stronger than in
other Swedish studies in children (45). Also a mother who
smoked was found to be a greater risk factor for asthma
than environmental tobacco smoke has been found to be in
previous studies (25,42), on the other hand, maternal smok-
ing did not appear as a risk factor for allergic sensitization
in this study, as has been shown in some other studies
(27,46). Dampness at home was associated with an
increased risk of asthma, which is in accordance with other
studies (25,27,28). The mechanism for this is unknown, but
a possible explanation may include poor ventilation that
concentrates indoor emissions. In the present study, mites
were not a likely explanation for the relationship between
asthma and dampness, since sensitization against mite was
uncommon. Bias from the reporting parents may occur.
Increased awareness or over reporting of exposure among
parents with children with asthma problems may, however,
affect the results for building exposure more than for
personal exposure, such as smoking at home.
The study results made it possible to define a group with
a considerably increased risk to develop asthma. Those
with a family history of asthma, past or present problems
with dampness in their homes, and a smoking mother had a
considerably greater risk (OR 8) of developing asthma than
those without these risks; in fact, without these risk factors,
the prevalence of asthma was very low.
Earlier studies have shown a north-south gradient of
asthma with higher prevalence in the north (37,38). The
present study found higher prevalence of symptoms, dis-
eases and use of asthma medicines in Kiruna in the north,
compared with Pitea and Lulea on the Baltic Sea, while the
prevalence of positive skin tests was similar in Kiruna and
Lulea. A hypothesis to explain the higher prevalence of
respiratory symptoms in the north may be that the longer
and colder winters may cause people to spend more time
indoors, so that they are more exposed to emissions from
buildings and to other indoor environmental exposures.
The prevalence of eczema and rhinitis were higher than
found in Kiruna in 1991 in somewhat older children (37).
As with the data for asthma, a higher prevalence of rhinitis
was found in boys, but girls had a higher prevalence of
eczema. This age-dependent gender distribution is consist-
ent with results from other studies (16,37,42,45). Wheezing,
rhinitis or eczema during the last 12 months was reported
ASTHMA,TYPE-I ALLERGY AND RELATED CONDITIONS
by 39% in this very young cohort. Studies in teenagers have
reported similar prevalence rates (45). For both rhinitis and
eczema, big differences were found in the prevalence of
children answering different questions screening the con-
ditions. This may reflect differences in degree of severity of
Twenty-one percent were sensitized to one or more of the
allergens used for testing. The most common allergen was
cat, while less than 1% were sensitized against mites. Many
studies have shown a close relation between allergic sensi-
tization to airborne allergens and asthma (10,12,18,44). Of
the asthmatic children in this study, around half were
sensitized against at least one of the tested allergens. The
corresponding proportion was 75% for the children with a
physician’s diagnosis of rhinitis. An interesting and import-
ant finding was that the risk factor patterns for asthma and
type-l allergy were different. This indicates that these
conditions have, at least in part, different aetiology even if
they are closely related and often occur together. The only
factor which was significantly associated with an increased
risk for type-l allergy, in addition to family history of
allergic disease, was urban living. This has been shown
previously in some Scandinavian studies (44) but not in
others (42). The decreased risk for asthma, as well as for
atopic sensitization associated with exposure to pets, may
be due to avoidance of pets in families with a history of
In conclusion, this study confirms that allergic diseases
are common among school children in Northern Sweden.
The prevalence of asthma was around 7%; that of rhinitis
was twice, type-l allergy thrice and of eczema four times
that of asthma. The risk of having asthma was eight (OR)
times greater if the children had a family history of asthma,
a smoking mother, and were or had been living in a house
with dampness problems compared with the children with-
out these risk factors.
The authors thank the school nurses Lena Gustafsson,
Lulea, and Kerstin Kemi-Bjiirnstriim, Kiruna, as well as
representatives for the municipalities of Kiruna, Lulea and
Pitea for work with the collection of the data. The Swedish
Asthma-Allergy Foundation and the Norrbottens Local
Health Authorities are acknowledged for financial support.
These studies were also supported by the U.S. Center For
Indoor Air Research and NIAID AI-20565.
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