Methods
Study design and
population
The study comprised data collected in the Espoo Cohort Study (ECS),
which included 2568 children delivered between 1 January 1984 and 31
March 1990 and living in 1990 in the city of Espoo in Southern Finland,
and who had been followed regularly since the birth. Espoo is an
urban–suburban municipality with a population of 289,731 (1/2020,https://www.espoo.fi/en) and it
is located across the western border of Helsinki, the capital of
Finland. A random sample of children, who were living in Espoo in 1991,
was taken from the roster of Statistics Finland. The baseline data
collection was conducted in 1991 (response rate, 80·3%) and the 6- and
20-year follow-ups were conducted in 1997 and 2010-11 (follow-up rates
77·3% and 63·2%, respectively).16-19 Ethical
approval for the study was obtained from the Ethics Committee of the
Northern Ostrobothnia Hospital District and signed informed consent was
obtained from all participants or their caregivers. The study was
conducted in accordance with The Code of Ethics of the World Medical
Association (Declaration of Helsinki) for studies involving humans.
The life-time home coordinates of the participating mothers and children
were retrieved from the Population Register Centre of Finland, processed
and georeferenced using ArcGIS Online World Geocoding Service, allowing
matching of participants to places and allowing tracking of cumulative
and spatially varying environmental exposures. The present study
included all the cohort members (n=2568).
Data collection
Health outcome
The outcome of interest was the development of allergic rhinitis during
the follow-up time – assessed in early childhood (up to six years of
age), later childhood (up to 12 years of age), and young adulthood (up
to 27 years of age). Information on allergic rhinitis was collected at
baseline and in the follow-up surveys by applying questions from the
1978 American Thoracic Society Division of Lung Disease questionnaire
for children. These had been modified and translated into Finnish and
Swedish.16 Allergic rhinitis ever was defined as a
parent- or self-reported answer ‘yes’ to “Has/have the child/you
ever had doctor diagnosed allergic rhinitis? ”. The age of the onset of
allergic rhinitis was asked in the 20-year follow-up questionnaire.
Exposure assessment