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