title: A clear urban-rural gradient of allergic rhinitis in a population-based study in Northern Europe.
abstract: BACKGROUND
  The protective effect of farm upbringing on allergic rhinitis is well known, but how upbringing in other environments influences the development of allergic rhinitis is scarcely investigated. The aim of this study was to investigate the association between place of upbringing and pet keeping in childhood and allergic rhinitis and nasal symptoms in adulthood.
  
  
  METHODS
  The population-based Respiratory Health in Northern Europe study includes subjects from Denmark, Norway, Sweden, Iceland, and Estonia born in 1945-1973. This paper analyses 13,376 participants of the third study wave. Six categories of place of upbringing were defined: farm with livestock, farm without livestock, village in rural area, small town, city suburb, and inner city. Pets in the home at birth and during childhood were recorded. Data were analysed using adjusted logistic regression models.
  
  
  RESULTS
  Livestock farm upbringing predicted less adult allergic rhinitis [odds ratio (OR) 0.68, 0.54-0.85] and nasal symptoms (OR 0.82, 0.68-0.99) than city upbringing, and an urban-rural gradient with decreasing risk per level of urbanisation was observed (OR 0.92, 0.88-0.94). Pets in the home at birth (OR 0.78, 0.68-0.88) and during childhood (OR 0.83, 0.74-0.93) were associated with less subsequent allergic rhinitis. Pet keeping did not explain the protective effect of place of upbringing.
  
  
  CONCLUSIONS
  Risk of allergic rhinitis and nasal symptoms in adulthood was inversely associated with the level of urbanisation during upbringing. Pets at birth decreased the risk further, but did not explain the urban-rural gradient. Persistent beneficial effects of microbial diversity in early life might be an explanation for the findings.
authors.affiliation: Department of Public Health, Section for Environment, Occupation and Health, Danish Ramazzini Centre, Aarhus University, Aarhus, Denmark.
authors.forename: Stine Holmegaard
authors.initials: SH
authors.lastname: Christensen
authors.affiliation: Department of Public Health, Section for Environment, Occupation and Health, Danish Ramazzini Centre, Aarhus University, Aarhus, Denmark.
authors.forename: Signe
authors.initials: S
authors.lastname: Timm
authors.affiliation: Department of Medical Sciences: Respiratory, Allergy and Sleep Research, Uppsala University, Uppsala, Sweden.
authors.forename: Christer
authors.initials: C
authors.lastname: Janson
authors.affiliation: Faculty of Medicine, University of Iceland, Reykjavik, Iceland.
authors.forename: Bryndis
authors.initials: B
authors.lastname: Benediktsdóttir
authors.affiliation: Division of Occupational and Environmental Medicine, Department of Public Health and Clinical Medicine, Umeå University, Umeå, Sweden.
authors.forename: Bertil
authors.initials: B
authors.lastname: Forsberg
authors.affiliation: Department of Occupational and Environmental Medicine, Sahlgrenska University Hospital, Gothenburg, Sweden.
authors.forename: Mathias
authors.initials: M
authors.lastname: Holm
authors.affiliation: Lung Clinic, Tartu University Hospital, Tartu, Estonia.
authors.forename: Rain
authors.initials: R
authors.lastname: Jogi
authors.affiliation: Centre for Clinical Research, Haukeland University Hospital, Bergen, Norway.
authors.forename: Ane
authors.initials: A
authors.lastname: Johannessen
authors.affiliation: Centre for Clinical Research, Haukeland University Hospital, Bergen, Norway.
authors.forename: Ernst
authors.initials: E
authors.lastname: Omenaas
authors.affiliation: Department of Public Health, Section for Environment, Occupation and Health, Danish Ramazzini Centre, Aarhus University, Aarhus, Denmark.
authors.forename: Torben
authors.initials: T
authors.lastname: Sigsgaard
authors.affiliation: Centre for Clinical Research, Haukeland University Hospital, Bergen, Norway.|Centre for International Health, Department of Global Public Health and Primary Care, University of Bergen, Bergen, Norway.|Department of Occupational Medicine, Haukeland University Hospital, Bergen, Norway.
authors.forename: Cecilie
authors.initials: C
authors.lastname: Svanes
authors.affiliation: Department of Public Health, Section for Environment, Occupation and Health, Danish Ramazzini Centre, Aarhus University, Aarhus, Denmark.|National Research Centre for the Working Environment, Copenhagen, Denmark; vs@ph.au.dk.
authors.forename: Vivi
authors.initials: V
authors.lastname: Schlünssen
keywords: allergic rhinitis; microbial diversity; microbial exposure; nasal symptoms; pets; place of upbringing