title: A systematic review of associations between environmental exposures and development of asthma in children aged up to 9 years.
abstract: OBJECTIVE
  Childhood asthma is a complex condition where many environmental factors are implicated in causation. The aim of this study was to complete a systematic review of the literature describing associations between environmental exposures and the development of asthma in young children.
  
  
  METHODS
  A systematic review of the literature up to November 2013 was conducted using key words agreed by the research team. Abstracts were screened and potentially eligible papers reviewed. Papers describing associations between exposures and exacerbation of pre-existing asthma were not included. Papers were placed into the following predefined categories: secondhand smoke (SHS), inhaled chemicals, damp housing/mould, inhaled allergens, air pollution, domestic combustion, dietary exposures, respiratory virus infection and medications.
  
  
  METHODS
  Children aged up to 9 years.
  
  
  METHODS
  Diagnosed asthma and wheeze.
  
  
  RESULTS
  14,691 abstracts were identified, 207 papers reviewed and 135 included in the present review of which 15 were systematic reviews, 6 were meta-analyses and 14 were intervention studies. There was consistent evidence linking exposures to SHS, inhaled chemicals, mould, ambient air pollutants, some deficiencies in maternal diet and respiratory viruses to an increased risk for asthma (OR typically increased by 1.5-2.0). There was less consistent evidence linking exposures to pets, breast feeding and infant dietary exposures to asthma risk, and although there were consistent associations between exposures to antibiotics and paracetamol in early life, these associations might reflect reverse causation. There was good evidence that exposures to house dust mites (in isolation) was not associated with asthma risk. Evidence from observational and intervention studies suggest that interactions between exposures were important to asthma causation, where the effect size was typically 1.5-3.0.
  
  
  CONCLUSIONS
  There are many publications reporting associations between environmental exposures and modest changes in risk for asthma in young children, and this review highlights the complex interactions between exposures that further increase risk.
authors.affiliation: Occupational and Environmental Medicine, University of Aberdeen, Aberdeen, UK.
authors.forename: S
authors.initials: S
authors.lastname: Dick
authors.affiliation: Department of Child Health, University of Aberdeen, Aberdeen, UK.
authors.forename: A
authors.initials: A
authors.lastname: Friend
authors.affiliation: Department of Child Health, University of Aberdeen, Aberdeen, UK.
authors.forename: K
authors.initials: K
authors.lastname: Dynes
authors.affiliation: Department of Child Health, University of Aberdeen, Aberdeen, UK.
authors.forename: F
authors.initials: F
authors.lastname: AlKandari
authors.affiliation: Institute of Occupational Medicine, Edinburgh, UK.
authors.forename: E
authors.initials: E
authors.lastname: Doust
authors.affiliation: Institute of Occupational Medicine, Edinburgh, UK.
authors.forename: H
authors.initials: H
authors.lastname: Cowie
authors.affiliation: Occupational and Environmental Medicine, University of Aberdeen, Aberdeen, UK Environmental and Respiratory Medicine, University of Birmingham, Birmingham, UK.
authors.forename: J G
authors.initials: JG
authors.lastname: Ayres
authors.affiliation: Department of Child Health, University of Aberdeen, Aberdeen, UK.
authors.forename: S W
authors.initials: SW
authors.lastname: Turner
mesh_terms: D000485:Allergens / Q000009:adverse effects; D001249:Asthma / Q000209:etiology*; D002648:Child; D002675:Child, Preschool; D004032:Diet / Q000009:adverse effects; D004781:Environmental Exposure / Q000009:adverse effects*; D006801:Humans; D007223:Infant; D014028:Tobacco Smoke Pollution / Q000009:adverse effects
chemical_list: D000485:Allergens; D014028:Tobacco Smoke Pollution
keywords: PAEDIATRICS; PUBLIC HEALTH