77 Review www.expert-reviews.com ISSN 1744-666X © 2009 Expert Reviews Ltd 10.1586/1744666X.5.1.77 Asthma is a chronic inflammatory condition of the airways characterized by recurrent symp- toms of variable airflow limitation. Risk fac- tors for the development of asthma have been studied with intensity due to the dramatic increase in its worldwide prevalence during the last half of the last century. Two major research initiatives among children and young adults, the International Study of Asthma and Allergies in Childhood (ISAAC) [1] and the European Community Respiratory Health Survey (ECRHS) [2] , have established inter- national and regional prevalence data. These studies show low prevalence rates (2–4%) in Asian countries (particularly China and India) and high rates (15–20%) in the UK, Canada, Australia, New Zealand and other ‘Westernized’ countries [1–4] . Repeated surveys using ISAAC [3] and ECRHS [4] allow the study of temporal trends within and across popula- tions. The regional variability and temporal trends of the ‘asthma epidemic’ have stimu- lated many studies, including observations in migrating populations [5] and in Germany after reunification [6] , which strongly suggest that environmental factors determine the expres- sion of asthma among genetically similar populations. These studies have also shown a continued increase in asthma prevalence in the younger age groups as well as in develop- ing countries, with a leveling off of asthma prevalence rates in countries where the baseline prevalence was already high [3] . Longitudinal studies have likewise examined risk factors predicting the development as well as the persistence, remission or relapse of asthma from infancy, through childhood, to middle-age. Mature population-based birth cohort studies, such as those commenced in the UK, Australia and New Zealand several decades ago, have pro- vided a useful understanding of adult outcomes of childhood asthma. More-recent birth cohort studies have focused on the early years of life, as evidence accrues suggesting that events and expo- sures in utero, in early infancy and during the pre- school years play a major role in the development of the various phenotypes of asthma. An extensive literature review was undertaken through 2006–2007 by Canadian investigators developing the recently launched Canadian Healthy Infant Longitudinal Development (CHILD) study (see Acknowledgements). That review has been updated with recent literature to summarize known and putative risk fac- tors associated with the development of child- hood allergy and asthma. After introducing the multiple phenotypes of asthma, we review the environmental (i.e., indoor air, outdoor air, nutrition and gut colonization, respiratory infec- tions, and psychosocial environment and stress), genetic (i.e., gene–environment and gene–gene interactions, gender and parent-of-origin effects and epigenetics) and host (i.e., immunity, airway inflammation, lung function and sex and gen- der effects) risk factors currently considered to influence the initiation and course of asthma. Padmaja Subbarao, Allan Becker, Jeffrey R Brook, Denise Daley, Piush J Mandhane, Gregory E Miller, Stuart E Turvey and Malcolm R Sears † ; on behalf of the CHILD study investigators † Author for correspondence St Joseph’s Healthcare, 50 Charlton Ave East, Hamilton, Ontario, L8N 4A6, Canada Tel.: +1 905 522 1155 ext. 33286 Fax: +1 905 521 6132 searsm@mcmaster.ca This comprehensive review of the recent literature was undertaken to determine the current state of knowledge of the risk factors involved in the development of asthma in order to focus investigations in a proposed new longitudinal birth cohort study. The origins of asthma appear to lie in the prenatal and early postnatal period, and renewed investigations in this period with long-term close follow-up and objective phenotypic characterization will help to unravel the role of the multiple putative environmental factors in the development of asthma. It is only after understanding these effects that one can hope to design rational prevention studies for asthma. KEYWORDS:฀allergy฀•฀asthma฀•฀child฀•฀cohort฀study฀•฀environment฀•฀epidemiology฀•฀genetics฀•฀lung฀function฀ •฀phenotype฀•฀risk฀factor Epidemiology of asthma: risk factors for development Expert Rev. Clin. Immunol. 5(1), 77–95 (2009) For reprint orders, please contact reprints@expert-reviews.com