ORIGINAL ARTICLE Coarse and fine particles but not ultrafine particles in urban air trigger hospital admission for asthma in children Amne Iskandar, 1 Zorana Jovanovic Andersen, 2 Klaus Bønnelykke, 1 Thomas Ellermann, 3 Klaus Kaae Andersen, 2 Hans Bisgaard 1 ABSTRACT Background Short-term exposure to air pollution can trigger hospital admissions for asthma in children, but it is not known which components of air pollution are most important. There are no available studies on the particular effect of ultrafine particles (UFPs) on paediatric admissions for asthma. Aim To study whether short-term exposure to air pollution is associated with hospital admissions for asthma in children. It is hypothesised that (1) the association between asthma admissions and air pollution is stronger with UFPs than with coarse (PM 10 ) and fine (PM 2.5 ) particles, nitrogen oxides (NO x ) or nitrogen dioxide (NO 2 ); and (2) infants are more susceptible to the effects of exposure to air pollution than older children. Method Daily counts of admissions for asthma in children aged 0e18 years to hospitals located within a 15 km radius of the central fixed background urban air pollution measurement station in Copenhagen between 2001 and 2008 were extracted from the Danish National Patient Registry. A time-stratified case crossover design was applied and data were analysed using conditional logistic regression to estimate the effect of air pollution on asthma admissions. Results A significant association was found between hospital admissions for asthma in children aged 0e18 years and NO x (OR 1.11; 95% CI 1.05 to 1.17), NO 2 (1.10; 95% CI 1.04 to 1.16), PM 10 (1.07; 95% CI 1.03 to 1.12) and PM 2.5 (1.09; 95% CI 1.04 to 1.13); there was no association with UFPs. The association was stronger in infants than in older children for all pollutants, but no statistically significant interaction was detected. Conclusion Short-term exposure to air pollution can trigger hospital admission for asthma in children, with infants possibly being most susceptible. These effects seemed to be mediated by larger particles and traffic- related gases, whereas UFPs showed no effect. INTRODUCTION Asthma is the most common chronic disease in children and the main cause of emergency room visits, 1 hospital admissions 2 and school absences. The incidence of childhood asthma has increased in recent decades in high-income countries, and envi- ronmental factors have gained increasing attention in attempts to understand the determinants of the asthma epidemic. 1 3 Exposure to air pollution from traffic is ubiquitous in urban environments, affecting millions of people. Children are possibly more susceptible to the effects of air pollution than adults because their lungs and immune systems are developing, they inhale higher doses of air pollution relative to adults due to differences in breathing rates and patterns, and because they are generally more active in environments with high levels of pollutants when participating in sports and outdoor play. 4 Chronic exposure to traffic-related air pollution in the early years of life has been suggested to contribute to the development of asthma in children, 5e7 although evidence is mixed. 8 Short-term exposures of several days to elevated levels of air pollution have been shown to trigger exacerbations of asthma leading to increased rates of emergency room visits 9 10 hospital admissions for asthma, 9 11e19 wheezing symptoms 20 and decreased peak expiratory flow. 21 However, many aspects of the relationship between air pollution Key messages What is the key question? < Is the association between asthma admissions in children and air pollution stronger with ultrafine particles (UFPs) than with coarse and fine particles (PM 10 , PM 2.5 ), as suggested by animal studies and toxicology? Are infants more susceptible to the effects of exposure to air pollution than older children? What is the bottom line? < Exposure to air pollution triggers hospital admission for asthma in children. This effect was mediated by coarse and fine particles and traffic-related gases and not by UFPs. Infants are slightly more susceptible to the effects of air pollution, but the effects or air pollution are not limited to this group of children. Why read on? < This is the first epidemiological study of hospital admissions for asthma in children with data on UFPs measured in the same way as larger particles and gases. It is also the first study of a link between air pollution and hospital admissions for asthma in children which also includes infants, and thus tests for effect modification in air pollution by age. < Additional tables are published online only. To view the file please visit the journal online (http://thorax.bmj.com/ content/early/recent). 1 Copenhagen Prospective Studies on Asthma in Childhood, Health Sciences, University of Copenhagen and Copenhagen University Hospital, Gentofte, Copenhagen, Denmark 2 Danish Cancer Society, Institute of Cancer Epidemiology, Copenhagen, Denmark 3 Aarhus University, National Environmental Research Institute, Roskilde, Denmark Correspondence to Professor Hans Bisgaard, Copenhagen Prospective Studies on Asthma in Childhood, Health Sciences, University of Copenhagen, Copenhagen University Hospital, Ledreborg Alle ´ 34, 2820 Gentofte, Denmark; bisgaard@copsac.com Received 11 April 2011 Accepted 10 November 2011 Iskandar A, Andersen ZJ, Bønnelykke K, et al. Thorax (2011). doi:10.1136/thoraxjnl-2011-200324 1 of 6 Paediatrics Thorax Online First, published on December 9, 2011 as 10.1136/thoraxjnl-2011-200324 Copyright Article author (or their employer) 2011. 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