Objective : as the results from epidemiological studies about the impact of outdoor air pollution on asthma in children are heterogeneous, our objective was to investigate the association between asthma exacerbation in children and exposure to air pollutants. Methods : a database of 1 264 585 paediatric visits during the 2010–2015 period to the emergency rooms from 20 emergency departments (EDs) of ‘Assistance Publique Hôpitaux de Paris (APHP)’, the largest hospital group in Europe, was used. A total of 47 107 visits were classified as asthma exacerbations. Concentration of air pollutants (nitrogen dioxide, ozone, fine particulate matter (PM) with an aerodynamic diameter smaller than 10  µm (PM10) and 2.5 µm (PM2.5)), as well as meteorological data, evolution of respiratory syncytial virus infection and pollen exposition, were collected on an hourly or daily basis for the same period using institutional databases. To assess the association between air pollution and asthma, mixed-effects quasi-Poisson regression modelling was performed. Results : the only compound independently associated with ED visits for asthma was PM2.5 (P<10−4). The association between asthma exacerbation and PM2.5 was not linear and a sigmoid function described the relationshipsatisfactorily. PM2.5 concentration which gives half the maximum effect was estimated at 13.5 µg/m3. Conclusions : we found an association between daily asthma exacerbation in paediatric visits to the ED and fine particulate air pollutants.