Asthma is a chronic inflammatory airway disease characterized by coughing, wheezing, chest tightness, variable airflow limitation and airway hyper-responsiveness (AHR) 1 to environmental specific (allergens such as house dust mite (HDM), pollen and animal dander) and nonspecific (eg tobacco smoke, air pollution) stimuli. Asthma is a heterogeneous disease with a complex aetiology. Allergen-induced asthma is the most common form, with atopy and allergic sensitization being identified as major risk factors. 2 Other risk factors include increased viral infections during early childhood, exposure to tobacco smoke and air pollution. 3 In