Exacerbations of asthma: Mild exacerbation defined as one of the following for 2 consecutive days: a drop in peak flow > 20% below baseline value, use of more than 3 additional puffs of reliever bronchodilator over 24 hours (excluding prophylactic puffs for exercise) as compared with baseline value or night awakening due to asthma. A severe exacerbation will be defined as any worsening of asthma control considered by the investigator or general practitioner (GP) to require a short course of oral corticosteroids/hospitalisation or decrease in morning peak flow to more than 30% below the baseline value on 2 consecutive days.