 All the treatments in the LUNG PHYSIOLOGY study will be non-radiolabelled. Salbutamol will be the drug aerosol administered to each subject at each visit. Subjects and investigators will be blinded to the fine-mist aerosol particle sizes given at 6 treatments. The control arm will be salbutamol drug aerosol delivered as a coarse mist standard metered dose inhaler 7th treatment visit. 