3. Sputum Induction Sputum induction will be performed in the Asthma Laboratory. The subject will inhale 3.5% saline at room temperature, nebulised via a De Vilbiss 99 Ultrasonic nebuliser set, at maximum output using a mouthpiece. After five minutes the subject will rinse his/her mouth thoroughly with water, and will be asked to cough deeply and expectorate sputum into a pot. Samples from this first five minute collection are discarded. This procedure will continue for a further ten minutes, and the resulting sample will be collected in a polypropylene pot.