Exhaled Hydrogen Sulphide as a Biomarker of Airways Disease in Asthma

The purpose of this study will be to determine whether there is any role for measuring Hydrogen Sulphide (H2S) in the exhaled breath in terms of management of the patient with asthma. The investigators will recruit patients with a range of severity of asthma and health volunteers. Levels of Hydrogen Sulphide will be measured in exhaled breath and blood. Also exhaled Nitric Oxide, Spirometry and asthma symptom scores will be measured at each study visit. Participants will attend either 2 or 4 separate visits. The investigators will determine whether there is a relationship between exhaled Hydrogen Sulphide and asthma severity.