This study will look at the permeability, or 'leakiness' of the airway epithelium (the inner lining of the lung) in asthmatic patients. Increased leakiness of this lining has been shown in asthmatic patients by other studies, not only in the lung but also possibly in the gut, perhaps reflecting a widespread defect. This leakiness may underline the interaction between the environment and a person's genetic make up, and may contribute to why some people get asthma, and how severe it is. Increased leakiness may allow increased exposure to inhaled allergic substances, helping to perpetuate the inflammation in the lungs that is a hallmark of asthma.