 Monitoring patients with chronic, inflammatory airways disease particularly in the early stages is hampered by the relative insensitivity of current outcome measures to detect subtle changes. Multiple breath washout is a potential sensitive test that is a useful readout of disease at these early stages but it lacks standardisation and knowledge of variability with reference to standard lung function measures. This is a Cross sectional and longitudinal observation study. The hypothesis is that multiple breath washout-derived indices will provide a robust signal of gas mixing inhomogeneity, correlating with conventional measures of airway disease severity. Multiple breath washout performed on different devices will generate indices which correlate but differ in value. 