<qa>  What is a cross sectional and longitudinal observation study?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. 