Occupational asthma is the commonest occupational lung disease, accounting for 5% of all cases of asthma, and incidence is rising. Sufferers lose on average half their total take-home income (including compensation) for four years after diagnosis. Whilst the clinical and laboratory features of asthma are generally well known, those seen in work-related asthma are not the same in all cases. Some individuals with work-related asthma have less variability in the size of their bronchial airways than usually found in this illness. The blood cells present in the phlegm of some work-related asthma patients may differ from those found in common asthma. Little is known about these variations, which may reflect a different disease process and hence long-term outcome for the patient. This project aims to investigate how the results of routine breathing tests relate to specialised tests of inflammation within the bronchial tubes (for example chemicals in the breath or cells in phlegm or blood). It will then evaluate whether any of these can predict prognosis in patients with occupational asthma. The pattern of breathing tests and/or phlegm tests may be useful in identifying workers at high risk of long-term health effects.