Introduction The aim of this study was to describe the demographic, clinical, exposure history and investigatory Results of hypersensitivity pneumonitis (HP) cases diagnosed by an expert regional NHS interstitial lung disease (ILD) board, to determine the extent and distribution of occupational aetiology.

Methods All HP cases identified from a database over a 13 year period were included. The demographic, clinical, exposure history and investigatory Results were reviewed from clinical notes. Cases were categorised in to three groups: occupational, non-occupational and no known cause. Hypothesis testing at the 95% confidence level was used to identify significant differences between the 3 groups.

Results A total of 127 cases (13 occupational; 34 non-occupational; 80 no known cause) of HP were identified. Men were more likely to experience occupational HP (p=0.029) compared to the other groups. Occupational HP cases were younger (p=0.002), more likely to experience weight loss (p=0.004), to have systemic symptoms (p=0.007) and a recurrence of symptoms (p<0.001). Occupational HP were due to metal working fluids (MWF), birds, mould/fungi, Farmer’s lung, and cleaning and treatment sprays. Non-occupational HP were due to birds, mould/fungi in the home and medication. Percentage lymphocyte count in broncho-alveolar lavage (BAL) was significantly raised in occupational HP (p=0.001). Where no causative agent was identified, there was a greater absence of exposure history (13.2%–32.5%).

Conclusion Occupations where there is exposure to birds or those working in trades where industrial processes and the use of chemical compounds predominate, are at risk of occupational HP. Birds remain an important cause of non-occupational HP. Clinical features such as weight loss, systemic symptoms and recurrent symptoms should raise a suspicion of an occupational cause of HP. BAL remains an important investigatory tool in HP, especially in occupational HP. An exposure history, especially an occupational history, is mandatory when assessing suspected cases of HP.