Chest x-ray may be normal but usually there is fine hazy opacification, often bilateral & patchy ground glass appearance can develop, with loss of definition of pulmonary vessels.

 CT in acute phase shows patchy areas of air space opacification (ill defined centrilobular nodules) with peripheral areas of decreased attenuation due to air trapping - a result of the response of the hyper reactive small airways. In severe cases air bronchogram may develop resembling pulmonary edema.

In the sub acute form, ground glass appearance & small well-defined centrilobular nodules are seen which may coalesce to give a homogeneous ground glass opacification.

 If repeated attacks persist, reticular pattern develops due to interstitial and alveolar fibrosis with traction bronchiectasis usually in the mid zone, though diffuse involvement may be seen. This is the chronic phase. HRCT differentiates this form cryptogenic fibrosing alveolitis (IPF) by demonstrating centrilobular nodules & lack of subpleural predominance of fibrosis.

Clinical features-
 In the acute phase, patient complains of cough & dyspnoea after 4-6 hours of exposure.

Pathogenesis-
This is a type III hypersensitivity reaction most commonly occurring due to organic antigens in individuals with asthma. The size of the particles is small (2 microns or less), and they penetrate the distal air spaces [alveoli] & cause allergic response. Most common variant is extrinsic allergic alveolitis.