Caused by disturbances in oncotic pressure (imbalances in the Starling forces)
Conditions that increase the capillary hydrostatic pressure (forcing fluid out of the pulmonary capillaries into the pleural space) include congestive cardiac failure
Conditions that reduce the capillary oncotic pressure (impairing the reabsorption of fluid from the pleural space into the pulmonary capillaries) include cirrhosis, nephrotic syndrome/chronic kidney disease, and gastrointestinal malabsorption/malnutrition (eg. Coeliac disease)
Exudative
High protein content (> 35g/L)
Caused by diseases which increase capillary permeability, allowing fluids to leave the circulatory system, including:
Infections such as pneumonia or TB
Malignancy such as bronchial carcinoma, mesothelioma, or lung metastases
Inflammatory conditions such as rheumatoid arthritis, lupus, or acute pancreatitis
Pulmonary infarct (for example secondary to a pulmonary embolism) and trauma