What causes pleural effusion?Pleural fluid is formed in the body in small amounts to lubricate the surfaces of the pleura, the thin membrane that lines the chest cavity and surrounds the lungs. A pleural effusion is an abnormal collection of this fluid. Two different types of effusions can develop: transudative and exudative. Transudative pleural effusions are usually caused by a disorder in thenormal pressure in the lung. Congestive heart failure is the most common cause of transudative effusion. Exudative effusions form as a result of inflammation (irritation and swelling) of the pleura, which is often caused by lung disease. Cancer, pneumonia, tuberculosis and other lung infections, drug reactions, collagen-vascular diseases, asbestosis, and sarcoidosis are some diseases that can cause exudative pleural effusions. The cause and type of pleural effusion is usually determined by thoracentesis (removal of a sample of fluid with a needle inserted between the ribs into the chest cavity). Blood in the pleural space (hemothorax) usually results from a chest injury. Rarely, a blood vessel ruptures into the pleural space when no injury has occurred, or a bulging area in the aorta (aortic aneurysm) leaks blood into the pleural space. Because blood in the pleural space does not clot fully, it is usually easy for a doctor to remove using a large-bore needle or a chest tube. Pus in the pleural space (empyema) can accumulate when pneumonia or a lung abscess spreads into the space. A wide range of bacteria as well as certain fungi and mycobacteria (especially the mycobacterium that causes tuberculosis) are the most common organisms causing pleural effusion. Empyema may also complicate an infection from chest wounds, chest surgery, rupture of the esophagus, or an abscess in the abdomen. Milky fluid in the pleural space (chylothorax) is caused by an injury to the main lymphatic duct in the chest (thoracic duct) or by a blockage of the duct by a tumor.