accumulation of watery fluid in the pleural cavity, between the membrane lining the thoracic cage and the membrane covering the lung. There are many causes of pleural effusion, including pneumonia, tuberculosis, and the spread of a malignant tumour from a distant site to the pleural surface. Pleural effusion often develops as a result of chronic heart failure because the heart cannot pump fluid away from the lungs, and fluid that seeps from the lungs places additional stress on the dysfunctioning heart. Large pleural effusions can cause disabling shortness of breath.

If symptoms of pleural effusion develop, a tube is inserted through the chest wall into the pleural space to drain the fluid. Under certain conditions, such as malignant disease of the pleura (i.e., mesothelioma), pleural effusion can be treated by introducing an irritating substance called a sclerosing agent into the pleural space in order to stimulate an inflammatory reaction of the pleural surfaces. As the inflammation heals, tissue adhesions obliterate the pleural space, thereby preventing the accumulation of more fluid. Examples of sclerosing agents that cause an inflammatory reaction of the pleural surfaces include talc, doxycycline, and bleomycin. Adhesions that form as the inflammation heals obliterate the pleural space, thereby preventing accumulation of more fluid.