What are the key symptoms of pleural effusion?

See below. sometimes there are no symptoms if the effusion is small or develops slowly. Other times and most often, patients are short of breath. They can also have pain due to the fluid irritating the lining of the lung - called pleurisy.
Shortness of breath. The key symptom of a pleural effusion is shortness of breath ... Since fluid is denser than air, and since the chest cavity is limited by the rib cage (cannot not expand much), then accummulation of pleural fluid (or effusion) will cause the lungs to be compressed ... As more and more pleural effusion accummulates, then more lung is compressed, and shortness of breath gets worse ...
Depends. What you feel depends on what is causing the effusion to begin with. An effusion may occur because of cardiac, pulmonary or pleural problems. The main symptom is varying degrees of shortness of breath. How much you are short of breath depends on how good your heart, and lung function was before it happened and the event causing the effusion.