Pleural effusion, also called “water on the lung,” is an excessive buildup of fluid between your lungs and chest cavity. There’s always a small amount of liquid inside your lungs.This fluid works to coat the membranes (pleura) that line the outside of your lungs and lubricate the chest cavity to aid in breathing.he pleura creates too much fluid when it’s irritated or infected. This fluid accumulates in the chest cavity outside the lung, causing what’s known as pleural effusion. Certain types of cancer, such as lung, breast, and ovarian cancers, can cause pleural effusions. Lymphoma and mesothelioma can cause them too.