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 - Northwestern Memorial Hospital - Chicago


The lungs are surrounded by two layers of tissue called the pleura. The outer layer, known as the parietal pleura, lines the chest wall, diaphragm and central zone of the chest (also called the mediastinum) while the inner layer, known as the visceral pleura, wraps the lungs. In healthy lungs, there is normally a small amount of fluid between the parietal and visceral pleura that enables the two layers of tissue to slide over each other as you inhale and exhale.

We all make nearly 8 liters of pleural fluid each day. It is produced on the parietal side and absorbed or drains through the visceral pleura into the lymph vessels of the lung. In certain disease states, excess pleural fluid is produced or the normal drainage pathways are blocked, both mechanisms can occur simultaneously.

Pleural effusion is a build-up of excess fluid in the pleural space between the two layers. It is sometimes referred to as “water on the lungs.” Pleural effusions are very common. According to the National Cancer Institute, approximately 100,000 cases are diagnosed in the United States every year.

There are two types of effusions—transudative (watery and protein-poor fluid) and exudative (protein-rich fluid). The type of effusion can help physicians determine what caused the fluid build-up. The most common causes of transudative pleural effusions include:

  • Heart failure
  • Post-operative fluid overload
  • Cirrhosis

The most common causes of exudative pleural effusions are:

  • Pneumonia
  • Cancer (including lung cancer, breast cancer and lymphoma)
  • Pulmonary embolism
  • Kidney disease
  • Inflammatory disease

Pleural effusions can also be caused by other conditions, such as:

  • Tuberculosis
  • Autoimmune disease (e.g. lupus)
  • Bleeding due to chest trauma
  • Chylothorax (lymphatic fluid accumulating in the pleural space) due to trauma or surgery
  • Rare chest and abdominal infections
  • Meig’s syndrome due to a non-cancerous ovarian tumor
  • Ovarian hyperstimulation syndrome
  • Certain medications
  • Abdominal surgery
  • Radiation therapy

The fluid that causes the pleural effusion can be both malignant (cancerous) and benign (non-cancerous).

How serious is the condition?

How serious pleural effusion is depends on a few things:

  • The primary cause of the effusion
  • Whether breathing is affected
  • Whether it can be effectively treated


If you are concerned about pleural effusion or other lung diseases, please contact your physician to discuss ways to reduce your risk and whether or not you should be screened. If you do not have a physician, call our Physician Referral Service at 1-877-926-4664 or request a first time appointment online.

Last UpdateJune 16, 2011


To make an appointment with a physician, please call 1-877-926-4664.

New patients can request an appointment online.