Pleural effusion, also called water on the lung, happens when fluid builds up in the space between your lungs and chest cavity. You may have symptoms, such as chest pain, depending on the cause.

Thin membranes, called pleura, cover the outside of the lungs and the inside of the chest cavity. There’s always a small amount of liquid within this lining to help lubricate the lungs as they expand within the chest during breathing. However, if too much fluid builds up, for example, because of a medical condition, problems can arise. Doctors call this pleural effusion.

Various conditions can lead to pleural effusion, but congestive heart failure is the most common cause.

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Illustration by Diego Sabogal

Some people with pleural effusion don’t have any symptoms. They may have symptoms of an underlying disease, such as a cough or fever.

You may find out you have pleural effusion through a chest X-ray or physical examination done for another reason.

When a doctor examines you, they may notice expansion on one side of the chest and a dull sound when they tap on that side.

Depending on the cause, a person with pleural effusion may also have:

See your doctor immediately if you have symptoms of pleural effusion.

Pleural effusion happens when fluid accumulates in the chest cavity outside the lung.

It can be either transudative or exudative.

Transudative pleural effusion happens when increased pressure on the small and large vessels of various organs causes them to be leaky, resulting in protein-filled fluid collections. It occurs with coronary heart disease or cirrhosis.

Exudative pleural effusion happens when there is irritation, inflammation, or infection. These can result in extra fluid production, reduced drainage, or both.

Causes of pleural effusion include:


Pleural effusion can be classified as transudative or exudative.

Transudative pleural effusions

This type is caused by fluid leaking into the pleural space due to increased pressure in the blood vessels.

It can happen if you:

Exudative effusions

This happens when a buildup of fluid occurs as the result of:

Conditions that could result in this type of pleural infusion include:

Some conditions, such as pulmonary embolism, can lead to either transudate or exudate pleural effusion.

Pleural effusions and cancer

Pleural effusions can occur when cancer cells spread to the pleura or block the flow of normal fluid within the pleura. Fluid may also build up due to certain cancer treatments, such as radiation therapy or chemotherapy.

Certain cancers are more likely to cause pleural effusions than others, including:

Signs and symptoms often include:

The doctor may drain the fluid or carry out pleurodesis if you’re likely to need repeated drainage. This involves insering a shunt that redirects the fluid away from the chest.

They may prescribe antibiotics if you have or are susceptible to infection. Steroids or other anti-inflammatory medications may reduce pain and inflammation. They will also discuss other treatment options for cancer.

People who are undergoing treatment for cancer may also have compromised immune systems, making them more prone to infections or other complications.

The treatment and outcome will depend on the cause of the pleural effusion.

Draining fluid

One way to treat pleural effusion is by draining the fluid from the chest cavity, either with a needle or by inserting a small tube into the chest.

You’ll receive a local anesthetic before this procedure, which will make the treatment more comfortable. You may feel some pain or discomfort at the incision site after the anesthetic wears off. Most doctors will prescribe medication to help relieve pain.

You may need this treatment more than once if fluid builds up again.

Other treatments may be necessary to manage fluid buildup if cancer is the cause of the pleural effusion.


If you have a bacterial infection, the doctor will likely prescribe antibiotics or administer them intravenously. They will usually do this alongside drainage.


Pleurodesis is a treatment that creates mild inflammation between the lung and chest cavity pleura. After drawing the excess fluid out of the chest cavity, a doctor injects a drug into the area. This drug causes the two layers of the pleura to stick together, which prevents the future buildup of fluid between them.

A doctor may decide to do this if pleural effusion is due to cancer. It reduces the need for frequent drainage.


If symptoms don’t improve with drainage and antibiotics, the doctor may recommend thoracoscopic decortication or thorascopic debridement. They will insert a thoracoscope into the pleural space then either remove any tissue that is causing a problem (decortication) or surgically clean a wound to enable it heal (debridement). A doctor may call this a medical thorocoscopy or a pleuroscopy.

In some cases, a doctor surgically inserts a shunt, or small tube, into the chest cavity. This helps redirect the fluid from the chest into the abdomen, where it can be more easily removed by the body. This may be an option for those who don’t respond to other treatments.

Pleurectomy, in which the surgeon removes part of the pleural lining, may also be an option in certain cases.

Your doctor will perform a physical examination and listen to your lungs with a stethoscope. They may also order a chest X-ray to help diagnose pleural effusion.

Other possible tests include:

Thoracentesis involves removing fluid from the pleural membrane area by inserting a needle into the chest cavity and suctioning the fluid into a syringe. The doctor will use ultrasound to guide the needle. At the same time, they may drain the excess fluid from the chest cavity. The fluid will then be tested to determine the cause.

Your doctor may also choose to perform a pleural biopsy, which involves taking a tissue sample from the pleura. They do this by inserting a small needle from outside the chest wall into the chest cavity.

If they discover you have a pleural effusion but are unable to diagnose which type, your doctor may schedule a thoracoscopy. This is a surgical procedure that lets the doctor see inside the chest cavity using a fiber-optic camera.

For this procedure, your doctor will make a few small incisions in the chest area while you’re under general anesthesia. Then they’ll insert the camera through one incision and the surgical tool through the other to extract a small amount of fluid or tissue for analysis.

Pleural effusions can be complicated or uncomplicated. Uncomplicated pleural effusions contain fluid without signs of infection or inflammation. They’re less likely to cause permanent lung problems.

Complicated pleural effusions, however, contain fluid with significant infection or inflammation. They require prompt treatment that frequently includes chest drainage.

Pleural effusion can be a sign of severe symptoms with some diseases. In 2021, some scientists found that people with COVID-19 who developed pleural effusion were more likely to have severe inflammation and complications, which could affect their chances of recovery.

Treatment can also lead to complications.

Minor complications from more invasive treatments can include slight pain and discomfort, which often go away with time. More serious complications will depend on the severity of the condition, the cause, and the treatment used.

Serious complications can include:

  • pulmonary edema or fluid in the lungs, which can result from draining fluid too quickly during thoracentesis
  • partial collapsed lung
  • infection or bleeding
  • empyema, when there is pus in the pleural space
  • trapped lung, when a layer forms around the lung that prevents it from expanding

These complications, while serious, are rare. Your doctor will help determine the most effective treatment option and discuss the benefits and risks of each procedure.

Pleural effusions can be a sign of a life-threatening condition, but the outlook and recovery time will depend on the cause, how severe the effusion is, and individual factors, such as your overall health.

If you seek treatment, you’re more likely to have a postive outcome. Without treatment, you may face complications such as empyema, sepsis, or a trapped lung. If you seek treatment, you’re also more likely to find out if you have an underlying condition, which a doctor can then treat.

Pleural effusion, sometimes called fluid on the lung, can happen for various reasons, ranging from an infection to cancer. It does not usually have specific symptoms, but you may have signs of an infection or experience chest pain or difficulty breathing.

Seek medical advice if you notice symptoms that affect your lungs or breathing or if you have other symptoms, such as a fever. A doctor can help identify the cause of the problem and provide appropriate treatment. Pleural effusion can indicate a serious underlying condition that may be essential to address.