What Is Phlegm?

Phlegm (pronounced flem; from Greek phlegma = inflammation) is an informal name for mucus coughed up from the throat. In this article, a term phlegm will be used only for mucus produced in the mucous layer of bronchi and windpipe (trachea). Medical term for phlegm is expectorated matter or sputum.

Phlegm appears as thick, jelly-like fluid of various colors and consistency, depending on the cause.

Phlegm is composed mainly from water and glycoproteins secreted by mucous glands.

Colors of Phlegm

Clear phlegm means there is no pus or blood in it. Yellow or green (not translucent) phlegm is mucus mixed with pus. Rusty or brown phlegm may be due to smoking, air pollution, blood or infection. Blood in phlegm may appear as read streaks. Pink phlegm may be from asthma. Frothy phlegm originates from the lungs. Read more about phlegm colors.

Symptoms Caused by Phlegm

Phlegm may irritate the lower airways and trigger cough. Cough and constant movements of tinny hairy-like projections (cilia) in the lining of the lower airways push phlegm into the throat, from where it can be coughed up or swallowed. Swallowed mucus is degraded by bowel bacteria that produce gas, what may result in abdominal bloating.

Acute Diseases with Phlegm

Phlegm may be produced in the following acute diseases:

  • Chest cold (acute bronchitis) - an infection of the windpipe (trachea) or airways that lead to the lungs (bronchi). It is often preceded by a common cold. Itchy throat, coughing up clear phlegm and moderate chest pain during coughing are main symptoms, which usually resolve within two weeks. There is usually no or only moderate fever.
  • Influenza (flu), including swine flu, can present with clear phlegm, high fever, runny nose, sore throat, headache and muscular pains.
  • Bacterial pneumonia, coughing up yellow or green frothy phlegm, high fever and rapid breathing are main symptoms. In viral pneumonia, dry cough is characteristic, though.

Chronic Diseases with Phlegm

Coughing up phlegm may be a long lasting problem:

  • In chronic bronchitis, mostly caused by smoking or air pollution, coughing up thick phlegm is the main symptom. In severe bronchitis, phlegm may obstruct the bronchi, resulting in lack of oxygen in the blood (hypoxemia), reflected in bluish lips and skin around the mouth (cyanosis).
  • In tuberculosis, yellow or green phlegm, often with blood, may be coughed up. Low grade fever, coughing, low appetite and generally feeling ill are main symptoms that usually lasts over 3 weeks.
  • In asthma, an allergic inflammation of the bronchi, a small amount of clear or pink phlegm may be produced and coughed up along with difficult breathing.
  • Certain types of lung cancer produce mucus. Chest pain, coughing up blood or yellow phlegm, low appetite and losing weight are main but late symptoms.

Phlegm in Small Children

  • Acute viral infections, like chest cold or influenza are main causes of coughing up phlegm in small children.
  • A baby with cystic fibrosis, a rare genetic disease, can cough up thick yellow or green phlegm, and have blocked nose, diarrhea, and respiratory infections shortly after birth and later in life. 

Does Drinking Milk Cause Phlegm?

Drinking milk does not cause phlegm, but it may make it thick and thus hard to expel.

Other Causes of Mucus in the Throat

Mucus produced in the nose or peri-nasal sinuses and dripped down the throat is informally called post nasal drip or, medically, catarrh.

Bacterial infection of tonsils or epiglottis (membrane that covers the voice box during swallowing) usually causes high fever, strong pain in the throat, enlarged lymph nodes in the neck and white-yellow pus that covers tonsils and the throat walls.


Article reviewed by Dr. Greg. Last updated on October 28, 2009