Mucus: Causes of Green, White, Yellow, Brown, Grey, Black Phlegm

Mucus is naturally produced by mucous cells that are present in the linings of various tracts and cavities in the body. One of the main reasons for this mucus is provide moisture and lubrication although it may have additional purposes like trapping dust and microbes (airways). Normally mucus is clear to slightly opaque but there are various reasons why mucus may become discolored.

Colors of Mucus

Phlegm may be clear (white), yellow/green, brown, grey, black, pink, orange, containing red streaks, or frothy. Coughing up phlegm is mucus (sputum) that originates from the airways or lungs. It reveals inflammation in the respiratory organs below the vocal cords (voice box, windpipe, bronchi and lungs). The color, thickeness and even odor of the mucus may be helpful in diagnosing the exact condition that may cause excessive mucus.


Clear or Thin White Mucus

Clear or cloudy (white), thin and translucent mucus means there is no (or not much) pus or blood in it. Causes of clear white phlegm include:

  • Acute viral bronchitis (chest cold) is inflammation of bronchi (airways between windpipe and lungs), sometimes affecting persons with common cold, influenza or other viral respiratory infection. Low-grade fever, runny nose, itchy throat, hoarseness, wheezing and initially dry and then productive cough with clear mucus may last from several days to several weeks. Mucus can, in some days, turn to yellow or green (1).
  • Acute bronchitis caused by airborrne irritant gases (environmental or occupational) resembles viral bronchitis but usually there is no fever.
  • Current or former smokers with chronic bronchitis may cough up white phlegm. White mucus in chronic bronchitis does not require antibiotic treatment (1).
  • Asthma is where there is narrowing of the airways (bronchi) due to spasm of the muscles in the bronchial walls as well as an accumulation of mucus. This is often related to an allergic (hypersensitive) response.

Thick White Mucus

Main causes of thick, non-translucent and white mucus includes:

  • Drinking milk or other thick drinks can make originally clear mucus thick. Milk may cover the back of the throat, but milk itself does not cause mucus.
  • In gastroesophageal reflux disease (GERD), gastric acid may back up from esophagus into the windpipe and throat. Burning stomach, chest (behind the breastbone), and throat, and coughing up white, thick, often frothy mucus are main symptoms. Often, white mucus is the only symptom in GERD.


Yellow Mucus

Mucus is yellow, when it contains inflammatory cells, mainly leukocytes. Thin yellow mucus can result from any cause of inflammation of respiratory tract: infection, allergy, asthma, irritation. Thick yellow mucus speaks for infection – viral or bacterial. It is a regular symptom in acute or chronic bronchitis or bacterial pneumonia.

Brown or Rusty Mucus

Brown mucus can be due to:

  • Chocolate, cocoa or other foods that stain mucus brown
  • Smoking – from resins and tars in smoke
  • Inhaling dust or smog
  • Infection, especially in chronic bronchitis
  • Blood, arising from the lungs, bronchi, windpipe or voice box

Grey Mucus

Grey mucus can be caused by air pollution , cigarette or marijuana smoking.

Black Mucus

Black mucus can be due to:

  • Smoking marijuana, cigarettes, cocaine or other substances. It is not possible to reliably say, if someone is smoking marijuana or cigarettes on the basis of the smoker mucus color. A smoker who has quit smoking may cough up mucus (black or other colors) for several days or weeks after quitting (4).
  • Air pollution
  • Coal workers pneumoconiosis or anthrosilicosis (black lung disease)
  • Fungal (mold) lung infection, like allergic broncopulmonary aspergillosis (ABPA), causing bronciectasis (3)
  • Old blood (5) from COPD (bronchitis or emphysema), tuberculosis, cancer or other bronchial or pulmonary disorder

NOTE 1: vomiting black liquid is a symptom of bleeding from the stomach and has to be checked by a doctor as soon as possible.

Blood in Mucus

Blood in mucus appears as red strikes, brown mesh or obviously red blood.

Pink Mucus

Pink mucus in asthma contains eosinophils (a type of white blood cells appearing in bronchial wall in certain allergies).

Frothy Mucus

Frothy mucus originates from lungs. In pneumonia or lung edema, fluid washes some surfactant (a substance that keeps lung vesicles dilated) from vesicular walls into inflammatory fluid and makes it frothy. Frothy mucus may be also caused by gastroesophageal reflux disease (GERD).

Green Mucus

The color of mucus may vary although it usually clear in color. Its if often mixed with saliva when passed out through the mouth (spitting or coughing).

Clear, thin and watery mucus (serous) or clear to white, thick mucus (mucoid) is often accepted as the normal color and may be seen in the early stages of an infection or other inflammatory conditions. Pink to red or rust colored mucus is a sign of blood within the mucus and is usually associated with a severe infection or inflammatory response.

Why is mucus green?

Green mucus is a result of dead neutrophils, a type of white blood cell active in infectious and inflammatory responses, within the sputum. When neutrophils die and burst (lysis), it releases a green pigment known as verdoperoxidase. This is an enzyme responsible for the perioxidase activity of pus.

Initially, in the acute stage of an infection or inflammation, the presence of live neutrophils causes the mucus to be yellow in color. Another type of white blood cell known as an eosinophil, which is often seen in allergy-related conditions like asthma, also causes mucus to be yellow in color. As the infection or inflammation becomes longer lasting (sub-acute or chronic), the lysis of neutrophils results in green mucus.

There is a common misconception that yellow to green mucus is associated specifically with bacterial infections. Neutrophil activity is seen in the presence of any invading pathogen, whether it is bacteria, viruses or fungi. Prolonged inflammatory responses, even if not due to infectious causes, will also result in green mucus.

Causes of Green Mucus

Mucus is often secreted in excess in certain conditions, especially in respiratory tract infections. It may originate from from the nose of mouth (catarrh) or from lower down the respiratory tract (sputum, phlegm). Rarely, it may emanate from the upper esophagus.

  • Colds
  • Flu (infleunza)
  • Hay fever
  • Sinusitis
  • Acute bronchitis
  • COPD , particularly in chronic bronchitis
  • Bronchiectasis
  • Pneumonia
  • Lung abscess
  • Cystic fibrosis


  1. Acute bronchitis.
  2. Airway mucus. PubMed