Phlegm (/ˈflɛm/; Greek: φλέγμα "inflammation, humour caused by heat") is a liquid secreted by the mucous membranes of mammalians. Its definition is limited to the mucus produced by the respiratory system, excluding that from the nasal passages, and particularly that which is expelled by coughing (sputum). Phlegm is in essence a water-based gel consisting of glycoproteins, immunoglobulins, lipids and other substances. Its composition varies depending on climate, genetics, and state of the immune system. Its color can vary from transparent to pale or dark yellow and green, from light to dark brown, and even to dark grey depending on the constituents.

Distinction between mucus and phlegm

Contrary to popular misconception and misuse, mucus and phlegm are not always the same.


Mucus is a normal protective layering around the airway, eye, nasal turbinate, and urogenital tract. Mucus is an adhesive viscoelastic gel produced in the airway by submucosal glands and goblet cells and is principally water. It also contains high-molecular weight mucous glycoproteins that form linear polymers.


Phlegm is more related to disease than is mucus. Phlegm is a secretion in the airway during disease and inflammation. Phlegm usually contains mucus with bacteria, debris, and sloughed-off inflammatory cells. Once phlegm has been expectorated by a cough it becomes sputum.[1]

Excessive phlegm creation

There are multiple factors that can contribute to an excess of phlegm in the throat or larynx.

  • Vocal abuse: Vocal abuse is the misuse or overuse of the voice in an unhealthy fashion such as clearing the throat, yelling, screaming, talking loudly, or singing incorrectly.
    • Clearing the throat: Clearing the throat removes or loosens phlegm but the vocal cords hit together causing inflammation and therefore more phlegm.[2]
    • Yelling/screaming: Yelling and screaming both cause the vocal chords to hit against each other causing inflammation and phlegm.[3][4][5][6][7]
    • Nodules: Excessive yelling, screaming, and incorrect singing as well as other vocal abusive habits can cause vocal nodules. See vocal fold nodule for more information on nodules.
  • Smoking: Smoke is hot, dry, polluted air which dries out the vocal cords. With each breath in of smoke, the larynx is polluted with toxins that inhibit it from rehydrating for about 3 hours.[8] The vocal cords need a fair amount of lubrication and swell from inflammation when they do not have enough of it. When the vocal folds swell and are inflamed, phlegm is often created to attempt to ease the dryness.[9][10]
    • Experiment on smoking correlations: In 2002, an experiment was done and published by the American College of Chest Physicians to find if there was a correlation of smokers with coughing and phlegm. In the study, 117 participants were studied, a mix of current smokers, ex-smokers, non-smokers, and a positive control of participants with a disease, COPD (Chronic Obstructive Pulmonary Disease.) At the end of the experiment, experimenters found that there was a high correlation between phlegm and cough with smoking of 0.49 (p<.001.)[11]
  • Illness: During illness like the flu and cold, phlegm becomes more excessive as an attempt to get rid of the bacteria or viral particles within the body. A major illness associated with excess phlegm is acute bronchitis. A major symptom of acute bronchitis is an excess amount of phlegm and is usually caused by a viral infection, and only bacterial infections, which are rare, are to be treated with an antibiotic.[12][13]
  • Hay fever, asthma: In hay fever and asthma, inner lining in bronchioles become inflamed and create an excess amount of phlegm that can clog up air pathways.[14]
  • Air pollution: In studies of children, air pollutants have been found to increase phlegm by drying out and irritating parts of the throat.[15]

Illnesses related to phlegm

Phlegm may be a carrier of larvae of intestinal parasites (see hookworm). Bloody sputum can be a symptom of serious disease (such as tuberculosis), but can also be a relatively benign symptom of a minor disease (such as bronchitis). In the latter case, the sputum is normally lightly streaked with blood. Coughing up any significant quantity of blood is always a serious medical condition, and any person who experiences this should seek medical attention.

Phlegm and humourism

Humourism is an ancient theory that the human body is filled with four basic substances, called the four humours, which are held in balance when a person is healthy. It is closely related to the ancient theory of the four elements and states that all diseases and disabilities result from an excess or deficit in black bile, yellow bile, phlegm, and blood. Hippocrates, an ancient Greek medical doctor, is credited for this theory, about 400 BC. It influenced medical thinking for more than 2,000 years, until finally discredited in the 1800s.

Phlegm was thought to be associated with apathetic behaviour; this old belief is preserved in the word "phlegmatic". This adjective always refers to behaviour, and is pronounced differently, giving full weight to the "g": not /ˈflɛmatɪk/ but /flɛgˈmatɪk/.[16]

The phlegm of Humourism far from the same thing as phlegm as it is defined today. Nobel laureate Charles Richet MD, when describing humorism's "phlegm or pituitary secretion" in 1910 asked rhetorically, "this strange liquid, which is the cause of tumours, of chlorosis, of rheumatism, and cacochymia - where is it? Who will ever see it? Who has ever seen it? What can we say of this fanciful classification of humours into four groups, of which two are absolutely imaginary?"[17]

Other concepts

Sir Johns Hopkins Medical Institutions."


Further reading

This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.