The Function of Phlegm

Phlegm is the mucus secreted by the glands in the respiratory passages of the lungs. There are 3 main parts to the mucus in the airways. The bottom layer, referred to as the sol phase or periciliary layer, is adjacent to the airways. This layer contains cilia, which are hair-like structures that beat the mucus towards the throat out of the airway. Surrounding the cilia is water and electrolytes.

A man coughing into a napkin
Daniel Allan / Getty Images

On top of the periciliary layer is a thin layer of lipids (oil) and protein. This layer known as surfactant is a substance that the airways secrete to reduce friction, which in this case helps the top layer of mucus to spread throughout the airways more easily.

The top layer of mucus is a gel made mostly of water combined with mucin, which makes the water gel-like. Mucin is secreted by goblet cells that are next to the cilia in the periciliary layer.

What Phlegm Is Not

While the body produces different kinds of mucus, phlegm is not snot or mucus produced and excreted from the nasal passages, as in the cases of rhinitis or postnasal drip. However, phlegm may contain some nasal secretions if phlegm is produced while post nasal drip is occurring.

Normal Function of Airway Mucus

When the airways are functioning normally, the mucus in the airways accumulates particles and debris. The cilia in the airway then propel the mucus towards the throat by beating in a wave-like motion. Once the mucus is at the level of the throat, you typically unknowingly swallow the mucus where it is digested in the stomach. This is one of the most important methods to protect the lungs.

What Does the Color of My Phlegm Tell Me?

There are some false rumors that the color of your phlegm could tell you if you need antibiotics or not. This is most certainly not the case. However, this is what the color of your phlegm can tell you:

  • Bright red or dark red colored phlegm, also known as hemoptysis, usually represents fresh or old blood respectively.
  • Pink colored phlegm that is frothy usually represents pulmonary edema (fluid and swelling in the lungs).
  • White or yellow colored phlegm happens when your body's immune system cells start to fight an infection, regardless of the source (bacterial or viral).
  • Green colored phlegm may occur as a result of infection.
  • Black colored phlegm, also known as melanoptysis, can be caused by coal worker's pneumoconiosis. This is caused by coal dust becoming trapped in the lung tissue.

More than a few teaspoons of blood is an emergency and you should seek medical attention immediately. More commonly, you will see blood-streaked sputum, which can represent an ongoing infection in the lungs.

Diseases That Can Affect Phlegm

Many diseases can affect phlegm production. This normally occurs by changes in the bottom layer of mucus which affects the movement of phlegm to the throat, or by changing the thickness (viscosity) of the phlegm. Disorders such as asthma, COPD, or cystic fibrosis can cause these changes.

When the phlegm becomes thick, it is important to have a good strong cough to assist in moving the phlegm into the throat. If you are unable to move thickened phlegm into the throat, you are at risk of having the phlegm occlude your airways. If you are experiencing increased difficulty breathing, you should seek medical attention immediately.

Medications That Can Help the Thickness of Phlegm

Thick phlegm can be difficult to cough up. You can try these types of medications to help:

  • Antibiotics: if the reason is related to a bacterial infection.
  • Expectorants: helps you to cough the phlegm up (i.e., guaifenesin)
  • Mucolytic: thins mucus (i.e., acetylcysteine)
Was this page helpful?
5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Lillehoj EP, Kato K, Lu W, Kim KC. Cellular and molecular biology of airway mucins. Int Rev Cell Mol Biol. 2013;303:139-202. doi:10.1016/B978-0-12-407697-6.00004-0

  2. Bills G, Rose C. Principles of Pharmacology for Respiratory Care. Sudbury, MA: Jones & Bartlett Learning; 2019.

  3. Fahy JV, Dickey BF. Airway mucus function and dysfunction. N Engl J Med. 2010;363(23):2233-47. doi:10.1056/NEJMra0910061

  4. Seaton A, Leitch G, Seaton D. Crofton and Douglas's Respiratory Diseases. Hoboken, NJ: Wiley; 2008.

  5. Scaglione F, Petrini O. Mucoactive agents in the therapy of upper respiratory airways infections: fair to describe them just as mucoactive?. Clin Med Insights Ear Nose Throat. 2019;12:1179550618821930. doi:10.1177/1179550618821930

Additional Reading
  • American Thoracic Society. (2015). What Are the Signs and Symptoms of COPD?

  • Centers for Disease Control and Prevention. (n.d.). Runny Nose (with green or yellow mucus).

  • Martínez-girón R, Mosquera-martínez J, Martínez-torre S. Black-Pigmented Sputum. J Cytol. 2013;30(4):274-5.

  • Nadel, J.A. (2016). Airway Epithelium and Mucous Secretion. Murray and Nadel's Textbook of Respiratory Medicine. 6th ed. 10, 157-167.

  • National Health Service. (2015). Coughing up blood (blood in phlegm).

  • Rogers, D.F. (2014). Airway Mucus and the Mucociliary System. Middleton's Allergy: Principles and Practice. 47, 739-753

  • Rubin, B.K. (2002). Physiology of Airway Mucus Clearance. Respir Care. 47(7), 761-8

  • Barnes, P.J., Drazen, J.M., Rennard, S.I. & Thomson, N.C. (2002). Asthma and COPD: Basic Mechanisms and Clinical Management. Academic Press : Elsevier