What Is Phlegm?

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Phlegm is the mucus secreted by the glands in the respiratory passages of the lungs. It is also called sputum.

This article will go over what phlegm is, why your body makes it, and why you might have more or less of it than usual.

A man coughing into a napkin
Daniel Allan / Getty Images

What Is Mucus?

There are three main parts, or layers, to the mucus in the airways.

The bottom layer is called the sol phase or periciliary layer. It’s next to the airways. There are hair-like structures that beat the mucus towards the throat out of the airway.

These structures are called cilia. The cilia are surrounded by water and minerals (electrolytes).

Then there is a thin layer of oil (lipids) and protein on top of the periciliary layer. This layer has a substance on it called surfactant, which reduces friction. Surfactant helps mucus spread through the airways more easily.

The top layer of mucus is mostly made of water and a substance called mucin. This substance makes the water gel-like.

Mucin is secreted in the periciliary layer by goblet cells next to the cilia.

Phlegm vs. Other Mucus

The body makes different kinds of mucus, but they are not all phlegm. The mucus that comes out of your nasal passages is sometimes confused with phlegm, but they are not the same thing.

The word “phlegm” is used to talk about mucus in your lungs.

For example, the “snot” from a runny nose or postnasal drip is not phlegm. However, if phlegm is being produced while a person also has postnasal drip, the secretions can mix.

What Mucus Does

When the airways are functioning normally, mucus gathers up particles and debris.

The cilia in the airway beat in a wave-like motion to push the mucus toward the throat. Once the mucus is in the throat, it gets swallowed and digested.

This process is one of the body’s most important strategies for protecting the lungs.

What Does Phlegm Color Mean?

It’s a myth that the color of your phlegm can tell you if you need antibiotics. However, there are some things that you can learn by looking at the color of your phlegm.

Here are a few examples of what different colored phlegm might mean:

  • Bright red or dark red phlegm is also called hemoptoic sputum. Bright red phlegm usually means there is fresh blood in the airway, while dark red phlegm means the blood is older.
  • Pink phlegm that is frothy can be a sign of fluid and swelling in the lungs (pulmonary edema).
  • White or yellow phlegm happens when your body’s immune system is fighting an infection, whether it’s bacterial or viral. It can also happen if you have allergies.
  • Green phlegm can occur if you have an infection. However, you won’t be able to tell what kind of infection you have just by looking at your phlegm. You’ll need to see your provider to find out what is causing the infection and to get the right treatment.
  • Brown phlegm often happens in people with chronic lung disease. It usually is caused by a mix of blood and bacteria.
  • Black or gray phlegm is also called melanoptysis. It can be caused by coal workers’ pneumoconiosis, which happens when coal dust gets trapped in the lung tissue.

Emergency Signs

If you have an infection, you might see blood-streaked phlegm when you cough into a tissue. While it might be startling to see, a small amount is not necessarily a sign of a serious problem.

The blood on a tissue might not be from your lungs—it could also be from your nose or even a cut in your mouth.

If the blood is coming from your lungs, it might look bubbly because it’s mixed with air.

However, if you’re coughing up more than a few teaspoons of blood, you need to seek medical attention right away.

If you’re also having trouble breathing or have signs of an infection (like a fever), you need emergency care.

Diseases That Affect Phlegm

Many diseases and certain medications can affect how your body makes phlegm.

Asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis can cause changes to your phlegm. They can increase the amount of phlegm and make it thicker and harder to clear.

Changes in the bottom layer of mucus affect how phlegm moves to the throat. They can also change the thickness (viscosity) of the phlegm.

If the phlegm becomes thick, you’ll need a strong cough to help move it into your throat.

When you can’t clear the phlegm, it can get stuck in your airway and make it harder for you to breathe.

Medications That Thin Phlegm

If you have thick phlegm that you can’t clear with coughing, you might have to take medication to help thin it out.

Here are a few examples of medications that can thin phlegm:

  • Antibiotics: If you have thick phlegm from a bacterial infection, your provider might prescribe a course of antibiotics.
  • Expectorants: These medications (such as guaifenesin) help you to cough the phlegm up. Many of them are available over the counter (OTC) at your local pharmacy.
  • Mucolytic: These medications (like acetylcysteine) thin mucus and make it easier to cough up.


The body makes different kinds of mucus. Phlegm is one important type of mucus. It is important to the normal functioning of your airways.

While they have some similarities, phlegm is not the same as the “snot” that you see when you have a runny nose. However, phlegm and other kinds of mucus can happen at the same time.

Sometimes, phlegm can create problems. For example, if you have an infection and the phlegm gets thick, it can be hard to cough up.

If this happens, it can affect your breathing. You might need to take medications to help thin the mucus and make it easier to clear.

10 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.
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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

By Kristin Hayes, RN
Kristin Hayes, RN, is a registered nurse specializing in ear, nose, and throat disorders for both adults and children.