1. What Is Phlegm Made Of?
Doctors often use the word sputum for phlegm. It means mucus (and whatever is mixed into it) that comes up from the lower airways, the tubes that lead down into the lungs, when you cough. Spit and the thin liquid from your nose are different things.
The starting material is airway mucus. A review in the New England Journal of Medicine describes it as a gel that is mostly water: normal mucus is about 97 percent water and about 3 percent solids. The solids include mucins (long, sticky protein molecules that give mucus its stretchiness), other proteins, salts, fats and bits of cells [2]. That 97 percent is for healthy mucus, not for the phlegm you cough up.
So phlegm is best thought of as healthy-style mucus plus whatever it has caught or collected along the way.
2. Why Do Healthy Airways Make Mucus All the Time?
The air we breathe can carry dust, soot and germs into the airways. The airways answer with a sticky layer of mucus that traps these particles. The cells lining the airways also have tiny, hairlike brushes called cilia. They beat more than 1,000 times a minute, pushing the mucus layer along like a very slow conveyor belt. The mucus, with its catch, is then either coughed out or moved up toward the mouth and swallowed [1].
The same review calls this cleaning job, done by cilia and by coughing, essential for removing inhaled particles and germs [2]. That is why you can have mucus all day and almost never see any: most of it is quietly swallowed.
3. What Protects the Deepest Part of the Lungs, Where There Is No Mucus?
The tiny air sacs at the very end of the airways, called alveoli, are where oxygen passes into the blood. They have no mucus layer and no cilia [1]. A thick coating there would get in the way of moving gases, which is a reasonable guess for why they are built this way, though that is an explanation to picture, not a quotation from the source.
Instead, white blood cells called alveolar macrophages patrol the air sacs. They find particles that land there, grab them, and swallow them. Macrophages can engulf and break down many particles, including some germs. When the threat is serious, more white blood cells, especially neutrophils, can be called in from the blood [1]. So the airway has two lines of defense: mucus and cilia in the tubes, and patrolling cells in the sacs.
4. What Does the Color of Phlegm Actually Show?
Many people learn that clear phlegm is fine, while yellow or green means bacteria. Researchers have tested this idea, and the picture is more modest.
A British team studied people with long-term airway diseases (chronic bronchitis and bronchiectasis). They matched each sputum sample against a nine-step color chart and compared the colors with chemicals in the sputum. The darker the color, the more of a neutrophil enzyme called myeloperoxidase and a signaling chemical called interleukin 8 it contained, and the stronger the other signs of inflammation [3]. In plain words, in these patients darker sputum went with stronger signs of inflammation, including signs tied to neutrophils, a kind of white blood cell. That is not the same as tracking bacteria, and these were people with long-term lung disease, so the result cannot simply be applied to a child with an ordinary cold.
The bacteria question was tested in a German study of 241 adults who went to a family doctor with a recent cough. A sample counted as a bacterial infection only if a lab culture grew bacteria and the sample also had a fair number of white blood cells. By that rule, 28 samples (12 percent) were bacterial. Yellow or green phlegm was linked to bacterial infection, but weakly. Of the 136 yellow or green samples, 22 (about 16 percent) were bacterial, compared with 6 of 105 colorless ones (about 6 percent). That means roughly 84 percent of yellow or green samples were not bacterial. The authors concluded that, for people without chronic lung disease, color should not decide whether to use antibiotics [4].
The percentages above come from dividing the study's counts. The study's own headline numbers (sensitivity 0.79, specificity 0.46) agree with them.
5. So How Should You Think About Phlegm and Color?
Healthy airways make mucus all day, cilia and coughs carry it up, and you swallow it. Phlegm is the part that comes out in a cough, with dust, germs, and cells mixed in. Color is a loose clue about how much inflammation is going on, not a test for bacteria.
If you want to dig deeper, good questions to chase: why does phlegm get thicker when you are dehydrated or ill? Why do some people cough up phlegm every morning for years? The review by Fahy and Dickey is a good place to begin, and the Merck Manual page on the lung's defenses is written for general readers. Whatever the color, a cough or phlegm that lasts or comes with trouble breathing is something to tell an adult and a doctor about. This article describes research, not medical advice.