Healthy phlegm is clear, thin, and slippery. Your body produces it constantly to keep your airways moist and trap dust, allergens, and germs before they reach your lungs. Most of the time you swallow it without noticing. When phlegm changes color, thickness, or volume, it’s your body signaling that something is going on, whether that’s a mild cold or something more serious.
What Phlegm Is Made Of
Phlegm is mostly water mixed with proteins, enzymes, antibodies, and white blood cells. A specific protein called mucin gives it that gel-like, slightly sticky consistency. These components work together as a defense system: antibodies and white blood cells neutralize bacteria and viruses, while the sticky texture traps particles so your airways can sweep them upward and out.
Clear Phlegm
Clear phlegm is the baseline. It means your respiratory lining is doing its normal job. You can produce large amounts of clear phlegm without being sick. Allergies are one of the most common causes of a sudden increase. Pollen, pet dander, or dust can trigger your airways to ramp up mucus production, giving you a runny nose or a feeling of drainage in the back of your throat. Early-stage viral infections also start with clear, watery phlegm before the color shifts over a day or two.
White Phlegm
White or cloudy phlegm usually means congestion. When your nasal passages or airways swell, mucus moves more slowly, loses water content, and thickens. This is common with upper respiratory infections, sinus congestion, and acid reflux. White phlegm on its own isn’t a sign of a bacterial infection. It typically signals that inflammation is building but your immune system hasn’t fully mobilized yet.
Yellow and Green Phlegm
Yellow and green are the colors people worry about most, and they’re often misunderstood. The color comes from white blood cells called neutrophils rushing to fight an infection. These cells contain a protein that makes up about 5% of their total mass. When neutrophils activate and break down at the infection site, this protein releases a green pigment. A lighter yellow means fewer neutrophils are present. Deeper green means more have arrived and the immune response is intense.
Here’s the important part: green phlegm does not automatically mean you have a bacterial infection or need antibiotics. The CDC states this directly. Your body produces green phlegm in response to viral infections just as readily as bacterial ones. A cold that turns your phlegm green for a few days and then clears up is following a completely normal course. The color alone isn’t enough for a diagnosis.
Red, Pink, or Rust-Colored Phlegm
Any shade of red in your phlegm means blood is mixing in somewhere along the respiratory tract. It often looks bubbly or frothy rather than solid red. Small amounts of pink or rust-colored streaks are common and usually not dangerous. Forceful coughing can rupture tiny blood vessels in your throat or airways, producing a few streaks. Dry air, especially in winter, can also irritate tissues enough to cause minor bleeding.
The most common medical causes of blood-tinged phlegm are bronchitis and pneumonia. Less common but more serious causes include tuberculosis, blood clots in the lung, and lung cancer, particularly in smokers over 40. If you’re coughing up more than a small amount of blood, or it happens repeatedly over several days, that warrants prompt medical evaluation.
Brown Phlegm
Brown phlegm typically means old blood or inhaled particles. For current or former smokers, this is one of the most recognizable colors. Within about a week of quitting, the tiny hair-like structures in your lungs (which smoke paralyzes) start working again and begin sweeping out accumulated tar. The result is brown or dark-flecked phlegm that can persist for weeks as the lungs clean themselves out.
Brown phlegm also shows up with fungal exposure. A common soil fungus called Aspergillus can inflame the lungs when inhaled by people who are allergic to it, producing brown-speckled mucus along with wheezing. Lung abscesses, which are pockets of infected tissue, can cause foul-smelling brown or blood-specked phlegm. Bacterial pneumonia can also produce rust-brown sputum as red blood cells break down in the infected area.
Very Dark Brown or Black Phlegm
Truly dark or black phlegm is rare and almost always tied to heavy environmental exposure. Coal miners, firefighters, and heavy smokers are the most likely to see it. Certain fungal infections can also darken phlegm significantly. People with chronic lung conditions like cystic fibrosis or bronchiectasis sometimes cough up very dark, sticky phlegm that looks different from the typical yellow or green. This thick, tar-like consistency reflects long-standing inflammation and repeated infections in the airways.
What Texture Tells You
Color gets the most attention, but thickness and texture carry information too. Thin, watery phlegm points to allergies or an early viral infection. Thick, sticky phlegm suggests dehydration of the mucus layer from congestion or prolonged inflammation. Frothy or foamy phlegm, especially if pink-tinged, can indicate fluid buildup in the lungs and is worth taking seriously.
One common piece of advice is to drink more water to thin your phlegm. The evidence for this is surprisingly weak. A study in the journal CHEST tested hydration levels in patients with chronic lung disease and found no significant difference in sputum volume, elasticity, or ease of coughing it up between hydrated and dehydrated conditions. Staying well-hydrated is good general advice, but it probably won’t change much about the phlegm you’re producing during an illness.
When Phlegm Color Matters
A temporary change in phlegm color during a cold is normal and resolves on its own. What matters more is the timeline and accompanying symptoms. Colored phlegm lasting longer than two weeks without improvement, phlegm paired with a persistent fever, or coughing up phlegm when you don’t otherwise feel sick are all patterns that deserve a closer look. Coughing up phlegm without any obvious illness can sometimes point to an underlying heart or lung condition that isn’t producing other obvious symptoms yet.