Phlegm is normally clear or white. When it changes color, it’s your immune system at work, and each shade offers a rough clue about what’s going on in your airways. Here’s what each color typically signals and when the change actually matters.
Clear or White Phlegm
Clear or white phlegm is normal. Your airways constantly produce mucus to trap dust, allergens, and other particles, and this baseline mucus is thin and colorless. You might notice more of it during allergy season, after exposure to cold air, or when your asthma flares up.
Several chronic conditions can increase the volume of clear or white phlegm without any infection present. Allergies, asthma, COPD, and chronic sinusitis all fall into this category. Acid reflux (GERD) is another common culprit: stomach acid irritates the upper airways, and the body responds by producing extra mucus as a protective barrier. If you have asthma and start coughing up hard, sticky chunks of white phlegm, that’s a sign your condition isn’t well controlled.
Why Phlegm Turns Yellow or Green
Yellow or green phlegm means your immune system has sent white blood cells called neutrophils to fight something off, usually an infection. These cells contain an enzyme with a vivid green pigment (originally named “verdeperoxidase” for its color). When neutrophils flood your airways and break down, they release this green-tinted enzyme into the surrounding mucus. A lower concentration looks yellow; a higher concentration looks deep green.
Here’s the important nuance: green or yellow phlegm does not automatically mean you need antibiotics. A large study published in the Scandinavian Journal of Primary Health Care found that the color of sputum cannot reliably distinguish a bacterial infection from a viral one in otherwise healthy adults. Among people coughing up discolored phlegm, only about 16% actually had a confirmed bacterial infection. The CDC’s clinical guidelines echo this, stating plainly that colored sputum does not indicate bacterial infection and that routine antibiotic treatment for uncomplicated acute bronchitis is not recommended, regardless of how long the cough lasts.
Most colds and cases of bronchitis are viral. Your phlegm will cycle through yellow and green shades as your immune response ramps up and then winds down, all without bacteria being involved.
Pink, Red, or Bloody Phlegm
Red or pink phlegm means blood is mixing with your mucus. The most common cause is simple irritation: a forceful cough can rupture tiny blood vessels in your airways, leaving streaks of blood in otherwise normal phlegm. Bronchitis and pneumonia can also produce blood-tinged mucus as inflamed tissue bleeds slightly.
Less common but more serious causes include lung cancer, tuberculosis, pulmonary embolism (a blood clot in the lung), COPD, and cystic fibrosis. A chest injury or inhaling a foreign object can also cause bloody phlegm. Even certain drug use, particularly cocaine, can damage airway tissue enough to produce it.
A single streak of blood after a bad coughing fit usually isn’t an emergency. But if you’re repeatedly coughing up red or pink phlegm, coughing up more than a teaspoon of blood, or the bleeding comes with fever, shortness of breath, or chest pain, that warrants prompt medical attention.
Brown Phlegm
Dark brown, sticky phlegm is often associated with old blood that has oxidized before being coughed up. It can appear a day or two after an episode of bleeding in the airways, once the blood has had time to break down.
In chronic lung diseases like cystic fibrosis and bronchiectasis, brown phlegm is more persistent. The color comes from a combination of blood and the intense, ongoing inflammation these conditions produce. Heavy smokers also sometimes cough up brownish mucus, as tar and resin from cigarette smoke accumulate in the airways and mix with phlegm over time.
Black or Gray Phlegm
Black phlegm, sometimes called melanoptysis, is uncommon and almost always tied to something you’ve been inhaling. Coal miners and factory workers exposed to heavy dust or soot develop it as dark particles deposit in the lungs and eventually get coughed out. The same mechanism explains the charcoal or gray phlegm seen in very heavy smokers and crack cocaine users, where carbonaceous material darkens both the airways and the mucus.
Rarely, black phlegm has an infectious cause. Certain fungal infections, particularly one caused by a black yeast that tends to affect people with cystic fibrosis, can produce dark-pigmented sputum. Another fungal organism can cause it in people with COPD. In very rare cases, a type of lung tumor that produces dark pigment or the drainage of a damaged, fibrosed mass in the lung can also result in blackish secretions.
What Phlegm Color Can and Cannot Tell You
Phlegm color is a useful signal, not a diagnosis. It tells you your body is responding to something, whether that’s an allergen, a virus, a bacterial infection, or an environmental irritant. But it can’t tell you exactly which one. The widespread belief that green phlegm means you need antibiotics has been studied and debunked: the positive predictive value of discolored sputum for bacterial infection is only about 16%, which is barely better than a coin flip.
What matters more than color alone is the combination of symptoms and how long they last. A cough lasting more than two weeks, phlegm that stays discolored rather than clearing up, a fever that won’t break, shortness of breath, or fatigue and leg weakness are all signs that something beyond a routine cold is happening. The color of your phlegm is one piece of that puzzle, not the whole picture.