Brown phlegm usually means one of three things: old blood, trapped tar from smoking, or ongoing inflammation deep in the lungs. It’s less common than clear, yellow, or green mucus, and the cause depends heavily on whether you smoke, how long the brown phlegm has lasted, and what other symptoms you’re experiencing.
Smoking and Quitting Smoking
The most common reason for brown phlegm is tobacco use. Cigarette smoke coats the inside of your airways with tar and resin, and your lungs have tiny hair-like structures that constantly sweep mucus and debris upward toward your throat. Smoking paralyzes these structures over time, so tar accumulates in your airways with no way to clear it out.
If you’ve recently quit smoking, you may actually notice more brown phlegm than when you were still lighting up. Within about a week of your last cigarette, those tiny sweepers start working again and begin pushing out the built-up tar. This “cleanup phase” can produce dark brown, sticky mucus for weeks. It’s a sign your lungs are healing, not a sign something is wrong. Current smokers can also cough up brown-tinged mucus, especially during a respiratory infection that increases mucus production and loosens trapped particles.
Old Blood in the Airways
Fresh blood in mucus looks red or pink. When blood sits in the lungs or airways for a while before being coughed up, it oxidizes and turns brown or rust-colored. This can happen after a particularly forceful coughing spell that irritates the lining of the airways, or it can signal a more serious issue like pneumonia or a lung abscess.
Rust-colored sputum is a hallmark of bacterial pneumonia. Along with the discolored phlegm, pneumonia typically comes on suddenly with fever, chills, chest pain that worsens when you breathe, shortness of breath, and a general feeling of weakness. If you’re coughing up rust-colored or brown phlegm alongside these symptoms, it needs prompt medical attention.
Acute Infections
Bronchitis and pneumonia are the two infections most likely to produce brown mucus. Acute bronchitis, which is inflammation of the airways leading into the lungs, is most often caused by a virus. It can also be triggered by bacteria, smoking, or chemical irritants. The mucus it produces ranges from clear to cloudy, yellow, green, or brown, depending on how much inflammation and old blood are involved.
A lung abscess is a rarer but more serious possibility. This is a pocket of pus that forms when bacteria from the mouth or throat (sometimes from gum disease) travel into the lungs and create a walled-off infection. The telltale sign is foul-smelling, brown or blood-specked phlegm, often alongside fever and chest pain.
Chronic Lung Conditions
If you’ve been coughing up brown phlegm for weeks or months, a chronic lung condition may be the cause. Two in particular produce the kind of dark, sticky, clingy brown mucus that stands out from a typical infection.
Bronchiectasis is a condition where the airways become permanently widened and damaged, trapping mucus and creating a cycle of repeated infections and chronic inflammation. The brown color comes from a combination of old blood and the intense, ongoing inflammatory response. Cystic fibrosis produces a similar pattern. Both conditions cause persistent coughing, heavy mucus production, and recurring chest infections. During flare-ups, the phlegm often darkens and thickens, and antibiotics may be needed to bring the infection under control.
COPD, or chronic obstructive pulmonary disease, is another possibility, especially in long-term smokers. If your mucus shifts from its usual color to brown, yellow, or green, it can be an early warning sign of a flare-up that needs treatment.
Fungal Allergies and Environmental Exposure
A fungus called aspergillus lives in soil, plants, and decaying vegetation. Most people breathe it in without any problem, but if you’re allergic, it can inflame your lungs and cause wheezing along with brown-flecked mucus. This is more common in people who already have asthma or other lung conditions.
Occupational dust exposure is another environmental cause. Workers exposed to coal dust, silica, iron dust, or other mineral particles can develop a group of lung diseases that cause increased phlegm production and coughing. Coal workers’ pneumoconiosis (sometimes called black lung) is the most well-known example. The color of the phlegm often reflects the particles trapped in the lungs, and brown or dark-tinged mucus can result from years of inhaling fine dust on the job.
What the Color Tells You
Phlegm color is a useful clue but not a diagnosis on its own. Light brown phlegm that shows up briefly during a cold or after exposure to dusty air is rarely serious. Dark brown, sticky phlegm that persists for more than a couple of weeks, especially with shortness of breath, fever, or chest pain, points toward something that needs medical evaluation.
If your doctor wants to identify the exact cause, they’ll likely start with a sputum culture. You’ll cough a sample into a cup, and the lab checks it for bacteria or fungi. A related test called a Gram stain helps narrow down the type of organism involved. In cases where you can’t produce enough of a sample by coughing, a procedure called bronchoscopy lets the doctor collect mucus directly from your airways using a thin, flexible tube with a camera.
The context around your brown phlegm matters more than the color alone. A former smoker coughing up brown mucus for a few weeks after quitting is experiencing something very different from someone with no smoking history who develops brown, foul-smelling sputum alongside a high fever. Pay attention to how long it’s been happening, whether it’s getting worse, and what other symptoms come with it.