Healthy phlegm is clear, thin, and slightly slippery. When it changes color, that shift usually reflects what your body is doing in response to irritation, infection, or inflammation. While color alone isn’t enough to diagnose a specific illness, it gives you a useful snapshot of what’s happening in your airways.
Clear Phlegm: The Healthy Baseline
When you’re not sick, your mucus is clear. Your body produces it constantly to line the airways and mucous membranes, where it traps germs, dust, and other particles before they reach deeper tissue. It also carries antibodies that mark pathogens for destruction by immune cells, and it keeps the lining of your nose, throat, and lungs moist.
Clear phlegm can increase in volume during allergies or mild irritation without changing color. If you’re coughing up clear mucus but feel fine otherwise, that’s generally your respiratory system doing its job.
White or Cloudy Phlegm
White, thicker phlegm often shows up with nasal congestion, early-stage colds, or conditions like acid reflux and asthma. The color change happens when mucus loses some of its water content and becomes more concentrated. Swollen, inflamed tissues in the airways slow mucus drainage, giving it time to thicken and turn opaque.
On its own, white phlegm is not a strong indicator of bacterial infection. In a large pooled analysis published in the European Respiratory Journal, white sputum had a high rate of being bacteria-free compared to green or yellow sputum.
Yellow Phlegm
Yellow mucus means your immune system has ramped up. The color comes from white blood cells that have rushed to the site of infection or inflammation. As these cells fight off invaders and die, they release enzymes that tint the mucus yellow. This is common in the first few days of a cold or sinus infection, and it doesn’t automatically mean you need antibiotics. Most upper respiratory infections are viral, and yellow phlegm during a typical cold will usually fade back to clear within a week or so.
Green Phlegm
Green phlegm follows the same mechanism as yellow, just further along. The more white blood cells accumulate and break down, the greener the mucus becomes. A thick green discharge that persists beyond 10 to 12 days, especially with facial pain, fever, or worsening symptoms, is more suggestive of a bacterial sinus infection or bronchitis.
That said, green phlegm is a better signal for what’s present than what’s absent. Research pooling data from chronic bronchitis patients found that green or yellow sputum had about 95% sensitivity for detecting bacteria, meaning it rarely missed a bacterial infection. But its specificity was only around 15%, meaning plenty of people with green or yellow mucus didn’t have harmful bacteria at all. Color is a clue, not a diagnosis.
Brown or Rusty Phlegm
Brown phlegm can come from old blood that has oxidized, giving it a rust-like tint rather than a fresh red. Smokers sometimes cough up brownish mucus as resin and tar mix with respiratory secretions. People with significant chronic lung conditions like cystic fibrosis or bronchiectasis commonly see dark brown, sticky phlegm. In those cases, the color reflects a combination of blood, chronic inflammation, and bacteria that have colonized the lungs over time, gradually changing the appearance and consistency of mucus.
If you’ve been a smoker and notice persistent brown phlegm, that’s worth flagging with a doctor. For people already living with chronic lung disease, a sudden shift in color or thickness from your personal baseline can signal a flare-up that may need treatment.
Red or Pink Phlegm
Red streaks in your phlegm usually mean small blood vessels in the airways have broken, often from forceful coughing, dry air, or minor irritation. A few streaks during a bad cold aren’t uncommon and typically resolve on their own.
Pink, frothy phlegm is a different story. The Mayo Clinic lists it as a hallmark symptom of pulmonary edema, a condition where fluid builds up in the lungs. Acute pulmonary edema is life-threatening and requires emergency care. Pink frothy sputum accompanied by severe shortness of breath, a rapid heartbeat, or a sense of drowning warrants calling 911 immediately. This can also occur at high altitude, where it’s known as high-altitude pulmonary edema.
Coughing up larger volumes of blood, more than a few teaspoons, is considered a serious warning sign regardless of other symptoms. A history of heavy smoking, unexplained weight loss, fatigue, or back pain alongside bloody phlegm makes urgent evaluation especially important.
Black Phlegm
Black sputum is rare. It’s most associated with heavy exposure to coal dust, soot, or other dark environmental particles, a pattern historically seen in miners and firefighters. Certain fungal infections can also produce dark-pigmented mucus. Fungi like Aspergillus niger and some Exophiala species naturally produce melanin, and when they infect the lungs, they can tint sputum dark brown to black. This is sometimes seen in people with underlying conditions like bronchiectasis. If you’re coughing up black phlegm without an obvious environmental explanation, that’s something to get checked promptly.
When Color Isn’t Enough
Phlegm color gives you a rough gauge of inflammation and immune activity, but it can’t tell you the specific cause. A cold, a bacterial infection, allergies, and air pollution can all produce similar-looking mucus. What matters more than color alone is the full picture: how long symptoms have lasted, whether they’re getting worse, and what other symptoms are present.
If your doctor suspects a lung infection based on symptoms like persistent productive cough, fever, chills, shortness of breath, or chest pain that worsens with breathing, they may order a sputum culture. This involves coughing a sample into a cup so a lab can identify specific bacteria or fungi. It’s most commonly used to check for pneumonia, bronchitis, and tuberculosis, and results help guide whether antibiotics or antifungal treatment is needed.
Volume and texture often matter as much as color. A sudden increase in how much phlegm you’re producing, a shift from thin to thick and sticky, or phlegm that smells unusually foul can all be meaningful signals even when the color hasn’t changed dramatically.