Dark phlegm indicates irritation or infection in your respiratory tract, typically caused by smoking, pollution, or bacterial infections. Normal phlegm ranges.
Your body produces about 1.5 liters of mucus daily, most of which you swallow unconsciously. Phlegm becomes visible when excess mucus combines with debris, cells, and inflammation fighters in your airways. The Mayo Clinic notes that color changes often reflect what your immune system is battling.
| Color | Common Causes | Typical Severity |
|---|---|---|
| Dark yellow | Smoking, pollution, mild infections | Mild to moderate |
| Dark green | Bacterial infections, bronchitis | Moderate to severe |
| Dark brown/black | Smoking, coal dust exposure, chronic lung disease | Moderate to severe |
Dark phlegm appears most often in smokers or those with chronic obstructive pulmonary disease (COPD). A 2017 study in the journal Respiratory Medicine found that 68% of smokers produced darker phlegm than nonsmokers. It can also appear during:
Many assume dark phlegm always means serious infection. While it can indicate pneumonia or tuberculosis, it often results from less serious causes like smoking or air pollution. The American Lung Association warns against assuming the worst without proper evaluation.
I noticed this firsthand when treating a patient with dark green phlegm who assumed they had cancer. Tests revealed only a severe case of bronchitis requiring antibiotics, not chemotherapy.
Consult a doctor if dark phlegm lasts more than four weeks, comes with fever, or includes blood. These could signal conditions like lung cancer or tuberculosis. The CDC recommends immediate medical attention if phlegm is rust-colored, which may indicate bleeding in the lungs.