Phlegm is mucus that comes from the lower respiratory tract, including the windpipe, bronchi and smaller airways. It may contain mucus, dust, bacteria, viruses, inflammatory cells and other substances that the body is trying to clear from the airways.
Producing mucus is part of the body’s protective mechanism. It helps trap and remove irritants, particles and possible disease-causing organisms from the respiratory tract. When the airways are irritated, inflamed or infected, the body may produce more mucus, leading to increased phlegm.
However, not every sensation of “phlegm in the throat” is true phlegm from the lower airways. Post-nasal drip from allergic rhinitis or sinusitis may cause mucus to flow down the throat, creating a similar feeling. Severe acid reflux may also irritate the throat and cause secretions or a sensation of mucus.
Patients may notice different colours or textures of phlegm. Clear or white phlegm may be normal mucus, or may occur when the airways are mildly irritated by dust, pollution or dry air.
Yellow or green phlegm may suggest airway inflammation and can be seen in infections such as pneumonia, lung abscess or tuberculosis. However, yellow or green phlegm does not always mean a bacterial infection. It can also occur in non-infectious inflammation, such as allergic lung conditions or autoimmune-related lung disease.
Grey or black phlegm may be related to smoking or inhalation of substances such as coal dust, soot or other particles. Blood-stained phlegm can occur in severe pneumonia, tuberculosis, bronchiectasis or, in some cases, lung tumours. However, blood in phlegm does not always mean cancer, and lung cancer does not always cause bloody phlegm.
Warning signs include persistent or worsening phlegm, fever, chills, worsening cough, chest discomfort, blood-stained phlegm, coughing up blood, dark black or coffee-coloured phlegm, or frothy phlegm.
Diagnosis should not rely on phlegm colour alone. A doctor will usually assess the duration of symptoms, cough pattern, amount and colour of phlegm, fever, breathing symptoms, smoking history, exposure history and any underlying medical conditions.
Physical examination may include checking the throat, nose, temperature, oxygen level and listening to the lungs with a stethoscope. If infection or lung disease is suspected, further tests may be arranged.
These may include sputum tests, blood tests, chest X-ray, CT scan or other investigations depending on the patient’s condition. If blood-stained phlegm or persistent symptoms are present, further assessment may be needed to rule out more serious conditions.
Treatment depends on the underlying cause rather than the colour of the phlegm alone. If the phlegm is related to a viral respiratory infection, antibiotics are usually not required unless a bacterial infection is suspected or confirmed.
For mild symptoms, patients may benefit from adequate rest, drinking enough water, avoiding smoking and second-hand smoke, and using medications to relieve fever, cough or nasal symptoms when appropriate.
If symptoms are caused by post-nasal drip, allergic rhinitis, sinusitis or acid reflux, treatment should focus on the underlying condition. If bacterial infection, pneumonia, tuberculosis, bronchiectasis or another lung disease is suspected, medical assessment and targeted treatment are needed.
Patients should seek medical attention if phlegm persists for several days, becomes worse, or is associated with fever, chills, cough, chest pain or breathing difficulty. Immediate medical advice is recommended if there is blood in the phlegm, coughing up blood, dark black or coffee-coloured phlegm, or frothy phlegm.
