Phlegm when you cough is produced by your own respiratory system and is not coming from nowhere—it’s mucus that’s accumulated and then expelled from the lower airways. Key points

• Source: Phlegm originates in the airways and lungs as part of your body’s mucus production, often in response to irritation, infection, or inflammation.

• What changes it: The composition and color of phlegm can change based on infections (viral or bacterial), allergies, irritants (smoke, pollution), or chronic lung conditions. Phlegm may contain bacteria, immune cells, and debris when there’s illness.

• Distinction: Mucus is a normal protective layer lining the airways; phlegm is a thicker, expectorated form associated with disease or irritation in the lower airways.

• Common causes: Acute infections like colds or flu, bronchitis, pneumonia, sinus infections, and exposure to environmental irritants can all increase phlegm production. Chronic conditions (asthma, COPD, allergies) can also cause ongoing phlegm, sometimes related to reflux or other factors.

• When to seek help: A sudden increase in phlegm, coughing up blood, chest pain, fever, shortness of breath, or phlegm that lasts many weeks warrant medical evaluation to rule out more serious conditions.

If you’d like, I can tailor this to your symptoms (e.g., color of phlegm, duration, associated symptoms) and suggest when to see a clinician or what home care steps might help.