Sputum, or phlegm, is produced when the airways in the lungs become inflamed or irritated. While some mucus production is normal (especially in the early morning), excessive, persistent, discolored, or foul-smelling sputum suggests something is wrong. Many different systems in the body can cause this — not just the lungs — and the cause often depends on how long it’s been happening, what the sputum looks like, and what other symptoms are present.
Causes of sputum
1. Infections (the most common cause):
Infections, especially of the respiratory tract, are probably the most frequent reason people produce sputum. Viral infections like the common cold, flu, COVID-19, and RSV often start with a dry cough but can lead to clear or yellow sputum as the airways react and mucus production increases. When bacteria are involved — like in pneumonia, bronchitis, or tuberculosis — the sputum becomes thicker, yellow or green, and may even contain blood.
Some people may also develop chronic infections. For example, tuberculosis can cause bloody, persistent sputum, while a lung abscess (a pocket of pus in the lung) might produce foul-smelling sputum. Fungal infections such as aspergillosis or histoplasmosis can do the same in people with weakened immune systems.
2. Chronic Lung Diseases:
There are conditions where the lungs themselves are damaged or inflamed over time, leading to constant or recurring sputum. The classic example is chronic bronchitis, a form of COPD, where patients bring up sputum every day, especially in the morning.
Another is bronchiectasis, where the airways become permanently widened and mucus pools inside them. These patients often cough up large amounts of green or smelly phlegm and may even feel better after doing so.
Asthma, although better known for wheezing and breathlessness, can also lead to the production of sticky, white mucus, especially during flare-ups. In a related condition, allergic bronchopulmonary aspergillosis (ABPA), the sputum might appear brown due to fungal plugs.
3. Lung Cancer and Tumors:
Cancer can cause sputum either directly by irritating the airways or indirectly through infection and bleeding. Lung cancer may present with a persistent cough and blood-streaked sputum, especially in smokers. Tumors like bronchial carcinoid or those pressing on the airways from nearby structures (like lymph nodes or the esophagus) can also lead to mucus buildup and cough.
4. Heart-Related Conditions:
Surprisingly, the heart can also play a role in sputum production. In heart failure, especially when fluid backs up into the lungs, the patient may cough up pink frothy sputum — a sign of pulmonary edema. This is a medical emergency. Sometimes, problems like mitral valve disease or pulmonary hypertension also lead to cough and sputum, occasionally tinged with blood.
5. Reflux and Aspiration:
When acid from the stomach rises into the throat or airway a condition called GERD (gastroesophageal reflux disease) it can lead to chronic throat clearing and a feeling of having to spit up mucus. While this isn’t sputum from the lungs, it feels very similar. In some cases, this acid is accidentally inhaled into the lungs (aspiration), causing inflammation or infection, which leads to true sputum production. This is especially common in elderly patients, people with strokes, or neurological diseases.
6. Autoimmune and Systemic Diseases:
Sometimes, the cause of sputum lies in autoimmune diseases that inflame the lungs or blood vessels. For example, rheumatoid arthritis and systemic lupus erythematosus can affect the lungs and make them prone to infection.
In Wegener’s disease (also called GPA), inflamed blood vessels can bleed, resulting in blood-streaked sputum.
Scleroderma, Sjogren’s syndrome, and polymyositis can also affect the lungs in rare cases, leading to excess mucus.
7. Medications and Treatment Side Effects
Some drugs can cause indirect sputum production. For instance, ACE inhibitors like ramipril can cause a dry cough that leads to throat clearing and mucus buildup.
Chemotherapy drugs like bleomycin and methotrexate can damage lung tissue, causing inflammation, infection, and phlegm. People who undergo radiation therapy to the chest may also develop radiation pneumonitis, which causes sputum in the healing or infected stage.
8. Neurological & Aspiration-Related Causes:
In people with conditions like Parkinson’s disease, stroke, or ALS, swallowing may be impaired. This leads to food, saliva, or acid entering the lungs, causing aspiration pneumonia, which results in foul-smelling, infected sputum. In children or mentally impaired individuals, accidentally inhaling a foreign object (like food or a toy) can also cause persistent phlegm.
9. Environmental, Lifestyle, and Occupational Causes:
Smoking, pollution, and indoor irritants like dust or incense can cause chronic sputum.
Workers exposed to dust, chemicals, or fibers (e.g., miners, farmers) may develop lung diseases.
Conditions like silicosis, asbestosis, and hypersensitivity pneumonitis produce mucus.
Pets and poor ventilation can worsen symptoms in sensitive individuals.
Lifestyle and environment are key triggers for long-term phlegm.
