Understanding Phlegm: Causes, Colors, and When to Worry

Phlegm is thick mucus produced by the lower respiratory tract (lungs, bronchi) to trap pathogens and particles. Coughing it up is a defense mechanism.

Common Causes

• Viral infections: Cold, flu, COVID-19 → clear/white phlegm initially.
• Bacterial infections: Bronchitis, pneumonia → yellow/green (neutrophils, enzymes).
• Sinusitis: Postnasal drip → throat phlegm.
• Allergies/asthma: Eosinophilic inflammation → white, stringy mucus.
• COPD/chronic bronchitis: Chronic mucus hypersecretion.
• Smoking: Irritant-induced mucus; brown/black from tar.
• GERD: Reflux triggers cough and mucus.

Red Flags—See a Doctor If Phlegm Is:

• Green/yellow with fever, chest pain, dyspnea
• Blood-tinged (hemoptysis)
• Foul-smelling
• Pink/frothy (pulmonary edema)
• Persistent >3 weeks

Color alone doesn't diagnose—clinical context matters. Hydration, humidification, and treating the cause are mainstays.