Overview

Phlegm is the thicker, often colored secretion produced in the respiratory tract and expelled by coughing. It is a form of mucus associated with the lower airways and lungs and serves to trap particles, pathogens and debris so they can be removed from the body.

Composition and production

Phlegm contains water, salts, mucin proteins, immune cells (for example neutrophils), cellular debris and sometimes microbes. It is generated by glandular tissue and the mucous membranes that line the nose, throat and bronchi; those membranes regulate the quantity and viscosity of the secretion. See mucous membranes for more on the lining tissues and related sources on respiratory secretions.

Characteristics and clinical clues

Unlike thin nasal mucus, phlegm is often thicker and produced deeper in the chest. Its color and consistency can change with different conditions: clear or white when viral or allergic; yellow or green when immune cells are present; brown or blood-tinged after smoking, pollution exposure or injury. Color alone is not a definitive diagnosis and must be interpreted with clinical context.

History and cultural context

Historically, bodily fluids including phlegm were central to ancient humoral medicine, where excess "phlegm" was linked to temperament and disease. Modern medicine discards humoral explanations but retains clinical attention to sputum as a diagnostic specimen.

Uses, examples and management

Clinically, expectorated phlegm (sputum) is sampled to identify infections or monitor chronic lung diseases. Management focuses on addressing the underlying cause: hydration, humidification, expectorants, airway clearance techniques and targeted treatments for infections or inflammation. Persistent, bloody, or very thick phlegm should prompt medical evaluation.

  • Clear/white: often viral or noninfectious.
  • Yellow/green: reflects immune cell activity but not always bacterial infection.
  • Brown/bloody: may follow smoking, pollution, or indicate bleeding—investigate if persistent.

Phlegm is a normal defensive secretion but changes in amount, color or smell can be important clinical signs. For further reading on related anatomy and secretions consult sources listed at external reference and on mucous membrane structure at related entry.