what is the cause of brown mucus

Brown mucus is usually caused by old blood, smoke/tar residue, or an infection/inflammation in the airways. It can also happen with chronic lung conditions such as bronchitis, COPD, bronchiectasis, cystic fibrosis, or after exposure to dust, pollution, or chemicals.

Common causes

• Old blood from irritated airways or a nosebleed that has drained back.
• Smoking or recent quitting , which can bring up trapped tar and debris.
• Respiratory infections like bronchitis or pneumonia.
• Chronic lung disease such as COPD or bronchiectasis.
• Environmental exposure to smoke, pollution, dust, coal, or other irritants.

When to get checked

See a clinician soon if the brown mucus is:

• Happening for more than a few weeks.
• Mixed with blood.
• Accompanied by fever, shortness of breath, chest pain, wheezing, or foul smell.
• Occurring with worsening cough or fatigue.

Simple note

If this is from the nose rather than coughing up phlegm, dryness, irritation, or old blood can also make mucus look brown.

If you want, I can also give you a quick “brown mucus vs. green/yellow mucus” explanation.