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Causes of Sinusitis

Front view of sinuses showing red, swollen lining and mucus.

Mucus helps keep your sinuses clean. But mucus may build up in the sinuses due to colds, allergies, or obstructions. These things interfere with the natural drainage of mucus. This may lead to sinusitis (sinus inflammation and infection).

  • Acute sinusitis comes on suddenly. It often happens right after an upper respiratory infection, such as a cold.

  • Chronic sinusitis is ongoing swelling of the sinus lining. This is often the result of allergies or chronic infections.

Colds and other infections

A cold or flu may cause your sinus and nasal linings to swell. Sinus openings can become blocked. This causes mucus to back up. This backed-up mucus becomes an ideal place for bacteria to grow. Thick, yellow, or discolored mucus is one sign of infection.

Allergic reactions

You may be sensitive to certain substances. This causes the release of histamine in the body. Histamine makes your sinus and nasal linings swell. Long-term swelling clogs your sinuses. It prevents the cilia (tiny hairs in the nasal lining) from sweeping away mucus. Allergy symptoms can be persistent. But they’re less severe than with colds.

Front view of sinuses showing polyps, mucus buildup, and deviated septum.


  • A polyp is a sac of swollen tissue. It can be the result of an allergy or infection. It may block the middle meatus (the opening where most of your sinuses drain). It may even grow large enough to block your nose.

  • A deviated septum is when the thin wall inside your nose is pushed to one side. It is often the result of injury. This can block your middle meatus.


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