There are a number of ways your sinuses can become inflamed and blocked, but the most common cause is a viral infection.
Infections
The most common cause of sinusitis is the common cold or influenza (flu). The cold or flu virus spreads to the sinuses from the upper airways.
Sometimes, a secondary bacterial infection can develop, leading to swelling inside the sinuses.
An infected tooth may also cause the sinuses to become infected.
Other causes
There are a number of other factors that can make the sinuses more vulnerable to infection. These include:
- Substances that may irritate the sinuses, such as air pollution, smoke, chemicals (such as pesticides), disinfectants and household detergents.
- Allergies, such as allergic rhinitis, asthma and hayfever.
- Anything that causes narrowing of the nose passages, such as facial injuries or nasal polyps (growths) inside the nose. Mucus can build up behind the narrowed areas, leading to sinus infection.
- Cystic fibrosis, a genetic condition that causes thick, sticky mucus to build up within the body, making you prone to infections.
