In healthy people, sinus secretions are always moving and draining into the nose. But if the movement of mucus out of the sinuses is blocked, rhinosinusitis - commonly known as sinusitis, or sinus inflammation - can develop.

When blockage occurs, mucus fails to drain properly, increases in thickness, and fills the sinus spaces. At the same time, oxygen levels in the affected sinuses are reduced. The cilia (hair-like structures that filter the air in the sinuses) slow down their sweeping and cleaning, making it even harder for mucus to drain.

There are many causes of sinusitis - these may vary depending on whether you have chronic or acute sinusitis.

Allergies and Chronic Sinusitis

Allergies, air quality, and anatomic variations in the nose all contribute to the development of sinus inflammation.

People with allergies or allergic rhinitis have higher rates of sinusitis than people without allergies. Constant exposure to allergens can thicken mucus and make your sinuses vulnerable to inflammation.

Allergens known to trigger allergic rhinitis and chronic sinusitis include:

  • Dust mites
  • Fungal spores (mold)
  • Animal dander
  • Cockroaches
  • Grass, tree, and weed pollen

Air pollution and environmental toxins may also trigger chronic sinusitis. Pollutants include cigarette smoke and secondhand smoke, carbon monoxide, nitrous dioxide, formaldehyde, and sulfur dioxide.

Additionally, anatomic variations in the nose, such as a deviated septum, can increase your risk for sinus obstruction and chronic sinusitis.

Narrow nasal passages caused by facial injury, tumors, foreign bodies in the nose, or polyps can also block the openings into your sinuses. (Polyps are small, grape-like growths that protrude into the sinuses and nasal passages, making it difficult for your sinuses to drain and for air to pass through your nose.)

Gastroesophageal reflux disease (GERD) is also linked to chronic sinusitis. According to the American Academy of Otolaryngology, when gastric fluids flow up out of the stomach into the esophagus and throat during acid reflux, acid can collect at the back of the nasal airway and in the sinuses. This acid can cause sinus inflammation.

Many other factors can increase the risk of developing chronic sinusitis, including:

  • Sensitivity to aspirin
  • Immune deficiencies (e.g., HIV infection)
  • Pregnancy
  • Bacteria
  • Viruses
  • Asthma and other lung disorders

Viral and Bacterial Sinus Infections

Although a sinus infection (acute sinusitis) can be viral or bacterial, it is usually triggered by a viral upper respiratory infection (such as a cold or flu). Rhinovirus, influenza, and parainfluenza are the most common viruses that cause acute sinusitis.

Dental problems involving infections can also cause acute sinusitis (odontogenic sinusitis). The roots of your upper back teeth are close to the maxillary sinus, and if they become infected, the infection can travel into your sinuses.

The following bacteria cause a small percentage (0.5 to 2 percent) of acute sinusitis:

  • Streptococcus pneumonia
  • Staphylococcus aureus
  • Haemophilus influenza
  • Moraxella catarrhalis

These bacteria are normally present in the nasopharynx (the uppermost part of your throat). If you have a cold, the flu, or an acute viral sinusitis, nose blowing can jolt bacteria from your nose into your sinuses. If mucus in the sinuses is slow moving or blocked, it provides a rich environment for bacteria to grow, which can lead to a sinus infection.

Fungal Sinusitis

Although fungal sinusitis is rare, fungal spores (i.e., mold) are often present in the sinuses. In fact, we breathe these spores in and out all the time. Fungus is more likely to be present in the warm, moist environment of the sinuses when the immune system is damaged or depressed, or in people with nasal polyps.

Some of the most common fungi that cause sinusitis include:

  • Aspergillus species
  • Cryptococcus neoformans
  • Candida species
  • Sporothrix schenckii
  • Altenaria species
  • Curvularia species.

Allergic fungal sinusitis is more common in southern than in northern states, and its symptoms are similar to viral and bacterial sinusitis. Fungal sinusitis does not respond to antibiotic treatment and can be difficult to treat - some types of fungal sinusitis may require surgical treatment to remove spores.

On rare occasions - especially among people who have uncontrolled diabetes, immune system problems, or hematological cancers such as leukemia - fungal sinusitis can become invasive and extend beyond sinus tissue and bone, eventually moving into the brain and eyes.