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There are many causes of sinusitis. Anything that interferes with airflow into the sinuses and mucus drainage out of the sinuses can cause inflammation and sinusitis.

Some of the medical conditions that can cause sinusitis include:

• Colds, viruses and other respiratory infections. These infections inflame the sinuses and cause the body to react to the infection by producing mucus and directing white blood cells to the lining of the nose. This causes congestion and swelling of the nasal passages and the adjacent mucous membranes of the sinuses. As openings of the sinuses narrow, air and mucus become trapped inside, creating the conditions that lead to sinusitis.
  
  
• Allergies. In some people, the immune system mistakenly recognizes harmless elements (e.g., pollen, pet dander, dust mites) as threats, triggering a reaction that resembles a cold with symptoms such as stuffy nose, itchy eyes and coughing. This can lead to the development of sinusitis.
  
  
• Exposure to fungi. Fungi can cause a particular form of sinusitis known as fungal sinusitis. While fungi can be found throughout the environment, they typically are harmless to people unless they have immune system problems or are allergic to fungi.
  
  
• Tumors or growths in the sinuses, such as nasal polyps (small, round growths).
  
  
• Aspirin sensitivity. Sinusitis may occur when people with sensitivity to aspirin take the medication.
  
  
• Environmental factors. Pollutants may lead to congestion that causes sinusitis. Other causes may include damp weather, particularly in northern climates.
  
  
• Overuse of decongestant nasal sprays. Using nasal sprays for longer than recommended can lead to “rebound congestion” that inflames the sinuses.
  
  
• Structural abnormalities and other blockages. These can be natural, such as a crookedness in the wall dividing the nostrils (deviated septum), or caused by trauma. An injury, such as a fractured facial bone, can obstruct one or more sinus cavities leading to sinusitis.
  
  
• Dental infections that spread to the sinuses.
  
  
• Use of illegal drugs that are snorted, such as cocaine.
  
  
• Asthma. People with asthma may have hyperactive nasal airways, which can lead to sinusitis. Also, pus inside an infected nasal cavity that manages to drain into the throat can sometimes trigger an asthma attack.
  
  
• Failure of the cilia to move mucus. The cilia are tiny hairs of mucus-producing cells that move mucus out of the sinuses and into the nose and throat. Tobacco smoke is known to damage the cilia. Several health disorders can also complicate this process, including cystic fibrosis (a hereditary disorder affecting the lungs and digestive tract), Kartagener’s syndrome (a congenital disorder that consists of three sinus afflictions), immotile cilia syndrome (a disorder that causes failure of the cilia to clear mucus), and immune deficiency diseases such as AIDS.
  
  As the sinuses become blocked, mucus inside the sinus cavity cannot drain to the back of the throat. This provides a breeding ground for viruses and bacteria to grow, resulting in infection. Pus then starts to form, and has nowhere to drain, further exacerbating the situation.