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Every bout of sinusitis can be classified according to the duration of the inflammation and whether or not it is caused by an infection. The length of inflammation is divided into two categories: chronic and acute. Most cases of sinusitis in the United States are chronic. Chronic sinusitis afflicts more than 33 million Americans annually, according to the U.S. Centers for Disease Control and Prevention. 

Acute sinusitis is characterized by fewer than three or four weeks of symptoms. Some experts also use the term subacute sinusitis to refer to an illness that lasts between four and eight weeks. A viral infection usually causes acute sinusitis, which often begins suddenly after a bout with a common cold. However, allergies can also be the source of acute sinusitis. Several episodes of acute sinusitis in the span of a year are referred to as recurrent sinusitis.

Acute sinusitis causes postnasal drainage and discomfort in the cheeks, forehead and eyes. Nasal congestion, cough, headache, toothache and fever are also typical symptoms. Many cases do not require medical treatment, but those caused by bacterial infection must be treated with antibiotics.

Chronic sinusitis is defined as two or more episodes per year of sinusitis. Other experts define chronic sinusitis as any case of the illness that lasts for at least eight weeks. Bouts of chronic sinusitis typically last longer than three months and can be caused by infections (treated with antibiotics) or a chronic inflammatory disorder similar to bronchial asthma (treated with corticosteroids).

Chronic sinusitis is often the result of untreated acute sinusitis. Symptoms are similar to those of acute sinusitis, except fever is usually not present. Nasal polyps (tissue growths) also occur in a minority of cases and cause additional cavity blockage. Allergies, such as hayfever  or mold allergies, often play a major role in chronic sinusitis by keeping the sinus membranes inflamed for extended periods of time. Sometimes, structural problems in the nose are the root cause of chronic sinusitis. Surgery may be needed to correct the problem in such situations.

Sinusitis may also be classified as either infectious or noninfectious.

Infectious sinusitis usually stems from a viral infection. Less frequently, it is the result of bacterial or fungal growth. Types of infectious sinusitis include:

• Viral sinusitis. Usually occurs due to an upper respiratory tract infection. Viruses attack the lining of the sinuses, which causes swelling of nasal tissues.
  
  
• Bacterial sinusitis. May be caused when bacteria grow inside the sinuses. Swelling or narrowing of the eustachian tube (passageway extending from the back of the throat and nose to the ears) due to inflammation prevents drainage from the middle ear, which may also cause bacterial sinusitis.
  
  
• Fungal sinusitis. Usually occurs in people with repressed immune systems, allergies to environmental fungi or those who have experienced injuries to the sinuses. Some studies have shown that fungal sinusitis may be more common than previously believed.

Noninfectious sinusitis is usually the result of allergies or other irritants. Types of noninfectious sinusitis include:

• Allergic sinusitis. Pollens, animal dander or other allergens trigger an allergic reaction, causing the lining of the nose and sinuses to become inflamed.
  
  
• Aspirin sensitivity sinusitis. Occurs in some individuals who have severe asthmatic responses to aspirin and other pain medications, such as ibuprofen.
  
  
• Cold air sinusitis. Produces symptoms when an individual is exposed to cold air.