Sinusitis is a condition where the sinuses become inflamed or infected.

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You may not ever think about them -- those little pockets of air that are part of our skull and in the bones of our head. They're your sinuses, and when they're working normally, they silently help keep the inside of your nose moist and help block foreign substances from being inhaled into your lungs. We have eight sinuses, in four pairs, called the paranasal sinuses, and they include:

  • The frontal sinuses, which are found just above our eyebrows
  • The maxillary sinuses, which are located inside our cheekbones
  • The ethmoid sinuses, which are between our eyes, behind the bridge of the nose
  • And the sphenoid sinuses, which are behind our eyes and upper part of the nose

Our sinuses are lined with the same mucus membranes that line the inside of our noses. These membranes produce mucus to keep the sinuses and respiratory tracts moist, and there are also microscopic hairs called cilia that move mucus around in our nasal passages. On average, when we're healthy we produce about 2 quarts (1.8 liters) of mucus every day, most of which we eventually swallow. When our sinuses become filled with mucus, such as from a cold, the cilia are unable to move properly. This can result in a build-up of mucus that blocks the sinus openings, and that blockage increases our risk for developing sinusitis.

Sinusitis is a condition where the sinuses become inflamed. It's a common infection, and there are two main types: acute and chronic. Acute sinusitis usually lasts less than a month and occurs no more than three times a year, while chronic sinusitis is a condition that can last upwards of three months or longer per episode and occurs at least four times a year, despite efforts to treat it.

It's estimated that roughly 31 million American adults were diagnosed with sinusitis in 2009, and about 12 percent of Americans age 45 or younger suffer from the chronic condition [sources: National Institute of Allergy and Infectious Diseases; American Academy of Allergy, Asthma & Immunology].