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The sinuses are hollow spaces located in the face and skull. They appear in pairs on either side of the face. Depending on the type, the sinuses vary in size from tiny to the size of a walnut. The four sets of sinuses are:

• Frontal sinuses. Located in the forehead. There are two of these sinuses, one per side of the forehead. The frontal sinuses vary greatly from person to person in both size and shape.
  
  
• Maxillary sinuses. Located in the cheeks between the teeth and the eyes. There are two of these sinuses, one in each cheek. These are the largest of the sinuses, and each can be roughly as large as a walnut.
  
  
• Ethmoid sinuses. Located on each side of the nose between the eyes. There are between 6 and 12 of these sinuses on each side of the face. These sinuses are very small.
  
  
• Sphenoid sinuses. Located deep behind the eyes, towards the middle of the skull. There are two of these sinuses, one per side. The size, shape and volume of these sinuses vary greatly from person to person.

The function of the sinuses is not entirely understood. Most physicians agree that the sinuses are useful for reducing the weight of the skull while preserving bone strength and shape. The shape of the sinuses and nasal cavity also serve to add resonance to the voice. In addition, sinus cavities may help reduce the damage sustained during a head trauma by absorbing some of the impact.

Like the nasal cavity, the sinuses are lined with mucous membranes – moist layers of tissue that secrete mucus. This mucus helps remove foreign particles that enter the sinuses. The sinuses also have cilia, tiny hairs which push the mucus back into the nasal cavity through small openings (ostia). This drainage is not based on gravity, but rather the efforts of the cilia. From the nasal cavity, the mucus can be removed from the body through either the nose, throat or mouth.

The ostia are very small, and can easily become blocked, preventing the normal drainage of mucus from the sinuses. This often occurs due to the inflammation produced by a cold or allergy (an exaggerated reaction of the immune system to certain foreign invaders that it mistakes as a threat to the body), excessive mucus production or growths such as polyps (bulging growths that develop in the lining of mucous membranes). After the ostia are blocked, inflammation or an infection can occur in the sinuses. This condition is called sinusitis. The air trapped in the sinuses during this condition can cause painful facial pressure, headaches or toothaches.

The ostia openings, mucus production and cilia are all interrelated. If one of the three mechanisms malfunctions, a cycle of problems can develop that often leads to a sinus problem (e.g., infection). This cycle can be difficult to break after it has started. For this reason, people should be aware of any sinus problems or abnormalities that may develop and have any conditions immediately checked out by a physician.