In humans, there are four pairs of cavities (air-filled spaces) known as paranasal sinuses. These small hollow spaces, which are located within the skull or bones of the head surrounding the nose, are named for the bones that contain them, as follows:

• Frontal sinuses over the eyes in the brow area
• Maxillary sinuses inside each cheekbone
• Ethmoid sinuses just behind the bridge of the nose, between the eyes
• Sphenoid sinuses behind the ethmoids in the upper region of the nose and behind the eyes

The paranasal sinuses open into the nasal cavity and are lined with cells that make mucus to keep the nose from drying out during breathing and to trap unwanted materials so that they do not reach the lungs.