The sinuses are small, air-filled cavities behind your cheekbones and forehead. The sinuses make mucus, which is a fluid that cleans the bacteria and other particles out of the air we breathe. Your sinuses open up into the cavity of your nose and help control the temperature and water content of the air reaching your lungs. The mucus that's naturally produced by your sinuses usually drains into your nose through small channels. These channels can become blocked when the sinuses are infected and inflamed. The sinuses behind the cheekbones (the largest ones) are most commonly affected. There are four different types of sinuses: Ethmoid sinus: Located inside the face, around the area of the bridge of the nose, it is is present at birth and continues to grow. Maxillary sinus: Located inside the face, around the area of the cheeks, it is also present at birth and continues to grow. Frontal sinus: Located inside the face, in the area of the forehead, it does not develop until around seven years of age. Sphenoid sinus: Located deep in the face, behind the nose, it does not develop until adolescence.