Sinusitis, (sinus infection or rhinosinusitis,) is inflammation of the sinuses . Common signs and symptoms include thick nasal mucus, a plugged nose, and pain in the face.Sinusitis commonly occurs when environmental pollens irritate the nasal passages, such as hay fever. Sinusitis can also result from irritants, such as chemicals and nasal sprays. A sinus is a hollow, air-filled cavity. Humans have - frontal sinus (in forehead),maxillary sinus (behind cheeks),ethmoid sinuses (between the eyes),and sphenoid sinuses.The four pair of sinuses are often described as "paranasal sinuses." Functions of the sinuses include humidifying and warming inspired air.