Respiratory system Upper respiratory tract infection Infection and inflammation of the sinuses is known as sinusitis. Sinuses are air field cavities located in the face and skull. Explain the Etiology/causes of sinusitis. Explain the Classification of sinusitis. Sinusitis is classified based on its duration and severity as follows. On the basis of duration: 1) Acute sinusitisAcute sinusitis is characterized by symptoms of sinusitis lasting less than four weeks. Which is