The paranasal sinuses are air-filled spaces located within the bones of the skull, surrounding the nasal cavity. They are named for the bones in which they are found: the frontal, maxillary, ethmoid, and sphenoid sinuses. These sinuses help to lighten the weight of the skull, improve voice resonance, and produce mucus that keeps the nasal passages moist. Each sinus is lined with a mucous membrane that helps trap dust, allergens, and pathogens. When the sinuses become inflamed or infected, a condition known as sinusitis can occur, leading to symptoms like facial pain, congestion, and headaches.