Each person has four pairs of sinus cavities in his or her head. The four pairs of sinus cavities are the ethmoid, frontal, maxillary and sphenoid sinuses.
The ethmoid sinuses are located between the eyes, right at the bridge of the nose. Humans are born with this pair of sinuses. The frontal sinuses develop at around 7 years old and are located right above the eyes. Maxillary sinuses are found on each side of the cheekbones and are present at birth, and the sphenoid sinuses lie behind the eyes and the ethmoid sinuses. These sinuses develop during adolescence.
When one or more of the eight sinus cavities become infected, the condition is called sinusitis.