The paranasal sinuses are spaces within the bones of the skull and face, which are filled with air. These open spaces serve the purpose of warming and humidifying the air we breathe, as well as giving resonance to the voice. They also decrease the weight of the skull, which would be significantly heavier without these open spaces. The cells that line the sinuses also produce a mucus that traps inhaled pathogens and keeps the inside of the nose from drying out. Humans have several paranasal sinuses, located in various places around the eyes and nose.
Usually, we refer to paranasal sinuses simply as our sinuses, for the sake of convenience. In reality, though, there are other types of sinuses in the skull as well. Small sinuses are present in the middle ear, for example, hence the term "paranasal sinuses" to differentiate them from the others. Because they only partially develop before birth, our paranasal sinuses continue to develop throughout our lifetime. This is true of nearly every organism with sinuses of any kind.
The paranasal sinuses are set up in four pairs that are more or less symmetrical from left to right. The maxillary sinuses are the largest of them, and are located under the eyes in the inner part of the cheekbones. The next largest are the frontal sinuses, located in the forehead above each eye. These are the last to develop, chronologically speaking; in some people, they don't develop at all.
The ethmoid sinuses are much smaller than the other two pairs, and are located behind either side of the bridge of the nose, near the eyes. The final pair are the sphenoid sinuses, located in the skull behind the inner nasal passage, above the throat. Each pair of sinuses is linked to the nasal passageway by small tubes called ostia. Normally, these tubes allow for mucus to drain harmlessly out of the sinuses. When a person catches a cold, the sinuses and ostia become inflamed, blocking proper drainage and leading to nasal congestion and sinus headaches.
Other, more serious ailments of these sinuses can also occur. In rare cases, cancer of the sinuses can develop. This is rare in Western nations but slightly more common in certain other areas of the world, including South Africa and Japan. Its exact causes have proven to be difficult to determine, but the use of snuff and long-term exposure to wood dust may be risk factors. Persistent sinus pain and blockage are the most common symptoms of this cancer, which, like other cancers, has the best prognosis if it is caught early and treated effectively.