Sinuses are hollow spaces in the bones surrounding the nasal cavity. Maxillary, frontal, sphenoid and ethmoid sinuses are four sets of sinuses found in the area around the nose. Their purpose is to reduce the weight of the bones without sacrificing the shape and strength of the face. Sinuses are connected to the nasal cavity through small openings called ostia. Swellings in the nasal cavity linings caused due to cold or an allergic inflammation can cause the blockage of the orifice of the sinus cavity resulting in the non drainage of the mucous produced in the sinus.
Nose and the sinus produce a large quantity of mucous every day which is drained through the nasal cavity and some of it passed through the throat and goes down to the stomach where the dangerous bacteria are destroyed by the acidic substances present in the stomach. This is normal activity which takes place every day and goes unnoticed.
Irritation of nasal passage by allergies, viral infections, and smoke cause increased secretions of the mucous. The purpose of this extra mucous is to wash away the allergic or irritating substances in the nose. This is the most common type of post nasal drip. When the air is dry and when enough moisture is not produced by the nasal membrane the post nasal drift produced will be thick and sticky. This mucous will be green in color as some puss will also be contained in this mucous.
Generally, sinusitis is not very dangerous and it can be managed very easily be proper medication. But as the sinuses are very close to the eyes and brain the possibility of the infection extending to the brain and eyes cannot be ruled out although this is a very rare possibility. The flowing in of infected mucous into the lungs is not allowable as it can aggravate asthma, chronic cough and bronchitis.