The nasal cavities are lined with the mucus membrane that secretes the mucus. The role of mucus is to wash the nasal passage and clean the bacteria, dust and other kinds of microbial organisms in an effort to prevent a sinusitis condition. Apart from that it also helps moisten the linings. Cleaning the air of all the allergens and micro-organisms is extremely important before it reaches the lungs. The mucus is then pushed by the cilia or hairs lining the passage into the throat where mucus with all the harmful foreign content is destroyed by the acids. This is part of the normal cleaning process of the body. However sinusitis is a condition where the quality and quantity of mucus varies due to multiple reasons leading to several different complications.

There are two types of sinusitis mucus discharge known as post nasal drip. In one case the mucus is watery produced in large quantities characterized by a running nose. The mucus linings secrete excess mucus as a bodily response to allergic substances in the atmosphere like dust particles, pollutants and fungi floating in the air. However, in cases where the body is dehydrated due to lack of water content, and the nasal passages are dry failing to moisten the mucus linings, the discharge is rather thick. This mucus discharge doesn't flow too easily. A similar kind of thick mucus with pus content is formed by the bacteria that have infected the mucus linings of the body too. The nasal cavity is very narrow and is one of the reasons why mucus shouldn't be thick else it will be blocked causing congestion, stuffiness and problem in breathing freely.

Mucus, along with cilia, form part of a great filtering process that clears the nasal passages of debris formed by air particles which include pollutants, dust and microbes. However, mucus build up in the nasal tract leads to blockage of the sinus cavities. Since the three sinuses are located behind the nose, cheekbones and eyebrows, mucus build up leads to facial swelling and pain. This is due to the pressure imbalance created of the excess fluid clogged in one cavity.

Even more problematic is the situation where this excess fluid affects the proper functioning of the inner ear. Inner ear is responsible for collecting bodily signals from all the nerves thus letting the brain know about the direction of movement viz. front, back, up or down. When the signals get mixed up, the inner ear cannot coordinate the direction of movement any longer. This is one of the reasons why people suffering from sinusitis also experience frequent nausea and dizziness.

Mucus, which itself is laden with bacteria collected from the nasal passages turns dangerous when it starts stagnating due to the blockage. The warm and humid environment in fact encourages the bacteria and fungi to thrive resulting in aggravation of the infection and increased inflammation. This infection is even more harmful when the body's immune system is relatively weak. The bacteria, fungi or viral infections further infect the mucus linings causing it to swell leading to further congestion in the already narrow nasal passages. Swelling of the linings causes pain and its infection leads to increased mucus secretion which is what explains excess mucus flow in sinusitis. Unless treated, the situation gets aggravated further owing to increased microbial action and a more severe illness.