KnowHOW team explains: Mucus is found all through the respiratory tract, including the sinus cavities, the space behind the nose and the passages to the lungs. Some of the same things that produce an excess of mucus, like allergy and irritation, can also make you sneeze. The explosive exhalation of the sneeze moves mucus out of the nose, whether it has swept down from nasal passages in the head or up lower respiratory passages.

Mucus is made by goblet cells which are scattered through the lining of the respiratory and digestive tracts and are also found in mucous glands under the lining. It consists of water, carbohydrates, lipids and glycoproteins, called mucins. In sputum, mucus mixes with dead cells and other body secretions, including saliva.

Normally, a thin layer of mucus is moved along by the flow of air and by cilia, or hairlike projections, on the cells of the respiratory tract. Mucus flow has a protective and housekeeping function, preventing dehydration and sweeping germs and dirt out of the airways into the pharynx and eventually down to the stomach, to be expelled by the digestive tract.

In infections and certain diseases, like cystic fibrosis and asthma, too much mucus is produced, and it may be thick ? thus defeating its main purpose ? blocking respiration and increasing the risk of infection.

The question was sent by Anasuya Mukherjee from Birati