Why do people sneeze? Is sneezing a sign that a person is ill? Why is it so difficult to stop a sneeze?
To answer these questions, we first have to understand that sneezing is a means of keeping dust, dirt and other irritants out of the nose. It is a natural reflex action intended to help in the expelling of such irritants from the nasal passages. When the sensitive nerve endings that line the nose are irritated, the brain transmits a signal to the lungs to draw in air; the message also tells the lungs to prepare for a sneeze. The air, which is propelled by the chest muscles, is then ejected through the nose and mouth at great speed.
The act of sneezing is involuntary. Much like coughing, sneezing is essentially a modified respiratory act, in which at first a strong forced expiratory movement is started with the vocal cords tightly apposed (that is, with the glottis closed). This greatly raises the pressure within the lungs, so that, when suddenly the glottis is opened, a blast of air is abruptly forced from the lungs through the nose in a sneeze (or through the mouth in a cough), tending to expel the irritant from the breathing passages.
The breathing passages, from the nose or mouth to alveoli (the small air-containing compartments of the lungs from which respiratory gases are exchanged with the pulmonary capillaries), are in a position open to attack by infections. After birth, the nose and mouth cavities always contain microorganisms, which may attack the respiratory system either in the upper breathing passages or deep down in the lungs. Particles or droplets of water laden with bacteria may also be drawn at any time.
Many of the respiratory tract disorders cause sneezing, apparently as a response to the presence of infections. These respiratory infectious diseases, whose names were derived from the site of the body they infect, include rhinitis (infection and inflammation of the nasal cavity), pharyngitis (infection and inflammation of the pharynx), laryngitis (infection and inflammation of the larynx), tracheitis (infection and inflammation of the trachea), and bronchitis (infection and inflammation of the bronchial tubes).
Certain special organisms may grow in the respiratory system and cause injury which may be fatal, either by the freeing of metabolic products which harm even remotely situated organs or by mechanical interference with breathing or blood circulation. Included here are the organisms of diphtheria (Corynebacterium diphtheriae), scarlet fever (Streptococcus pyogenes), and tuberculosis (Mycobacterium tuberculosis), with the first two being localized in the throat and the last one most often localizing and extending through the deeper tissues of the lungs.
Several body mechanisms have been evolved for the defense of these organisms against such infections. For example, the hair which lines the nasal cavity and the thick mucous secretion on the linings of the respiratory tract ensnare some of the bacteria-laden particles. Checked in their progress, the irritants may then be expelled by a reflex sneeze. In a different instance, chilling of the skin of the lower extremities can cause reflex congestion of blood in the nasal cavities, thus effecting sneezing.
1. "Sneezing", on Your Total Health (online) - http://yourtotalhealth.ivillage.com/sneezing.html
2. "Sneeze", on the Wikipedia - http://en.wikipedia.org/wiki/Sneeze
3. "Why Do We Sneeze?" by Pamela Georgeson D.O., a member of the American Osteopathic Association and a board-certified osteopathic physician in pediatrics - http://www.scientificamerican.com/article.cfm?id=why-do-we-sneeze