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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

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Previous Week's Question Published: 13 December, 2006 Next Week's Question
Sneezing occurs due to irritation of the nose - or bright light
Question
How is it physically possible for a sneeze to efficiently clear out our sinuses? What happens during a sneeze?

Question
A sneeze is the body's reflexive response to irritation in the nose. The sneeze begins with an inhalation (or several) to fill the lungs—the "Ah" part of "Ah-Choo!" Air flows into the lungs down its pressure gradient as contraction of the diaphragm and rib muscles increases the volume (therefore reducing the pressure) of the lungs. Sometimes these involuntary contractions are spasmodic and repeated—as in "Ah- ah- ah-" with each "ah" filling the lungs a little more.

When the lungs are filled, a forceful exhalation begins as the diaphragm and rib inhalation muscles relax, while other muscles squeeze the ribcage smaller, reducing the volume and increasing the pressure in the lungs. This squeezing action is why we often bend forward a little when we sneeze or cough. At the same time, however, the glottis (voice box or larynx) is held closed so that air cannot escape and great pressure is built up.

The "Choo!" phase begins as the glottis opens. Pressurized air bursts from the windpipe and flows first up the back of the throat, into the nasal passages and through the nose. As the glottis begins to close again, some air flows through the mouth. The unusually high pressure and volume of the exhalation results in the high speed of the sneezed air, which sweeps up dust, pollen, mucus, saliva and their contents from the airways, particularly the cavity of the nose, and propels them from the nose and mouth as an aerosol. Frequently this is sufficient to remove, or at least displace, the irritant which triggered the sneeze, although it does not really clear the sinuses.

Some kinds of irritation are not so easily relieved. Allergens or microbes which elicit an immune response can cause a sense of irritation in the nose as small blood vessels dilate in response to histamine released by the immune system. Since the irritation is due to the body's own products, sneezing cannot remove the cause and only adds to the discomfort of the allergy or cold sufferer.

About a quarter of the US population experience "photic sneezing", in which sneezing is induced by bright light, typically when the person goes from a building out into the sun. Photic sneezing is inherited as a dominant trait. It is thought to be due to leakage, or crosstalk, of signals from the optic nerve to the nearby trigeminal nerve which controls sneezing.

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