A sneeze (or sternutation) is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa. Sneezing can be triggered through sudden exposure to bright light, a particularly full stomach, or viral infection. Sneezing may spread disease.
The function of sneezing is to expel mucus containing foreign particles or irritants and cleanse the nasal cavity. During a sneeze, the soft palate and uvula depress while the back of the tongue elevates to partially close the passage to the mouth so that air ejected from the lungs may be expelled through the nose. Because the closing of the mouth is partial, a considerable amount of this air is usually also expelled from the mouth. The force and extent of the expulsion of the air through the nose varies.
Sneezing typically occurs when foreign particles or sufficient external stimulants pass through the nasal hairs to reach the nasal mucosa. This triggers the release of histamines, which irritate the nerve cells in the nose, resulting in signals being sent to the brain to initiate the sneeze through the trigeminal nerve network. The brain then relates this initial signal, activates the pharyngeal and tracheal muscles and creates a large opening of the nasal and oral cavities, resulting in a powerful release of air and bioparticles. The powerful sneeze is attributed to its involvement of numerous organs of the upper body – it is a reflexive response involving the face, throat, and chest muscles
A cause of sternutation is sudden exposure to bright light - known as the photic sneeze reflex.
Sneezing is also triggered by sinus nerve stimulation caused by sinus infection and allergies.
A rarer trigger, observed in some individuals, is the fullness of the stomach immediately after a large meal. This is known as snatiation and is regarded a medical disorder passed along genetically as an autosomal dominant trait.
Sneezing cannot occur during sleep due to REM atonia - a bodily state wherein motor neurons are not stimulated and reflex signals are not relayed to the brain. Sufficient external stimulants, however, may cause a person to wake from their sleep for the purpose of sneezing, although sneezing occurring afterwards would take place when partially awake.
Conservative estimates place the speed of release at 75 kilometers per hour (47 mph) or 21 meters per second (69 ft/s). The highest estimates — such as by the Health World Museum in Barrington, Illinois — propose speeds as fast as 85% of the speed of sound, corresponding to 1,045 kilometers per hour (649 mph) or 290 meters per second (950 ft/s).
Examples of preventive techniques are: the deep exhalation of the air in the lungs that would otherwise be used in the act of sneezing, holding the breath in while counting to ten, and crinkling the nose.
Proven methods to reduce sneezing generally advocate reducing interaction with irritants, such as keeping pets out of the house to avoid animal dander; ensuring the timely and continuous removal of dirt and dust particles through proper housekeeping; replacing filters for furnaces and air-handling units; employing air filtration devices and humidifiers; and staying away from industrial and agricultural zones.
In Ancient Greece, sneezes were believed to be prophetic signs from the gods. In 410 BC, for instance, the Athenian general Xenophon gave a dramatic oration exhorting his fellow soldiers to follow him to liberty or to death against the Persians. He spoke for an hour motivating his army and assuring them of a safe return to Athens until a soldier underscored his conclusion with a sneeze. Thinking that this sneeze was a favorable sign from the gods, the soldiers bowed before Xenophon and followed his command. Another divine moment of sneezing for the Greeks occurs in the story of Odysseus. When Odysseus returns home disguised as a beggar and talks with his waiting wife Penelope, she says to Odysseus, not knowing to whom she speaks, that "[her husband] will return safely to challenge her suitors"". At that moment, their son sneezes loudly and Penelope laughs with joy, reassured that it is a sign from the gods (Odyssey 17: 541-550).
In Europe, principally around the early Middle Ages, it was believed that one's life was in fact tied to one's breath - a belief reflected in the word "expire" (originally meaning "to exhale") gaining the additional meaning of "to come to an end" or "to die". This connection, coupled with the significant amount of breath expelled from the body during a sneeze, had likely led people to believe that sneezing could easily be fatal. This theory, if proven conclusively, could in turn explain the reasoning behind the traditional "God bless you" response to a sneeze, the origins of which are currently unclear. (see "Traditional Responses To A Sneeze" below for alternative explanations). Sir Raymond Henry Payne Crawfurd, for instance, the late registrar of the Royal College of Physicians, in his 1909 book "The Last Days of Charles II", states that, when the controversial monarch was on his deathbed, his medical attendants administered a concoction of cowslips and extract of ammonia to promote sneezing. However, it is not known if this promotion of sneezing was done to hasten his death (as coup de grace), or as an ultimate attempt at treatment.
In certain parts of Eastern Asia, particularly in Japanese culture and Vietnamese culture, a sneeze without an obvious cause was generally perceived as a sign that someone was talking about the sneezer at that very moment - a belief that is still depicted in present-day manga and anime. In China, Vietnam and Japan, for instance, there is a superstition that if talking behind someone's back causes the person being talked about to sneeze; as such, the sneezer can tell if something good is being said (one sneeze), something bad is being said (two sneezes in a row), or if this is a sign that they are about to catch a cold (multiple sneezes).
Parallel beliefs are known to exist around the world, particularly in contemporary Greek, Celtic, English, French, and Indian cultures. Similarly, in Nepal, sneezers are believed to be remembered by someone at that particular moment.
In Indian culture, especially in northern parts of India, it has been a common superstition that a sneeze taking place before the start of any work was a sign of impending bad interruption. It was thus customary to pause in order to drink water or break any work rhythm before resuming the job at hand in order to prevent any misfortune from occurring.
The practice among Islamic culture, in turn, has largely been based on various Prophetic traditions and the teachings of Muhammad. An example of this is Al-Bukhaari's narrations from Abu Hurayrah that the Islamic prophet once said:
When one of you sneezes, let him say, "Al-hamdu-Lillah" (Praise be to Allah), and let his brother or companion say to him, "Yarhamuk Allah" (May Allah have mercy on you). If he says, "Yarhamuk-Allah", then let [the sneezer] say, "Yahdeekum Allah wa yuslihu baalakum" (May Allah guide you and rectify your condition).
In English-speaking countries, the common verbal response to another person's sneeze is "Bless You" or the less common "Gesundheit" (the German word for "health"). There are several proposed origins for the use "Bless you" in the context of sneezing: