A sneeze (or sternutation) is a semi-autonomous, convulsive expulsion of air from the lungs, most commonly caused by foreign particles irritating the nasal mucosa. Sneezing can further be triggered through sudden exposure to bright light, a particularly full stomach, or as a result of unexpected, climatic variations in one's environment such as rapid changes in temperature and humidity levels.
Sneezing is widely considered to be an agent of disease dissemination in many cultures.
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 cavity, resulting in a powerful release of air and bioparticles. The reason behind the particularly powerful nature of a sneeze is attributed to its involvement of not simply the nose and mouth, but numerous organs of the upper body – it is a reflectory response that involves the muscles of the face, throat, and chest.
An alternative cause of sternutation is sudden exposure to bright light - a bodily attribute known as the photic sneeze reflex. Up to 37 percent of individuals are believed to have this particular genetic trait, which is most commonly related to exposure to direct sunlight.
A rarer alternative 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 reflectory 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 any sneezing that would occur afterwards would take place with at least a partially awake state of mind.
While it is usually thought that the "function" of sneezing is to expel irritating particles, this may not be the case. In many people, a large part or even the whole of the expulsion of breath may be through the mouth rather than the nose, so that little or no expulsory action occurs. The fact that some individuals sneeze in response to the stimulus of cold air on the upper chest, or to bright sunlight, is not easily related to any expulsory finction. As in the case of yawning, itching, stretching and hiccups, functional aspects of sneezing have not been definitively identified.
The speed of human sternal release has been the source of much speculation, with the most conservative estimates placing it around 150 kilometers/hour (42 meters/second) or roughly 95 mph (135 feet/second), and the highest estimates -such as the JFK Health World Museum in Barrington, Illinois- which propose a speed as fast as 85% of the speed of sound, corresponding to approximately 1045 kilometers per hour (290 meters/second) or roughly 650 mph (950 feet/second).
Proven significant tips to reduce sneezing generally advocate reducing interaction with potential irritants, such as keeping pets out of the house to avoid animal dander; taking measures to ensure 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.
A similar linguistic approach has been taken with several other languages; in French, the sound "Atchoum!" is used; in Finnish "Atsiuh!"; in Swedish "Atjo"; in Danish "Atju!"; in Dutch "Hatsjoe!" or "Hatsjie!"; in Hebrew "Apchi!"; in German "Hatschie!"; in Hungarian "Hapci!"; in Polish, "Apsik!"; in Russian , "Apchkhi!"; in Turkish, "Hapşu!"; in Italian, "Etciù!"; in Spanish "¡Achú!"; in Portuguese, "Atchim!"; in Romanian "Hapciu!" ; in Filipino "Hatsing!"; in Japanese, "Hakushon!" and in [Tamil] "thummal" and in Korean, "Achee!". In Cypriot Greek, the word is "Apshoo!",incidentally also the name of a village, which is the cause of much mirth locally.
In the Hellenistic cultures of Classical Antiquity, 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 (Od. 17: 539-551).
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, a sneeze without an obvious cause was generally perceived as a sign that someone was talking about the sneezer at that very moment. In China and Japan, for instance, there is a superstition that if you talk behind someone's back, the person in question will 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 certain Islamic cultures, 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).Also, In Islam- it is considered good to sneeze. Sneezing brings a person the blessing of relief by releasing vapours that were trapped in the head which, if they were to remain there, would cause him pain and sickness. For this reason, Islam tells him to praise Allah for this blessing and for the fact that his body is still intact after this jolt that shook him like an earthquake.
Today, it is said mostly in the spirit of good manners and not saying thank you is considered rude.