This is an accepted version of this page A sneeze (also known as 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.

In addition to covering the mouth, looking down is also recommended to change the direction of the droplets spread and avoid high concentration in the human breathing heights.

Sneezing with the mouth closed does expel mucus through the nose but is not recommended because it creates a very high pressure in the head and is potentially harmful.

Sneezing cannot occur during sleep due to REM atonia – a bodily state where motor neurons are not stimulated and reflex signals are not relayed to the brain.

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 neural regions involved in the sneeze reflex are located in the brainstem along the ventromedial part of the spinal trigeminal nucleus and the adjacent pontine-medullary lateral reticular formation.

[5] Other than irritating foreign particles, allergies or possible illness, another stimulus is sudden exposure to bright light – a condition known as photic sneeze reflex (PSR).

Walking out of a dark building into sunshine may trigger PSR, or the ACHOO (autosomal dominant compulsive helio-ophthalmic outbursts of sneezing) syndrome as it is also called.

[8] While generally harmless in healthy individuals, sneezes spread disease through the infectious aerosol droplets, commonly ranging from 0.5 to 5 μm.

[9] To reduce the possibility of thus spreading disease (such as the flu), one holds the forearm, the inside of the elbow, a tissue or a handkerchief in front of one's mouth and nose when sneezing.

Using one's hand for that purpose has recently fallen into disuse[10] as it is considered inappropriate, since it promotes spreading germs through human contact (such as handshaking) or by commonly touched objects (most notably doorknobs).

[12] 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; air filtration devices and humidifiers; and staying away from industrial and agricultural zones.

It may be because this belief survived through the centuries, that in certain parts of Greece today, when someone is asserting something and the listener sneezes promptly at the end of the assertion, the former responds "bless you and I am speaking the truth", or "bless you and here is the truth" ("γεια σου κι αλήθεια λέω", ya sou ki alithia leo, or "γεια σου και να κι η αλήθεια", ya sou ke na ki i alithia).

[citation needed] Such a theory could explain the reasoning behind the traditional English phrase, "God bless you", in response to a sneeze, the origins of which are not entirely clear.

Sir Raymond Henry Payne Crawfurd, for instance, the 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.

This can be seen in the Book of Songs (a collection of Chinese poems)[22] in ancient China as early as 1000 BC, and in Japan this belief is still depicted in present-day manga and anime.

[citation needed] Parallel beliefs are known to exist around the world, particularly in contemporary Greek, Slavic, Celtic, English, French, and Indian cultures.

Doctors suspect that the phenomenon might arise from a case of crossed wires in the autonomic nervous system, which regulates a number of functions in the body, including, but not limited to, rousing the genitals during sexual arousal.