A sneeze seems like a simple thing—especially since it’s so common—but it’s a complex neurological phenomenon that occurs for a multitude of different reasons.



Sneezing, technically known as sternutation, is triggered by anything that irritates the nerve endings in the mucous membranes of the sinuses. The irritation sets off a reaction that sends signals to the brain stem, which controls most of the body’s essential functions, like breathing. The brain then tells the body to leap into action. The eyes close, the whole body contracts, and the glottis—a slit-like opening between the vocal cords—snaps shut as the body forcefully inhales. That’s followed by a powerful exhalation that, because the glottis is closed, has to go up through the nasal passage—the “achoo.” The exhalation expels the irritants.



A sneeze can contain 40,000 droplets (often packed with bacterial and viral particles), rushing out at an average speed of 100 miles per hour, but can range up to 600 mph. That’s a fast and furious way to spread disease, which is why doctors urge people to sneeze into their elbow.



But there’s still much that’s not known about what happens in the brain and why the sneeze mechanism developed.



Sneezing is most commonly associated with allergies, and colds, but can also be a non-allergic reaction to dust, perfume, mold, smoke, or even a change in temperature or the moisture content of the air. And, sneezing “may be seen with exercise, with a full stomach, and even after sex,” says Andy Nish, chief of allergy and asthma for the Northeast Georgia Physicians Group in Gainesville, Georgia.



Why after sex? Interestingly, erectile tissue is found not only in the genital area, but also in the mucous membranes of the nose, says Nish. The association between sneezing and sex was first written about in the 1880s, but was not more fully described until the 1970s. Although there seems to be a physiological cause for sneezing after orgasm, it’s still considered to be relatively rare.



Sneezing after a good meal has been reported even less often, but it still managed to get its own medical term—“snatiation”—for sneezing combined with satiation.



Some people sneeze in reaction to sunlight or bright light. Nish says the photic sneeze reflex is a genetic trait that affects about a quarter of humans. The reflex is also known as the ACHOO (autosomal dominant compelling helio-ophthalmic outburst) syndrome.



Almost everyone sneezes at some point—and often in clusters, which is likely a way for the body to fully clear the irritant, says Nish. “But an inability to sneeze over a prolonged period of time might raise concern for problems with the nervous system, since intact nerve pathways to and from the brain are necessary,” he says.



Sneezing is only rarely associated with serious disease. There have been reports of sneezing accompanying seizures, and preceding a stroke. So-called intractable—or incessant—sneezing has been identified as a potential psychiatric disorder in adolescent girls. Contrary to popular myth, sneezing does not cause the heart to stop.



It’s okay to try to prevent a sneeze, but it might lead to an unpleasant feeling, says Nish. If you feel a sneeze coming on and try to suppress or contain it, that may temporarily increase the pressure in your ears and upper airway, causing a popping sensation.



Humans are not the only creatures to sneeze. Nish says that cats, dogs, chickens and iguanas all sneeze. “Most of these sneezes are probably for the same reason that we sneeze, but in iguanas, the sneeze is actually an important means of getting rid of excess salt that its body no longer needs,” he says.



Throughout human history, sneezing has been interpreted as both good and bad. “The Greeks and the Romans took sneezing as a sign of wellness and expressed their good wishes to the person who sneezed using the phrase ‘Live long’ or ‘May Jupiter bless you,’” wrote Turkish researchers Murat Songu and Cemal Cingi in an overview article about the sneeze.



In the Talmud, it is considered to be a favorable omen if someone sneezes while praying, and it is common in China and Japan to believe that if a person sneezes without a reason, this means that somebody else is talking about him, according to Songu and Cingi.



Pagans thought that sneezing got rid of the devil, but that it also created an opening for “invasion by Satan and evil spirits, or even caused part of one’s soul being ‘thrown out of the body,’” say Songu and Cingi, who trace the phrase “God bless you,” to this Pagan belief.



While you might bless someone for sneezing, “the sneeze is actually a protective mechanism for them, but maybe not for you,” says Nish.

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