SCIENCE FACT: Why Do People Sneeze? Everything You Need to Know By: Anne Marie Helmenstine, Ph.D

Everyone sneezes, but there are different reasons why we do it. The technical term for sneezing is sternutation. It's an involuntary, convulsive expulsion of air from the lungs through the mouth and nose. Although it may be embarrassing, sneezing is beneficial. The primary purpose of a sneeze is to expel foreign particles or irritants from the nasal mucosa.


How Sneezing Works
Usually, sneezing occurs when irritants aren't caught by nasal hairs and touch the nasal mucosa. Irritation may also occur from an infection or allergic reaction. Motor neurons in the nasal passage send an impulse to the brain via the trigeminal nerve. The brain responds with a reflex stimulus that contracts muscles in the diaphragm, pharynx, larnyx, mouth, and face. In the mouth, the soft palate and uvula depress while the back of the tongue rises. Air is convulsively expelled from the lungs, but because the passage to the mouth is only partially closed, a sneeze exits both the nose and mouth.


You cannot sneeze while sleeping because of REM atonia, in which motor neurons stop relaying reflex signals to the brain. However, an irritant may wake you up to sneeze. A sneeze does not temporarily stop your heart or cause it to skip a beat. The heart rhythm may slow slightly from vagus nerve stimulation as you take a deep breath, but the effect is minor.


Sneezing in Bright Light

About one in three people sneezes when first exposed to bright light. Imgorthand / Getty Images


If bright lights make you sneeze, you're not alone. Scientists estimate 18 to 35 percent of people experience photic sneezing. The photic sneeze response or PSR is an autosomal dominant trait, which accounts for its other name: Autosomal Dominant Compelling Helio-Ophthalmic Outburst Syndrome or ACHOO (seriously). If you experience photic sneezing, one or both of your parents experienced it too! Sneezing in response to bright light does not indicate an allergy to the Sun. Scientists think the signal sent to the brain to shrink pupils in response to light may cross paths with the signal to sneeze.



More Reasons for Sneezes



Plucking eyebrows can stimulate facial nerves and cause a sneeze. PeopleImages / Getty Images


A reaction to irritants or bright light are common reasons for sneezing, but there are other causes. Some people sneeze when they feel a cold draft. Others sneeze when they pluck their eyebrows. Sneezing immediately following a big meal is called snatiation. Snatiation, like photic sneezing, is an autosomal dominant (inherited) trait. Sneezing can also occur either at the beginning or climax of sexual arousal. Scientists speculate sexual sneezing indicates erectile tissue in the nose may react to stimulation, possibly to enhance pheromone reception.

Sneezing and Your Eyes


     No, sneezing with your eyes open will not make them pop out. LindaMarieB / Getty Images
It's true you generally can't keep your eyes open when you sneeze. Cranial nerves link both the eyes and the nose to the brain, so the stimulus to sneeze also triggers the eyelids to close.
    However, the reason for the response isn't to protect your eyes from popping out of your head! Sneezing is powerful, but there isn't any muscle behind the eye that could contract to eject your peepers. Mythbusters proved it's possible to keep your eyes open during a sneeze (although not easy) and that if you sneeze with your eyes open, you won't lose them.


Sneezing More Than Once
It's perfectly normal to sneeze twice or multiple times in a row. This is because it may take more than one sneeze to dislodge and eject irritating particles. How many times you sneeze in a row varies from person to person and depends on the reason for the sneeze.


Sneezing in Animals

This tiger is sneezing underwater. Buck Forester / Getty Images


Humans aren't the only creatures that sneeze. Other mammals sneeze, such as cats and dogs. Some non-mammalian vertebrates sneeze, such as iguanas and chickens. Sneezing serves the same purpose as in humans, plus it may be used for communication. For example, African wild dogs sneeze to vote on whether or not the pack should hunt.


What Happens When You Hold in a Sneeze?

If you stifle a sneeze, pressurized air enters the Eustachian tube and may rupture your eardrum. LEONELLO CALVETTI / Getty Images


While holding in a sneeze won't eject your eyeballs, you can still hurt yourself. According to Dr. Allison Woodall, an audiologist at the University of Arkansas for Medical Sciences, holding your nose and mouth closed to stifle a sneeze can cause vertigo, rupture your eardrums, and lead to hearing loss. The pressure from the sneeze affects the Eustachian tube and middle ear. It can also injure your diaphragm, rupture blood vessels in your eyes, and even weaken or rupture blood vessels in your brain! It's best to let a sneeze out.


How to Stop a Sneeze

Pinching the bridge of your nose may help prevent a sneeze. travenian / Getty Images


While you shouldn't stifle a sneeze, you may be able to stop one before it happens. Of course, the easiest way is to avoid triggers, such as pollen, pet dander, sunlight, overeating, dust, and infections. Good housekeeping can reduce particulates in the home. Filters on vacuums, heaters, and air conditioners also help.


If you feel a sneeze coming on, try a physical preventative method:

Gently pinch the bridge of your nose until the urge to sneeze passes.
Press your tongue on the roof of your mouth.
Hold your breath and count to ten.
Deeply exhale the air in your lungs so it won't be available to support a sneeze.
Look away from bright light (if you're a photic sneezer).


If you can't stop the sneeze, you should use a tissue or at the very least turn away from others. According to Mayo Clinic, a sneeze expels mucous, irritants, and infectious agents at a speed of 30 to 40 miles per hour up to 100 miles per hour. Residue from the sneeze may travel up to 20 feet and include 100,000 germs.


Key Points About Sneezing
Sneezing or sternutation is a beneficial involuntary process characterized by forcible expulsion of air from the lungs through the mouth and nose.
The primary reason for sneezing is to remove irritants from the nasal mucosa. However, sneezing may also be a reaction to sudden bright light, overeating, or sexual arousal.
Stifling a sneeze is not recommended. It can damage your hearing, lead to an ear infection, and rupture blood vessels in the eyes and brain.
It is possible to keep your eyes open while sneezing. If you do so, there is no risk of popping your eyes out.
Sneezing does not stop your heart.

Sources
Nonaka S, Unno T, Ohta Y, Mori S (March 1990). "Sneeze-evoking region within the brainstem". Brain Res. 511 (2): 265–70. Walker,
Reena H., et al. "Sneeze to leave: African wild dogs (Lycaon pictus) use variable quorum thresholds facilitated by sneezes in collective decisions." Proc. R. Soc. B. Vol. 284. No. 1862. The Royal Society, 2017.