Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of the reflex. The models are simplified to show the idea.
Tap the buttons and see what happens.
Pick a trigger and watch the nerve endings light up. Then switch on a cold or allergy. What changes?
Send in different irritants, then add histamine (a cold or allergy) and compare how easily the nerve endings fire.
Tap dust, flower dust or pepper. Watch the alarm buttons light up!
Tap "Tickle" once, then several times fast. How many tickles does it take to set off a sneeze?
In this simplified model, each signal adds to an alarm level that slowly falls again; crossing the threshold triggers the model sneeze.
Tap "Tickle" lots of times, fast. Can you make the alarm ring?
Step through the program one stage at a time. When does the pressure build up?
Step through inhalation, eye closure, glottis closure and compression, then expulsion.
Tap "Next stage" and watch what your body does.
Drag the slider from the dark doorway into the sun. Then switch off "sun-sneezer" and try again.
Move from dark to bright light: only the photic sneezers (about 1 in 4) respond.
A picture of the idea: nobody knows exactly how the photic sneeze reflex works.
Sneeze with and without an elbow. Where do the droplets go?
Large droplets fall out close by; the smallest ride the warm cloud much farther. Compare with an elbow.
Tap "Sneeze!", then tap "Use elbow" and sneeze again. Which is better?
Simplified: real sneezes vary a lot in speed and distance.
A sneeze is your nose cleaning itself. Your body does it all by itself. You don't choose to sneeze!
Inside, your nose is soft and wet. It has tiny alarm buttons all over. Dust, flower dust, pepper, cold air or smoke can press the buttons. A cold can make them extra touchy.
The buttons send a message along a wire, up to your brain. One little tickle may not be enough. But a strong enough tickle, or lots of them, and the alarm rings!
Your body follows the same steps. A big breath in. Your eyes close. Your throat shuts and your tummy squeezes. Then whoosh! The air pushes the dust out.
A sneeze sprays lots of tiny drops. Some can float across the whole room! So sneeze into your elbow or a tissue, not your hand. Then wash your hands.
Sneezing is caused by irritation of the lining inside your nose. Dust, pollen, pepper, smoke, cold air or a cold virus tickles nerve endings in that lining.12 They send an alarm along the trigeminal nerve to a "sneeze centre" in your brainstem,145 which runs an automatic program: a deep breath in, eyes shut, throat closes, pressure builds, and then a burst of air shoots out through your nose and mouth.34 The job of all this is to sweep the irritant out of your nose.1
This page explains how the body works; it isn't medical advice.
A sneeze is a sudden, forceful burst of air from your lungs out through your nose and mouth.12 It's a reflex: an automatic action that your body does without you deciding to. You can't choose when a sneeze happens.1 Doctors sometimes call it sternutation.2 One allergist puts it neatly: a sneeze is to the nose what a cough is to the lungs.12
The inside of your nose is lined with a soft, damp layer called the mucous membrane. It warms and moistens the air you breathe and traps particles floating in it.3 The lining is full of nerve endings. When something irritates them, you get that tickly, itchy feeling, and a sneeze may follow.315
| Kind of trigger | Examples |
|---|---|
| Particles in the air | Dust, pollen, mold, pet dander, powders2 |
| Germs | Colds, flu and other infections21 |
| Irritating smells and air | Smoke, pollution, strong perfume, cold or dry air32 |
| Spicy things | Pepper and spicy food62 |
| Surprises | Bright light (in some people), strong emotions, some medicines122 |
Pepper works so well that scientists have used it to make volunteers sneeze for experiments.6 It's also why the trigger buttons in lab 1 include pepper, and why pollen season means sneezing season for many people. (Curious where that pollen comes from? See how pollination works.)
When you have an allergy or a cold, cells in your nose lining release chemicals such as histamine.315 Histamine makes blood vessels leak fluid (hello, runny nose) and makes the nerve endings extra sensitive, so they fire more easily.3 That's why antihistamine medicines can reduce allergy sneezing: they block histamine's effect.3 Allergies can affect the lungs too; see what causes asthma.
A sneeze has two halves: a sensing half and an action half.4
Sensing. The irritated nerve endings send signals along the trigeminal nerve, the large nerve that carries feeling from your face.14 The signals travel to the brainstem, the part at the base of your brain that runs automatic jobs like breathing. Experiments in cats found a "sneeze-evoking region" there, alongside the trigeminal nerve's own relay station.5 In humans the exact spot hasn't been pinned down, but the evidence points to the lowest part of the brainstem, the medulla.16
The sneeze centre works a bit like a smoke alarm: a faint tickle may not set it off, but once the signal is strong enough, the sneeze is on its way and there's no turning back. That "threshold" is a simple way to picture it, and it's how lab 2 works.
Action. Once triggered, the brainstem runs its usual sequence:3415
The blast carries irritants out of your nose.1 The fluid part of a sneeze is over in less than a fifth of a second.9 Sneezing may even give your nose a little "reset": in one lab study, the tiny hairs that sweep mucus along (cilia) beat faster after a sneeze.15
Sneezes are close cousins of other reflexes your body runs on autopilot, like hiccups and yawning. A cough is triggered from the throat and airways by a different nerve (the vagus nerve), while a sneeze starts in the nose.4
Closing your eyes is simply part of the reflex. It's possible to keep them open with a lot of effort, and no, your eyes won't pop out if you do.115
Step out of a dark room into sunshine and some people sneeze, sometimes several times. This is the photic sneeze reflex, nicknamed ACHOO syndrome (short for "autosomal dominant compelling helio-ophthalmic outburst", a name clearly chosen for the joke).12 It happens most when you go from dark to bright, like driving out of a tunnel.12
It's harmless in itself, but researchers point out that a sudden sneeze could be dangerous in some situations, such as driving out of a tunnel.11
You'll often read that a sneeze blasts out at 100 miles per hour. When scientists actually measured sneezes, they mostly found slower air, and big differences from sneeze to sneeze:
| Measurement | Speed |
|---|---|
| Popular claim | 100 mph (about 45 m/s) |
| High-speed filming of pepper sneezes (Tang et al., 2013)6 | up to 4.5 m/s (about 10 mph) |
| Peak speeds in MIT gas-cloud studies (Bourouiba)7 | 10 to 30 m/s (about 22 to 67 mph) |
So a sneeze is fast, but in most measurements well under 100 mph. The study with the slowest result said its sneezes were "not extremely high" in speed, unlike some older estimates.6
A single sneeze can send out up to 40,000 tiny droplets.1 MIT's high-speed videos show two parts: larger droplets that fall to the ground within about 2 meters, and a warm, moist cloud of air carrying the smaller ones much farther.108 Depending on the room, that cloud can travel about 7 to 8 meters (23 to 27 feet).7
Let it out, but cover it. Sneeze into a tissue or the crook of your elbow, not your hands, then wash your hands.14 Covering breaks up the sneeze cloud so germs don't spread as far.10 Holding a sneeze in completely isn't a good idea: in rare cases it can hurt your ears or throat.13
Often it's an allergy to something around you, such as pollen, dust or a pet, or a cold that's on its way.12 Sneezing that's not caused by an allergy usually stops once the cold or the trigger goes away.2 Newborn babies sneeze a lot, and that's normal.1 If sneezing is getting in the way of your life, or you also have a fever or trouble breathing, it's a good idea to see a doctor.2
Something irritates the soft, damp lining inside your nose, such as dust, pollen, pepper or a cold virus. Nerve endings there send an alarm along the trigeminal nerve to your brainstem, which runs an automatic program: a deep breath in, eyes shut, throat closes, pressure builds, then a burst of air out through your nose and mouth.
Plenty of sneeze triggers have nothing to do with germs: dust, pollen, pet dander, smoke, perfume, cold or dry air, spicy food and even bright light. An allergy, such as hay fever, is a common reason for lots of sneezing. If sneezing is bothering you or comes with a fever, talk to a doctor.
Usually just that something irritated your nose and your body is clearing it out. A sneeze is to your nose what a cough is to your lungs: a cleaning reflex.
Scientists aren't sure. It may be that the first sneezes didn't clear the irritant, or that the nose lining stays irritated, for example during an allergy.
No. A sneeze can change your heartbeat very briefly, but your heart doesn't stop.
Closing your eyes is part of the reflex, and it's very hard to override, but your eyes won't pop out if they stay open. You can't sneeze during REM sleep, because the muscles involved are temporarily inactive, though you can in lighter sleep.
That's the photic sneeze reflex, nicknamed ACHOO syndrome. One study found it in about one person in four; other estimates range from about 11% to 35%. It runs in families, and nobody knows exactly how it works.
More reflexes your body runs on autopilot: what causes hiccups and why people yawn.