A sneeze is your body's way of blasting something annoying out of your nose. When dust, pollen or a sprinkle of pepper drifts up your nostrils, it tickles tiny nerve endings inside. Those nerves send a message to your brain that says, roughly, get this out of here. The tickle has to build up to a certain point first, like filling a cup until it overflows. Once it crosses that line, your brain fires off a reflex you cannot stop: you take a big breath in, your chest squeezes, and then air rushes out in an explosive blast, carrying the dust and pollen with it. That blast leaves your nose at around 50 kilometres an hour, fast enough to spray tiny droplets across a room. Some people even sneeze when they step into bright sunshine, which is just their wiring getting a little crossed. Add irritants in the simulator and watch the meter fill.
Most people think a sneeze is something you simply do, like a cough you choose. In fact it is an automatic reflex that fires only once irritation crosses a threshold, and once started it cannot be stopped.
What's actually happening
Everyone knows the feeling: a tickle deep in the nose, a sharp intake of breath, the helpless pause, and then the explosion you could not have stopped if your life depended on it. A sneeze feels almost violent, and it should, because it is your body forcibly ejecting something it has decided does not belong in your airway. It is one of the most powerful reflexes you have, and like all reflexes it bypasses your conscious control entirely.
The story starts with irritation. The lining of your nose is laced with sensitive nerve endings, branches of the trigeminal nerve, whose job is to keep watch for anything that should not be there. When dust, pollen, pepper, smoke or even a stray hair brushes against them, they fire off a warning signal. But one small tickle is not enough. The signals have to accumulate, building toward a threshold, rather like water filling a cup. A faint irritation might make your nose itch and go no further. A strong one, or many small ones stacking up, pushes the total past the trigger point. The moment it does, a region of your brainstem sometimes called the sneeze centre takes over and commits to the whole sequence; once it starts, there is no calling it off.
What follows is beautifully coordinated. You draw in a deep breath, your vocal cords snap shut to seal the airway, and pressure builds in your chest. Then everything releases at once: the cords fly open and air erupts out through your nose and mouth at around 50 kilometres an hour, blasting the irritant clear and spraying a fine mist of droplets that can travel several metres. It is a superb cleaning mechanism, which is also exactly why sneezes spread colds and flu so efficiently. There is one charming quirk in the wiring. Roughly a quarter of people sneeze when they suddenly step into bright light, a quirk passed down in families called the photic sneeze reflex. Nobody is completely sure why, but the leading idea is simple crossed wiring: the nerve that reacts to bright light runs close to the nerve that triggers sneezing, and a strong signal in one spills over into the other.
A sneeze is a reflex that clears the nose: irritation builds to a threshold, then fires a blast of tens of km/h you cannot call back.
- 1Next time you feel a sneeze building, notice the slow tickle that rises before it fires, that is the irritation climbing toward the threshold.
- 2On a bright day, glance from shade toward the sky (not at the Sun) and see whether you are one of the people who sneezes at light.
- 3Notice how a sneeze always comes with a deep breath in first, that inhale is the reflex loading up the pressure before the blast.
Common questions
Because a sneeze is a reflex controlled by the brainstem, not a conscious action. Once the irritation crosses the trigger threshold, the whole sequence, breath in, pressure up, blast out, runs automatically and cannot be halted.
This is the photic sneeze reflex, which runs in families and affects roughly a quarter of people. The likely cause is crossed wiring: the nerve reacting to sudden bright light sits close to the one that triggers sneezing, so the signal spills over.
Because the blast sprays a fine mist of tiny droplets at around 50 km/h, which can travel several metres. If you are carrying a cold or flu virus, that mist carries it into the air around you.