A Tiny Wind Tunnel in Your Head
A sneeze is your body's way of cleaning out your nose. When dust or other bits irritate the lining inside, the body fires air out of the lungs in a sudden, explosive burst. The goal is to push out the mucus that holds the irritants.
The clever part happens at the back of your mouth. The soft palate and the little uvula drop, while the back of the tongue rises to partly close the way to the mouth. This creates a venturi, much like the one in a car's carburetor. Air from the lungs speeds up through the mouth and leaves low pressure at the back of the nose. So air rushes in through the front of the nose, and the mucus is launched out of the mouth.
Sneezing with your mouth shut still clears the nose, but it is not advised. It builds very high pressure in the head and could cause harm.
How Your Brain Runs a Sneeze
A sneeze is a two-part reflex. First comes the sensing part. Special nerve cells in the lining of the nose or sinuses pick up an irritant and send signals along the trigeminal nerve to the brainstem.
Then comes the action part. The medulla oblongata sends orders to muscle groups all over the body. The diaphragm and the muscles between the ribs pull in a big breath. Then the belly and chest muscles squeeze hard and force the air out in one burst.
Your eyelids also shut by reflex during that burst. Still, the popular idea that you cannot sneeze with your eyes open is wrong, because the reflex can be held back. And you cannot sneeze in your sleep at all. During REM atonia, motor nerves are not stimulated and reflex signals are not passed on. A strong enough tickle can wake you, though, and then you sneeze once you are at least partly awake.
Sneezing at the Sun
Some people sneeze when they suddenly face bright light. Walking out of a dark building into sunshine is a classic trigger. This is called the photic sneeze reflex, and it has a playful second name: ACHOO syndrome, short for autosomal dominant compulsive helio-ophthalmic outbursts of sneezing.
It runs in families as an autosomal dominant trait, and it affects 18 to 35% of people. An even odder trigger shows up in some people: a very full stomach right after a big meal. This one is called snatiation, and it is passed down in families the same way.
Light has fooled scientists before. In 1884, biologist Henry Walter Bates wrote about how light affects sneezing. He believed people could only sneeze when they felt in control of their whole surroundings, so he decided nobody could sneeze in the dark. That idea was later shown to be wrong.
40,000 Droplets in One Blast
A single sneeze can throw out 40,000 droplets, most of them between 0.5 and 5 micrometers across. These droplets and aerosols are how sneezes can help spread illnesses like the flu.
For a long time, the farthest anyone had seen a sneeze cloud travel was 0.6 meters, with a top speed of about 4.5 meters per second, or roughly 10 miles per hour. Then in 2020, sneezes were recorded sending clouds as far as 8 meters, about 26 feet.
The advice is to sneeze into your forearm, the inside of your elbow, a tissue or a handkerchief. Using your hand has fallen out of favor, because germs then spread through handshakes and shared things like doorknobs. Looking down while you sneeze also helps, since it sends droplets away from the height where people breathe. Holding a sneeze in is a bad idea too. One computer simulation found it creates a burst of pressure about 24 times that of a normal sneeze.
Chickens, Iguanas and Pack Decisions
Humans are not the only sneezers, and not even mammals are. Cats, dogs, chickens and iguanas all sneeze. African wild dogs take it further. They use sneezing to communicate, especially when the pack is deciding together whether or not to go hunting.
Sneezing can also simply feel good. Some people enjoy sneezes so much that they would not want to stop them. Holly Boyer of the University of Minnesota explains that muscle tension builds up in your chest, and when you sneeze and those muscles relax, the pressure is released. There is also some evidence that the body releases endorphins. Because a sneeze happens in one quick burst, so does the pleasure.
If you ever do want to stop one, tickling the roof of your mouth with your tongue can do it. And however you sneeze, in English the sound is usually spelled achoo.
