Why Do We Sneeze? The Sneeze Reflex, Sun Sneezing and the Risk of Holding It In

First a tickle in the nose, then your eyes close by themselves and — “achoo!” A sneeze is an ordinary moment in all our lives, yet it contains one of the body’s most powerful and ancient defence reflexes. And some people sneeze not because anything irritates their nose, but simply because they look at the sun.

The guardians of the nose

The nose is not just for smelling; it is the lungs’ first line of defence. Its inner lining is covered with a sticky layer of mucus and tiny hair-like cilia that trap most of the dust, pollen and germs in the air we breathe.

The lining is also packed with nerve endings linked to the trigeminal nerve, which handles touch and pain in the face. When something irritates these endings enough, the alarm goes off.

A sneeze, step by step

The sneeze reflex is an involuntary sequence of movements lasting a few seconds:

1. Irritation: a trigger such as dust, pollen or inflammation from a virus stimulates nerve endings in the nose.
2. Signal: the trigeminal nerve carries the message to the sneeze centre in the brainstem.
3. Preparation: the brainstem makes you take a deep breath — that familiar “ah… ah…” moment.
4. Pressure: the throat closes briefly while the muscles of the chest, abdomen and diaphragm contract, building high pressure in the lungs.
5. Explosion: the throat suddenly opens and air bursts out through the nose and mouth. The tongue rises slightly to direct part of the air through the nose, and the eyelids close reflexively.

All of this fires many tightly linked muscles at once — which is why a big sneeze can leave a slight strain in your neck, back or stomach muscles.

How powerful is a sneeze?

You often read online that sneezes reach 160 kilometres (100 miles) per hour. Measurements with high-speed cameras suggest that figure is exaggerated: real speeds vary a lot from person to person but generally fall well below it.

What is really surprising is how far a sneeze spreads. When Lydia Bourouiba and colleagues at MIT studied coughs and sneezes with high-speed cameras, they found that:

• large droplets fall to the ground within a few metres;
• but small droplets are carried inside a warm, moist cloud of air released with the sneeze;
• this cloud lets droplets travel much further than expected and stay suspended for a while.

The findings matter for understanding how illnesses spread — and explain why covering your mouth and nose with a tissue or the inside of your elbow is so strongly recommended.

What triggers a sneeze?

• Allergens and dust: pollen, house dust mites and pet dander are the most common triggers. In people with allergies, the immune system overreacts and releases histamine, which stimulates nerve endings and causes repeated sneezing.
• Infections: cold and flu viruses inflame the inside of the nose. Sneezing is the body’s attempt to expel the virus — and, unfortunately, the virus’s way of reaching new people.
• Irritants: black pepper, smoke, strong smells and perfume.
• Cold air: stepping suddenly into the cold can stimulate the nasal lining.
• Plucking eyebrows: this stimulates branches of the trigeminal nerve in the face, which in some people turns into a sneeze.

Sneezing at the sun

If you sneeze when you step from a dark room into a sunny street, you are not alone. This is the photic sneeze reflex, also nicknamed “ACHOO syndrome”. Studies suggest roughly a quarter of people experience it.

The reflex is known to be inherited, running in families, though its cause is not fully understood. The leading explanation is that the optic nerve, which signals the pupils to constrict, and the trigeminal nerve, which carries sensation from the nose, work very close together in the brain. The strong signal from bright light may partly be misread as “the nose is irritated”.

The reflex is usually harmless, though some experts point out it could pose a risk for pilots and drivers at high speed when emerging from a tunnel.

Is holding in a sneeze dangerous?

In a crowded room it can be tempting to stifle a sneeze by pinching your nose and closing your mouth. But that traps the sneeze’s explosive energy inside the airways with nowhere to go.

Rare cases show this can have serious consequences. In 2023, doctors in Scotland reported a man who developed a tear in his windpipe after trying to sneeze with his nose and mouth closed. Injuries to the throat and eardrum have also been recorded in the past.

Pressing a finger under your nose or pushing your tongue against the roof of your mouth before a sneeze starts can sometimes prevent it. Once it has begun, though, it is best to let it out safely.

Why do we say “bless you”?

Wishing someone well after a sneeze is a tradition in many cultures. In English people say “Bless you!”, in German “Gesundheit!” (health), and in Turkish “Çok yaşa!” (live long), to which the sneezer replies “Sen de gör!” (may you see it too).

The origins of the custom are uncertain. Some historians link it to times when epidemics were common and a sneeze could be seen as a sign of illness. Whatever its roots, a blessing makes no scientific difference — but it lives on as a kind habit.

In short

A sneeze is a powerful defence reflex triggered by irritated nerve endings in the nose and run by the trigeminal nerve and the brainstem. A deep breath, a closed throat and an explosive exhalation expel foreign material from the nose. About a quarter of people also sneeze when they look at the sun. Because forcibly holding in a sneeze can, rarely, cause injury, it is best to sneeze into a tissue or your elbow.