What is ACHOO syndrome?
The short version: some people sneeze when they move suddenly from dark into bright light, especially sunlight. It's usually harmless, sometimes risky, and still not fully explained.
The name
Doctors usually call it the photic sneeze reflex: photic meaning light. In 1978 a group of four doctors, Collie, Pagon, Hall and Shokeir, gave it a catchier label, ACHOO syndrome, short for Autosomal dominant Compelling Helio-Ophthalmic Outburst.[1] Yes, it was on purpose.
You'll also see the letters expanded as Autosomal Cholinergic Helio-Ophthalmic Outburst.[9] Same sneeze, different backronym.
People have been writing about light-triggered sneezing for a long time. A 2025 review of the research describes a literature going back centuries, including a German-language self-report on one person's own sneezing, published in 1904.[10]
What sets it off
Sunlight is the usual trigger, but a camera flash or any bright light can do it. It isn't an allergy.[7] What seems to matter is the change from dark to light, more than how bright the light is.[7] Testing for aviation research found the same thing: a change in light intensity set off the sneezing, not a particular colour of light.[5]
One man with the reflex logged his sneezes and his light exposure for 30 days. He recorded 82 sneezing episodes, and the light around him had typically jumped about tenfold just before a sneeze. In the lab, more than 150 bright light tests gave him a tickle but never a sneeze.[19] That's one person, so it proves little on its own, but it fits a familiar story: the reflex is hard to produce on demand, and a doctor usually recognises it from your description rather than from a test.[6]
Why does it happen?
Nobody knows for sure.[7] Researchers have proposed several explanations, and none has been proven yet.[10]
- Crossed wires. One idea, called optic-trigeminal summation, is crosstalk in the midbrain between the nerve that carries vision (the optic nerve) and the nerve that carries feeling from the face (the trigeminal nerve). That could explain the prickle in the nose that many photic sneezers describe.[10]
- A spreading reflex. Bright light already makes your pupils shrink and can make your eyes water. Another idea, parasympathetic generalisation, is that the same burst of nerve activity spreads to nearby branches that control the nose.[10, 9]
- A more sensitive system. A third idea is that photic sneezers' nerve systems are simply more sensitive, so intense light sets off nasal nerves along with others.[10, 7]
In 2010 a Swiss team recorded brain activity (EEG) in photic sneezers and non-sneezers. The sneezers' visual cortex, the part of the brain that handles sight, reacted more strongly to visual stimuli. A stronger prickle in the nose went with more activity in areas that handle touch and body sensation. The authors concluded that the reflex isn't purely a brainstem reflex: parts of the brain's cortex seem to be involved.[9]
Does it run in families?
Often, yes. Family trees in early studies fit an autosomal dominant pattern, which is where the "A" in ACHOO comes from.[1, 2] With that pattern, a child of a parent who has it has a 50 percent chance of inheriting it.[6]
The genetics turn out to be messier than one gene. Large genetic studies have linked the reflex to several spots in our DNA: a 23andMe study found variants near two genes, ZEB2 and NR2F2,[14] and later studies in China and Japan found overlapping and new locations.[3, 12] The Chinese study's authors described the trait as polygenic, meaning many genes each play a small part.[3] None of this gives you a test for it.
How common is it?
It depends a lot on how you ask, and who.
| Where | Who was asked | Result |
|---|---|---|
| Britain | A sample of the population | 25 percent[2] |
| China | 3,417 people in a genetic study | 25.6 percent[3] |
| Germany | 1,042 patients at a university ear, nose and throat clinic | 57 percent reported regular light-induced sneezing[11] |
| Japan | More than 11,000 people answering a web-based survey | 3.1 to 3.2 percent[12, 13] |
Why so wide? The German figure dropped back in line with other studies once people who only sneezed occasionally were left out, and it came from clinic patients rather than the general public.[11] The Japanese authors thought their low figure may reflect a questionnaire that didn't pick the reflex up well.[13] Aviation researchers quoted a range of 18 to 35 percent.[5] The NCBI summary adds a useful wrinkle: about one in four people who already get a prickle in the nose will sneeze in sunlight, but "pure" photic sneezing is far less common.[6]
Our honest summary: somewhere between a few percent and about a third of people, depending on the study. "About one in four" is the figure you'll see most often.
If light-triggered sneezing is bothering you, or you're not sure that's what it is, talk to your GP. This site can't diagnose anything.