Photic sneeze reflex (ACHOO)

The photic sneeze reflex (ACHOO) is the urge to sneeze when stepping into bright light. This variant is linked to how likely you are to have it.

What the photic sneeze reflex is

Somewhere between one in six and one in three people sneeze when they move suddenly into bright light. It is harmless, entirely involuntary and runs in families, and it has an acronym that is almost certainly a joke made to fit: ACHOO, for autosomal dominant compelling helio-ophthalmic outburst.

The mechanism is not settled. The leading explanation is crosstalk between the optic and trigeminal nerve pathways, so that a strong visual signal spills over into the reflex that triggers sneezing, which is why the reflex can also be set off by other trigeminal stimuli in some people.

How strong the evidence is

This variant, near ZEB2, was identified in the same 2010 web-based study of roughly 10,000 people that found the asparagus association, and a second signal near NR2F2 was reported alongside it. This site's evidence file links five publications to the variant.

It is an association marker rather than a demonstrated cause: the variant sits near a gene involved in nervous system development, and no mechanism connecting it to the reflex has been established. The region has also turned up in genome-wide studies of other traits, which is a reminder that a single position can carry several associations.

How to read your result

Lightly, and empirically. This is one of the few traits on the site you can check in seconds by stepping outdoors on a bright day. If the result does not match your experience, your experience is right.

Where it occasionally matters

Mostly nowhere, but there are two documented contexts. Pilots and drivers emerging from tunnels have been discussed as a minor safety consideration, since a sneeze closes the eyes for a moment. And the reflex can be triggered during eye examinations and some procedures, which is why ophthalmologists occasionally ask about it.

What each rs10427255 genotype means

rs10427255 has three possible genotypes: CC, CT and TT.

Likely a sun sneezer (rs10427255 CC). You carry two copies of the version most linked to the photic sneeze reflex, the urge to sneeze when you step into bright sunlight. If you've ever sneezed walking out of a dark room into the sun, this may be why.

Maybe a sun sneezer (rs10427255 CT). You carry one copy of the variant tied to the photic sneeze reflex (sneezing in bright light), so you may or may not be a light-triggered sneezer. It's a harmless quirk either way.

Unlikely to sun-sneeze (rs10427255 TT). You don't carry the variant most tied to the photic sneeze reflex, so you're less likely to sneeze when stepping into bright light. It's a harmless trait that varies a lot between people.

Evidence & sources

Across global populations, about 21% of people carry two copies of this variant, and about 30% carry none.

This variant appears in 5 published genetic studies: PubMed 20585627 · PubMed 30617275 · PubMed 30899065 · PubMed 34446935 · PubMed 36477530.

Common questions

Why do I sneeze when I look at the sun? The photic sneeze reflex, probably caused by signal crossover between the optic and trigeminal nerves. It is harmless and inherited in families, and this variant is one of two markers associated with it.

Is the photic sneeze reflex dangerous? No, though a sneeze briefly closes the eyes, which has been raised as a small consideration for pilots and for drivers leaving tunnels. It needs no treatment.

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