Walk out of a dark building into bright sunlight and, for some people, the response is immediate: achoo!

It is called the photic sneeze reflex, and estimates suggest it affects roughly 10 to 35 percent of people.

Scientists do not know exactly why it happens, but the leading explanation involves crossed signals between nerves. Bright light entering the eyes triggers a response through the optic nerve, including constriction of the pupils. 

In people with the photic sneeze reflex, that sudden burst of activity may also stimulate the nearby trigeminal nerve, which carries sensations from the face and nose and is involved in sneezing. The result may be a sneeze even though nothing is actually irritating the nose.

The tendency also runs in families and appears to have a strong genetic component.

And scientists have given it an acronym almost too perfect to be true: ACHOO syndrome, short for Autosomal Dominant Compelling Helio-Ophthalmic Outburst syndrome.