Why the Sun Makes Noses Go Ah, Ah, Ah ...choo

Painting of a child sneezing

Franz Lefler was one smart painter. In 'Frolicking Putti,' the 19th-century Czech artist illustrates the principle of the sun-triggered sneeze. Corbis hide caption

itoggle caption Corbis

We say "God bless you" to our listeners, and not just because they've sneezed. Science Out of the Box has received many queries from the NPR audience, but we're greedy — we want more. Send us your questions about puzzling scientific phenomena. We can't reply to all of the the submissions but will answer some of the questions in the weeks to come, like this one about nasal matters.

My four-year-old son, Logan, asked me the other day why he sneezes when he goes out into the bright sun. I sneeze similarly but my daughter and wife do not. Why do some people sneeze because of bright light? — Bill Bolduc

You know, Aristotle wondered the same thing.

"Why is it that one sneezes more after one has looked at the sun?" he asked in Problems, Book XXXIII, in a section called "Problems of the Nose." "Is it because the sun engenders heat and so causes movement, just as does tickling the nose with a feather?"

Sorry, Aristotle, you're sneezing up the wrong tree.

A sneeze provoked by sudden exposure to intense light is known as the photic sneeze reflex — or, more whimsically, ACHOO (for Autosomal-dominant Compelling Helio-Ophthalmic Outburst). It occurs in about a quarter of the population, according to researchers at the University of Texas. And it has nothing to do with the heat of the sun.

The reflex is probably the result of a malfunction in the fifth cranial nerve, called the trigeminal nerve. The trigeminal nerve is responsible for facial sensation and such motor functions as biting, swallowing and, yes, sneezing. Scientists believe that in some people, the trigeminal nerve is linked to the optic nerve, which transmits visual impulses to the brain. So when someone with ACHOO syndrome sees bright light, their optic nerve is overstimulated, triggering the trigeminal nerve. Ergo: achoo!

Researchers say some people can also sneeze when they suddenly breathe in cold air or eat strong mints, like Altoids. These sensations likely overstimulate other nerves close to the trigeminal nerve, launching the sneeze.

But why doesn't the family that sees together sneeze together? ACHOO syndrome is passed along genetically, as an "autosomal dominant trait." This means only one parent has to have a set of ACHOO genes pass on the trait. In your case, Mr. Bolduc, you probably have one set of DNA with the genes for ACHOO and one sneezeless set of DNA. Each of your children had a 50 percent chance of inheriting this trait from you. In your family's case, genetic inheritance worked just as expected: two kids, and exactly 50 percent of them sneeze when they look at the sun.

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