Why does the sun make some people sneeze?

You walk out into the sunlight after a movie, and you sneeze once. Or maybe you don't sneeze just once, but the sun brings about a fit of sneezing.

It's not an allergy to the sun and it's not a cold: It's the photic sneeze reflex.

The human body has all sorts of reflexes — your stomach expands when you eat; if you touch a hot stove, you move your hand away; your pupils dilate when it's dark.

Sneezing is a reflex that we've developed to help clean out our airways. If an irritant — like pepper or pollen or dust — gets in our nose, the phrenic nerve tells the diaphragm to contract and vigorously expel air. We call that a sneeze.

For people who have photic sneeze reflex, sneezes are not only triggered by irritants, they're also triggered by bright light.

So why does this reflex exist? Even though it's a question that's been posed for millennia (Aristotle wondered about sun-triggered sneezing in his "Book of Problems"), scientists still aren't sure.

Since photic sneeze reflex doesn't cause any harm, not much research has been done into the cause. But Louis Ptacek, a neurologist and human geneticist at University of California, San Francisco, has looked into photic sneeze reflex because he thinks it can help us better understand epilepsy and other movement disorders.

Ptacek has found that photic sneeze reflex is genetic and that it runs in families. "If I have photic sneeze reflex, there's a 50/50 chance that each child will inherit the gene from me," Ptacek said.

If you're one of the people who finds themselves sneezing at the sun, you may think this happens to everyone. You might exhort your friends to look at the light to help them sneeze. But you're actually in the minority — only 10 percent of people have this reflex.

"It doesn't seem to have any benefit as pulling our hand away from hot stove does," said Ptacek. "We know that if some patients with epiliepsy are exposed to a flashing strobe light, that can induce seizure. It is my thought that if we can find out what causes photic sneeze reflex it might teach us about some of these other reflex phenomena."

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