Why does the sun make some people sneeze?

If your find yourself succumbing to a sneeze — or a fit of sneezes — as you stare up into the sunlight, you might be among those suffering from a rare, but benign, syndrome.

It's not an allergy to the sun and it's not a cold.

It's the photic sneeze reflex, or Autosomal dominant Compelling Helio-Ophthalmic Outburst (ACHOO) syndrome.

While it may be annoying, it's also a very real phenomenon and relatively rare.

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.

Likewise, 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 triggered not only by irritants, but also 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.

There is some concern that photic sneeze reflex could cause problems for drivers or pilots, but for the most part it's believed to be harmless.

Louis Ptacek, a neurologist and human geneticist at University of California, San Francisco, has looked into photic sneeze reflex. 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 those who finds themselves sneezing at the sun, you may think it's a common reaction.

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 epilepsy 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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