Sometimes, a funny thing happens when Dr. Roberta Pagon looks directly into the sun. She sneezes. Not just once though, but usually three times.

She's not the only one in her family who sneezes when sunlight hits their face -- two of her children also react by sneezing three times in a row. And now a grandchild does it, but only sneezes once.

Odd coincidence? Not really, says Pagon, a pediatrician in the division of genetic medicine at Seattle Children's Hospital. This tendency to sneeze at sunlight is known as the "photic sneeze reflex," and it's hereditary.

Not only is there a genetic basis for "sun sneezing," Pagon says the number of times people sneeze in response to light also appears to run in families.

This scientific discovery happened in a very unscientific way. Pagon and her genetics colleagues were sitting at the same table during a birth defects conference when the conversation shifted to discussing the sun and sneezing. Much to their surprise, they learned that 4 out of 10 of them were affected by this strange reaction. "One person said it was common for people in his family to sneeze five times; in my family it was three times, and another person said once," recalls Pagon.

They quickly did what years of medical training had taught them -- they coined an acronym for it: ACHOO syndrome, or Autosomal Dominant Compelling Helio-Ophthalmic Outburst. Autosomal dominant stands for the way the 'sneeze gene' is inherited; an individual has a 50-50 chance of passing this trait on to a child. "Compelling" because it was well, interesting, or at the very least, quirky. "Helio" meant sun, "ophthalmic" meant eye, and "outburst" the end result. They even wrote up a paper about the peculiar phenomenon.

Although this reaction might seem unusual, it's not that uncommon. By one estimate,18% to 35% of people get a tickling sensation in their nose when their eyes meet intense sunlight. Some folks may also get this weird response to bright artificial light, such as the eye doctor's or dentist's light or a photographer's flash.

Driving out of a tunnel may trigger the reflex or leaving a movie theatre on a summer's day, says Nicolas Langer, PhD, a neuropsychology researcher the University of Zurich, who has studied the photic sneeze reflex. Often "it's just the change of a dark location to a bright (very sun exposed) location" that brings on the reflex, he explains.

In his research published in the journal PLoS ONE, Langer compared the visual reactions of 10 sun sneezers to 10 people without this reflex. Volunteers were hooked up to an EEG machine so the scientists could measure their brain and neural responses when exposed to bright light.

Their results suggest that "the 'photic sneeze reflex' is not a classical reflex that occurs only at a brainstem or spinal cord level," says Langer. "It seems to involve other cortical areas" of the brain.

As for why it happens, Langer offers two theories. One is that the visual system in the brain is more sensitive in photic sneezers. When it gets overstimulated by light, this co-activates regions representing the nose (the somatosensory system), which then triggers a sneeze.

A second possibility is that two nerves (the optic nerve and trigeminal nerve) are too close together in photic sneezers. Langer says light may cause stimulation of the optic nerve in the eye, which then coactivates the trigeminal nerve in the face, and results in an achoo reaction.

Solar sneezes could be an occupational hazard if you're an airplane pilot, baseball outfielder, sky diver, punt-return specialist, or high-wire acrobat. But for roughly one of out four people, it's merely something curious that makes them a little different from the next guy or gal.

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