WEBVTT - How Can Sunlight Make You Sneeze?

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<v Speaker 1>Welcome to brain Stuff from how Stuff Works. Hey, brain stuff.

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<v Speaker 1>Lauren vog obam here with today's question, why do some

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<v Speaker 1>people sneeze when suddenly exposed to bright light like sunlight?

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<v Speaker 1>Only about ten to of people do this. I happen

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<v Speaker 1>to be one of them, And no, we're not allergic

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<v Speaker 1>to the sun. We may be the proud owner of

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<v Speaker 1>a photic sneeze reflex or PSR, also known as at

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<v Speaker 1>Chew syndrome. And yes, that's an acronym. It stands for

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<v Speaker 1>autosomal dominant compulsive helio ophthalmic outbursts of sneezing. And don't worry,

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<v Speaker 1>we'll get back to some of those terms in a minute.

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<v Speaker 1>People have been trying to figure out the cause of

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<v Speaker 1>the photic sneeze for thousands of years. The ancient Greek

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<v Speaker 1>philosopher Aristotle or an unknown author writing in his name, asked,

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<v Speaker 1>why does the heat of the sun provoke sneezing and

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<v Speaker 1>not the heat of the fire? Good question, Aristotle or

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<v Speaker 1>fake Aristotle's answer to himself was because the heat of

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<v Speaker 1>the sun doth dissolve but not consume, and therefore the

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<v Speaker 1>vapor dissolved is expelled by sneezing, So His theory is

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<v Speaker 1>that the heat of the sun makes the inside of

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<v Speaker 1>your nose wet, and you've got to purge that liquid

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<v Speaker 1>with a good sneeze. Unfortunately, in the seventeenth century, the

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<v Speaker 1>English science pioneer Francis Bacon decided to put Aristotle to

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<v Speaker 1>the test. He showed that if a photic sneezer walks

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<v Speaker 1>into the sun with his or her eyes closed, the

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<v Speaker 1>reflex is not triggered. Therefore, we know it's not caused

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<v Speaker 1>by the heat. It's caused by your eyes reacting to light.

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<v Speaker 1>So what do we know about the photic sneeze today, Well,

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<v Speaker 1>one study in found that it's not any particular wavelength

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<v Speaker 1>of light that causes the problem, So you can't protect

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<v Speaker 1>against it by just filtering out red light or violet

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<v Speaker 1>light or anything in between. Instead, it has to do

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<v Speaker 1>with the intensity of the light. So the good news

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<v Speaker 1>is that if you want to avoid the photic sneeze,

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<v Speaker 1>there's a pretty easy solution that works most of the time,

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<v Speaker 1>plain old sunglasses. Also more good news. If you're one

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<v Speaker 1>of the seventy to nine of people listening and thinking,

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<v Speaker 1>what on earth is she talking about? Am I going

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<v Speaker 1>to catch this sneeze? Disease. Someday you can rest easy.

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<v Speaker 1>You can't acquire the photic sneeze reflex after birth. It's

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<v Speaker 1>a genetic trait. A study in the nineteen sixties found

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<v Speaker 1>that this trait is inherited through an autosomal dominant mechanism.

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<v Speaker 1>This means that it doesn't matter which of your parents

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<v Speaker 1>has the photox sneezing gene. If one of them has it,

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<v Speaker 1>you have a fifty percent chance of being a photo

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<v Speaker 1>sneezer yourself. But the question of why remains. No absolutely

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<v Speaker 1>conclusive answer has been reached, But there's one very interesting theory.

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<v Speaker 1>Photox sneezing happens because some wires are crossed in your head. Normally,

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<v Speaker 1>a sneeze is your noses version of the passenger eject

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<v Speaker 1>seat in James Bond's car. Sneezing begins when irritation in

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<v Speaker 1>the nose is detected by an organ called the trigeminal nerve.

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<v Speaker 1>When that nerve senses an unpleasant foreign substance, it sends

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<v Speaker 1>the eject signal to the brain. But inside your head,

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<v Speaker 1>this nerve runs very close to the optic nerve, which

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<v Speaker 1>senses light in the eyes. So let's say the eyes

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<v Speaker 1>of a photo sneezer are suddenly exposed to bright sunlight.

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<v Speaker 1>The optic nerve detects this and sends another reflix signal

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<v Speaker 1>too bright drink by pupils. But if the optic nerve

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<v Speaker 1>runs a little too close to the trigeminal nerve, the

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<v Speaker 1>pupil constriction signal also gets interpreted as a something's in

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<v Speaker 1>my nose signal and Bam sneezing fit. Today's episode was

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<v Speaker 1>written by Joe McCormick and produced by Tyler Clang. For

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<v Speaker 1>more on this and lots of other topics about our

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<v Speaker 1>weird bodies, visit our home planet, how Stuff Works dot com.