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Why Do Some People Sneeze In Sunlight?


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 Biology   |   Health   |   Environmental   |   Science
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You walk outside on a sunny day and BAM! A sneezing fit hits you. But why? Join Cristen as she explains the photic sneeze reflex.

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OK, pop quiz: When you’ve been locked in the trunk of some smuggler’s Buick in total darkness for a seven-hour trip across the border, and suddenly they pop the trunk open and that noonday desert sun is beating down on your face: Do you start sneezing? If so... why is that?

Why do some people sneeze when suddenly exposed to bright sunlight?

If you’re one of the 10 to 35 percent of people who start sneezing when exposed to a bright light, you may be the proud owner of a photic sneeze reflex (PSR), also known as ACHOO syndrome – and yes, you bet your nose hair that’s an acronym!

It stands for Autosomal dominant Compulsive Helio-Ophthalmic Outbursts of sneezing. Don’t worry; we’ll get back to some of those terms in a minute.

So what do we know about the photic sneeze today? Well, one study in 1993 found that it’s not any particular wavelength of light that causes the problem – so you can’t protect against it just by filtering out red light, or violet light or anything in between.

Instead, it has to do with the intensity of the light. So the good news is that if you want to avoid the photic sneeze, there’s a pretty easy solution that works most of the time: plain old sunglasses.

Also, more good news: If you’re one of the 70 to 90 percent of people who are watching this video and thinking, “What are they talking about? I only sneeze when I do the cinnamon challenge!” YOU CAN REST EASY. You can’t acquire the photic sneeze reflex after birth – it’s a genetic trait.

But the question of WHY remains! No absolutely conclusive answer has been reached, but there’s one very interesting theory: Photic sneezing happens because some wires are crossed in your head. Normally, a sneeze is your nose’s version of the “Passenger Seat Eject” button in James Bond’s car.

Sneezing begins when irritation in the nose is detected by an organ called the trigeminal nerve. When that nerve senses an unpleasant foreign substance, it sends the “eject” signal to the brain. But inside your head, the trigeminal nerve runs very close to the optic nerve, which senses light in the eyes.

So let’s say the eyes of a photic sneezer are suddenly exposed to bright sunlight. The optic nerve detects this, and sends another reflex signal: Ah! Too bright! Shrink my pupils!

But because the optic nerve runs a little too close to the trigeminal nerve, the pupil constriction signal also gets interpreted as a “something’s in my nose” signal, and BAM! Sneezing fit.

Bonus: Did you know that sneezing is also known as “sternutation”? I guess that’s for people who have a hard time pronouncing the word “sneeze.”


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