You're not sick and you don't have allergies, yet you're still sneezing—and it always seems to happen when you're suddenly exposed to sunlight. What's up with that?
You could be among the 10 to 35 percent of people who have photic sneeze reflex (PSR). The condition is a genetic trait some people are born with, so if you don't already have it, you won't develop it as you age — unless, of course, you experience a genetic mutation or a superhero-style industrial accident.
Photic sneeze reflex also goes by the clever acronym ACHOO syndrome, which stands for Autosomal-dominant Compulsive Helio-Ophthalmic Outbursts of sneezing.
So why do some people have PSR? Well, science still hasn't ascertained exactly why some have the trait and some don't. But understanding how a sneeze starts in the first place might put us on the right track. As our BrainStuff video above explains, a sneeze starts when the trigeminal nerve detects irritation in the nose and sends an "eject" signal to the brain. One theory is that, in some people, the trigeminal nerve runs a little too close to the optic nerve, which senses light in the eyes. So when the pupil suddenly constricts in bright sunlight, a signal meant for the optic nerve is misinterpreted by trigeminal nerve as "something's in my nose" and the trigeminal nerve sends orders for a sneezing fit.
Scientists do at least know this sneeze reflex is not triggered by certain wavelengths of light, such as red light or violet light. Rather, it's the intensity of light that causes a sneeze to erupt. These findings were published in a 1993 Military Medicine Journal study exploring whether uncontrolled sneezing brought on by bright sunlight posed a danger to pilots during flight, and whether specific wavelengths had an effect.
As it turns out, figher pilots — and you — can simply wear sunglasses to prevent sneezing fits.