When you feel a sneeze building up but it doesn’t seem to want to come out, you may have heard the advice to look into a light. The idea is that it’s supposed to trigger a reflex that makes you sneeze, and it turns out this happens to be based more in science than urban legend.
Seventeen to 35 percent of the population is estimated to be prone to the photic sneeze reflex (PSR), also known as—no joke—ACHOO (autosomal dominant compulsive helio-ophthalmic outbursts of sneezing) syndrome. PSR is reflexive sneezing set off by light, especially light from the sun. Why this occurs has been puzzling scientists for millennia. Aristotle suspected that it was the heat from the sun on someone’s nose that brought on the sneezing. Francis Bacon tested this theory 2,000 years later by walking into the sunlight with his eyes closed to find that heat alone wasn’t enough to cause the reaction. He surmised that when the light made someone's eyes water, that moisture leaked into the nose and irritated it to the point of sneezing. This hypothesis wasn’t too far-out, considering that sneezes are usually induced by irritants in the nose, but today scientists believe that the phenomenon has more to do with our brains than our noses.
You sneeze when your brain’s trigeminal nerve, the nerve responsible for your face’s sensations and movements, senses irritants like dust or hair in your nose. This nerve lies close to the optic nerve, which senses vision. If your optic nerve senses a sudden transition from dim light to bright light, it responds by constricting the eye’s pupils. For people affected by PSR, it’s believed that this signal gets misinterpreted by your trigeminal nerve, resulting in a sneeze. Individuals sensitive to light-induced sneezing have their parents to thank. The trait is autosomal-dominant, meaning it doesn’t show up on the X or Y chromosome. All it takes is one copy of the gene for the trait to be expressed, so if one parent has PSR there’s a 50/50 chance their child will too.
ACHOO may sound like a silly condition (for several reasons), but it can have some real-life implications. Because a sneeze is accompanied by momentary loss of vision, a sunny day could prove problematic for tightrope walkers, outdoor athletes, and even automobile drivers with the syndrome. One 1993 essay published in the journal Military Medicine brought up the point that this phenomenon could be life-threatening to fighter pilots. Fortunately, it was discovered that this could be avoided with a basic pair of sunglasses.