Did you know that 25% of people sneeze when exposed to the sun? The phenomenon of sun-sneezing is also known as Autosomal-Dominant Compelling Helio-Ophthalmic Outburst (ACHOO). In Aristotle’s Book XXXIII “Problems Concerning the Nose”, the Greek philosopher questions why “one sneezes more after one has looked at the sun,” and hypothesizes that the sun’s heat on his nose caused an “escape of” this ”breath.” Although his hypothesis was disproved by English philosopher Francis Bacon when he stepped out into the sun with his eyes closed, our best guess at the reason for this occurrence is a reflex mix-up in the brain.

Sneezing is a reflex, just like when the doctor taps your knee cap and you kick your leg upwards. A sneeze first begins when an irritation is sensed in the nose, which triggers the trigeminal nerve to send a signal to the part of the brain responsible for sneezing. The brain then commands the nose to release a fluid in order to remove the irritation. The muscles in your abdomen and chest are then told to take a quick, deep breath and let it out rapidly. By rapidly, I mean up to 100 miles per hour!

Another reflex in the body is the pupillary light reflex, more commonly known as the pupil-shrinking reflex. Encased in the iris of your eye is the pupil–the black part of your eye–and it’s important in controlling the amount of light that goes into your eyes. When light is shined into your eye, your pupil automatically shrinks in order to capture less light in comparison to having no light-exposure. Since so many nerves are packed especially close together in your brain, the nerves that control sneezing and pupil-shrinking most likely crossed over each other. This hypothesis was proposed by Henry Everett, who was a psychiatrist at Johns Hopkins University Hospital.

But why do only some people sneeze in the sun, while others don’t even know sun-sneezing exists? Well, ACHOO is an Autosomal dominant trait. Autosomal, meaning that the gene isn’t on the X or Y chromosome. Dominant, meaning that you only need one copy of that gene in order for it to be “visible” on a person. This means that if one parent has the sun-sneezing gene, there is a 50% chance that their child will as well.

Being a sun-sneezer isn’t anything dangerous, even though there were some speculations that it might endanger fighter pilots. Those who have it see it as an odd, but harmless little quirk. It even made its way into the Berenstain Bears franchise!