The Scientist: Jeffrey Demain, M.D., Director of the Allergy, Asthma & Immunology Center of Alaska

The Answer: For some people, looking directly into the light can trigger a sneeze, either helping them force out one that won’t come, or catching them completely off guard. Between 18 and 35 percent of the population carries the gene for the photic sneeze reflex, or as it’s called by some clever otolaryngologists, Autosomal dominant Compulsive Helio-Ophthalmic Outbursts of sneezing (ACHOO).

Doctors aren’t entirely sure why light—from the sun or a light bulb—has this effect, but the top theories implicate the trigeminal nerve, which is the main sensory control panel for your face and mucous membranes. The optic nerve that connects your eyes to your brain runs really close to the trigeminal nerve, and it’s possible that when it is stimulated it accidentally tickles its neighbor, setting off a sneeze response in the mucous glands. Or, perhaps the light waves directly stimulate the branch of the trigeminal nerve that is involved with the eyes, resulting in the same end. Either way, the path seems to be light, eye, trigeminal nerve, brain, snot.

Why on Earth we have evolved this bizarre reflex is still a mystery. Animals sneeze to enhance their ability to smell, so it’s possible that there is some rudimentary protective adaptation at work here. Or, it’s just a weird quirk we keep passing through the generations.

The trait is genetically dominant, so if one or both of your parents sneezes in the sun, there’s a good chance you do, too. The characteristics of how the reflex plays out are often shared by family members, as well. Some people can use it as a way to release a single stubborn sneeze, while others walk out into sunlight and are uncontrollably bombarded by two, three or as many as a dozen sneezes in a row. That can be dangerous if you’re in the middle of driving, operating machinery or flying a jumbo jet to LAX.

You can prevent the reflex reaction by wearing polarized sunglasses that filter out light going directly into your eye. Or, game the system: Most people have a refractory period of up to 24 hours after an attack, during which it won’t happen again. So, purposely looking at the light to induce a sneezing fit will give you at least a few hours of safety in the sun.

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