DEAR DOCTOR K:
Often when I step outside from a dark room into the sunlight, I sneeze. Why does this happen?
My colleague, Dr. Robert Shmerling, looked into this question once. I learned from him that there are many people like you, and even more who suddenly sneeze when they move suddenly from a warm environment to a cold one (or vice versa).
Why do we sneeze in the first place? It’s a complex reflex we all are born with, as it protects our lungs. It begins with nerve endings in the soft membranes that line the nose. When something irritating enters the nose with the air we breathe in and lands on the membranes, the nerve endings react. These triggers send messages to the throat, chest and abdomen to contract to forcefully rid the nasal passage of its contents. The sneeze forcibly ejects nasal air back out of the body (and away from the lungs).
It’s similar to vomiting when we’ve eaten something that the stomach really doesn’t like. The irritating substance that came down through the mouth gets ejected right back out through the mouth.
If sneezing is supposed to protect the lungs, why would anyone sneeze when he or she steps into the sunlight? How does sunlight threaten the lungs? It doesn’t. Something has gone wrong with the reflex: It is triggered for no good reason.
No one knows why some people sneeze at the sight of bright light. It’s possible that bright light triggers the other nerves involved in sneezing. Maybe the light flooding into the eye, or squinting in reaction to bright light, causes a crossed signal of sorts, making the body think a sneeze is in order.
Another unknown is why the muscles of the face, including the eyelid muscles, are also involved in sneezing. Scientists think the eyes might shut during a sneeze to keep out flying particles. (Though the idea that you cannot keep your eyes open during a sneeze is a myth. If you tried hard enough, you could probably do it.)
Humans actually expel more material from our mouths when we sneeze than from our noses. That’s why it’s important to cover both your mouth and nose when sneezing.
Dr. Shmerling tells me that he once saw an Internet video of a woman sneezing every time she walked from the shadowy spot of a room into the bright light near a window. Apparently she was using this sensitivity to light — called “photic” sneezing — to audition for an allergy medication commercial. She found a profitable use for her “talent.”
Photic sneezing is harmless and can actually be useful. Ever have that annoying “need-to-sneeze” feeling, but the sneeze just won’t come? Look briefly at a light. Often that will encourage the sneeze.
If you really want to avoid sneezing next time you come out of a dark space, keep your sunglasses handy. One study found they could prevent the photic sneeze.