Why do some people sneeze when they look at the sun?

Have you ever stepped out of a dim subway station into the sunshine and felt that telltale tickle in your nose—the unmistakable need to sneeze? Sneezing in the sudden presence of bright light, especially sunlight, is a phenomenon known as sun sneezing or the photic sneeze reflex. It affects anywhere between 10 to 35 percent of the population, depending on which survey you read. A 1987 study in the Cleveland Clinic Journal of Medicine, for example, estimated its prevalence at 17 to 35 percent of the population. A 1983 study in Human Heredity found a 24 percent prevalence among 460 blood donors.

Although most of us aren’t sun sneezers, it’s a common enough curiosity to get lots of people wondering: What’s going on here?

There’s still no hard evidence to fully explain sun sneezing, but scientific and popular attention has largely focused on a particular hypothesis proposed in 1964 by Henry Everett when he was a psychiatrist at Johns Hopkins University Hospital. According to the hypothesis, the photic sneeze reflex is caused by a confusion of nerve signals in pathways very near one another. Since sneezing is such a sudden and involuntary reflex, the cause is probably located in the nervous system, which is capable of transmitting signals very quickly.

Researchers suspect that two important reflexes may play a key role in sun sneezing. The first is the pupillary light reflex. In this reflex, bright light entering the eyes sends signals along the optic nerve to the brain, which sends signals back to the eyes to constrict the pupils—a means of adjusting to differently lit environments. The second is the sneeze reflex, in which a cranial nerve called the trigeminal nerve detects a tickling in the nose and alerts the brain, which in turn stimulates the chest, nose, mouth and other muscles involved in sneezing.

For most of us, the pathways involved in these two reflexes—though physically close—do not directly interact. But in sun sneezers, the hypothesis claims, one pathway stimulates the other. The result? Exposure to bright light sends a signal to the brain to constrict the pupils, as usual, but the crossed wires rouse a sneeze as well. “While this is an interesting hypothesis, there’s no data supporting it or any other hypothesis for that matter,” said Louis Ptácek via email. Ptácek is a neurologist at the University of California, San Francisco who studies the photic sneeze reflex.

An alternative hypothesis attempts to explain sun sneezing and other strange sneezing behaviors by singling out the medulla oblongata, a part of the brainstem that helps regulate many involuntary processes, including breathing, heart rate and sneezing. Believe it or not, some people always sneeze after eating a large meal—a condition called snatiation—while others sneeze during orgasm. Constriction of the pupils, the feeling of being stuffed, and orgasm are exactly the kind of reflexes mediated by the medulla. The implication is that, for some individuals, all these signals flowing to the same area of the brainstem might be getting a bit mixed up.

The specific genes responsible for sun sneezing have not yet been identified, but scientists can guess your chances of having the photic sneeze reflex because of the way it’s inherited—it’s an autosomal dominant trait. This means that if just one of your parents has one copy of the culprit gene, you have a 50 percent chance of being a sun sneezer. In 1978, a group of witty eggheads pounced on the new genetic evidence as an opportunity to create the following acronym for the photic sneeze reflex: Autosomal-dominant Compelling Helio-Ophthalmic Outburst syndrome—ACHOO!

Although most sun sneezers accept their condition as an odd but harmless quirk, there’s been plenty of speculation about harmful consequences. According to a 1993 issue of Military Medicine, sun sneezing could threaten combat pilots by interfering with their vision, leading to potentially fatal situations. Similar fears have been raised about drivers emerging from dark tunnels into bright light. Some researchers have even expressed concerns over baseball players searching the sunny skies for a fly ball.

So much for the gloom and doom. Are there any benefits to the photic sneeze reflex—anything at all? Some have theorized that sun sneezing is a gift of evolution, passed down from our cavemen forefathers. According to the theory, after hanging out in dark, dirty caves all day, our ancestors’ noses and throats would become full of dust and need a little forceful cleaning. When the cavemen emerged from their dwellings into the sun, they would sneeze, thereby clearing their noses and throats of cave must. Unfortunately, this theory is an old wives’ tale, about as verifiable as the Area 51 conspiracy.

The photic sneeze reflex has largely eluded our attempts to understand it, remaining a mystery for neuroscientists and sun sneezers alike. “There is so little known about the photic sneeze reflex that I think the jury is completely out at this point,” said Ptácek.

Sun sneezing is, however, becoming more well known. The photic sneeze reflex recently attracted the attention of 23andMe, a company that will analyze the DNA in your saliva to predict your chances of having certain heritable traits and diseases. Sun sneezing also found its way into the popular Berenstain Bears series of children’s books. And anecdotal evidence suggests that some people take advantage of the reflex, training themselves to hasten an imminent sneeze by directing their attention to the sun. There’s even an online support group for those with the photic sneeze reflex.

If you would like to help scientists specify the genetic factors involved in sun sneezing, you can apply to participate in ongoing research at the University of California, San Francisco, where Ptácek and his colleagues work. “We’ve collected some interesting families,” Ptácek said, but they will need many more volunteers before they find something conclusive.