Black pepper, dust, or pollen are a few of the answers you might expect if you ask people what makes them sneeze. But it likely won't be long into your poll that you get a more mysterious response: sunlight.
As the sneezing season dies down for those of us who are plagued by pollen in the spring, it is just getting started for those with a not-so-rare genetic condition called photic sneeze reflex. Sudden exposure to any kind of bright light can trigger sneezing for people who have the condition, but sunlight is the usual culprit.
This is not a kind of sun allergy, although "sun allergies" do exist, and the only symptom is sneezing. Much about the disorder remains a mystery, but one study published back in 1964 found that just under a quarter of Johns Hopkins medical students had it, and other studies suggest that as many as one-third of the population suffers from these sun-spurred sneezes.
This curious phenomenon has been going on for millennia and has even captured the interest of some of history's most famous minds. Aristotle posed the question, "Why doth the heat of the sun provoke sneezing?" in his work The Book of Problems, though experts are now fairly certain that it is not the heat of sunlight that provokes sneezing, but the brightness.
When you finally decide to emerge from the dark cave that is your bedroom and step into the morning sun, for example, your eyes adjust to the change in light by making their pupils larger or smaller. It's your optic nerve that controls this routine adjustment, and this nerve happens to be located very close to your trigeminal nerve, the one that causes sneezing when you, say, get pepper up your nose. According to an article from Scientific American, experts suspect that when signals go through the optic nerve in response to bright light, they get picked up by the nearby trigeminal nerve and misinterpreted as signaling an irritant in the nose.
The exact gene responsible for the photic sneeze reflex has yet to be identified, but both anecdotal information and a study conducted in the 1960s show the trait is inherited in a particular manner, known as autosomal dominant. About half of a sun-sneezer's children will also have this quirky condition even if the other parent does not have it. And if both parents have it, all of their children are almost certain to have it, too. Fortunately, of all of the genetic conditions you could end up with, this is a fairly harmless one. Though you might get called a weirdo for it.
Photo by mcfarlandmo | Flickr
